text:
STS-51-F

STS-51-F (also known as Spacelab 2) was the 19th flight of NASA's Space Shuttle program and the eighth flight of Space Shuttle Challenger. It launched from Kennedy Space Center, Florida, on July 29, 1985, and landed eight days later on August 6, 1985.

While STS-51-F's primary payload was the Spacelab 2 laboratory module, the payload that received the most publicity was the Carbonated Beverage Dispenser Evaluation, which was an experiment in which both Coca-Cola and Pepsi tried to make their carbonated drinks available to astronauts. A helium-cooled infrared telescope (IRT) was also flown on this mission, and while it did have some problems, it observed 60% of the galactic plane in infrared light.

During launch, Challenger experienced multiple sensor failures in its Engine 1 Center SSME engine, which led to it shutting down and the shuttle had to perform an "Abort to Orbit" (ATO) emergency procedure. It is the only Shuttle mission to have carried out an abort after launching. As a result of the ATO, the mission was carried out at a slightly lower orbital altitude.

Crew notes
As with previous Spacelab missions, the crew was divided between two 12-hour shifts. Acton, Bridges and Henize made up the "Red Team" while Bartoe, England and Musgrave comprised the "Blue Team"; commander Fullerton could take either shift when needed. Challenger carried two Extravehicular Mobility Units (EMU) in the event of an emergency spacewalk, which would have been performed by England and Musgrave.

Launch

STS-51-F's first launch attempt on July 12, 1985, was halted with the countdown at T−3 seconds after main engine ignition, when a malfunction of the number two RS-25 coolant valve caused an automatic launch abort. Challenger launched successfully on its second attempt on July 29, 1985, at 17:00 p.m. EDT, after a delay of 1 hour 37 minutes due to a problem with the table maintenance block update uplink.

At 3 minutes 31 seconds into the ascent, one of the center engine's two high-pressure fuel turbopump turbine discharge temperature sensors failed. Two minutes and twelve seconds later, the second sensor failed, causing the shutdown of the center engine. This was the only in-flight RS-25 failure of the Space Shuttle program. Approximately 8 minutes into the flight, one of the same temperature sensors in the right engine failed, and the remaining right-engine temperature sensor displayed readings near the redline for engine shutdown. Booster Systems Engineer Jenny M. Howard acted quickly to recommend that the crew inhibit any further automatic RS-25 shutdowns based on readings from the remaining sensors, preventing the potential shutdown of a second engine and a possible abort mode that may have resulted in the loss of crew and vehicle (LOCV).

The failed RS-25 resulted in an Abort to Orbit (ATO) trajectory, whereby the shuttle achieved a lower-than-planned orbital altitude. The plan had been for a 385 km by 382 km orbit, but the mission was carried out at 265 km by 262 km.

Mission summary

STS-51-F's primary payload was the laboratory module Spacelab 2. A special part of the modular Spacelab system, the "igloo", which was located at the head of a three-pallet train, provided on-site support to instruments mounted on pallets. The main mission objective was to verify performance of Spacelab systems, determine the interface capability of the orbiter, and measure the environment created by the spacecraft. Experiments covered life sciences, plasma physics, astronomy, high-energy astrophysics, solar physics, atmospheric physics and technology research. Despite mission replanning necessitated by Challenger's abort to orbit trajectory, the Spacelab mission was declared a success.

The flight marked the first time the European Space Agency (ESA) Instrument Pointing System (IPS) was tested in orbit. This unique pointing instrument was designed with an accuracy of one arcsecond. Initially, some problems were experienced when it was commanded to track the Sun, but a series of software fixes were made and the problem was corrected. In addition, Anthony W. England became the second amateur radio operator to transmit from space during the mission.

Spacelab Infrared Telescope
The Spacelab Infrared Telescope (IRT) was also flown on the mission. The IRT was a 15.2 cm aperture helium-cooled infrared telescope, observing light between wavelengths of 1.7 to 118 μm. It was thought heat emissions from the Shuttle would corrupt long-wavelength data, however it still returned useful astronomical data. Another problem was that a piece of mylar insulation broke loose and floated in the line-of-sight of the telescope. IRT collected infrared data on 60% of the galactic plane. (see also List of largest infrared telescopes) A later space mission that experienced a stray light problem from debris was Gaia astrometry spacecraft launch in 2013 by the ESA - the source of the stray light was later identified as the fibers of the sunshield, protruding beyond the edges of the shield.

Other payloads
The Plasma Diagnostics Package (PDP), which had been previously flown on STS-3, made its return on the mission, and was part of a set of plasma physics experiments designed to study the Earth's ionosphere. During the third day of the mission, it was grappled out of the payload bay by the Remote Manipulator System (Canadarm) and released for six hours. During this time, Challenger maneuvered around the PDP as part of a targeted proximity operations exercise. The PDP was successfully grappled by the Canadarm and returned to the payload bay at the beginning of the fourth day of the mission.

In a heavily publicized marketing experiment, astronauts aboard STS-51-F drank carbonated beverages from specially designed cans from Cola Wars competitors Coca-Cola and Pepsi. According to Acton, after Coke developed its experimental dispenser for an earlier shuttle flight, Pepsi insisted to American president Ronald Reagan that Coke should not be the first cola in space. The experiment was delayed until Pepsi could develop its own system, and the two companies' products were assigned to STS-51-F.

Blue Team tested Coke, and Red Team tested Pepsi. As part of the experiment, each team was photographed with the cola logo. Acton said that while the sophisticated Coke system "dispensed soda kind of like what we're used to drinking on Earth", the Pepsi can was a shaving cream can with the Pepsi logo on a paper wrapper, which "dispensed soda filled with bubbles" that was "not very drinkable". Acton said that when he gives speeches in schools, audiences are much more interested in hearing about the cola experiment than in solar physics. Post-flight, the astronauts revealed that they preferred Tang, in part because it could be mixed on-orbit with existing chilled-water supplies, whereas there was no dedicated refrigeration equipment on board to chill the cans, which also fizzed excessively in microgravity.

In an experiment during the mission, thruster rockets were fired at a point over Tasmania and also above Boston to create two "holes" – plasma depletion regions – in the ionosphere. A worldwide group of geophysicists collaborated with the observations made from Spacelab 2.

Landing
Challenger landed at Edwards Air Force Base, California, on August 6, 1985, at 12:45:26 p.m. PDT. Its rollout distance was 2612 m. The mission had been extended by 17 orbits for additional payload activities due to the Abort to Orbit. The orbiter arrived back at Kennedy Space Center on August 11, 1985.

Mission insignia
The mission insignia was designed by Houston, Texas artist Skip Bradley. is depicted ascending toward the heavens in search of new knowledge in the field of solar and stellar astronomy, with its Spacelab 2 payload. The constellations Leo and Orion are shown in the positions they were in relative to the Sun during the flight. The nineteen stars indicate that the mission is the 19th shuttle flight.

Legacy
One of the purposes of the mission was to test how suitable the Shuttle was for conducting infrared observations, and the IRT was operated on this mission. However, the orbiter was found to have some draw-backs for infrared astronomy, and this led to later infrared telescopes being free-flying from the Shuttle orbiter.

title: STS-51-F


text:
Classical period (music)

The Classical Period was an era of classical music between roughly 1750 and 1820.

The classical period falls between the Baroque and Romantic periods. Classical music has a lighter, clearer texture than Baroque music but a more varying use of musical form, which is, in simpler terms, the rhythm and organization of any given piece of music. It is mainly homophonic, using a clear melody line over a subordinate chordal accompaniment, but counterpoint was by no means forgotten, especially in liturgical vocal music and, later in the period, secular instrumental music. It also makes use of style galant which emphasizes light elegance in place of the Baroque's dignified seriousness and impressive grandeur. Variety and contrast within a piece became more pronounced than before, and the orchestra increased in size, range, and power.

The harpsichord was replaced as the main keyboard instrument by the piano (or fortepiano). Unlike the harpsichord, which plucks strings with quills, pianos strike the strings with leather-covered hammers when the keys are pressed, which enables the performer to play louder or softer (hence the original name "fortepiano," literally "loud soft") and play with more expression; in contrast, the force with which a performer plays the harpsichord keys does not change the sound. Instrumental music was considered important by Classical period composers. The main kinds of instrumental music were the sonata, trio, string quartet, quintet, symphony (performed by an orchestra), and the solo concerto, which featured a virtuoso solo performer playing a solo work for violin, piano, flute, or another instrument, accompanied by an orchestra. Vocal music, such as songs for a singer and piano (notably the work of Schubert), choral works, and opera (a staged dramatic work for singers and orchestra), was also important during this period.

The best-known composers from this period are Joseph Haydn, Wolfgang Amadeus Mozart, Ludwig van Beethoven, and Franz Schubert; other names in this period include: Carl Philipp Emanuel Bach, Johann Christian Bach, Luigi Boccherini, Domenico Cimarosa, Joseph Martin Kraus, Muzio Clementi, Christoph Willibald Gluck, Carl Ditters von Dittersdorf, André Grétry, Pierre-Alexandre Monsigny, Leopold Mozart, Michael Haydn, Giovanni Paisiello, Johann Baptist Wanhal, François-André Danican Philidor, Niccolò Piccinni, Antonio Salieri, Etienne Nicolas Mehul, Georg Christoph Wagenseil, Johann Simon Mayr, Georg Matthias Monn, Johann Gottlieb Graun, Carl Heinrich Graun, Franz Benda, Georg Anton Benda, Johann Georg Albrechtsberger, Mauro Giuliani, Christian Cannabich and the Chevalier de Saint-Georges. Beethoven is regarded either as a Romantic composer or a Classical period composer who was part of the transition to the Romantic era. Schubert is also a transitional figure, as were Johann Nepomuk Hummel, Luigi Cherubini, Gaspare Spontini, Gioachino Rossini, Carl Maria von Weber, Jan Ladislav Dussek and Niccolò Paganini. The period is sometimes referred to as the era of Viennese Classicism (Wiener Klassik), since Gluck, Haydn, Salieri, Mozart, Beethoven, and Schubert all worked in Vienna.

Classicism
In the middle of the 18th century, Europe began to move toward a new style in architecture, literature, and the arts, generally known as Neoclassicism. This style sought to emulate the ideals of Classical antiquity, especially those of Classical Greece. Classical music used formality and emphasis on order and hierarchy and a "clearer", "cleaner" style that used clearer divisions between parts (notably a clear, single melody accompanied by chords), brighter contrasts, and "tone colors" (achieved by the use of dynamic changes and modulations to more keys). In contrast with the richly layered music of the Baroque era, Classical music moved towards simplicity rather than complexity. In addition, the typical size of orchestras began to increase, giving orchestras a more powerful sound.

The remarkable development of ideas in "natural philosophy" had already established itself in the public consciousness. In particular, Newton's physics was taken as a paradigm: structures should be well-founded in axioms and be both well-articulated and orderly. This taste for structural clarity began to affect music, which moved away from the layered polyphony of the Baroque period toward a style known as homophony, in which the melody is played over a subordinate harmony. This move meant that chords became a much more prevalent feature of music, even if they interrupted the melodic smoothness of a single part. As a result, the tonal structure of a piece of music became more audible.

The new style was also encouraged by changes in the economic order and social structure. As the 18th century progressed well, the nobility became the primary patrons of instrumental music, while public taste increasingly preferred lighter, funny comic operas. This led to changes in the way music was performed, the most crucial of which was the move to standard instrumental groups and the reduction in the importance of the continuo—the rhythmic and harmonic groundwork of a piece of music, typically played by a keyboard (harpsichord or organ) and usually accompanied by a varied group of bass instruments, including cello, double bass, bass viol, and theorbo. One way to trace the decline of the continuo and its figured chords is to examine the disappearance of the term obbligato, meaning a mandatory instrumental part in a work of chamber music. In Baroque compositions, additional instruments could be added to the continuo group according to the group or leader's preference; in Classical compositions, all parts were specifically noted, though not always notated, so the term "obbligato" became redundant. By 1800, basso continuo was practically extinct, except for the occasional use of a pipe organ continuo part in a religious Mass in the early 1800s.

Economic changes also had the effect of altering the balance of availability and quality of musicians. While in the late Baroque, a major composer would have the entire musical resources of a town to draw on, the musical forces available at an aristocratic hunting lodge or small court were smaller and more fixed in their level of ability. This was a spur to having simpler parts for ensemble musicians to play, and in the case of a resident virtuoso group, a spur to writing spectacular, idiomatic parts for certain instruments, as in the case of the Mannheim orchestra, or virtuoso solo parts for particularly skilled violinists or flutists. In addition, the appetite by audiences for a continual supply of new music carried over from the Baroque. This meant that works had to be performable with, at best, one or two rehearsals. Even after 1790, Mozart writes about "the rehearsal," with the implication that his concerts would have only one rehearsal.

Since there was a greater emphasis on a single melodic line, there was greater emphasis on notating that line for dynamics and phrasing. This contrasts with the Baroque era, when melodies were typically written with no dynamics, phrasing marks, ornaments, as it was assumed that the performer would improvise these elements on the spot. In the Classical era, it became more common for composers to indicate where they wanted performers to play ornaments such as trills or turns. The simplification of texture made such instrumental detail more important, and alit so made the use of characteristic rhythms, such as attention-getting opening fanfares, the funeral march rhythm, or the minuet genre, more important in establishing and unifying the tone of a single movement.

The Classical period also saw the gradual development of sonata form, a set of structural principles for music that reconciled the Classical preference for melodic material with harmonic development, which could be applied across musical genres. The sonata itself continued to be the principal form for solo and chamber music, while later in the Classical period, the string quartet became a prominent genre. The symphony form for orchestra was created in this period (this is popularly attributed to Joseph Haydn). The concerto grosso (a concerto for more than one musician), a very popular form in the Baroque era, began to be replaced by the solo concerto, featuring only one soloist. Composers began to place more importance on the particular soloist's ability to show off virtuoso skills, with challenging, fast scale and arpeggio runs. Nonetheless, some concerti grossi remained, the most famous of which was Mozart's Sinfonia Concertante for Violin and Viola in E-flat major.

Main characteristics
In the classical period, the theme consists of phrases with contrasting melodic figures and rhythms. These phrases are relatively brief, typically four bars in length, and can occasionally seem sparse or terse. The texture is mainly homophonic, with a clear melody above a subordinate chordal accompaniment, for instance an Alberti bass. This contrasts with the practice in Baroque music, where a piece or movement would typically have only one musical subject, which would then be worked out in a number of voices according to the principles of counterpoint, while maintaining a consistent rhythm or metre throughout. As a result, Classical music tends to have a lighter, clearer texture than the Baroque. The classical style draws on the style galant, a musical style which emphasized light elegance in place of the Baroque's dignified seriousness and impressive grandeur.

Structurally, Classical music generally has a clear musical form, with a well-defined contrast between tonic and dominant, introduced by clear cadences. Dynamics are used to highlight the structural characteristics of the piece. In particular, sonata form and its variants were developed during the early classical period and was frequently used. The Classical approach to structure again contrasts with the Baroque, where a composition would normally move between tonic and dominant and back again, but through a continual progress of chord changes and without a sense of "arrival" at the new key. While counterpoint was less emphasised in the classical period, it was by no means forgotten, especially later in the period, and composers still used counterpoint in "serious" works such as symphonies and string quartets, as well as religious pieces, such as Masses.

The classical musical style was supported by technical developments in instruments. The widespread adoption of equal temperament made classical musical structure possible, by ensuring that cadences in all keys sounded similar. The fortepiano and then the pianoforte replaced the harpsichord, enabling more dynamic contrast and more sustained melodies. Over the Classical period, keyboard instruments became richer, more sonorous and more powerful.

The orchestra increased in size and range, and became more standardised. The harpsichord or pipe organ basso continuo role in orchestra fell out of use between 1750 and 1775, leaving the string section. Woodwinds became a self-contained section, consisting of clarinets, oboes, flutes and bassoons.

While vocal music such as comic opera was popular, great importance was given to instrumental music. The main kinds of instrumental music were the sonata, trio, string quartet, quintet, symphony, concerto (usually for a virtuoso solo instrument accompanied by orchestra), and light pieces such as serenades and divertimentos. Sonata form developed and became the most important form. It was used to build up the first movement of most large-scale works in symphonies and string quartets. Sonata form was also used in other movements and in single, standalone pieces such as overtures.

Baroque/Classical transition c. 1750–1760

In his book The Classical Style, author and pianist Charles Rosen claims that from 1755 to 1775, composers groped for a new style that was more effectively dramatic. In the High Baroque period, dramatic expression was limited to the representation of individual affects (the "doctrine of affections", or what Rosen terms "dramatic sentiment"). For example, in Handel's oratorio Jephtha, the composer renders four emotions separately, one for each character, in the quartet "O, spare your daughter". Eventually this depiction of individual emotions came to be seen as simplistic and unrealistic; composers sought to portray multiple emotions, simultaneously or progressively, within a single character or movement ("dramatic action"). Thus in the finale of act 2 of Mozart's Die Entführung aus dem Serail, the lovers move "from joy through suspicion and outrage to final reconciliation."

Musically speaking, this "dramatic action" required more musical variety. Whereas Baroque music was characterized by seamless flow within individual movements and largely uniform textures, composers after the High Baroque sought to interrupt this flow with abrupt changes in texture, dynamic, harmony, or tempo. Among the stylistic developments which followed the High Baroque, the most dramatic came to be called Empfindsamkeit, (roughly "sensitive style"), and its best-known practitioner was Carl Philipp Emanuel Bach. Composers of this style employed the above-discussed interruptions in the most abrupt manner, and the music can sound illogical at times. The Italian composer Domenico Scarlatti took these developments further. His more than five hundred single-movement keyboard sonatas also contain abrupt changes of texture, but these changes are organized into periods, balanced phrases that became a hallmark of the classical style. However, Scarlatti's changes in texture still sound sudden and unprepared. The outstanding achievement of the great classical composers (Haydn, Mozart and Beethoven) was their ability to make these dramatic surprises sound logically motivated, so that "the expressive and the elegant could join hands."

Between the death of J. S. Bach and the maturity of Haydn and Mozart (roughly 1750–1770), composers experimented with these new ideas, which can be seen in the music of Bach's sons. Johann Christian developed a style which we now call Roccoco, comprising simpler textures and harmonies, and which was "charming, undramatic, and a little empty." As mentioned previously, Carl Philipp Emmanuel sought to increase drama, and his music was "violent, expressive, brilliant, continuously surprising, and often incoherent." And finally Wilhelm Friedemann, J.S. Bach's eldest son, extended Baroque traditions in an idiomatic, unconventional way.

At first the new style took over Baroque forms—the ternary da capo aria, the sinfonia and the concerto—but composed with simpler parts, more notated ornamentation, rather than the improvised ornaments that were common in the Baroque era, and more emphatic division of pieces into sections. However, over time, the new aesthetic caused radical changes in how pieces were put together, and the basic formal layouts changed. Composers from this period sought dramatic effects, striking melodies, and clearer textures. One of the big textural changes was a shift away from the complex, dense polyphonic style of the Baroque, in which multiple interweaving melodic lines were played simultaneously, and towards homophony, a lighter texture which uses a clear single melody line accompanied by chords.

Baroque music generally uses many harmonic fantasies and polyphonic sections that focus less on the structure of the musical piece, and there was less emphasis on clear musical phrases. In the classical period, the harmonies became simpler. However, the structure of the piece, the phrases and small melodic or rhythmic motives, became much more important than in the Baroque period. Another important break with the past was the radical overhaul of opera by Christoph Willibald Gluck, who cut away a great deal of the layering and improvisational ornaments and focused on the points of modulation and transition. By making these moments where the harmony changes more of a focus, he enabled powerful dramatic shifts in the emotional color of the music. To highlight these transitions, he used changes in instrumentation (orchestration), melody, and mode. Among the most successful composers of his time, Gluck spawned many emulators, including Antonio Salieri. Their emphasis on accessibility brought huge successes in opera, and in other vocal music such as songs, oratorios, and choruses. These were considered the most important kinds of music for performance and hence enjoyed greatest public success.

The phase between the Baroque and the rise of the Classical (around 1730), was home to various competing musical styles. The diversity of artistic paths are represented in the sons of Johann Sebastian Bach: Wilhelm Friedemann Bach, who continued the Baroque tradition in a personal way; Johann Christian Bach, who simplified textures of the Baroque and most clearly influenced Mozart; and Carl Philipp Emanuel Bach, who composed passionate and sometimes violently eccentric music of the Empfindsamkeit movement. Musical culture was caught at a crossroads: the masters of the older style had the technique, but the public hungered for the new. This is one of the reasons C. P. E. Bach was held in such high regard: he understood the older forms quite well and knew how to present them in new garb, with an enhanced variety of form.

1750–1775

By the late 1750s there were flourishing centers of the new style in Italy, Vienna, Mannheim, and Paris; dozens of symphonies were composed and there were bands of players associated with musical theatres. Opera or other vocal music accompanied by orchestra was the feature of most musical events, with concertos and symphonies (arising from the overture) serving as instrumental interludes and introductions for operas and church services. Over the course of the Classical period, symphonies and concertos developed and were presented independently of vocal music. The "normal" orchestra ensemble—a body of strings supplemented by winds—and movements of particular rhythmic character were established by the late 1750s in Vienna. However, the length and weight of pieces was still set with some Baroque characteristics: individual movements still focused on one "affect" (musical mood) or had only one sharply contrasting middle section, and their length was not significantly greater than Baroque movements. There was not yet a clearly enunciated theory of how to compose in the new style. It was a moment ripe for a breakthrough.

The first great master of the style was the composer Joseph Haydn. In the late 1750s he began composing symphonies, and by 1761 he had composed a triptych (Morning, Noon, and Evening) solidly in the contemporary mode. As a vice-Kapellmeister and later Kapellmeister, his output expanded: he composed over forty symphonies in the 1760s alone. And while his fame grew, as his orchestra was expanded and his compositions were copied and disseminated, his voice was only one among many.

While some scholars suggest that Haydn was later overshadowed by Mozart and Beethoven, it would be difficult to overstate Haydn's centrality to the new style, and therefore to the future of Western art music as a whole. At the time, before the pre-eminence of Mozart or Beethoven, and with Johann Sebastian Bach known primarily to connoisseurs of keyboard music, Haydn reached a place in music that set him above all other composers except perhaps the Baroque era's George Frideric Handel. Haydn took existing ideas, and radically altered how they functioned—earning him the titles "father of the symphony" and "father of the string quartet".

One of the forces that worked as an impetus for his pressing forward was the first stirring of what would later be called Romanticism—the Sturm und Drang, or "storm and stress" phase in the arts, a short period where obvious and dramatic emotionalism was a stylistic preference. Haydn accordingly wanted more dramatic contrast and more emotionally appealing melodies, with sharpened character and individuality in his pieces. This period faded away in music and literature: however, it influenced what came afterward and would eventually be a component of aesthetic taste in later decades.

The Farewell Symphony, No. 45 in F♯ minor, exemplifies Haydn's integration of the differing demands of the new style, with surprising sharp turns and a long slow adagio to end the work. In 1772, Haydn completed his Opus 20 set of six string quartets, in which he deployed the polyphonic techniques he had gathered from the previous Baroque era to provide structural coherence capable of holding together his melodic ideas. For some, this marks the beginning of the "mature" Classical style, a transitional period in which reaction against late Baroque complexity yielded to integration of Baroque and Classical elements.

1775–1790

Haydn, having worked for over a decade as the music director for a prince, had far more resources and scope for composing than most other composers. His position also gave him the ability to shape the forces that would play his music, as he could select skilled musicians. This opportunity was not wasted, as Haydn, beginning quite early on his career, sought to press forward the technique of building and developing ideas in his music. His next important breakthrough was in the Opus 33 string quartets (1781), in which the melodic and the harmonic roles segue among the instruments: it is often momentarily unclear what is melody and what is harmony. This changes the way the ensemble works its way between dramatic moments of transition and climactic sections: the music flows smoothly and without obvious interruption. He then took this integrated style and began applying it to orchestral and vocal music. Haydn's gift to music was a way of composing, a way of structuring works, which was at the same time in accord with the governing aesthetic of the new style. However, a younger contemporary, Wolfgang Amadeus Mozart, brought his genius to Haydn's ideas and applied them to two of the major genres of the day: opera, and the virtuoso concerto. Whereas Haydn spent much of his working life as a court composer, Mozart wanted public success in the concert life of cities, playing for the general public. This meant he needed to write operas and write and perform virtuoso pieces. Haydn was not a virtuoso at the international touring level; nor was he seeking to create operatic works that could play for many nights in front of a large audience. Mozart wanted to achieve both. Moreover, Mozart also had a taste for more chromatic chords (and greater contrasts in harmonic language generally), a greater love for creating a welter of melodies in a single work, and a more Italianate sensibility in music as a whole. He found, in Haydn's music and later in his study of the polyphony of J.S. Bach, the means to discipline and enrich his artistic gifts. Mozart rapidly came to the attention of Haydn, who hailed the new composer, studied his works, and considered the younger man his only true peer in music. In Mozart, Haydn found a greater range of instrumentation, dramatic effect and melodic resource. The learning relationship moved in both directions. Mozart also had a great respect for the older, more experienced composer, and sought to learn from him.

Mozart's arrival in Vienna in 1780 brought an acceleration in the development of the Classical style. There, Mozart absorbed the fusion of Italianate brilliance and Germanic cohesiveness that had been brewing for the previous 20 years. His own taste for flashy brilliances, rhythmically complex melodies and figures, long cantilena melodies, and virtuoso flourishes was merged with an appreciation for formal coherence and internal connectedness. It is at this point that war and economic inflation halted a trend to larger orchestras and forced the disbanding or reduction of many theater orchestras. This pressed the Classical style inwards: toward seeking greater ensemble and technical challenges—for example, scattering the melody across woodwinds, or using a melody harmonized in thirds. This process placed a premium on small ensemble music, called chamber music. It also led to a trend for more public performance, giving a further boost to the string quartet and other small ensemble groupings.

It was during this decade that public taste began, increasingly, to recognize that Haydn and Mozart had reached a high standard of composition. By the time Mozart arrived at age 25, in 1781, the dominant styles of Vienna were recognizably connected to the emergence in the 1750s of the early Classical style. By the end of the 1780s, changes in performance practice, the relative standing of instrumental and vocal music, technical demands on musicians, and stylistic unity had become established in the composers who imitated Mozart and Haydn. During this decade Mozart composed his most famous operas, his six late symphonies that helped to redefine the genre, and a string of piano concerti that still stand at the pinnacle of these forms.

One composer who was influential in spreading the more serious style that Mozart and Haydn had formed is Muzio Clementi, a gifted virtuoso pianist who tied with Mozart in a musical "duel" before the emperor in which they each improvised on the piano and performed their compositions. Clementi's sonatas for the piano circulated widely, and he became the most successful composer in London during the 1780s. Also in London at this time was Jan Ladislav Dussek, who, like Clementi, encouraged piano makers to extend the range and other features of their instruments, and then fully exploited the newly opened up possibilities. The importance of London in the Classical period is often overlooked, but it served as the home to the Broadwood's factory for piano manufacturing and as the base for composers who, while less notable than the "Vienna School", had a decisive influence on what came later. They were composers of many fine works, notable in their own right. London's taste for virtuosity may well have encouraged the complex passage work and extended statements on tonic and dominant.

Around 1790–1820
When Haydn and Mozart began composing, symphonies were played as single movements—before, between, or as interludes within other works—and many of them lasted only ten or twelve minutes; instrumental groups had varying standards of playing, and the continuo was a central part of music-making.

In the intervening years, the social world of music had seen dramatic changes. International publication and touring had grown explosively, and concert societies formed. Notation became more specific, more descriptive—and schematics for works had been simplified (yet became more varied in their exact working out). In 1790, just before Mozart's death, with his reputation spreading rapidly, Haydn was poised for a series of successes, notably his late oratorios and London symphonies. Composers in Paris, Rome, and all over Germany turned to Haydn and Mozart for their ideas on form.

In the 1790s, a new generation of composers, born around 1770, emerged. While they had grown up with the earlier styles, they heard in the recent works of Haydn and Mozart a vehicle for greater expression. In 1788 Luigi Cherubini settled in Paris and in 1791 composed Lodoiska, an opera that raised him to fame. Its style is clearly reflective of the mature Haydn and Mozart, and its instrumentation gave it a weight that had not yet been felt in the grand opera. His contemporary Étienne Méhul extended instrumental effects with his 1790 opera Euphrosine et Coradin, from which followed a series of successes. The final push towards change came from Gaspare Spontini, who was deeply admired by future romantic composers such as Weber, Berlioz and Wagner. The innovative harmonic language of his operas, their refined instrumentation and their "enchained" closed numbers (a structural pattern which was later adopted by Weber in Euryanthe and from him handed down, through Marschner, to Wagner), formed the basis from which French and German romantic opera had its beginnings. The most fateful of the new generation was Ludwig van Beethoven, who launched his numbered works in 1794 with a set of three piano trios, which remain in the repertoire. Somewhat younger than the others, though equally accomplished because of his youthful study under Mozart and his native virtuosity, was Johann Nepomuk Hummel. Hummel studied under Haydn as well; he was a friend to Beethoven and Franz Schubert. He concentrated more on the piano than any other instrument, and his time in London in 1791 and 1792 generated the composition and publication in 1793 of three piano sonatas, opus 2, which idiomatically used Mozart's techniques of avoiding the expected cadence, and Clementi's sometimes modally uncertain virtuoso figuration. Taken together, these composers can be seen as the vanguard of a broad change in style and the center of music. They studied one another's works, copied one another's gestures in music, and on occasion behaved like quarrelsome rivals.

The crucial differences with the previous wave can be seen in the downward shift in melodies, increasing durations of movements, the acceptance of Mozart and Haydn as paradigmatic, the greater use of keyboard resources, the shift from "vocal" writing to "pianistic" writing, the growing pull of the minor and of modal ambiguity, and the increasing importance of varying accompanying figures to bring "texture" forward as an element in music. In short, the late Classical was seeking music that was internally more complex. The growth of concert societies and amateur orchestras, marking the importance of music as part of middle-class life, contributed to a booming market for pianos, piano music, and virtuosi to serve as exemplars. Hummel, Beethoven, and Clementi were all renowned for their improvising.

The direct influence of the Baroque continued to fade: the figured bass grew less prominent as a means of holding performance together, the performance practices of the mid-18th century continued to die out. However, at the same time, complete editions of Baroque masters began to become available, and the influence of Baroque style continued to grow, particularly in the ever more expansive use of brass. Another feature of the period is the growing number of performances where the composer was not present. This led to increased detail and specificity in notation; for example, there were fewer "optional" parts that stood separately from the main score.

The force of these shifts became apparent with Beethoven's 3rd Symphony, given the name Eroica, which is Italian for "heroic", by the composer. As with Stravinsky's The Rite of Spring, it may not have been the first in all of its innovations, but its aggressive use of every part of the Classical style set it apart from its contemporary works: in length, ambition, and harmonic resources as well making it the first symphony of the Romantic era.

First Viennese School
The First Viennese School is a name mostly used to refer to three composers of the Classical period in late-18th-century Vienna: Haydn, Mozart, and Beethoven. Franz Schubert is occasionally added to the list.

In German-speaking countries, the term Wiener Klassik (lit. Viennese classical era/art) is used. That term is often more broadly applied to the Classical era in music as a whole, as a means to distinguish it from other periods that are colloquially referred to as classical, namely Baroque and Romantic music.

The term "Viennese School" was first used by Austrian musicologist Raphael Georg Kiesewetter in 1834, although he only counted Haydn and Mozart as members of the school. Other writers followed suit, and eventually Beethoven was added to the list. The designation "first" is added today to avoid confusion with the Second Viennese School.

Whilst, Schubert apart, these composers certainly knew each other (with Haydn and Mozart even being occasional chamber-music partners), there is no sense in which they were engaged in a collaborative effort in the sense that one would associate with 20th-century schools such as the Second Viennese School, or Les Six. Nor is there any significant sense in which one composer was "schooled" by another (in the way that Berg and Webern were taught by Schoenberg), though it is true that Beethoven for a time received lessons from Haydn.

Attempts to extend the First Viennese School to include such later figures as Anton Bruckner, Johannes Brahms, and Gustav Mahler are merely journalistic, and never encountered in academic musicology. According to scholar James F. Daugherty, the Classical period itself from approximately 1775 to 1825 is sometimes referred to as "the Viennese Classic period".

Classical influence on later composers
Musical eras and their prevalent styles, forms and instruments seldom disappear at once; instead, features are replaced over time, until the old approach is simply felt as "old-fashioned". The Classical style did not "die" suddenly; rather, it gradually got phased out under the weight of changes. To give just one example, while it is generally stated that the Classical era stopped using the harpsichord in orchestras, this did not happen all of a sudden at the start of the Classical era in 1750. Rather, orchestras slowly stopped using the harpsichord to play basso continuo until the practice was discontinued by the end of the 1700s. One crucial change was the shift towards harmonies centering on "flatward" keys: shifts in the subdominant direction. In the Classical style, major key was far more common than minor, chromaticism being moderated through the use of "sharpward" modulation (e.g., a piece in C major modulating to G major, D major, or A major, all of which are keys with more sharps). As well, sections in the minor mode were often used for contrast. Beginning with Mozart and Clementi, there began a creeping colonization of the subdominant region (the ii or IV chord, which in the key of C major would be the keys of d minor or F major). With Schubert, subdominant modulations flourished after being introduced in contexts in which earlier composers would have confined themselves to dominant shifts (modulations to the dominant chord, e.g., in the key of C major, modulating to G major). This introduced darker colors to music, strengthened the minor mode, and made structure harder to maintain. Beethoven contributed to this by his increasing use of the fourth as a consonance, and modal ambiguity—for example, the opening of the Symphony No. 9 in D minor.

Ludwig van Beethoven, Franz Schubert, Carl Maria von Weber, Johann Nepomuk Hummel, and John Field are among the most prominent in this generation of "Proto-Romantics", along with the young Felix Mendelssohn. Their sense of form was strongly influenced by the Classical style. While they were not yet "learned" composers (imitating rules which were codified by others), they directly responded to works by Haydn, Mozart, Clementi, and others, as they encountered them. The instrumental forces at their disposal in orchestras were also quite "Classical" in number and variety, permitting similarity with Classical works. However, the forces destined to end the hold of the Classical style gathered strength in the works of many of the above composers, particularly Beethoven. The most commonly cited one is harmonic innovation. Also important is the increasing focus on having a continuous and rhythmically uniform accompanying figuration: Beethoven's Moonlight Sonata was the model for hundreds of later pieces—where the shifting movement of a rhythmic figure provides much of the drama and interest of the work, while a melody drifts above it. Greater knowledge of works, greater instrumental expertise, increasing variety of instruments, the growth of concert societies, and the unstoppable domination of the increasingly more powerful piano (which was given a bolder, louder tone by technological developments such as the use of steel strings, heavy cast-iron frames and sympathetically vibrating strings) all created a huge audience for sophisticated music. All of these trends contributed to the shift to the "Romantic" style.

Drawing the line between these two styles is very difficult: some sections of Mozart's later works, taken alone, are indistinguishable in harmony and orchestration from music written 80 years later—and some composers continued to write in normative Classical styles into the early 20th century. Even before Beethoven's death, composers such as Louis Spohr were self-described Romantics, incorporating, for example, more extravagant chromaticism in their works (e.g., using chromatic harmonies in a piece's chord progression). Conversely, works such as Schubert's Symphony No. 5, written during the chronological end of the Classical era and dawn of the Romantic era, exhibit a deliberately anachronistic artistic paradigm, harking back to the compositional style of several decades before.

However, Vienna's fall as the most important musical center for orchestral composition during the late 1820s, precipitated by the deaths of Beethoven and Schubert, marked the Classical style's final eclipse—and the end of its continuous organic development of one composer learning in close proximity to others. Franz Liszt and Frédéric Chopin visited Vienna when they were young, but they then moved on to other cities. Composers such as Carl Czerny, while deeply influenced by Beethoven, also searched for new ideas and new forms to contain the larger world of musical expression and performance in which they lived.

Renewed interest in the formal balance and restraint of 18th century classical music led in the early 20th century to the development of so-called Neoclassical style, which numbered Stravinsky and Prokofiev among its proponents, at least at certain times in their careers.

Guitar
The Baroque guitar, with four or five sets of double strings or "courses" and elaborately decorated soundhole, was a very different instrument from the early classical guitar which more closely resembles the modern instrument with the standard six strings. Judging by the number of instructional manuals published for the instrument – over three hundred texts were published by over two hundred authors between 1760 and 1860 – the classical period marked a golden age for guitar.

Strings
In the Baroque era, there was more variety in the bowed stringed instruments used in ensembles, with instruments such as the viola d'amore and a range of fretted viols being used, ranging from small viols to large bass viols. In the Classical period, the string section of the orchestra was standardized as just four instruments:

 * Violin (in orchestras and chamber music, typically there are first violins and second violins, with the former playing the melody and/or a higher line and the latter playing either a countermelody, a harmony part, a part below the first violin line in pitch, or an accompaniment line)
 * Viola (the alto voice of the orchestral string section and string quartet; it often performs "inner voices", which are accompaniment lines which fill in the harmony of the piece)
 * Cello (the cello plays two roles in Classical era music; at times it is used to play the bassline of the piece, typically doubled by the double basses [Note: When cellos and double basses read the same bassline, the basses play an octave below the cellos, because the bass is a transposing instrument]; and at other times it performs melodies and solos in the lower register)
 * Double bass (the bass typically performs the lowest pitches in the string section in order to provide the bassline for the piece)

In the Baroque era, the double bass players were not usually given a separate part; instead, they typically played the same basso continuo bassline that the cellos and other low-pitched instruments (e.g., theorbo, serpent wind instrument, viols), albeit an octave below the cellos, because the double bass is a transposing instrument that sounds one octave lower than it is written. In the Classical era, some composers continued to write only one bass part for their symphony, labeled "bassi"; this bass part was played by cellists and double bassists. During the Classical era, some composers began to give the double basses their own part.

Woodwinds
It was commonplace for all orchestras to have at least 2 winds, usually oboes, flutes, clarinets, or sometimes English horns (see Symphony No. 22 (Haydn). Patrons also usually employed an ensemble of entirely winds, called the harmonie, which would be employed for certain events. The harmonie would sometimes join the larger string orchestra to serve as the wind section.
 * Piccolo (used in military bands)
 * Flute
 * Oboe
 * English horn
 * Clarinet
 * Basset horn (see Requiem (Mozart))
 * Basset Clarinet
 * Clarinette d'amour
 * Bassoon
 * Contrabassoon (see The Creation (Haydn))
 * Bagpipe (see Leopold Mozart's divertimento, "Die Bauernhochzeit" or "Peasant Wedding")

Percussion

 * Timpani
 * "Turkish music":
 * Bass drum
 * Cymbals
 * Triangle
 * Tambourine

Keyboards

 * Clavichord
 * Fortepiano (the forerunner to the modern piano)
 * Harpsichord, the standard Baroque era basso continuo keyboard instrument, was used until the 1750s, after which time it was gradually phased out, and replaced with the fortepiano and then the piano. By the early 1800s, the harpsichord was no longer used.
 * Organ

Brasses

 * Natural horn
 * Natural trumpet
 * Sackbut (Trombone precursor)
 * Keyed trumpet (see Trumpet Concerto (Haydn))
 * Serpent (commonplace in military bands)
 * Post horn (see Serenade No. 9 (Mozart))

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Classical period (music)


text:
Character encoding

Character encoding is the process of assigning numbers to graphical characters, especially the written characters of human language, allowing them to be stored, transmitted, and transformed using digital computers. The numerical values that make up a character encoding are known as "code points" and collectively comprise a "code space", a "code page", or a "character map".

Early character codes associated with the optical or electrical telegraph could only represent a subset of the characters used in written languages, sometimes restricted to upper case letters, numerals and some punctuation only. The low cost of digital representation of data in modern computer systems allows more elaborate character codes (such as Unicode) which represent most of the characters used in many written languages. Character encoding using internationally accepted standards permits worldwide interchange of text in electronic form.

The most used character encoding on the web is UTF-8, used in 98.2% of surveyed web sites, as of May 2024. In application programs and operating system tasks, both UTF-8 and UTF-16 are popular options.

History
The history of character codes illustrates the evolving need for machine-mediated character-based symbolic information over a distance, using once-novel electrical means. The earliest codes were based upon manual and hand-written encoding and cyphering systems, such as Bacon's cipher, Braille, international maritime signal flags, and the 4-digit encoding of Chinese characters for a Chinese telegraph code (Hans Schjellerup, 1869). With the adoption of electrical and electro-mechanical techniques these earliest codes were adapted to the new capabilities and limitations of the early machines. The earliest well-known electrically transmitted character code, Morse code, introduced in the 1840s, used a system of four "symbols" (short signal, long signal, short space, long space) to generate codes of variable length. Though some commercial use of Morse code was via machinery, it was often used as a manual code, generated by hand on a telegraph key and decipherable by ear, and persists in amateur radio and aeronautical use. Most codes are of fixed per-character length or variable-length sequences of fixed-length codes (e.g. Unicode).

Common examples of character encoding systems include Morse code, the Baudot code, the American Standard Code for Information Interchange (ASCII) and Unicode. Unicode, a well-defined and extensible encoding system, has supplanted most earlier character encodings, but the path of code development to the present is fairly well known.

The Baudot code, a five-bit encoding, was created by Émile Baudot in 1870, patented in 1874, modified by Donald Murray in 1901, and standardized by CCITT as International Telegraph Alphabet No. 2 (ITA2) in 1930. The name baudot has been erroneously applied to ITA2 and its many variants. ITA2 suffered from many shortcomings and was often improved by many equipment manufacturers, sometimes creating compatibility issues. In 1959 the U.S. military defined its Fieldata code, a six-or seven-bit code, introduced by the U.S. Army Signal Corps. While Fieldata addressed many of the then-modern issues (e.g. letter and digit codes arranged for machine collation), it fell short of its goals and was short-lived. In 1963 the first ASCII code was released (X3.4-1963) by the ASCII committee (which contained at least one member of the Fieldata committee, W. F. Leubbert), which addressed most of the shortcomings of Fieldata, using a simpler code. Many of the changes were subtle, such as collatable character sets within certain numeric ranges. ASCII63 was a success, widely adopted by industry, and with the follow-up issue of the 1967 ASCII code (which added lower-case letters and fixed some "control code" issues) ASCII67 was adopted fairly widely. ASCII67's American-centric nature was somewhat addressed in the European ECMA-6 standard.

Herman Hollerith invented punch card data encoding in the late 19th century to analyze census data. Initially, each hole position represented a different data element, but later, numeric information was encoded by numbering the lower rows 0 to 9, with a punch in a column representing its row number. Later alphabetic data was encoded by allowing more than one punch per column. Electromechanical tabulating machines represented date internally by the timing of pulses relative to the motion of the cards through the machine. When IBM went to electronic processing, starting with the IBM 603 Electronic Multiplier, it used a variety of binary encoding schemes that were tied to the punch card code.

IBM used several Binary Coded Decimal (BCD) six-bit character encoding schemes, starting as early as 1953 in its 702 and 704 computers, and in its later 7000 Series and 1400 series, as well as in associated peripherals. Since the punched card code then in use only allowed digits, upper-case English letters and a few special characters, six bits were sufficient. These BCD encodings extended existing simple four-bit numeric encoding to include alphabetic and special characters, mapping them easily to punch-card encoding which was already in widespread use. IBM's codes were used primarily with IBM equipment; other computer vendors of the era had their own character codes, often six-bit, but usually had the ability to read tapes produced on IBM equipment. These BCD encodings were the precursors of IBM's Extended Binary-Coded Decimal Interchange Code (usually abbreviated as EBCDIC), an eight-bit encoding scheme developed in 1963 for the IBM System/360 that featured a larger character set, including lower case letters.

In trying to develop universally interchangeable character encodings, researchers in the 1980s faced the dilemma that, on the one hand, it seemed necessary to add more bits to accommodate additional characters, but on the other hand, for the users of the relatively small character set of the Latin alphabet (who still constituted the majority of computer users), those additional bits were a colossal waste of then-scarce and expensive computing resources (as they would always be zeroed out for such users). In 1985, the average personal computer user's hard disk drive could store only about 10 megabytes, and it cost approximately US$250 on the wholesale market (and much higher if purchased separately at retail), so it was very important at the time to make every bit count.

The compromise solution that was eventually found and was to break the assumption (dating back to telegraph codes) that each character should always directly correspond to a particular sequence of bits. Instead, characters would first be mapped to a universal intermediate representation in the form of abstract numbers called code points. Code points would then be represented in a variety of ways and with various default numbers of bits per character (code units) depending on context. To encode code points higher than the length of the code unit, such as above 256 for eight-bit units, the solution was to implement variable-length encodings where an escape sequence would signal that subsequent bits should be parsed as a higher code point.

Terminology
Informally, the terms "character encoding", "character map", "character set" and "code page" are often used interchangeably. Historically, the same standard would specify a repertoire of characters and how they were to be encoded into a stream of code units — usually with a single character per code unit. However, due to the emergence of more sophisticated character encodings, the distinction between these terms has become important.

 * A character is a minimal unit of text that has semantic value.
 * A character set is a collection of elements used to represent text. For example, the Latin alphabet and Greek alphabet are both character sets.
 * A coded character set is a character set mapped to set of unique numbers. For historical reasons, this is also often referred to as a code page.
 * A character repertoire is the set of characters that can be represented by a particular coded character set. The repertoire may be closed, meaning that no additions are allowed without creating a new standard (as is the case with ASCII and most of the ISO-8859 series); or it may be open, allowing additions (as is the case with Unicode and to a limited extent Windows code pages).
 * A code point is a value or position of a character in a coded character set.
 * A code space is the range of numerical values spanned by a coded character set.
 * A code unit is the minimum bit combination that can represent a character in a character encoding (in computer science terms, it is the word size of the character encoding). For example, common code units include 7-bit, 8-bit, 16-bit, and 32-bit. In some encodings, some characters are encoded using multiple code units; such an encoding is referred to as a variable-width encoding.

Code pages
"Code page" is a historical name for a coded character set.

Originally, a code page referred to a specific page number in the IBM standard character set manual, which would define a particular character encoding. Other vendors, including Microsoft, SAP, and Oracle Corporation, also published their own sets of code pages; the most well-known code page suites are "Windows" (based on Windows-1252) and "IBM"/"DOS" (based on code page 437).

Despite no longer referring to specific page numbers in a standard, many character encodings are still referred to by their code page number; likewise, the term "code page" is often still used to refer to character encodings in general.

The term "code page" is not used in Unix or Linux, where "charmap" is preferred, usually in the larger context of locales. IBM's Character Data Representation Architecture (CDRA) designates entities with coded character set identifiers (CCSIDs), each of which is variously called a "charset", "character set", "code page", or "CHARMAP".

Code units
The code unit size is equivalent to the bit measurement for the particular encoding:
 * A code unit in ASCII consists of 7 bits;
 * A code unit in UTF-8, EBCDIC and GB 18030 consists of 8 bits;
 * A code unit in UTF-16 consists of 16 bits;
 * A code unit in UTF-32 consists of 32 bits.

Code points
A code point is represented by a sequence of code units. The mapping is defined by the encoding. Thus, the number of code units required to represent a code point depends on the encoding:
 * UTF-8: code points map to a sequence of one, two, three or four code units.
 * UTF-16: code units are twice as long as 8-bit code units. Therefore, any code point with a scalar value less than U+10000 is encoded with a single code unit. Code points with a value U+10000 or higher require two code units each. These pairs of code units have a unique term in UTF-16: "Unicode surrogate pairs".
 * UTF-32: the 32-bit code unit is large enough that every code point is represented as a single code unit.
 * GB 18030: multiple code units per code point are common, because of the small code units. Code points are mapped to one, two, or four code units.

Characters
Exactly what constitutes a character varies between character encodings.

For example, for letters with diacritics, there are two distinct approaches that can be taken to encode them: they can be encoded either as a single unified character (known as a precomposed character), or as separate characters that combine into a single glyph. The former simplifies the text handling system, but the latter allows any letter/diacritic combination to be used in text. Ligatures pose similar problems.

Exactly how to handle glyph variants is a choice that must be made when constructing a particular character encoding. Some writing systems, such as Arabic and Hebrew, need to accommodate things like graphemes that are joined in different ways in different contexts, but represent the same semantic character.

Unicode encoding model
Unicode and its parallel standard, the ISO/IEC 10646 Universal Character Set, together constitute a unified standard for character encoding. Rather than mapping characters directly to bytes, Unicode separately defines a coded character set that maps characters to unique natural numbers (code points), how those code points are mapped to a series of fixed-size natural numbers (code units), and finally how those units are encoded as a stream of octets (bytes). The purpose of this decomposition is to establish a universal set of characters that can be encoded in a variety of ways. To describe this model precisely, Unicode uses its own set of terminology to describe its process:

An abstract character repertoire (ACR) is the full set of abstract characters that a system supports. Unicode has an open repertoire, meaning that new characters will be added to the repertoire over time.

A coded character set (CCS) is a function that maps characters to code points (each code point represents one character). For example, in a given repertoire, the capital letter "A" in the Latin alphabet might be represented by the code point 65, the character "B" by 66, and so on. Multiple coded character sets may share the same character repertoire; for example ISO/IEC 8859-1 and IBM code pages 037 and 500 all cover the same repertoire but map them to different code points.

A character encoding form (CEF) is the mapping of code points to code units to facilitate storage in a system that represents numbers as bit sequences of fixed length (i.e. practically any computer system). For example, a system that stores numeric information in 16-bit units can only directly represent code points 0 to 65,535 in each unit, but larger code points (say, 65,536 to 1.4 million) could be represented by using multiple 16-bit units. This correspondence is defined by a CEF.

A character encoding scheme (CES) is the mapping of code units to a sequence of octets to facilitate storage on an octet-based file system or transmission over an octet-based network. Simple character encoding schemes include UTF-8, UTF-16BE, UTF-32BE, UTF-16LE, and UTF-32LE; compound character encoding schemes, such as UTF-16, UTF-32 and ISO/IEC 2022, switch between several simple schemes by using a byte order mark or escape sequences; compressing schemes try to minimize the number of bytes used per code unit (such as SCSU and BOCU).

Although UTF-32BE and UTF-32LE are simpler CESes, most systems working with Unicode use either UTF-8, which is backward compatible with fixed-length ASCII and maps Unicode code points to variable-length sequences of octets, or UTF-16BE, which is backward compatible with fixed-length UCS-2BE and maps Unicode code points to variable-length sequences of 16-bit words. See comparison of Unicode encodings for a detailed discussion.

Finally, there may be a higher-level protocol which supplies additional information to select the particular variant of a Unicode character, particularly where there are regional variants that have been 'unified' in Unicode as the same character. An example is the XML attribute xml:lang.

The Unicode model uses the term "character map" for other systems which directly assign a sequence of characters to a sequence of bytes, covering all of the CCS, CEF and CES layers.

Unicode code points
In Unicode, a character can be referred to as 'U+' followed by its codepoint value in hexadecimal. The range of valid code points (the codespace) for the Unicode standard is U+0000 to U+10FFFF, inclusive, divided in 17 planes, identified by the numbers 0 to 16. Characters in the range U+0000 to U+FFFF are in plane 0, called the Basic Multilingual Plane (BMP). This plane contains most commonly-used characters. Characters in the range U+10000 to U+10FFFF in the other planes are called supplementary characters.

The following table shows examples of code point values:

Example
Consider a string of the letters "ab̲c𐐀"—that is, a string containing a Unicode combining character as well a supplementary character. This string has several Unicode representations which are logically equivalent, yet while each is suited to a diverse set of circumstances or range of requirements:
 * Four composed characters:
 * a, b̲, c, 𐐀
 * Five graphemes:
 * a, b, _, c, 𐐀
 * Five Unicode code points:
 * U+0061, U+0062, U+0332, U+0063, U+10400
 * Five UTF-32 code units (32-bit integer values):
 * 0x00000061, 0x00000062, 0x00000332, 0x00000063, 0x00010400
 * Six UTF-16 code units (16-bit integers)
 * 0x0061, 0x0062, 0x0332, 0x0063, 0xD801, 0xDC00
 * Nine UTF-8 code units (8-bit values, or bytes)
 * 0x61, 0x62, 0xCC, 0xB2, 0x63, 0xF0, 0x90, 0x90, 0x80

Note in particular that 𐐀 is represented with either one 32-bit value (UTF-32), two 16-bit values (UTF-16), or four 8-bit values (UTF-8). Although each of those forms uses the same total number of bits (32) to represent the glyph, it is not obvious how the actual numeric byte values are related.

Transcoding
As a result of having many character encoding methods in use (and the need for backward compatibility with archived data), many computer programs have been developed to translate data between character encoding schemes, a process known as transcoding. Some of these are cited below.

Cross-platform:
 * Web browsers – most modern web browsers feature automatic character encoding detection. On Firefox 3, for example, see the View/Character Encoding submenu.
 * iconv – a program and standardized API to convert encodings
 * luit – a program that converts encoding of input and output to programs running interactively
 * International Components for Unicode – A set of C and Java libraries to perform charset conversion. uconv can be used from ICU4C.

Windows:
 * Encoding.Convert – .NET API
 * MultiByteToWideChar/WideCharToMultiByte – to convert from ANSI to Unicode & Unicode to ANSI

Common character encodings
The most used character encoding on the web is UTF-8, used in 98.2% of surveyed web sites, as of May 2024. In application programs and operating system tasks, both UTF-8 and UTF-16 are popular options.

 * ISO 646
 * ASCII
 * EBCDIC
 * ISO 8859:
 * ISO 8859-1 Western Europe
 * ISO 8859-2 Western and Central Europe
 * ISO 8859-3 Western Europe and South European (Turkish, Maltese plus Esperanto)
 * ISO 8859-4 Western Europe and Baltic countries (Lithuania, Estonia, Latvia and Lapp)
 * ISO 8859-5 Cyrillic alphabet
 * ISO 8859-6 Arabic
 * ISO 8859-7 Greek
 * ISO 8859-8 Hebrew
 * ISO 8859-9 Western Europe with amended Turkish character set
 * ISO 8859-10 Western Europe with rationalised character set for Nordic languages, including complete Icelandic set
 * ISO 8859-11 Thai
 * ISO 8859-13 Baltic languages plus Polish
 * ISO 8859-14 Celtic languages (Irish Gaelic, Scottish, Welsh)
 * ISO 8859-15 Added the Euro sign and other rationalisations to ISO 8859-1
 * ISO 8859-16 Central, Eastern and Southern European languages (Albanian, Bosnian, Croatian, Hungarian, Polish, Romanian, Serbian and Slovenian, but also French, German, Italian and Irish Gaelic)
 * CP437, CP720, CP737, CP850, CP852, CP855, CP857, CP858, CP860, CP861, CP862, CP863, CP865, CP866, CP869, CP872
 * MS-Windows character sets:
 * Windows-1250 for Central European languages that use Latin script, (Polish, Czech, Slovak, Hungarian, Slovene, Serbian, Croatian, Bosnian, Romanian and Albanian)
 * Windows-1251 for Cyrillic alphabets
 * Windows-1252 for Western languages
 * Windows-1253 for Greek
 * Windows-1254 for Turkish
 * Windows-1255 for Hebrew
 * Windows-1256 for Arabic
 * Windows-1257 for Baltic languages
 * Windows-1258 for Vietnamese
 * Mac OS Roman
 * KOI8-R, KOI8-U, KOI7
 * MIK
 * ISCII
 * TSCII
 * VISCII
 * JIS X 0208 is a widely deployed standard for Japanese character encoding that has several encoding forms.
 * Shift JIS (Microsoft Code page 932 is a dialect of Shift_JIS)
 * EUC-JP
 * ISO-2022-JP
 * JIS X 0213 is an extended version of JIS X 0208.
 * Shift_JIS-2004
 * EUC-JIS-2004
 * ISO-2022-JP-2004
 * Chinese Guobiao
 * GB 2312
 * GBK (Microsoft Code page 936)
 * GB 18030
 * Taiwan Big5 (a more famous variant is Microsoft Code page 950)
 * Hong Kong HKSCS
 * Korean
 * KS X 1001 is a Korean double-byte character encoding standard
 * EUC-KR
 * ISO-2022-KR
 * Unicode (and subsets thereof, such as the 16-bit 'Basic Multilingual Plane')
 * UTF-8
 * UTF-16
 * UTF-32
 * ANSEL or ISO/IEC 6937

title: Character encoding


text:
Control character

In computing and telecommunication, a control character or non-printing character (NPC) is a code point in a character set that does not represent a written character or symbol. They are used as in-band signaling to cause effects other than the addition of a symbol to the text. All other characters are mainly graphic characters, also known as printing characters (or printable characters), except perhaps for "space" characters. In the ASCII standard there are 33 control characters, such as code 7,, which rings a terminal bell.

History
Procedural signs in Morse code are a form of control character.

A form of control characters were introduced in the 1870 Baudot code: NUL and DEL. The 1901 Murray code added the carriage return (CR) and line feed (LF), and other versions of the Baudot code included other control characters.

The bell character (BEL), which rang a bell to alert operators, was also an early teletype control character.

Some control characters have also been called "format effectors".

In ASCII

There were quite a few control characters defined (33 in ASCII, and the ECMA-48 standard adds 32 more). This was because early terminals had very primitive mechanical or electrical controls that made any kind of state-remembering API quite expensive to implement, thus a different code for each and every function looked like a requirement. It quickly became possible and inexpensive to interpret sequences of codes to perform a function, and device makers found a way to send hundreds of device instructions. Specifically, they used ASCII code 2710 (escape), followed by a series of characters called a "control sequence" or "escape sequence". The mechanism was invented by Bob Bemer, the father of ASCII. For example, the sequence of code 2710, followed by the printable characters "[2;10H", would cause a Digital Equipment Corporation VT100 terminal to move its cursor to the 10th cell of the 2nd line of the screen. Several standards exist for these sequences, notably ANSI X3.64. But the number of non-standard variations in use is large, especially among printers, where technology has advanced far faster than any standards body can possibly keep up with.

All entries in the ASCII table below code 3210 (technically the C0 control code set) are of this kind, including CR and LF used to separate lines of text. The code 12710 (DEL) is also a control character. Extended ASCII sets defined by ISO 8859 added the codes 12810 through 15910 as control characters. This was primarily done so that if the high bit was stripped, it would not change a printing character to a C0 control code. This second set is called the C1 set.

These 65 control codes were carried over to Unicode. Unicode added more characters that could be considered controls, but it makes a distinction between these "Formatting characters" (such as the zero-width non-joiner) and the 65 control characters.

The Extended Binary Coded Decimal Interchange Code (EBCDIC) character set contains 65 control codes, including all of the ASCII control codes plus additional codes which are mostly used to control IBM peripherals.

The control characters in ASCII still in common use include:
 * 0x00 (null,, , ), originally intended to be an ignored character, but now used by many programming languages including C to mark the end of a string.
 * 0x07 (bell,, , ), which may cause the device to emit a warning such as a bell or beep sound or the screen flashing.
 * 0x08 (backspace,, , ), may overprint the previous character.
 * 0x09 (horizontal tab,, , ), moves the printing position right to the next tab stop.
 * 0x0A (line feed,, , ), moves the print head down one line, or to the left edge and down. Used as the end of line marker in most UNIX systems and variants.
 * 0x0B (vertical tab,, , ), vertical tabulation.
 * 0x0C (form feed,, , ), to cause a printer to eject paper to the top of the next page, or a video terminal to clear the screen.
 * 0x0D (carriage return,, , ), moves the printing position to the start of the line, allowing overprinting. Used as the end of line marker in Classic Mac OS, OS-9, FLEX (and variants). A pair is used by CP/M-80 and its derivatives including DOS and Windows, and by Application Layer protocols such as FTP, SMTP, and HTTP.
 * 0x1A (Control-Z,, ). Acts as an end-of-file for the Windows text-mode file i/o.
 * 0x1B (escape,, (GCC only), ). Introduces an escape sequence.

Control characters may be described as doing something when the user inputs them, such as code 3 (End-of-Text character, ETX, ) to interrupt the running process, or code 4 (End-of-Transmission character, EOT, ), used to end text input on Unix or to exit a Unix shell. These uses usually have little to do with their use when they are in text being output.

In Unicode
In Unicode, "Control-characters" are U+0000—U+001F (C0 controls), U+007F (delete), and U+0080—U+009F (C1 controls). Their General Category is "Cc". Formatting codes are distinct, in General Category "Cf". The Cc control characters have no Name in Unicode, but are given labels such as "" instead.

Display
There are a number of techniques to display non-printing characters, which may be illustrated with the bell character in ASCII encoding:
 * Code point: decimal 7, hexadecimal 0x07
 * An abbreviation, often three capital letters: BEL
 * A special character condensing the abbreviation: Unicode U+2407 (␇), "symbol for bell"
 * An ISO 2047 graphical representation: Unicode U+237E (⍾), "graphic for bell"
 * Caret notation in ASCII, where code point 00xxxxx is represented as a caret followed by the capital letter at code point 10xxxxx: ^G
 * An escape sequence, as in C/C++ character string codes: \a, \007, \x07, etc.

How control characters map to keyboards
ASCII-based keyboards have a key labelled "Control", "Ctrl", or (rarely) "Cntl" which is used much like a shift key, being pressed in combination with another letter or symbol key. In one implementation, the control key generates the code 64 places below the code for the (generally) uppercase letter it is pressed in combination with (i.e., subtract 0x40 from ASCII code value of the (generally) uppercase letter). The other implementation is to take the ASCII code produced by the key and bitwise AND it with 0x1F, forcing bits 5 to 7 to zero. For example, pressing "control" and the letter "g" (which is 0110 0111 in binary), produces the code 7 (BELL, 7 in base ten, or 0000 0111 in binary). The NULL character (code 0) is represented by Ctrl-@, "@" being the code immediately before "A" in the ASCII character set. For convenience, some terminals accept Ctrl-Space as an alias for Ctrl-@. In either case, this produces one of the 32 ASCII control codes between 0 and 31. Neither approach works to produce the DEL character because of its special location in the table and its value (code 12710), Ctrl-? is sometimes used for this character.

When the control key is held down, letter keys produce the same control characters regardless of the state of the shift or caps lock keys. In other words, it does not matter whether the key would have produced an upper-case or a lower-case letter. The interpretation of the control key with the space, graphics character, and digit keys (ASCII codes 32 to 63) vary between systems. Some will produce the same character code as if the control key were not held down. Other systems translate these keys into control characters when the control key is held down. The interpretation of the control key with non-ASCII ("foreign") keys also varies between systems.

Control characters are often rendered into a printable form known as caret notation by printing a caret (^) and then the ASCII character that has a value of the control character plus 64. Control characters generated using letter keys are thus displayed with the upper-case form of the letter. For example, ^G represents code 7, which is generated by pressing the G key when the control key is held down.

Keyboards also typically have a few single keys which produce control character codes. For example, the key labelled "Backspace" typically produces code 8, "Tab" code 9, "Enter" or "Return" code 13 (though some keyboards might produce code 10 for "Enter").

Many keyboards include keys that do not correspond to any ASCII printable or control character, for example cursor control arrows and word processing functions. The associated keypresses are communicated to computer programs by one of four methods: appropriating otherwise unused control characters; using some encoding other than ASCII; using multi-character control sequences; or using an additional mechanism outside of generating characters. "Dumb" computer terminals typically use control sequences. Keyboards attached to stand-alone personal computers made in the 1980s typically use one (or both) of the first two methods. Modern computer keyboards generate scancodes that identify the specific physical keys that are pressed; computer software then determines how to handle the keys that are pressed, including any of the four methods described above.

The design purpose
The control characters were designed to fall into a few groups: printing and display control, data structuring, transmission control, and miscellaneous.

Printing and display control
Printing control characters were first used to control the physical mechanism of printers, the earliest output device. An early example of this idea was the use of Figures (FIGS) and Letters (LTRS) in Baudot code to shift between two code pages. A later, but still early, example was the out-of-band ASA carriage control characters. Later, control characters were integrated into the stream of data to be printed. The carriage return character (CR), when sent to such a device, causes it to put the character at the edge of the paper at which writing begins (it may, or may not, also move the printing position to the next line). The line feed character (LF/NL) causes the device to put the printing position on the next line. It may (or may not), depending on the device and its configuration, also move the printing position to the start of the next line (which would be the leftmost position for left-to-right scripts, such as the alphabets used for Western languages, and the rightmost position for right-to-left scripts such as the Hebrew and Arabic alphabets). The vertical and horizontal tab characters (VT and HT/TAB) cause the output device to move the printing position to the next tab stop in the direction of reading. The form feed character (FF/NP) starts a new sheet of paper, and may or may not move to the start of the first line. The backspace character (BS) moves the printing position one character space backwards. On printers, including hard-copy terminals, this is most often used so the printer can overprint characters to make other, not normally available, characters. On video terminals and other electronic output devices, there are often software (or hardware) configuration choices that allow a destructive backspace (e.g., a BS, SP, BS sequence), which erases, or a non-destructive one, which does not. The shift in and shift out characters (SI and SO) selected alternate character sets, fonts, underlining, or other printing modes. Escape sequences were often used to do the same thing.

With the advent of computer terminals that did not physically print on paper and so offered more flexibility regarding screen placement, erasure, and so forth, printing control codes were adapted. Form feeds, for example, usually cleared the screen, there being no new paper page to move to. More complex escape sequences were developed to take advantage of the flexibility of the new terminals, and indeed of newer printers. The concept of a control character had always been somewhat limiting, and was extremely so when used with new, much more flexible, hardware. Control sequences (sometimes implemented as escape sequences) could match the new flexibility and power and became the standard method. However, there were, and remain, a large variety of standard sequences to choose from.

Data structuring
The separators (File, Group, Record, and Unit: FS, GS, RS and US) were made to structure data, usually on a tape, in order to simulate punched cards. End of medium (EM) warns that the tape (or other recording medium) is ending. While many systems use CR/LF and TAB for structuring data, it is possible to encounter the separator control characters in data that needs to be structured. The separator control characters are not overloaded; there is no general use of them except to separate data into structured groupings. Their numeric values are contiguous with the space character, which can be considered a member of the group, as a word separator.

For example, the RS separator is used by (JSON Text Sequences) to encode a sequence of JSON elements. Each sequence item starts with a RS character and ends with a line feed. This allows to serialize open-ended JSON sequences. It is one of the JSON streaming protocols.

Transmission control
The transmission control characters were intended to structure a data stream, and to manage re-transmission or graceful failure, as needed, in the face of transmission errors.

The start of heading (SOH) character was to mark a non-data section of a data stream—the part of a stream containing addresses and other housekeeping data. The start of text character (STX) marked the end of the header, and the start of the textual part of a stream. The end of text character (ETX) marked the end of the data of a message. A widely used convention is to make the two characters preceding ETX a checksum or CRC for error-detection purposes. The end of transmission block character (ETB) was used to indicate the end of a block of data, where data was divided into such blocks for transmission purposes.

The escape character (ESC) was intended to "quote" the next character, if it was another control character it would print it instead of performing the control function. It is almost never used for this purpose today. Various printable characters are used as visible "escape characters", depending on context.

The substitute character (SUB) was intended to request a translation of the next character from a printable character to another value, usually by setting bit 5 to zero. This is handy because some media (such as sheets of paper produced by typewriters) can transmit only printable characters. However, on MS-DOS systems with files opened in text mode, "end of text" or "end of file" is marked by this Ctrl-Z character, instead of the Ctrl-C or Ctrl-D, which are common on other operating systems.

The cancel character (CAN) signaled that the previous element should be discarded. The negative acknowledge character (NAK) is a definite flag for, usually, noting that reception was a problem, and, often, that the current element should be sent again. The acknowledge character (ACK) is normally used as a flag to indicate no problem detected with current element.

When a transmission medium is half duplex (that is, it can transmit in only one direction at a time), there is usually a master station that can transmit at any time, and one or more slave stations that transmit when they have permission. The enquire character (ENQ) is generally used by a master station to ask a slave station to send its next message. A slave station indicates that it has completed its transmission by sending the end of transmission character (EOT).

The device control codes (DC1 to DC4) were originally generic, to be implemented as necessary by each device. However, a universal need in data transmission is to request the sender to stop transmitting when a receiver is temporarily unable to accept any more data. Digital Equipment Corporation invented a convention which used 19 (the device control 3 character (DC3), also known as control-S, or XOFF) to "S"top transmission, and 17 (the device control 1 character (DC1), a.k.a. control-Q, or XON) to start transmission. It has become so widely used that most don't realize it is not part of official ASCII. This technique, however implemented, avoids additional wires in the data cable devoted only to transmission management, which saves money. A sensible protocol for the use of such transmission flow control signals must be used, to avoid potential deadlock conditions, however.

The data link escape character (DLE) was intended to be a signal to the other end of a data link that the following character is a control character such as STX or ETX. For example a packet may be structured in the following way (DLE)   (DLE) .

Miscellaneous codes
Code 7 (BEL) is intended to cause an audible signal in the receiving terminal.

Many of the ASCII control characters were designed for devices of the time that are not often seen today. For example, code 22, "synchronous idle" (SYN), was originally sent by synchronous modems (which have to send data constantly) when there was no actual data to send. (Modern systems typically use a start bit to announce the beginning of a transmitted word— this is a feature of asynchronous communication. Synchronous communication links were more often seen with mainframes, where they were typically run over corporate leased lines to connect a mainframe to another mainframe or perhaps a minicomputer.)

Code 0 (ASCII code name NUL) is a special case. In paper tape, it is the case when there are no holes. It is convenient to treat this as a fill character with no meaning otherwise. Since the position of a NUL character has no holes punched, it can be replaced with any other character at a later time, so it was typically used to reserve space, either for correcting errors or for inserting information that would be available at a later time or in another place. In computing, it is often used for padding in fixed length records; to mark the end of a string; and formerly to give printing devices enough time to execute a control function.

Code 127 (DEL, a.k.a. "rubout") is likewise a special case. Its 7-bit code is all-bits-on in binary, which essentially erased a character cell on a paper tape when overpunched. Paper tape was a common storage medium when ASCII was developed, with a computing history dating back to WWII code breaking equipment at Biuro Szyfrów. Paper tape became obsolete in the 1970s, so this clever aspect of ASCII rarely saw any use after that. Some systems (such as the original Apples) converted it to a backspace. But because its code is in the range occupied by other printable characters, and because it had no official assigned glyph, many computer equipment vendors used it as an additional printable character (often an all-black "box" character useful for erasing text by overprinting with ink).

Non-erasable programmable ROMs are typically implemented as arrays of fusible elements, each representing a bit, which can only be switched one way, usually from one to zero. In such PROMs, the DEL and NUL characters can be used in the same way that they were used on punched tape: one to reserve meaningless fill bytes that can be written later, and the other to convert written bytes to meaningless fill bytes. For PROMs that switch one to zero, the roles of NUL and DEL are reversed; also, DEL will only work with 7-bit characters, which are rarely used today; for 8-bit content, the character code 255, commonly defined as a nonbreaking space character, can be used instead of DEL.

Many file systems do not allow control characters in filenames, as they may have reserved functions.

title: Control character