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Surround sound

Surround sound is a technique for enriching the fidelity and depth of sound reproduction by using multiple audio channels from speakers that surround the listener (surround channels). Its first application was in movie theaters. Prior to surround sound, theater sound systems commonly had three screen channels of sound that played from three loudspeakers (left, center, and right) located in front of the audience. Surround sound adds one or more channels from loudspeakers to the side or behind the listener that are able to create the sensation of sound coming from any horizontal direction (at ground level) around the listener.

The technique enhances the perception of sound spatialization by exploiting sound localization: a listener's ability to identify the location or origin of a detected sound in direction and distance. This is achieved by using multiple discrete audio channels routed to an array of loudspeakers. Surround sound typically has a listener location (sweet spot) where the audio effects work best and presents a fixed or forward perspective of the sound field to the listener at this location.

Surround sound formats vary in reproduction and recording methods, along with the number and positioning of additional channels. The most common surround sound specification, the ITU's 5.1 standard, calls for 6 speakers: Center (C), in front of the listener; Left (L) and Right (R), at angles of 60°; Left Surround (LS) and Right Surround (RS) at angles of 100–120°; and a subwoofer, whose position is not critical.

Fields of application
Though cinema and soundtracks represent the major uses of surround techniques, its scope of application is broader than that, as surround sound permits creation of an audio-environment for all sorts of purposes. Multichannel audio techniques may be used to reproduce contents as varied as music, speech, natural or synthetic sounds for cinema, television, broadcasting, or computers. In terms of music content for example, a live performance may use multichannel techniques in the context of an open-air concert, of a musical theatre performance or for broadcasting; for a film, specific techniques are adapted to movie theater or to home (e.g. home cinema systems). The narrative space is also a content that can be enhanced through multichannel techniques. This applies mainly to cinema narratives, for example the speech of the characters of a film,  but may also be applied to plays performed in a theatre, to a conference, or to integrate voice-based comments in an archeological site or monument. For example, an exhibition may be enhanced with topical ambient sound of water, birds, train or machine noise. Topical natural sounds may also be used in educational applications. Other fields of application include video game consoles, personal computers and other platforms. In such applications, the content would typically be synthetic noise produced by the computer device in interaction with its user. Significant work has also been done using surround sound for enhanced situation awareness in military and public safety application.

Types of media and technologies
Commercial surround sound media include videocassettes, DVDs, and SDTV broadcasts encoded as analog matrixed Dolby Surround compressed Dolby Digital and DTS, and lossless audio such as DTS HD Master Audio and Dolby TrueHD on HDTV Blu-ray Disc and HD DVD, which are identical to the studio master. Other commercial formats include the competing DVD-Audio (DVD-A) and Super Audio CD (SACD) formats, and MP3 Surround. Cinema 5.1 surround formats include Dolby Digital and DTS. Sony Dynamic Digital Sound (SDDS) is an 8 channel cinema configuration which features 5 independent audio channels across the front with two independent surround channels, and a Low-frequency effects channel. Traditional 7.1 surround speaker configuration introduces two additional rear speakers to the conventional 5.1 arrangement, for a total of four surround channels and three front channels, to create a more 360° sound field.

Most surround sound recordings are created by film production companies or video game producers; however some consumer camcorders have such capability either built-in or available separately. Surround sound technologies can also be used in music to enable new methods of artistic expression. After the failure of quadraphonic audio in the 1970s, multichannel music has slowly been reintroduced since 1999 with the help of SACD and DVD-Audio formats. Some AV receivers, stereophonic systems, and computer sound cards contain integral digital signal processors or digital audio processors to simulate surround sound from a stereophonic source (see fake stereo).

In 1967, the rock group Pink Floyd performed the first-ever surround sound concert at "Games for May", a lavish affair at London’s Queen Elizabeth Hall where the band debuted its custom-made quadraphonic speaker system. The control device they had made, the Azimuth Co-ordinator, is now displayed at London's Victoria and Albert Museum, as part of their Theatre Collections gallery.

History
The first documented use of surround sound was in 1940, for the Disney studio's animated film Fantasia. Walt Disney was inspired by Nikolai Rimsky-Korsakov's operatic piece Flight of the Bumblebee to have a bumblebee featured in his musical Fantasia and also sound as if it was flying in all parts of the theatre. The initial multichannel audio application was called 'Fantasound', comprising three audio channels and speakers. The sound was diffused throughout the cinema, controlled by an engineer using some 54 loudspeakers. The surround sound was achieved using the sum and the difference of the phase of the sound. However, this experimental use of surround sound was excluded from the film in later showings. In 1952, "surround sound" successfully reappeared with the film "This is Cinerama", using discrete seven-channel sound, and the race to develop other surround sound methods took off.

In the 1950s, the German composer Karlheinz Stockhausen experimented with and produced ground-breaking electronic compositions such as Gesang der Jünglinge and Kontakte, the latter using fully discrete and rotating quadraphonic sounds generated with industrial electronic equipment in Herbert Eimert's studio at the Westdeutscher Rundfunk (WDR). Edgar Varese's Poème électronique, created for the Iannis Xenakis-designed Philips Pavilion at the 1958 Brussels World's Fair, also used spatial audio with 425 loudspeakers used to move sound throughout the pavilion.

In 1957, working with artist Jordan Belson, Henry Jacobs produced Vortex: Experiments in Sound and Light - a series of concerts featuring new music, including some of Jacobs' own, and that of Karlheinz Stockhausen, and many others - taking place in the Morrison Planetarium in Golden Gate Park, San Francisco. Sound designers commonly regard this as the origin of the (now standard) concept of "surround sound." The program was popular, and Jacobs and Belson were invited to reproduce it at the 1958 World Expo in Brussels. There are also many other composers that created ground-breaking surround sound works in the same time period.

In 1978, a concept devised by Max Bell for Dolby Laboratories called "split surround" was tested with the movie Superman. This led to the 70mm stereo surround release of Apocalypse Now, which became one of the first formal releases in cinemas with three channels in the front and two in the rear. There were typically five speakers behind the screens of 70mm-capable cinemas, but only the Left, Center and Right were used full-frequency, while Center-Left and Center-Right were only used for bass-frequencies (as it is currently common). The Apocalypse Now encoder/decoder was designed by Michael Karagosian, also for Dolby Laboratories. The surround mix was produced by an Oscar-winning crew led by Walter Murch for American Zoetrope. The format was also deployed in 1982 with the stereo surround release of Blade Runner.

The 5.1 version of surround sound originated in 1987 at the famous French Cabaret Moulin Rouge. A French engineer, Dominique Bertrand used a mixing board specially designed in cooperation with Solid State Logic, based on 5000 series and including six channels. Respectively: A left, B right, C centre, D left rear, E right rear, F bass. The same engineer had already achieved a 3.1 system in 1974, for the International Summit of Francophone States in Dakar, Senegal.

Creating surround sound
Surround sound is created in several ways. The first and simplest method is using a surround sound recording technique—capturing two distinct stereo images, one for the front and one for the back or by using a dedicated setup, e.g., an augmented Decca tree —or mixing-in surround sound for playback on an audio system using speakers encircling the listener to play audio from different directions. A second approach is processing the audio with psychoacoustic sound localization methods to simulate a two-dimensional (2-D) sound field with headphones. A third approach, based on Huygens' principle, attempts reconstructing the recorded sound field wave fronts within the listening space; an "audio hologram" form. One form, wave field synthesis (WFS), produces a sound field with an even error field over the entire area. Commercial WFS systems, currently marketed by companies sonic emotion and Iosono, require many loudspeakers and significant computing power. The 4th approach is using three mics, one for front, one for side and one for rear, also called Double MS recording.

The Ambisonics form, also based on Huygens' principle, gives an exact sound reconstruction at the central point; however, it is less accurate away from the central point. There are many free and commercial software programs available for Ambisonics, which dominates most of the consumer market, especially musicians using electronic and computer music. Moreover, Ambisonics products are the standard in surround sound hardware sold by Meridian Audio. In its simplest form, Ambisonics consumes few resources, however this is not true for recent developments, such as Near Field Compensated Higher Order Ambisonics. Some years ago it was shown that, in the limit, WFS and Ambisonics converge.

Finally, surround sound can also be achieved by mastering level, from stereophonic sources as with Penteo, which uses digital signal processing analysis of a stereo recording to parse out individual sounds to component panorama positions, then positions them, accordingly, into a five-channel field. However, there are more ways to create surround sound out of stereo, for instance with the routines based on QS and SQ for encoding Quad sound, where instruments were divided over 4 speakers in the studio. This way of creating surround with software routines is normally referred to as "upmixing", which was particularly successful on the Sansui QSD-series decoders that had a mode where it mapped the L ↔ R stereo onto an ∩ arc.

Standard configurations
There are many alternative setups available for a surround sound experience, with a 3-2 (3 front, 2 back speakers and a Low Frequency Effects channel) configuration (more commonly referred to as 5.1 surround) being the standard for most surround sound applications, including cinema, television and consumer applications. This is a compromise between the ideal image creation of a room and that of practicality and compatibility with two-channel stereo. Because most surround sound mixes are produced for 5.1 surround (6 channels), larger setups require matrixes or processors to feed the additional speakers.

The standard surround setup consists of three front speakers LCR (left, center and right), two surround speakers LS and RS (left and right surround respectively) and a subwoofer for the Low Frequency Effects (LFE) channel, that is low-pass filtered at 120 Hz. The angles between the speakers have been standardized by the ITU (International Telecommunication Union) recommendation 775 and AES (Audio Engineering Society) as follows: 60 degrees between the L and R channels (allows for two-channel stereo compatibility) with the center speaker directly in front of the listener. The Surround channels are placed 100–120 degrees from the center channel, with the subwoofer's positioning not being critical due to the low directional factor of frequencies below 120 Hz. The ITU standard also allows for additional surround speakers, that need to be distributed evenly between 60 and 150 degrees.

Surround mixes of more or fewer channels are acceptable, if they are compatible, as described by the ITU-R BS. 775-1, with 5.1 surround. The 3-1 channel setup (consisting of one monophonic surround channel) is such a case, where both LS and RS are fed by the monophonic signal at an attenuated level of -3 dB.

The function of the center channel is to anchor the signal so that any central panned images do not shift when a listener is moving or is sitting away from the sweet spot. The center channel also prevents any timbral modifications from occurring, which is typical for 2-channel stereo, due to phase differences at the two ears of a listener. The centre channel is especially used in films and television, with dialogue primarily feeding the center channel. The function of the center channel can either be of a monophonic nature (as with dialogue) or it can be used in combination with the left and right channels for true three-channel stereo. Motion Pictures tend to use the center channel for monophonic purposes with stereo being reserved purely for the left and right channels. Surround microphones techniques have however been developed that fully use the potential of three-channel stereo.

In 5.1 surround, phantom images between the front speakers are quite accurate, with images towards the back and especially to the sides being unstable. The localisation of a virtual source, based on level differences between two loudspeakers to the side of a listener, shows great inconsistency across the standardised 5.1 setup, also being largely affected by movement away from the reference position. 5.1 surround is therefore limited in its ability to convey 3D sound, making the surround channels more appropriate for ambience or effects. )

7.1 channel surround is another setup, most commonly used in large cinemas, that is compatible with 5.1 surround, though it is not stated in the ITU-standards. 7.1 channel surround adds two additional channels, center-left (CL) and center-right (CR) to the 5.1 surround setup, with the speakers situated 15 degrees off centre from the listener. This convention is used to cover an increased angle between the front loudspeakers as a product of a larger screen.

Surround microphone techniques
Most 2-channel stereophonic microphone techniques are compatible with a 3-channel setup (LCR), as many of these techniques already contain a center microphone or microphone pair. Microphone techniques for LCR should, however, try to obtain greater channel separation to prevent conflicting phantom images between L/C and L/R for example. Specialised techniques have therefore been developed for 3-channel stereo. Surround microphone techniques largely depend on the setup used, therefore being biased towards the 5.1 surround setup, as this is the standard.

Surround recording techniques can be differentiated into those that use single arrays of microphones placed in close proximity, and those treating front and rear channels with separate arrays. Close arrays present more accurate phantom images, whereas separate treatment of rear channels is usually used for ambience. For accurate depiction of an acoustic environment, such as a halls, side reflections are essential. Appropriate microphone techniques should therefore be used, if room impression is important. Although the reproduction of side images are very unstable in the 5.1 surround setup, room impressions can still be accurately presented.

Some microphone techniques used for coverage of three front channels, include double-stereo techniques, INA-3 (Ideal Cardioid Arrangement), the Decca Tree setup and the OCT (Optimum Cardioid Triangle). Surround techniques are largely based on 3-channel techniques with additional microphones used for the surround channels. A distinguishing factor for the pickup of the front channels in surround is that less reverberation should be picked up, as the surround microphones will be responsible for the pickup of reverberation. Cardioid, hypercardioid, or supercardioid polar patterns will therefore often replace omnidirectional polar patterns for surround recordings. To compensate for the lost low-end of directional (pressure gradient) microphones, additional omnidirectional (pressure microphones), exhibiting an extended low-end response, can be added. The microphone's output is usually low-pass filtered. A simple surround microphone configuration involves the use of a front array in combination with two backward-facing omnidirectional room microphones placed about 10–15 meters away from the front array. If echoes are notable, the front array can be delayed appropriately. Alternatively, backward facing cardioid microphones can be placed closer to the front array for a similar reverberation pickup.

The INA-5 (Ideal Cardioid Arrangement) is a surround microphone array that uses five cardioid microphones resembling the angles of the standardised surround loudspeaker configuration defined by the ITU Rec. 775. Dimensions between the front three microphone as well as the polar patterns of the microphones can be changed for different pickup angles and ambient response. This technique therefore allows for great flexibility.

A well established microphone array is the Fukada Tree, which is a modified variant of the Decca Tree stereo technique. The array consists of five spaced cardioid microphones, three front microphones resembling a Decca Tree and two surround microphones. Two additional omnidirectional outriggers can be added to enlarge the perceived size of the orchestra or to better integrate the front and surround channels. The L, R, LS and RS microphones should be placed in a square formation, with L/R and LS/RS angled at 45 degrees and 135 degrees from the center microphone respectively. Spacing between these microphones should be about 1.8 meters. This square formation is responsible for the room impressions. The center channel is placed a meter in front of the L and R channels, producing a strong center image. The surround microphones are usually placed at the critical distance (where the direct and reverberant field is equal), with the full array usually situated several meters above and behind the conductor.

The NHK (Japanese broadcasting company) developed an alternative technique also involving five cardioid microphones. Here a baffle is used for separation between the front left and right channels, which are 30 cm apart. Outrigger omnidirectional microphones, low-pass filtered at 250 Hz, are spaced 3 meters apart in line with the L and R cardioids. These compensate for the bass roll-off of the cardioid microphones and also add expansiveness. A 3-meter spaced microphone pair, situated 2–3 meters behind front array, is used for the surround channels. The centre channel is again placed slightly forward, with the L/R and LS/RS again angled at 45 and 135 degrees respectively.

The OCT-Surround (Optimum Cardioid Triangle-Surround) microphone array is an augmented technique of the stereo OCT technique using the same front array with added surround microphones. The front array is designed for minimum crosstalk, with the front left and right microphones having supercardioid polar patterns and angled at 90 degrees relative to the center microphone. It is important that high quality small diaphragm microphones are used for the L and R channels to reduce off-axis coloration. Equalization can also be used to flatten the response of the supercardioid microphones to signals coming in at up to about 30 degrees from the front of the array. The center channel is placed slightly forward. The surround microphones are backwards facing cardioid microphones, that are placed 40 cm back from the L and R microphones. The L, R, LS and RS microphones pick up early reflections from both the sides and the back of an acoustic venue, therefore giving significant room impressions. Spacing between the L and R microphones can be varied to obtain the required stereo width.

Specialized microphone arrays have been developed for recording purely the ambience of a space. These arrays are used in combination with suitable front arrays, or can be added to above mentioned surround techniques. The Hamasaki square (also proposed by NHK) is a well established microphone array used for the pickup of hall ambience. Four figure-eight microphones are arranged in a square, ideally placed far away and high up in the hall. Spacing between the microphones should be between 1–3 meters. The microphones nulls (zero pickup point) are set to face the main sound source with positive polarities outward facing, therefore very effectively minimizing the direct sound pickup as well as echoes from the back of the hall The back two microphones are mixed to the surround channels, with the front two channels being mixed in combination with the front array into L and R.

Another ambient technique is the IRT (Institut für Rundfunktechnik) cross. Here, four cardioid microphones, 90 degrees relative to one another, are placed in square formation, separated by 21–25 cm. The front two microphones should be positioned 45 degrees off axis from the sound source. This technique therefore resembles back to back near-coincident stereo pairs. The microphones outputs are fed to the L, R and LS, RS channels. The disadvantage of this approach is that direct sound pickup is quite significant.

Many recordings do not require pickup of side reflections. For Live Pop music concerts a more appropriate array for the pickup of ambience is the cardioid trapezium. All four cardioid microphones are backward facing and angled at 60 degrees from one another, therefore similar to a semi-circle. This is effective for the pickup of audience and ambience.

All the above-mentioned microphone arrays take up considerable space, making them quite ineffective for field recordings. In this respect, the double MS (Mid Side) technique is quite advantageous. This array uses back to back cardioid microphones, one facing forward, the other backwards, combined with either one or two figure-eight microphone. Different channels are obtained by sum and difference of the figure-eight and cardioid patterns. When using only one figure-eight microphone, the double MS technique is extremely compact and therefore also perfectly compatible with monophonic playback. This technique also allows for postproduction changes of the pickup angle.

Bass management
Surround replay systems may make use of bass management, the fundamental principle of which is that bass content in the incoming signal, irrespective of channel, should be directed only to loudspeakers capable of handling it, whether the latter are the main system loudspeakers or one or more special low-frequency speakers called subwoofers.

There is a notation difference before and after the bass management system. Before the bass management system there is a Low Frequency Effects (LFE) channel. After the bass management system there is a subwoofer signal. A common misunderstanding is the belief that the LFE channel is the "subwoofer channel". The bass management system may direct bass to one or more subwoofers (if present) from any channel, not just from the LFE channel. Also, if there is no subwoofer speaker present then the bass management system can direct the LFE channel to one or more of the main speakers.

Low frequency effects channel
Because the low-frequency effects (LFE) channel requires only a fraction of the bandwidth of the other audio channels, it is referred to as the .1 channel; for example 5.1 or 7.1.

The LFE channel is a source of some confusion in surround sound. It was originally developed to carry extremely low sub-bass cinematic sound effects (e.g., the loud rumble of thunder or explosions) on their own channel. This allowed theaters to control the volume of these effects to suit the particular cinema's acoustic environment and sound reproduction system. Independent control of the sub-bass effects also reduced the problem of intermodulation distortion in analog movie sound reproduction.

In the original movie theater implementation, the LFE was a separate channel fed to one or more subwoofers. Home replay systems, however, may not have a separate subwoofer, so modern home surround decoders and systems often include a bass management system that allows bass on any channel (main or LFE) to be fed only to the loudspeakers that can handle low-frequency signals. The salient point here is that the LFE channel is not the subwoofer channel; there may be no subwoofer and, if there is, it may be handling a good deal more than effects.

Some record labels such as Telarc and Chesky have argued that LFE channels are not needed in a modern digital multichannel entertainment system. They argue that, given loudspeakers that have low frequency response to 30Hz, all available channels have a full-frequency range and, as such, there is no need for an LFE in surround music production, because all the frequencies are available in all the main channels. These labels sometimes use the LFE channel to carry a height channel. The label BIS Records generally uses a 5.0 channel mix.

Channel notation
Channel notation indicates the number of discrete channels encoded in the audio signal, not necessarily the number of channels reproduced for playback. The number of playback channels can be increased by using matrix decoding. The number of playback channels may also differ from the number of speakers used to reproduce them if one or more channels drives a group of speakers. Notation represents the number of channels, not the number of speakers.

The first digit in "5.1" is the number of full range channels. The ".1" reflects the limited frequency range of the LFE channel.

For example, two stereo speakers with no LFE channel = 2.0 5 full-range channels + 1 LFE channel = 5.1

An alternative notation shows the number of full-range channels in front of the listener, separated by a slash from the number of full-range channels beside or behind the listener, with a decimal point marking the number of limited-range LFE channels.

E.g. 3 front channels + 2 side channels + an LFE channel = 3/2.1

The notation can be expanded to include Matrix Decoders. Dolby Digital EX, for example, has a sixth full-range channel incorporated into the two rear channels with a matrix. This is expressed:

3 front channels + 2 rear channels + 3 channels reproduced in the rear in total + 1 LFE channel = 3/2:3.1

The term stereo, although popularised in reference to two channel audio, historically also referred to surround sound, as it strictly means "solid" (three-dimensional) sound. However this is no longer common usage and "stereo sound" almost exclusively means two channels, left and right.

Channel identification
In accordance with ANSI/CEA-863-A
 * {|class="wikitable sortable"

! colspan=3 | Zero-based channel index ! rowspan=2 | Channel name ! rowspan=2 colspan=2 | Color-coding on commercial receiver and cabling ! MP3/WAV/FLAC ! DTS/AAC ! Vorbis/Opus
 * +ANSI/CEA-863-A identification for surround sound channels
 * 0
 * 1
 * 0
 * Front left
 * style="background:white"| ||White
 * 1
 * 2
 * 2
 * Front right
 * style="background:red"| ||Red
 * 2
 * 0
 * 1
 * Center
 * style="background:green"| ||Green
 * 3
 * 5
 * 7
 * Subwoofer
 * style="background:purple"| ||Purple
 * 4
 * 3
 * 3
 * Side left
 * style="background:blue"| ||Blue
 * 5
 * 4
 * 4
 * Side right
 * style="background:gray"| ||Grey
 * 6
 * 6
 * 5
 * Rear left
 * style="background:brown"| ||Brown
 * 7
 * 7
 * 6
 * Rear right
 * style="background:khaki"| ||Khaki
 * }
 * style="background:brown"| ||Brown
 * 7
 * 7
 * 6
 * Rear right
 * style="background:khaki"| ||Khaki
 * }
 * }

 * {|class="wikitable"

 * +Diagram
 * style="background:white;color:black"|Front left
 * style="background:green;color:white"|Center
 * style="background:red;text-align:right;color:white"|Front right
 * style="background:blue;color:white"|Side left
 * style="text-align:center;font-size:200%;" rowspan="2"| Person at the cinema with popcorn.svg
 * style="background:gray;text-align:right;color:black"|Side right
 * style="background:brown;color:white"|Rear left
 * style="background:khaki;text-align:right;color:black"|Rear right
 * style="background:purple;text-align:center;color:white" colspan="3"|Subwoofer
 * }
 * style="background:purple;text-align:center;color:white" colspan="3"|Subwoofer
 * }
 * }

 * {|class="wikitable"

!Index !Channel name !colspan=2|Color-coding on commercial receiver and cabling
 * +Height channels
 * 8
 * Left height 1
 * style="background:yellow"| ||Yellow
 * 9
 * Right height 1
 * style="background:orange"| ||Orange
 * 10
 * Left height 2
 * style="background:pink"| ||Pink
 * 11
 * Right height 2
 * style="background:magenta"| ||Magenta
 * }
 * 11
 * Right height 2
 * style="background:magenta"| ||Magenta
 * }

Sonic Whole Overhead Sound
In 2002, Dolby premiered a master of We Were Soldiers which featured a Sonic Whole Overhead Sound soundtrack. This mix included a new ceiling-mounted height channel.

Ambisonics
Ambisonics is a recording and playback technique using multichannel mixing that can be used live or in the studio and which recreates the soundfield as it existed in the space, in contrast to traditional surround systems, which can only create illusion of the soundfield if the listener is located in a very narrow sweetspot between speakers. Any number of speakers in any physical arrangement can be used to recreate a sound field. With 6 or more speakers arranged around a listener, a 3-dimensional ("periphonic", or full-sphere) sound field can be presented. Ambisonics was invented by Michael Gerzon.

Binaural recording
Binaural recording is a method of recording sound that uses two microphones, arranged with the intent to create the 3-D stereo experience of being present in the room with the performers or instruments. The idea of a three dimensional or "internal" form of sound has developed into technology for stethoscopes creating "in-head" acoustics and IMAX movies creating a three dimensional acoustic experience.

Panor-Ambiophonic (PanAmbio) 4.0/4.1
PanAmbio combines a stereo dipole and crosstalk cancellation in front and a second set behind the listener (total of four speakers) for 360° 2D surround reproduction. Four channel recordings, especially those containing binaural cues, create speaker-binaural surround sound. 5.1 channel recordings, including movie DVDs, are compatible by mixing C-channel content to the front speaker pair. 6.1 can be played by mixing SC to the back pair.

Standard speaker channels
Several speaker configurations are commonly used for consumer equipment. The order and identifiers are those specified for the channel mask in the standard uncompressed WAV file format (which contains a raw multichannel PCM stream) and are used according to the same specification for most PC connectible digital sound hardware and PC operating systems capable of handling multiple channels. While it is possible to build any speaker configuration, there is little commercial movie or music content for alternative speaker configurations. However, source channels can be remixed for the speaker channels using a matrix table specifying how much of each content channel is played through each speaker channel.

Most channel configuration may include a low frequency effects (LFE) channel (the channel played through the subwoofer.) This makes the configuration ".1" instead of ".0". Most modern multichannel mixes contain one LFE, some use two.

7.1 surround sound
7.1 surround sound is a popular format in theaters & Home cinema including Blu-rays with Dolby and DTS being major players.

7.1.2/7.1.4 immersive sound
7.1.2 and 7.1.4 immersive sound along with 5.1.2 and 5.1.4 format adds either 2 or 4 overhead speakers to enable sound objects and special effect sounds to be panned overhead for the listener. Introduced for theatrical film releases in 2012 by Dolby Laboratories under the trademark name Dolby Atmos.

Dolby Atmos (and other Microsoft Spatial Sound engines; see AudioObjectType in SpatialAudioClient.h) additionally support a virtual "8.1.4.4" configuration, to be rendered by a HRTF. The configuration adds to 7.1.4 with a center speaker behind the listener and 4 speakers below.

10.2 surround sound
10.2 is the surround sound format developed by THX creator Tomlinson Holman of TMH Labs and University of Southern California (schools of Cinema/Television and Engineering). Developed along with Chris Kyriakakis of the USC Viterbi School of Engineering, 10.2 refers to the format's promotional slogan: "Twice as good as 5.1". Advocates of 10.2 argue that it is the audio equivalent of IMAX.

11.1 surround sound
11.1 sound is supported by BARCO with installations in theaters worldwide.

22.2 surround sound
22.2 is the surround sound component of Ultra High Definition Television, developed by NHK Science & Technical Research Laboratories. As its name suggests, it uses 24 speakers. These are arranged in three layers: A middle layer of ten speakers, an upper layer of nine speakers, and a lower layer of three speakers and two sub-woofers. The system was demonstrated at Expo 2005, Aichi, Japan, the NAB Shows 2006 and 2009, Las Vegas, and the IBC trade shows 2006 and 2008, Amsterdam, Netherlands.

title: Surround sound


text:
Operation Samwest

During World War II, Operation Samwest (5–12 June 1944) was a large raid conducted by 116 Free French paratroops of the 4th Special Air Service Regiment. Their objective was to hinder movement of German troops from west Brittany to the Normandy beaches via ambush and sabotage attempts.

The first phase of the mission was to establish a secure base on the Breton Peninsula, near St. Brieuc in Duault in the Bretagne Region. Their base was heavily attacked by German troops on 12 June and they were forced to disperse.

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

title: Operation Samwest


text:
Apple ProDOS

ProDOS is the name of two similar operating systems for the Apple II series of personal computers. The original ProDOS, renamed ProDOS 8 in version 1.2, is the last official operating system usable by all 8-bit Apple II series computers, and was distributed from 1983 to 1993. The other, ProDOS 16, was a stop-gap solution for the 16-bit Apple II that was replaced by GS/OS within two years.

ProDOS was marketed by Apple as meaning Professional Disk Operating System, and became the most popular operating system for the Apple II series of computers 10 months after its release in January 1983.

Background
ProDOS was released to address shortcomings in the earlier Apple operating system (called simply DOS), which was beginning to show its age.

Apple DOS only has built-in support for 5.25" floppy disks and requires patches to use peripheral devices such as hard disk drives and non-Disk-II floppy disk drives, including 3.5" floppy drives. ProDOS adds a standard method of accessing ROM-based drivers on expansion cards for disk devices, expands the maximum volume size from about 400 kilobytes to 32 megabytes, introduces support for hierarchical subdirectories (a vital feature for organizing a hard disk's storage space), and supports RAM disks on machines with 128 KB or more of memory. ProDOS addresses problems with handling hardware interrupts, and includes a well-defined and documented programming and expansion interface, which Apple DOS had always lacked. Although ProDOS also includes support for a real-time clock (RTC), this support went largely unused until the release of the Apple II, the first in the Apple II series to include an RTC on board. Third-party clocks were available for the II Plus, IIe, and IIc, however.

ProDOS, unlike earlier Apple DOS versions, has its developmental roots in SOS, the operating system for the ill-fated Apple III computer released in 1980. Pre-release documentation for ProDOS (including early editions of Beneath Apple ProDOS) documented SOS error codes, notably one for switched disks, that ProDOS itself could never generate. Its disk format and programming interface are completely different from those of Apple DOS, and ProDOS cannot read or write DOS 3.3 disks except by means of a conversion utility; while the low-level track-and-sector format of DOS 3.3 disks was retained for 5.25-inch disks, the high-level arrangement of files and directories is completely different. For this reason, most machine-language programs that run under Apple DOS will not work under ProDOS. However, most BASIC programs work, though they sometimes require minor changes. A third-party program called DOS.MASTER enables users to have multiple virtual DOS 3.3 partitions on a larger ProDOS volume.

With the release of ProDOS came the end of support for Integer BASIC and the original Apple II model, which had long since been effectively supplanted by Applesoft BASIC and the Apple II Plus. Whereas DOS 3.3 always loads built-in support for BASIC programming, under ProDOS this job is given to a separate system program called BASIC.SYSTEM, which one launches to run and write Applesoft BASIC programs. BASIC itself continued to be built into the Apple ROMs; BASIC.SYSTEM is merely a command interpreter enhancement that allows BASIC programs to access ProDOS by means of the same "Control-D" text output they had used under DOS 3.3. BASIC.SYSTEM alone requires about as much memory as the whole of DOS 3.3. Since the ProDOS kernel itself is stowed away in the "Language Card" RAM, the usable amount of RAM for BASIC programmers remains the same under ProDOS as it had been under DOS 3.3.

Despite ProDOS's many advantages, many users and programmers resisted it for a time because of their investment in learning the ins and outs of Apple DOS and in Apple-DOS-based software and data formats. A contributing reason was that ProDOS allows only 15 characters in a filename compared to Apple DOS's 30. But Apple's integrated software package AppleWorks, released in 1984, proved a compelling reason to switch, and by the end of 1985 few new software products were being released for the older operating system. Apple IIs continued to be able to boot the older DOS (even the Apple II GS can boot the older DOS floppies) but as 3.5" floppies and hard disks became more prevalent, most users spent the bulk of their time in ProDOS.

The Apple IIe, also released in 1983, was the first Apple II computer to have 64 KB of memory built in. For a while, Apple shipped both DOS 3.3 and ProDOS with new computers.

The original ProDOS was renamed ProDOS 8 when ProDOS 16 was released to support the 16-bit Apple II computer, although ProDOS 16 (which was 8-bit at its core) was soon replaced by GS/OS.

Requirements
All editions of ProDOS require an Apple II series computer or compatible.

ProDOS 8 requires 64 KB of memory to run. The original ProDOS (8) 1.0 through 1.0.2 requires only 48 KB for the kernel, but nearly all programs, including the BASIC.SYSTEM needed to use Applesoft BASIC, require 64 KB, making a 48 KB system useless for ProDOS as a practical matter, and support for 48 KB machines was removed in version 1.1.

ProDOS 8 version 2.x requires a 65C02 or later (65802, 65816) CPU. ProDOS 8 2.x runs in 64 KB, but the utility programs on the system disk require 128 KB. Systems with a 6502 CPU instead of a 65C02 must use ProDOS 8 versions prior to version 2.0.

ProDOS 16 requires an Apple IIGS.

Unlicensed Apple II clones
With the release of ProDOS version 1.01 and higher, a check was added to see if it was running on an official Apple-manufactured computer. If the word "Apple" is found in the computer's ROM firmware, ProDOS will load up as normal. If anything else is found (e.g. "Golden", "Franklin", "Elite") ProDOS refuses to run, locking up at the boot splash screen. This measure was taken by Apple Computer to discourage use of unlicensed Apple II clones. It is still possible to run newer versions of ProDOS on clones; however, users have to apply a small byte patch to every successive version of ProDOS. Some users go as far as replacing their physical ROM chip(s) with an illegal copied version of Apple's own ROM; or, failing that, a custom patched ROM with "Apple" added in the name.

Disk support
ProDOS 8 natively supports Disk II-compatible floppy drives, a RAM drive of approximately 59 KB on computers having 128 KB or more RAM, and block devices whose controllers support the Pascal firmware protocol, a standardized method of accepting block reads and writes originally introduced for use with the UCSD p-System. This latter category includes 3.5" floppy drives and hard drives. Custom block device drivers can be hooked into the OS as well.

File system
ProDOS uses the same file system as the earlier Apple SOS for the Apple III. The SOS/ProDOS file system is native to Apple SOS, ProDOS 8, ProDOS 16, and GS/OS. Some classic Mac OS versions also come with a file system translator to handle this file system.

A volume is allocated in 512-byte blocks. (5.25" floppy disks are still formatted using 256-byte sectors, as this is the format required by the controller ROM to boot the disk. ProDOS simply treats pairs of 256-byte sectors as a single block on such drives.) A volume can have a capacity of up to 32 megabytes, and each file can be up to 16 megabytes. Each volume (floppy disk or hard drive partition) has a "volume name", a filename which is used as the base directory name; having two volumes with the same volume name can result in conflicts. If necessary, ProDOS searches all available drives to find a named volume. Subdirectories are supported, and the concept of a "prefix" (working directory or current path) was provided to make working with subdirectories easier.

File, directory, and volume names can be 1 to 15 characters, starting with a letter, then containing more letters, digits or periods. Each file entry also contains the 16-bit (2-byte) pointer to the block containing the beginning of the file (or its block index); a 16-bit block count; a 24-bit (3-byte) file size; an 8-bit (1-byte) filetype; a 16-bit auxiliary type (the meaning of which depends upon the filetype); creation and modification timestamps; and data related to how the file is stored on the volume. Sparse files are supported, but files are never "sparsified" by removing zero-filled blocks. The volume header contains similar information as relevant to volumes.

Directories (including the root directory) are sequentially indexed, with each block starting with the address of the previous block (or zero if none) and the subsequent block (or zero if none). The root directory on most disks is initialized to 4 blocks, allowing 51 entries (excluding the volume header). It never changes in size, except by manual intervention with special tools. Subdirectories begin at one block, and grow automatically as needed.

Normal files are progressively indexed. Single-block files (under 513 bytes) have no index block; the directory entry points directly to the block of file data. Files with between 2 and 256 blocks (513 bytes to 128 KB) of data have a single index block, to which the directory entry points, which contains a list of up to 256 data block addresses. Larger files have a master index block containing a list of up to 256 index block addresses. When the Apple IIGS was introduced, a new storage format was introduced for files with two forks, as was typical for IIGS system and program files; the directory entry points to an informational block that tells the computer the storage format of the two forks. These files cannot be read or written natively by ProDOS 8, though the volume itself remains compatible.

The volume has a bitmap of used blocks. Other than this, there is no central file allocation table.

A ProDOS 8 volume formatted by Apple's tools has a boot sector that supports booting both ProDOS and SOS depending on what computer it is booted on. Block 0 is the Apple II boot block and block 1 boots SOS. This allows a disk to be used to boot on either Apple II or Apple III computers by putting both operating system kernels in the top directory: the Apple II boot sector looks for the file PRODOS and the Apple III boot sector looks for the file SOS.KERNEL. Third-party formatting utilities often did not provide the SOS boot block, and some would even mark block 1 available for user data.

ProDOS has no kernel support for other file systems. If necessary, a conversion utility on the main system disk is used to transfer files individually between ProDOS and older Apple DOS 3.3 disks. Because they use a different low-level disk format than DOS 3.3 and ProDOS, transferring data from DOS 3.2 disks to ProDOS is a two-step process using a DOS 3.3 disk as an intermediary (utilizing the DOS 3.3 utility MUFFIN or similar).

title: Apple ProDOS


text:
Operation Cooney

Operation Cooney was the deployment of elements of the 4ème Bataillon d'Infanterie de l'Air - the 4th Free French Parachute Battalion (later renamed 2ème Régiment de Chasseurs Parachutistes) - also known as 4th Special Air Service.

On 7 June 1944, the 9 aircraft of 38 Group (including two from No. 297 Squadron RAF), dropped the parachutists.

These men were to disrupt enemy communications between West Brittany and the remainder of France, and in all 58 Free French soldiers were dropped on no fewer than 18 undefended drop zones between St. Malo and Vannes. Their goal was to impair the German Army's response to the unfolding invasion of Normandy, Operation Overlord.

Breaking into 18 three-man or five-man SAS teams, the commandos scattered throughout Brittany destroying railroad targets. As a sign they had passed through they tied railroad ties around trees.

Some raiders then joined the base established by the Dingson team in Saint-Marcel, Morbihan or the base established by the Samwest team in Duault, Côtes d'Armor.

Dramatization

 * The Longest Day, a 1962 American film produced by Darryl F. Zanuck : 3 Free French SAS paratroopers, 1 woman (Janine Boitard) and 1 Resistance fighter against 2 german soldiers on the railroad near the bridge before the explosion.

title: Operation Cooney


text:
Bus station

A bus station or a bus interchange is a structure where city buses or intercity buses stop to pick up and drop off passengers. While the term bus depot can also be used to refer to a bus station, it can also refer to a bus garage. A bus station is larger than a bus stop, which is usually simply a place on the roadside, where buses can stop. It may be intended as a terminal station for a number of routes, or as a transfer station where the routes continue.

Bus station platforms may be assigned to fixed bus lines, or variable in combination with a dynamic passenger information system. The latter requires fewer platforms, but does not provide consistent locations for passengers.

Largest bus stations
Kilambakkam bus terminus in Chennai is spread over an area of 88.52 acre, making it the largest bus station in the world.

The Woodlands Bus Interchange in Singapore is one of the busiest bus interchanges in the world, handling up to 400,000 passengers daily across 42 bus services. Other Singaporean bus interchanges such as Bedok Bus Interchange, Tampines Bus Interchange and Yishun Bus Interchange handle similar number of passengers daily.

The largest underground bus station in Europe is Kamppi Centre in Helsinki, Finland completed in 2006. The terminal cost 100 million Euro to complete and took 3 years to design and build. Today, the bus terminal, which covers 25,000 square meters, is the busiest bus terminal in Finland. Every day, the terminal has around 700 bus departures, transporting approximately 170,000 passengers.

Preston Bus Station in Preston, England, built in 1969 and later heritage-listed, was described in 2014 as "depending on how you measure it, the largest bus station in the world, the second-biggest in Europe, and the longest in Europe". It was fully refurbished in 2018.

The largest bus terminal in North America is the Port Authority Bus Terminal located in New York City. The terminal is located in Midtown at 625 Eighth Avenue between 40th Street and 42nd Street, one block east of the Lincoln Tunnel and one block west of Times Square. The terminal is the largest in the Western Hemisphere and the busiest in the world by volume of traffic, serving about 8,000 buses and 225,000 people on an average weekday and more than 65 million people a year. It has 223 gates. It operates intercity bus routes all over the United States and some routes with international destinations, mostly in Canada, and mostly operated by Greyhound Lines.

The largest bus terminal in the southern hemisphere is the Tietê Bus Terminal located in São Paulo, Brazil. It is also the 2nd busiest in the world, serving about 90,000 people per weekday in 300 bus lines on its 89 platforms (72 for boarding and 17 for deboarding), with services to over 1,000 cities over the country and South America. The terminal is also linked to Portuguesa-Tietê, an adjacent metro station.

title: Bus station


text:
Board foot

The board foot or board-foot is a unit of measurement for the volume of lumber in the United States and Canada. It equals the volume of a board that is 1 ft in length, 1 ft in width, and 1 in in thickness.

Board foot can be abbreviated as FBM (for "foot, board measure"), BDFT, or BF. A thousand board feet can be abbreviated as MFBM, MBFT, or MBF. Similarly, a million board feet can be abbreviated as MMFBM, MMBFT, or MMBF.

Until the 1970s, in Australia and New Zealand, the terms super foot and superficial foot were used with the same meaning.

Description
One board foot equals:
 * 1 ft × 1 ft × 1 in
 * 12 in × 12 in × 1 in
 * 144 in3
 * 1/12 ft3
 * ≈ 1 bdft
 * ≈ 1 bdft
 * ≈ 1 bdft or steres
 * 1/1980 Petrograd Standard of board

Usage
The board foot is used to measure rough lumber (before drying and planing with no adjustments) or planed/surfaced lumber. An example of planed lumber is softwood "two by four" lumber sold by large lumber retailers, nominally 2 x. The 2 × 4 is actually only 1+1/2 ×, but the dimensions for the lumber when purchased wholesale could still be represented as full 2 × 4 lumber, although the "standard" can vary between vendors. This means that nominal lumber includes air space around the physical board when calculating board feet in some situations, while the true measurement of "board feet" should be limited to the actual dimensions of the board.

For planed lumber, board feet refer to the nominal thickness and width of lumber, calculated in principle on its size before drying and planing. Here, the actual length is used.

Note: see dimensional lumber for a full discussion of the relationship of actual and nominal dimensions. Briefly, for softwoods, to convert nominal to actual, subtract 1/4 in for dimensions under 2 inches; subtract 1/2 in for dimensions over 2 inches and under 8 inches; and subtract 3/4 in for larger measurements. The system is more complicated for hardwoods.

title: Board foot


text:
Temporal

Temporal may refer to:

Entertainment

 * Temporal (band), an Australian metal band
 * Temporal (Radio Tarifa album), 1997
 * Temporal (Love Spirals Downwards album), 2000
 * Temporal (Isis album), 2012
 * Temporal (video game), a 2008 freeware platform and puzzle game
 * Temporal (film), a 2022 Sri Lankan short film

Philosophy

 * Temporality
 * Temporal actual entity, see

Other

 * Temporal (anatomy), An alternative for lateral, in the head; towards the temporal bone
 * Temporality (ecclesiastical), or temporal goods, secular possessions of the Church
 * Temporal database

title: Temporal


text:
Apple DOS

Apple DOS is the disk operating systems for the Apple II computers from late 1978 through early 1983. It was superseded by ProDOS in 1983. Apple DOS has three major releases: DOS 3.1, DOS 3.2, and DOS 3.3; each one of these three releases was followed by a second, minor "bug-fix" release, but only in the case of Apple DOS 3.2 did that minor release receive its own version number, Apple DOS 3.2.1. The best-known and most-used version is Apple DOS 3.3 in the 1980 and 1983 releases. Prior to the release of Apple DOS 3.1, Apple users had to rely on audio cassette tapes for data storage and retrieval.

Version history
When Apple Computer introduced the Apple II in April 1977, the new computer had no disk drive or disk operating system (DOS). Although Apple co-founder Steve Wozniak designed the Disk II controller late that year, and believed that he could have written a DOS, his co-founder Steve Jobs decided to outsource the task. The company considered using Digital Research's CP/M, but Wozniak sought an operating system that was easier to use. On 10 April 1978 Apple signed a $13,000 contract with Shepardson Microsystems to write a DOS and deliver it within 35 days. Apple provided detailed specifications, and early Apple employee Randy Wigginton worked closely with Shepardson's Paul Laughton as the latter wrote the operating system with punched cards and a minicomputer.

There was no Apple DOS 1 or 2. Versions 0.1 through 2.8 were serially enumerated revisions during development, which might as well have been called builds 1 through 28. Apple DOS 3.0, a renamed issue of version 2.8, was never publicly released due to bugs. Apple published no official documentation until release 3.2.

Apple DOS 3.1 was publicly released in June 1978, slightly more than one year after the Apple II was introduced, becoming the first disk-based operating system for any Apple computer. A bug-fix release came later, addressing a problem by means of its MASTER CREATE utility, which was used to create Apple DOS master (bootable) disks: The built-in INIT command created disks that could be booted only on machines with at least the same amount of memory as the one that had created them. MASTER CREATE includes a self-relocating version of DOS that boots on Apples with any memory configuration.

Apple DOS 3.2 was released in 1979 to reflect changes in computer booting methods that were built into the successor of the Apple II, the Apple II Plus. New firmware included an auto-start feature which automatically found a disk controller and booted from it when the system was powered up—earning it the name "Autostart ROM". DOS 3.2.1 was then released in July 1979 with some minor bug fixes.

Apple DOS 3.3 was released in 1980. It improves various functions of release 3.2, while allowing for large gains in available floppy disk storage. The newer P5A/P6A PROMs in the disk controller enable the reading and writing of data at a higher density, so 16 sectors (4 KiB) can be stored per track instead of 13 sectors (3.25 KiB), increasing capacity from 113.75 KB to 140 KB per side – 16 KB of which is used by filesystem overhead and a copy of DOS, leaving 124 KB for user programs and data. DOS 3.3 is, however, not backward compatible; it cannot read or write DOS 3.2 disks. To address this problem, Apple Computer released "MUFFIN", a utility to migrate Apple DOS 3.2 files and programs to version 3.3 disks. Apple never offered a utility to copy in the other direction. To migrate Apple DOS 3.3 files back to version 3.2 disks, someone wrote a "NIFFUM" utility. There are also commercial utilities (such as Copy II Plus) that can copy files between either format (and eventually ProDOS as well). Release 3.3 also improves the ability to switch between Integer BASIC and Applesoft BASIC, if the computer has a language card (RAM expansion) or firmware card.

Technical details
Apple DOS 3.1 disks use 13 sectors of data per track, each sector being 256 bytes. It uses 35 tracks per disk side, and can access only one side of the floppy disk, unless the user flipped the disk over. This gives the user a total storage capacity of 113.75 KB per side, of which about 10 KB are used to store DOS itself and the disk directory, leaving about 100 KB for user programs.

The first layer of the operating system is called RWTS, which stands for "read/write track sector". This layer consists of subroutines for track seeking, sector reading and writing, and disk formatting. An API called the File Manager was built on top of this, and implements functions to open, close, read, write, delete, lock (i.e. write-protect), unlock (i.e. write-enable), and rename files, and to verify a file's structural integrity. There is also a CATALOG function, for listing files on the diskette, and an INIT function, which formats a disk for use with DOS, storing a copy of DOS on the first three tracks, and storing a startup program (usually called HELLO) that is auto-started when this disk is booted from. On top of the File Manager API, the main DOS routines are implemented which hook into the machine's BASIC interpreter and intercept all disk commands. It provides BLOAD, BSAVE, and BRUN for storing, loading, and running binary executables. LOAD, RUN, and SAVE are provided for BASIC programs, and an EXEC was provided for running text-based batch files consisting of BASIC and DOS commands. Finally, four types of files exist, identified by letters in a catalog listing:
 * I – Integer BASIC programs (stored in a compact format, not plain-text)
 * A – Applesoft BASIC programs (also stored in a packed, space-saving format)
 * B – Binary files, either executable machine-language programs, or data files
 * T – ASCII text files (or plain-text, unpacked batch files)

There are four additional file types; "R", "S", and an additional "A" and "B", none of which are fully supported. DOS recognizes these types for catalog listings only, and there are no direct ways to manipulate these types of files. The "R" type found some use for relocatable binary executable files. A few programs support the "S" type as data files.

A call vector table in the region of $03D0–03FF$16$ allows programs to find DOS wherever it is loaded in the system memory. For example, if the DOS hooked into the BASIC CLI stops functioning, it can be reinitialized by calling location $03D0$16$ (976$10$) – hence the traditional 3D0G ("3D0 go") command to return to BASIC from the System Monitor.

Boot loader
The process of loading Apple DOS involves a series of very tiny programs, each of which carries the loading process forward a few steps before passing control to the next program in the chain.

 * Originally, the Apple II ROM did not support disk booting at all. At power-up it would display the System Monitor prompt. Both the Monitor and Integer BASIC have commands to redirect printing to a printer driver in a designated slot, so the conventional way to boot from disk then was to command the computer to start "printing" to the disk interface card, typically installed in slot 6, using the command (from the ML monitor) or PR#6 (from BASIC). When the monitor or BASIC issued the next prompt character, the computer would call the ROM routines on the disk card to "print" to it, which would then proceed with the boot sequence. (One could use input redirection to similar ends.) Alternatively, from the ML monitor, the user could invoke the controller's boot code directly with a command like C600G.
 * When the Apple II Plus was introduced, it included the ability to scan each expansion slot (working downward from slot 7 to slot 1) for a bootable expansion card ROM, and automatically call it.
 * The expansion card ROM boot code attempts to boot from drive 1 of the controller by moving the read/write arm to track zero and attempting to read 256 bytes from sector zero of that track. (If no readable disk is available, the drive spins indefinitely until one is provided and the drive door is closed.)
 * Sector zero contains a small program which instructs the computer to read sectors 0 through 9 of track zero into memory using part of the ROM boot code (rereading sector 0 in the process).
 * The program in sectors 1–9 of track 0, including the complete RWTS code, then proceeds to load tracks 1 and 2, which contain the rest of DOS. On a system master disk, code is also included to determine the computer's RAM configuration and relocate DOS as high into system memory as possible, up to the 48 KB limit of the Apple II's main memory ($BFFF$16$).
 * Once DOS is loaded into memory, it attempts to load and execute a startup program as indicated in the DOS program code. This is commonly a BASIC language program named HELLO (or some other name) but DOS can be modified to run other types of programs at startup, such as an executable binary file. The appearance of the right-hand bracket (]) on the screen is an indication to the user that an Applesoft BASIC startup program is loading, while a greater-than symbol (>) indicates that an Integer BASIC program is loading. (These are the prompts for the respective versions of BASIC, which are being initialized at this point.)
 * The startup program then begins executing.

Integer BASIC and Applesoft BASIC support
The original Apple II included BASIC interpreter in ROM known originally as Apple BASIC and later as Integer BASIC. Variables in this language can only handle integer numbers ranging from −32,768 to +32,767 (16-bit binary values); floating point numbers are not supported. Apple commissioned Microsoft to develop Applesoft BASIC, capable of handling floating-point numbers. Applesoft BASIC cannot run Integer BASIC programs, causing some users to resist upgrading to it.

DOS 3.3 was released when Applesoft BASIC was standard in ROM on the Apple II Plus, so Apple designed it to support switching back and forth between the two BASIC interpreters. Integer BASIC is loaded into RAM on the language card of Apple IIs (if present) and by typing FP or INT from BASIC, the user can switch between either version.

Decline
After 1980, Apple DOS entered into a state of stagnation as Apple concentrated its efforts on the ill-fated Apple III computer and its SOS operating system. Two more versions of Apple DOS, both still called DOS 3.3 but with some bug fixes and better support for the new Apple IIe model, were released in early and mid-1983.

Without third-party patches, Apple DOS can only read floppy disks running in a 5.25-inch Disk II drive and cannot access any other media, such as hard disk drives, virtual RAM drives, or 3.5-inch floppy disk drives. The structure of Apple DOS disks (particularly the free sector map, which was restricted to part of a single sector) is such that it is not possible to have more than 400 KB available at a time per drive without a major rewrite of almost all sections of the code; this is the main reason Apple abandoned this iteration of DOS in 1983, when Apple DOS was entirely replaced by ProDOS.

ProDOS retains the 16-sector low-level format of DOS 3.3 for 5.25 inch disks, but introduces a new high-level format that is suitable for devices of up to 32 MB; this makes it suitable for hard disks from that era and 3.5-inch floppies. All the Apple computers from the II Plus onward can run both DOS 3.3 and ProDOS, the Plus requiring a "Language Card" memory expansion to use ProDOS; the e and later models have built-in Language Card hardware, and so can run ProDOS straight. ProDOS includes software to copy files from Apple DOS disks. However, many people who had no need for the improvements of ProDOS (and who did not like its much higher memory footprint) continued using Apple DOS or one of its clones long after 1983. The Apple convention of storing a bootable OS on every single floppy disk means that commercial software can be used no matter what OS the user owns. A program called DOS.MASTER enables users to have multiple virtual DOS 3.3 partitions on a larger ProDOS volume, which allows the use of many floppy-based DOS programs with a hard disk.

Shortly after ProDOS came out, Apple withdrew permission from third parties to redistribute DOS 3.3, but granted one company, Syndicomm, an exclusive license to resell DOS 3.3.

Commercial games usually did not use Apple DOS, instead having their own custom disk routines for copy protection purposes as well as for performance.

Performance improvements
DOS's RWTS routine can read or write a track in two revolutions with proper interleaving. A sector of the spinning disk passes under the read/write head while the RWTS routine is decoding the just-read sector (or encoding the next one to be written), and if this missed sector is the next one needed, DOS needs to wait nearly an entire revolution of the disk for the sector to come around again. This is called "blowing a rev" and is a well-understood performance bottleneck in disk systems. To avoid this, the sectors on a DOS disk are arranged in an interleaved order:

0 7 e 6 d 5 c 4 b 3 a 2 9 1 8 f

Later, ProDOS arranged the sectors in this order:

0 8 1 9 2 a 3 b 4 c 5 d 6 e 7 f

When reading and decoding sector 0, then sector 8 passes by, so that sector 1, the next sector likely to be needed, will be available without waiting. When reading sector 7, two unneeded sectors, f and 0, pass by before sector 8 is available, and when reading sector 15, the drive will always have to wait an extra revolution for sector 0 on the same track. However, the sector 0 actually needed in most cases will be on the next-higher track, and that track can be arranged relative to the last one to allow the needed time to decode the just-read sector and move the head before sector 0 comes around. On average, a full track can be read in two revolutions of the disk.

The early DOS File Manager subverted this efficiency by copying bytes read from or written to a file one at a time between a disk buffer and main memory, requiring more time and resulting in DOS constantly blowing revs when reading or writing files. Programs became available early on to format disks with modified sector interleaves; these disks give DOS more time between sectors to copy the data, ameliorating the problem.

Later, programmers outside Apple rewrote the File Manager routines to avoid making the extra copy for most sectors of a file; RWTS was instructed to read or write sectors directly to or from main memory rather than from a disk buffer whenever a full sector was to be transferred. An early patch to provide this functionality was published in Call-A.P.P.L.E.. Speedups in the LOAD command of three to five times were typical.

This functionality soon appeared in commercial products, such as Pronto-DOS, Diversi-DOS, Hyper-DOS, and David-DOS, along with additional features, but it was never used in an official Apple DOS release. Similar functionality was, however, employed by Apple's successor operating system, ProDOS. The Apple IIGS-specific operating system GS/OS would eventually employ an even more efficient "scatter read" technique that would read any sector that happened to be passing under the read head if it was needed for the file being read.

Source code release
In 2013, more than 35 years after the Apple II debuted, the original Apple DOS source code was released by the Computer History Museum on its website. It was donated by the original author, Paul Laughton.

title: Apple DOS