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Voice-operated switch

In telecommunications, a voice operated switch, also known as VOX or voice-operated exchange, is a switch that operates when sound over a certain threshold is detected. It is usually used to turn on a transmitter or recorder when someone speaks and turn it off when they stop speaking. It is used instead of a push-to-talk button on transmitters or to save storage space on recording devices. On cell phones, it is used to save battery life. Intercom systems that use a speaker in a room as both a speaker and a microphone will often use VOX on the main console to switch the audio direction during a conversation. The circuit usually includes a delay between the sound stopping and switching direction, to avoid the circuit turning off during short pauses in speech.

A special case exists, if there is enough energy to power the system directly. For example, a microphone may send a voltage high enough to directly operate a transmitter.

Comparison with push-to-talk
Unlike manual push-to-talk (PTT) operation, VOX is automatic; the user can keep his or her hands free while talking. But VOX also has some significant disadvantages that explain why PTT is still common.

Most VOX circuits have a sensitivity adjustment, but unwanted (and sometimes undetected) VOX triggering can still occur on background noise, heavy breathing or a side conversation. Conversely, it may not activate when desired on speech that is too weak.

The VOX in a two-way radio can also be triggered by the loudspeaker carrying the other side of the conversation. This problem can be minimized with an "anti vox" feature to decrease VOX sensitivity when the receiver is active.

Transmitters and recorders have short but finite activation times that may clip the beginnings of phrases. Some modern VOX circuits eliminate this problem by recording or transmitting a delayed version of the input signal. An older way of overcoming this, used by pilots, and astronauts, as some of the first users of VOX, was to habitually preface every transmission with "uh" in place of keying the microphone.

VOX uses a "hang" timer, typically 1–3 seconds, to remain engaged during brief speech pauses. This means the last several seconds of each transmission or recorded segment are always silence. A VOX-activated recorder can delete the end of each segment but the user of a VOX-activated half duplex radio must wait for the timer to expire before he or she can receive again.

title: Voice-operated switch


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Wafer (electronics)

In electronics, a wafer (also called a slice or substrate) is a thin slice of semiconductor, such as a crystalline silicon (c-Si, silicium), used for the fabrication of integrated circuits and, in photovoltaics, to manufacture solar cells.

The wafer serves as the substrate for microelectronic devices built in and upon the wafer. It undergoes many microfabrication processes, such as doping, ion implantation, etching, thin-film deposition of various materials, and photolithographic patterning. Finally, the individual microcircuits are separated by wafer dicing and packaged as an integrated circuit.

History
In the semiconductor industry, the term wafer appeared in the 1950s to describe a thin round slice of semiconductor material, typically germanium or silicon. The round shape characteristic of these wafers comes from single-crystal ingots usually produced using the Czochralski method. Silicon wafers were first introduced in the 1940s.

By 1960, silicon wafers were being manufactured in the U.S. by companies such as MEMC/SunEdison. In 1965, American engineers Eric O. Ernst, Donald J. Hurd, and Gerard Seeley, while working under IBM, filed Patent US3423629A for the first high-capacity epitaxial apparatus.

Silicon wafers are made by companies such as Sumco, Shin-Etsu Chemical, Hemlock Semiconductor Corporation and Siltronic.

Formation

Wafers are formed of highly pure, nearly defect-free single crystalline material, with a purity of 99.9999999% (9N) or higher. One process for forming crystalline wafers is known as the Czochralski method, invented by Polish chemist Jan Czochralski. In this process, a cylindrical ingot of high purity monocrystalline semiconductor, such as silicon or germanium, called a boule, is formed by pulling a seed crystal from a melt. Donor impurity atoms, such as boron or phosphorus in the case of silicon, can be added to the molten intrinsic material in precise amounts in order to dope the crystal, thus changing it into an extrinsic semiconductor of n-type or p-type.

The boule is then sliced with a wafer saw (a type of wire saw), machined to improve flatness, chemically etched to remove crystal damage from machining steps and finally polished to form wafers. The size of wafers for photovoltaics is 100–200 mm square and the thickness is 100–500 μm. Electronics use wafer sizes from 100 to 450 mm diameter. The largest wafers made have a diameter of 450 mm, but are not yet in general use.

Cleaning, texturing and etching
Wafers are cleaned with weak acids to remove unwanted particles. There are several standard cleaning procedures to make sure the surface of a silicon wafer contains no contamination. One of the most effective methods is the RCA clean. When used for solar cells, the wafers are textured to create a rough surface to increase surface area and so their efficiency. The generated PSG (phosphosilicate glass) is removed from the edge of the wafer in the etching.

Silicon
Silicon wafers are available in a variety of diameters from 25.4 mm (1 inch) to 300 mm (11.8 inches). Semiconductor fabrication plants, colloquially known as fabs, are defined by the diameter of wafers that they are tooled to produce. The diameter has gradually increased to improve throughput and reduce cost with the current state-of-the-art fab using 300 mm, with a proposal to adopt 450 mm. Intel, TSMC, and Samsung were separately conducting research to the advent of 450 mm "prototype" (research) fabs, though serious hurdles remain.

Wafers grown using materials other than silicon will have different thicknesses than a silicon wafer of the same diameter. Wafer thickness is determined by the mechanical strength of the material used; the wafer must be thick enough to support its own weight without cracking during handling. The tabulated thicknesses relate to when that process was introduced, and are not necessarily correct currently, for example the IBM BiCMOS7WL process is on 8-inch wafers, but these are only 200 μm thick. The weight of the wafer goes up along with its thickness and diameter.

Historical increases of wafer size
A unit of wafer fabrication step, such as an etch step, can produce more chips proportional to the increase in wafer area, while the cost of the unit fabrication step goes up more slowly than the wafer area. This was the cost basis for increasing wafer size. Conversion to 300 mm wafers from 200 mm wafers began in early 2000, and reduced the price per die for about 30–40%. Larger diameter wafers allow for more die per wafer.

Photovoltaic
M1 wafer size (156.75 mm) is in the process of being phased out in China as of 2020. Various nonstandard wafer sizes have arisen, so efforts to fully adopt the M10 standard (182 mm) are ongoing. Like other semiconductor fabrication processes, driving down costs has been the main driving factor for this attempted size increase, in spite of the differences in the manufacturing processes of different types of devices.

Crystalline orientation

Wafers are grown from crystal having a regular crystal structure, with silicon having a diamond cubic structure with a lattice spacing of 5.430710 Å (0.5430710 nm). When cut into wafers, the surface is aligned in one of several relative directions known as crystal orientations. Orientation is defined by the Miller index with (100) or (111) faces being the most common for silicon. Orientation is important since many of a single crystal's structural and electronic properties are highly anisotropic. Ion implantation depths depend on the wafer's crystal orientation, since each direction offers distinct paths for transport.

Wafer cleavage typically occurs only in a few well-defined directions. Scoring the wafer along cleavage planes allows it to be easily diced into individual chips ("dies") so that the billions of individual circuit elements on an average wafer can be separated into many individual circuits.

Crystallographic orientation notches
Wafers under 200 mm diameter have flats cut into one or more sides indicating the crystallographic planes of the wafer (usually a {110} face). In earlier-generation wafers a pair of flats at different angles additionally conveyed the doping type (see illustration for conventions). Wafers of 200 mm diameter and above use a single small notch to convey wafer orientation, with no visual indication of doping type. 450 mm wafers are notchless, relying on a laser scribed structure on the wafer surface for orientation.

Impurity doping
Silicon wafers are generally not 100% pure silicon, but are instead formed with an initial impurity doping concentration between 1013 and 1016 atoms per cm3 of boron, phosphorus, arsenic, or antimony which is added to the melt and defines the wafer as either bulk n-type or p-type. However, compared with single-crystal silicon's atomic density of 5×1022 atoms per cm3, this still gives a purity greater than 99.9999%. The wafers can also be initially provided with some interstitial oxygen concentration. Carbon and metallic contamination are kept to a minimum. Transition metals, in particular, must be kept below parts per billion concentrations for electronic applications.

Challenges
There is considerable resistance to the 450 mm transition despite the possible productivity improvement, because of concern about insufficient return on investment. There are also issues related to increased inter-die / edge-to-edge wafer variation and additional edge defects. 450mm wafers are expected to cost 4 times as much as 300mm wafers, and equipment costs are expected to rise by 20 to 50%. Higher cost semiconductor fabrication equipment for larger wafers increases the cost of 450 mm fabs (semiconductor fabrication facilities or factories). Lithographer Chris Mack claimed in 2012 that the overall price per die for 450 mm wafers would be reduced by only 10–20% compared to 300 mm wafers, because over 50% of total wafer processing costs are lithography-related. Converting to larger 450 mm wafers would reduce price per die only for process operations such as etch where cost is related to wafer count, not wafer area. Cost for processes such as lithography is proportional to wafer area, and larger wafers would not reduce the lithography contribution to die cost.

Nikon planned to deliver 450-mm lithography equipment in 2015, with volume production in 2017. In November 2013 ASML paused development of 450-mm lithography equipment, citing uncertain timing of chipmaker demand.

In 2012, a group consisting of New York State (SUNY Poly/College of Nanoscale Science and Engineering (CNSE)), Intel, TSMC, Samsung, IBM, Globalfoundries and Nikon companies has formed a public-private partnership called Global 450mm Consortium (G450C, similar to SEMATECH) who made a 5-year plan (expiring in 2016) to develop a "cost effective wafer fabrication infrastructure, equipment prototypes and tools to enable coordinated industry transition to 450mm wafer level". In the mid of 2014 CNSE has announced that it will reveal first fully patterned 450mm wafers at SEMICON West. In early 2017, the G450C began to dismantle its activities over 450mm wafer research due to undisclosed reasons. Various sources have speculated that demise of the group came after charges of bid rigging made against Alain E. Kaloyeros, who at the time was a chief executive at the SUNY Poly. The industry realization of the fact that the 300mm manufacturing optimization is more cheap than costly 450mm transition may also have played a role.

The timeline for 450 mm has not been fixed. In 2012, it was expected that 450mm production would start in 2017, which never realized. Mark Durcan, then CEO of Micron Technology, said in February 2014 that he expects 450 mm adoption to be delayed indefinitely or discontinued. "I am not convinced that 450mm will ever happen but, to the extent that it does, it's a long way out in the future. There is not a lot of necessity for Micron, at least over the next five years, to be spending a lot of money on 450mm."

"There is a lot of investment that needs to go on in the equipment community to make that happen. And the value at the end of the day – so that customers would buy that equipment – I think is dubious." As of March 2014, Intel Corporation expected 450 mm deployment by 2020 (by the end of this decade). Mark LaPedus of semiengineering.com reported in mid-2014 that chipmakers had delayed adoption of 450 mm "for the foreseeable future." According to this report some observers expected 2018 to 2020, while G. Dan Hutcheson, chief executive of VLSI Research, didn't see 450mm fabs moving into production until 2020 to 2025.

The step up to 300 mm required major changes, with fully automated factories using 300 mm wafers versus barely automated factories for the 200 mm wafers, partly because a FOUP for 300 mm wafers weighs about 7.5 kilograms when loaded with 25 300 mm wafers where a SMIF weighs about 4.8 kilograms when loaded with 25 200 mm wafers, thus requiring twice the amount of physical strength from factory workers, and increasing fatigue. 300mm FOUPs have handles so that they can be still be moved by hand. 450mm FOUPs weigh 45 kilograms when loaded with 25 450 mm wafers, thus cranes are necessary to manually handle the FOUPs and handles are no longer present in the FOUP. FOUPs are moved around using material handling systems from Muratec or Daifuku. These major investments were undertaken in the economic downturn following the dot-com bubble, resulting in huge resistance to upgrading to 450 mm by the original timeframe. On the ramp-up to 450 mm, the crystal ingots will be 3 times heavier (total weight a metric ton) and take 2–4 times longer to cool, and the process time will be double. All told, the development of 450 mm wafers requires significant engineering, time, and cost to overcome.

Analytical die count estimation
In order to minimize the cost per die, manufacturers wish to maximize the number of dies that can be made from a single wafer; dies always have a square or rectangular shape due to the constraint of wafer dicing. In general, this is a computationally complex problem with no analytical solution, dependent on both the area of the dies as well as their aspect ratio (square or rectangular) and other considerations such as the width of the scribeline or saw lane, and additional space occupied by alignment and test structures. Note that gross die per wafer (DPW) formulas account only for wafer area that is lost because it cannot be used to make physically complete dies; gross DPW calculations do not account for yield loss due to defects or parametric issues.

Nevertheless, the number of gross DPW can be estimated starting with the first-order approximation or floor function of wafer-to-die area ratio,
 * $$DPW = \left\lfloor\frac{\pi r^2}{S}\right\rfloor = \left\lfloor\frac{\pi d^2}{4S}\right\rfloor$$,

where
 * $$d$$ is the wafer diameter (typically in mm)
 * $$S$$ the size of each die (mm2) including the width of the scribeline ( or in the case of a saw lane, the kerf plus a tolerance).

This formula simply states that the number of dies which can fit on the wafer cannot exceed the area of the wafer divided by the area of each individual die. It will always overestimate the true best-case gross DPW, since it includes the area of partially patterned dies which do not fully lie on the wafer surface (see figure). These partially patterned dies don't represent complete ICs, so they cannot be sold as functional parts.

Refinements of this simple formula typically add an edge correction, to account for partial dies on the edge, which in general will be more significant when the area of the die is large compared to the total area of the wafer. In the other limiting case (infinitesimally small dies or infinitely large wafers), the edge correction is negligible.

The correction factor or correction term generally takes one of the forms cited by De Vries:

 * $$DPW = \frac{\displaystyle \pi d^2}{4S} - \frac{\displaystyle \pi d}{\sqrt{2S}}$$ (area ratio – circumference/(die diagonal length))
 * or $$DPW = \left(\frac{\displaystyle \pi d^2}{4S}\right) \exp(-2 \sqrt{S}/d)$$ (area ratio scaled by an exponential factor)
 * or $$DPW = \frac{\displaystyle \pi d^2}{4S} \left(1 - \frac{\displaystyle 2\sqrt{S}}{d} \right)^2$$ (area ratio scaled by a polynomial factor).

Studies comparing these analytical formulas to brute-force computational results show that the formulas can be made more accurate, over practical ranges of die sizes and aspect ratios, by adjusting the coefficients of the corrections to values above or below unity, and by replacing the linear die dimension $$\sqrt{S}$$ with $$(H+W)/2$$ (average side length) in the case of dies with large aspect ratio:

 * $$DPW = \frac{\displaystyle \pi d^2}{4S} - 0.58^{*} \frac{\displaystyle \pi d}{\sqrt{S}}$$
 * or $$DPW = \left(\frac{\displaystyle \pi d^2}{4S}\right) \exp(-2.32^{*} \sqrt{S}/d)$$
 * or $$DPW = \frac{\displaystyle \pi d^2}{4S} \left(1 - \frac{\displaystyle 1.16^{*} \sqrt{S}}{d} \right)^2$$.

Compound semiconductors
While silicon is the prevalent material for wafers used in the electronics industry, other compound III-V or II-VI materials have also been employed. Gallium arsenide (GaAs), a III-V semiconductor produced via the Czochralski method, gallium nitride (GaN) and silicon carbide (SiC) are also common wafer materials, with GaN and sapphire being extensively used in LED manufacturing. The hard carbon thin films elevate silicon durability against high-contact load applications.

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

title: Wafer (electronics)


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Wide area information server

Wide Area Information Server (WAIS) is a client–server text searching system that uses the ANSI Standard Z39.50 Information Retrieval Service Definition and Protocol Specifications for Library Applications" (Z39.50:1988) to search index databases on remote computers. It was developed in 1990 as a project of Thinking Machines, Apple Computer, Dow Jones, and KPMG Peat Marwick.

WAIS did not adhere to either the standard nor its OSI framework (adopting instead TCP/IP) but created a unique protocol inspired by Z39.50:1988.

History
The WAIS protocol and servers were promoted by Thinking Machines Corporation (TMC) of Cambridge, Massachusetts. TMC-produced WAIS servers ran on their massively parallel CM-2 (Connection Machine) and SPARC-based CM-5 MP supercomputers. WAIS clients were developed for various operating systems and windowing systems including Microsoft Windows, Macintosh, NeXT, X, GNU Emacs, and character terminals. TMC released a free open source software version of WAIS for Unix in 1991.

Inspired by the WAIS project on full-text databases and emerging SGML projects, Z39.50 version 2 (Z39.50:1992) was released. Unlike its 1988 predecessor, it was a compatible superset of the international ISO 10162/10163 standard.

With the advent of Z39.50:1992, the termination of support for free WAIS by Thinking Machines and the establishment of WAIS Inc as a commercial venture, the U.S. National Science Foundation funded the Clearinghouse for Networked Information Discovery and Retrieval (CNIDR) to promote Internet search and discovery systems, open source and standards. CNIDR created a new, free open-source WAIS. This was the first freeWAIS based on the wais-8-b5 codebase of TMC, with a wholly new software suite Isite based upon Z39.50:1992 using Isearch as its full-text search engine.

Ulrich Pfeifer and Norbert Gövert of the computer science department of the University of Dortmund extended the CNIDR freeWAIS code to become freeWAIS-sf with structured fields as its main improvement. Ulrich Pfeifer rewrote freeWAIS-sf in Perl, becoming WAIT.

Inspired by WAIS' "Directory of Servers", Eliot Christian of USGS envisioned GILS: Government Information Locator Service. GILS (based upon Z39.50:1992 with some extensions) became a U.S. Federal mandate as part of the Paperwork Reduction Act of 1995.

Directory of Servers
Thinking Machines Corp provided a service called the Directory of Servers. It was a WAIS server like any other information source except containing information about the other WAIS servers on the Internet. A WAIS server with TMC WAIS code creates a special record containing metadata plus some common words describing its indexed content. The record is uploaded to the central server and indexed along with the records from other public servers. The directory can be searched to find servers that might have content relevant to a specific field of interest. This model of searching for (WAIS) servers to search became the model for GILS and Peter Deutsch's WHOIS++ distributed white pages directory.

People
Two of the developers of WAIS, Brewster Kahle and Harry Morris, left Thinking Machines to found WAIS Inc in Menlo Park, California, with Bruce Gilliat. WAIS Inc. was originally a joint project between Apple Computer, Peat Marwick, Dow Jones, and Thinking Machines. In 1992, the presidential campaign of Ross Perot used the WAIS product as a campaign wide information system, connecting the field offices to the national office. Later, Perot Systems adopted WAIS to better access the information in its corporate databases. Other early clients were the Environmental Protection Agency, Library of Congress, and the Department of Energy and later the Wall Street Journal and Encyclopædia Britannica.

WAIS Inc was sold to AOL in May 1995 for $15 million. Following the sale, Margaret St. Pierre left WAIS Inc to start Blue Angel Technologies. Her WAIS variant formed the basis of MetaStar. Georgios Papadopoulos left to found Atypon. François Schiettecatte left Human Genome Project at Johns Hopkins Hospital and started FS-Consult and developed his own variant of WAIS which eventually became ScienceServer, which was later sold to Elsevier Science. Kahle and Gilliat went on to found the Internet Archive and Alexa Internet.

WAIS and Gopher
Public WAIS is often used as a full-text search engine for individual Internet Gopher servers, supplementing the popular Veronica system which only searches the menu titles of Gopher sites. WAIS and Gopher share the World Wide Web's client–server architecture and a certain amount of its functionality. The WAIS protocol is influenced largely by the z39.50 protocol designed for networking library catalogs. It allows a text-based search, and retrieval following a search. Gopher provides a free text search mechanism, but principally uses menus. A menu is a list of titles, from which the user may pick one. While Gopher Space is a web containing many loops, the menu system gives the user the impression of a tree.

The Web's data model is similar to the gopher model, except that menus are generalized to hypertext documents. In both cases, simple file servers generate the menus or hypertext directly from the file structure of a server. The Web's hypertext model permits the author more freedom to communicate the options available to the reader, as it can include headings and various forms of list structure.

title: Wide area information server


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Waveguide

A waveguide is a structure that guides waves by restricting the transmission of energy to one direction. Common types of waveguides include acoustic waveguides which direct sound, optical waveguides which direct light, and radio-frequency waveguides which direct electromagnetic waves other than light like radio waves.

Without the physical constraint of a waveguide, waves would expand into three-dimensional space and their intensities would decrease according to the inverse square law.

There are different types of waveguides for different types of waves. The original and most common meaning is a hollow conductive metal pipe used to carry high frequency radio waves, particularly microwaves. Dielectric waveguides are used at higher radio frequencies, and transparent dielectric waveguides and optical fibers serve as waveguides for light. In acoustics, air ducts and horns are used as waveguides for sound in musical instruments and loudspeakers, and specially-shaped metal rods conduct ultrasonic waves in ultrasonic machining.

The geometry of a waveguide reflects its function; in addition to more common types that channel the wave in one dimension, there are two-dimensional slab waveguides which confine waves to two dimensions. The frequency of the transmitted wave also dictates the size of a waveguide: each waveguide has a cutoff wavelength determined by its size and will not conduct waves of greater wavelength; an optical fiber that guides light will not transmit microwaves which have a much larger wavelength. Some naturally occurring structures can also act as waveguides. The SOFAR channel layer in the ocean can guide the sound of whale song across enormous distances. Any shape of cross section of waveguide can support EM waves. Irregular shapes are difficult to analyse. Commonly used waveguides are rectangular and circular in shape.

Uses
The uses of waveguides for transmitting signals were known even before the term was coined. The phenomenon of sound waves guided through a taut wire have been known for a long time, as well as sound through a hollow pipe such as a cave or medical stethoscope. Other uses of waveguides are in transmitting power between the components of a system such as radio, radar or optical devices. Waveguides are the fundamental principle of guided wave testing (GWT), one of the many methods of non-destructive evaluation.

Specific examples:
 * Optical fibers transmit light and signals for long distances with low attenuation and a wide usable range of wavelengths.
 * In a microwave oven a waveguide transfers power from the magnetron, where waves are formed, to the cooking chamber.
 * In a radar, a waveguide transfers radio frequency energy to and from the antenna, where the impedance needs to be matched for efficient power transmission (see below).
 * Rectangular and circular waveguides are commonly used to connect feeds of parabolic dishes to their electronics, either low-noise receivers or power amplifier/transmitters.
 * Waveguides are used in scientific instruments to measure optical, acoustic and elastic properties of materials and objects. The waveguide can be put in contact with the specimen (as in a medical ultrasonography), in which case the waveguide ensures that the power of the testing wave is conserved, or the specimen may be put inside the waveguide (as in a dielectric constant measurement, so that smaller objects can be tested and the accuracy is better.
 * A transmission line is a commonly used specific type of waveguide.

History
The first structure for guiding waves was proposed by J. J. Thomson in 1893, and was first experimentally tested by Oliver Lodge in 1894. The first mathematical analysis of electromagnetic waves in a metal cylinder was performed by Lord Rayleigh in 1897. For sound waves, Lord Rayleigh published a full mathematical analysis of propagation modes in his seminal work, "The Theory of Sound". Jagadish Chandra Bose researched millimeter wavelengths using waveguides, and in 1897 described to the Royal Institution in London his research carried out in Kolkata.

The study of dielectric waveguides (such as optical fibers, see below) began as early as the 1920s, by several people, most famous of which are Rayleigh, Sommerfeld and Debye. Optical fiber began to receive special attention in the 1960s due to its importance to the communications industry.

The development of radio communication initially occurred at the lower frequencies because these could be more easily propagated over large distances. The long wavelengths made these frequencies unsuitable for use in hollow metal waveguides because of the impractically large diameter tubes required. Consequently, research into hollow metal waveguides stalled and the work of Lord Rayleigh was forgotten for a time and had to be rediscovered by others. Practical investigations resumed in the 1930s by George C. Southworth at Bell Labs and Wilmer L. Barrow at MIT. Southworth at first took the theory from papers on waves in dielectric rods because the work of Lord Rayleigh was unknown to him. This misled him somewhat; some of his experiments failed because he was not aware of the phenomenon of waveguide cutoff frequency already found in Lord Rayleigh's work. Serious theoretical work was taken up by John R. Carson and Sallie P. Mead. This work led to the discovery that for the TE01 mode in circular waveguide losses go down with frequency and at one time this was a serious contender for the format for long-distance telecommunications.

The importance of radar in World War II gave a great impetus to waveguide research, at least on the Allied side. The magnetron, developed in 1940 by John Randall and Harry Boot at the University of Birmingham in the United Kingdom, provided a good power source and made microwave radar feasible. The most important centre of US research was at the Radiation Laboratory (Rad Lab) at MIT but many others took part in the US, and in the UK such as the Telecommunications Research Establishment. The head of the Fundamental Development Group at Rad Lab was Edward Mills Purcell. His researchers included Julian Schwinger, Nathan Marcuvitz, Carol Gray Montgomery, and Robert H. Dicke. Much of the Rad Lab work concentrated on finding lumped element models of waveguide structures so that components in waveguide could be analysed with standard circuit theory. Hans Bethe was also briefly at Rad Lab, but while there he produced his small aperture theory which proved important for waveguide cavity filters, first developed at Rad Lab. The German side, on the other hand, largely ignored the potential of waveguides in radar until very late in the war. So much so that when radar parts from a downed British plane were sent to Siemens & Halske for analysis, even though they were recognised as microwave components, their purpose could not be identified. "At that time, microwave techniques were badly neglected in Germany. It was generally believed that it was of no use for electronic warfare, and those who wanted to do research work in this field were not allowed to do so." German academics were even allowed to continue publicly publishing their research in this field because it was not felt to be important.

Immediately after World War II waveguide was the technology of choice in the microwave field. However, it has some problems; it is bulky, expensive to produce, and the cutoff frequency effect makes it difficult to produce wideband devices. Ridged waveguide can increase bandwidth beyond an octave, but a better solution is to use a technology working in TEM mode (that is, non-waveguide) such as coaxial conductors since TEM does not have a cutoff frequency. A shielded rectangular conductor can also be used and this has certain manufacturing advantages over coax and can be seen as the forerunner of the planar technologies (stripline and microstrip). However, planar technologies really started to take off when printed circuits were introduced. These methods are significantly cheaper than waveguide and have largely taken its place in most bands. However, waveguide is still favoured in the higher microwave bands from around Ku band upwards.

Propagation modes and cutoff frequencies
A propagation mode in a waveguide is one solution of the wave equations, or, in other words, the form of the wave. Due to the constraints of the boundary conditions, there are only limited frequencies and forms for the wave function which can propagate in the waveguide. The lowest frequency in which a certain mode can propagate is the cutoff frequency of that mode. The mode with the lowest cutoff frequency is the fundamental mode of the waveguide, and its cutoff frequency is the waveguide cutoff frequency.

Propagation modes are computed by solving the Helmholtz equation alongside a set of boundary conditions depending on the geometrical shape and materials bounding the region. The usual assumption for infinitely long uniform waveguides allows us to assume a propagating form for the wave, i.e. stating that every field component has a known dependency on the propagation direction (i.e. $$z$$). More specifically, the common approach is to first replace all unknown time-varying fields $$u(x,y,z,t)$$ (assuming for simplicity to describe the fields in cartesian components) with their complex phasors representation $$U(x,y,z)$$, sufficient to fully describe any infinitely long single-tone signal at frequency $$f$$, (angular frequency $$\omega=2\pi f$$), and rewrite the Helmholtz equation and boundary conditions accordingly. Then, every unknown field is forced to have a form like $$U(x,y,z)=\hat{U}(x,y)e^{-\gamma z}$$, where the $$\gamma$$ term represents the propagation constant (still unknown) along the direction along which the waveguide extends to infinity. The Helmholtz equation can be rewritten to accommodate such form and the resulting equality needs to be solved for $$\gamma$$ and $$\hat{U}(x,y)$$, yielding in the end an eigenvalue equation for $$\gamma$$ and a corresponding eigenfunction $$\hat{U}(x,y)_\gamma$$for each solution of the former.

The propagation constant $$\gamma$$ of the guided wave is complex, in general. For a lossless case, the propagation constant might be found to take on either real or imaginary values, depending on the chosen solution of the eigenvalue equation and on the angular frequency $$\omega$$. When $$\gamma$$ is purely real, the mode is said to be "below cutoff", since the amplitude of the field phasors tends to exponentially decrease with propagation; an imaginary $$\gamma$$, instead, represents modes said to be "in propagation" or "above cutoff", as the complex amplitude of the phasors does not change with $$z$$.

Impedance matching
In circuit theory, the impedance is a generalization of electrical resistance in the case of alternating current, and is measured in ohms ($$\Omega$$). A waveguide in circuit theory is described by a transmission line having a length and characteristic impedance. In other words, the impedance indicates the ratio of voltage to current of the circuit component (in this case a waveguide) during propagation of the wave. This description of the waveguide was originally intended for alternating current, but is also suitable for electromagnetic and sound waves, once the wave and material properties (such as pressure, density, dielectric constant) are properly converted into electrical terms (current and impedance for example).

Impedance matching is important when components of an electric circuit are connected (waveguide to antenna for example): The impedance ratio determines how much of the wave is transmitted forward and how much is reflected. In connecting a waveguide to an antenna a complete transmission is usually required, so an effort is made to match their impedances.

The reflection coefficient can be calculated using: $$\Gamma=\frac{Z_2-Z_1}{Z_2+Z_1}$$, where $$\Gamma$$ (Gamma) is the reflection coefficient (0 denotes full transmission, 1 full reflection, and 0.5 is a reflection of half the incoming voltage), $$Z_1$$ and $$Z_2$$ are the impedance of the first component (from  which the wave enters) and the second component, respectively.

An impedance mismatch creates a reflected wave, which added to the incoming waves creates a standing wave. An impedance mismatch can be also quantified with the standing wave ratio (SWR or VSWR for voltage), which is connected to the impedance ratio and reflection coefficient by: $$\mathrm{VSWR}=\frac{|V|_{\rm max}}{|V|_{\rm min}}=\frac{1+|\Gamma|}{1-|\Gamma|}$$, where $$\left|V\right|_{\rm min/max}$$ are the minimum and maximum values of the voltage absolute value, and the VSWR is the voltage standing wave ratio, which value of 1 denotes full transmission, without reflection and thus no standing wave, while very large values mean high reflection and standing wave pattern.

Radio-frequency waveguides
Waveguides can be constructed to carry waves over a wide portion of the electromagnetic spectrum, but are especially useful in the microwave and optical frequency ranges. Depending on the frequency, they can be constructed from either conductive or dielectric materials. Waveguides are used for transferring both power and communication signals.

Optical waveguides
Waveguides used at optical frequencies are typically dielectric waveguides, structures in which a dielectric material with high permittivity, and thus high index of refraction, is surrounded by a material with lower permittivity. The structure guides optical waves by total internal reflection. An example of an optical waveguide is optical fiber.

Other types of optical waveguide are also used, including photonic-crystal fiber, which guides waves by any of several distinct mechanisms. Guides in the form of a hollow tube with a highly reflective inner surface have also been used as light pipes for illumination applications. The inner surfaces may be polished metal, or may be covered with a multilayer film that guides light by Bragg reflection (this is a special case of a photonic-crystal fiber). One can also use small prisms around the pipe which reflect light via total internal reflection —such confinement is necessarily imperfect, however, since total internal reflection can never truly guide light within a lower-index core (in the prism case, some light leaks out at the prism corners).

Acoustic waveguides
An acoustic waveguide is a physical structure for guiding sound waves. Sound in an acoustic waveguide behaves like electromagnetic waves on a transmission line. Waves on a string, like the ones in a tin can telephone, are a simple example of an acoustic waveguide. Another example are pressure waves in the pipes of an organ. The term acoustic waveguide is also used to describe elastic waves guided in micro-scale devices, like those employed in piezoelectric delay lines and in stimulated Brillouin scattering.

Mathematical waveguides
Waveguides are interesting objects of study from a strictly mathematical perspective. A waveguide (or tube) is defined as type of boundary condition on the wave equation such that the wave function must be equal to zero on the boundary and that the allowed region is finite in all dimensions but one (an infinitely long cylinder is an example.) A large number of interesting results can be proven from these general conditions. It turns out that any tube with a bulge (where the width of the tube increases) admits at least one bound state that exist inside the mode gaps. The frequencies of all the bound states can be identified by using a pulse short in time. This can be shown using the variational principles. An interesting result by Jeffrey Goldstone and Robert Jaffe is that any tube of constant width with a twist, admits a bound state.

Sound synthesis
Sound synthesis uses digital delay lines as computational elements to simulate wave propagation in tubes of wind instruments and the vibrating strings of string instruments.

title: Waveguide


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Wave impedance

The wave impedance of an electromagnetic wave is the ratio of the transverse components of the electric and magnetic fields (the transverse components being those at right angles to the direction of propagation). For a transverse-electric-magnetic (TEM) plane wave traveling through a homogeneous medium, the wave impedance is everywhere equal to the intrinsic impedance of the medium. In particular, for a plane wave travelling through empty space, the wave impedance is equal to the impedance of free space. The symbol Z is used to represent it and it is expressed in units of ohms. The symbol η (eta) may be used instead of Z for wave impedance to avoid confusion with electrical impedance.

Definition
The wave impedance is given by

 * $$Z = {E_0^-(x) \over H_0^-(x)}$$

where $$E_0^-(x)$$ is the electric field and $$H_0^-(x)$$ is the magnetic field, in phasor representation. The impedance is, in general, a complex number.

In terms of the parameters of an electromagnetic wave and the medium it travels through, the wave impedance is given by

 * $$Z = \sqrt {j \omega \mu \over \sigma + j \omega \varepsilon} $$

where μ is the magnetic permeability, ε is the (real) electric permittivity and σ is the electrical conductivity of the material the wave is travelling through (corresponding to the imaginary component of the permittivity multiplied by omega). In the equation, j is the imaginary unit, and ω is the angular frequency of the wave. Just as for electrical impedance, the impedance is a function of frequency. In the case of an ideal dielectric (where the conductivity is zero), the equation reduces to the real number

 * $$Z = \sqrt {\mu \over \varepsilon }.$$

In free space
In free space the wave impedance of plane waves is:

 * $$Z_0 = \sqrt{\frac{\mu_0} {\varepsilon_0}}$$

(where ε0 is the permittivity constant in free space and μ0 is the permeability constant in free space). Now, since

 * $$c_0 = \frac{1}{\sqrt{\mu_0 \varepsilon_0}} = 299,792,458\text{ m/s}$$ (by the SI definition of the metre),

 * $$Z_0 = \mu_0 c_0$$.

Hence the value essentially depends on $$\mu_0$$. Until May 20th, 2019, $$\mu_0 = 4\pi \times 10^{-7} \text{H/m}$$, hence

 * $$Z_0 = 376.73031346177\ldots~\Omega \approx 120 \pi~\Omega$$.

The currently accepted value of $$Z_0$$ is

 * $$Z_0 = 376.730313668(57)~\Omega$$.

In an unbounded dielectric
In an isotropic, homogeneous dielectric with negligible magnetic properties, i.e. $$\mu = \mu_0 = 4 \pi \times 10^{-7} $$ H/m and $$\varepsilon = \varepsilon_r \times 8.854\times 10^{-12}$$ F/m. So, the value of wave impedance in a perfect dielectric is

 * $$Z = \sqrt {\mu \over \varepsilon} = \sqrt {\mu_0 \over \varepsilon_0 \varepsilon_r} = {Z_0 \over \sqrt{\varepsilon_r}} \approx {377 \over \sqrt {\varepsilon_r} }\,\Omega$$,

where $$\varepsilon_r$$ is the relative dielectric constant.

In a waveguide
For any waveguide in the form of a hollow metal tube, (such as rectangular guide, circular guide, or double-ridge guide), the wave impedance of a travelling wave is dependent on the frequency $$f$$, but is the same throughout the guide. For transverse electric (TE) modes of propagation the wave impedance is:

 * $$Z = \frac{Z_{0}}{\sqrt{1 - \left( \frac{f_{c}}{f}\right)^{2}}} \qquad \mbox{(TE modes)},$$

where fc is the cut-off frequency of the mode, and for transverse magnetic (TM) modes of propagation the wave impedance is:

 * $$Z = Z_{0} \sqrt{1 - \left( \frac{f_{c}}{f}\right)^{2}} \qquad \mbox{(TM modes)}$$

Above the cut-off (f > fc), the impedance is real (resistive) and the wave carries energy. Below cut-off the impedance is imaginary (reactive) and the wave is evanescent. These expressions neglect the effect of resistive loss in the walls of the waveguide. For a waveguide entirely filled with a homogeneous dielectric medium, similar expressions apply, but with the wave impedance of the medium replacing Z0. The presence of the dielectric also modifies the cut-off frequency fc.

For a waveguide or transmission line containing more than one type of dielectric medium (such as microstrip), the wave impedance will in general vary over the cross-section of the line.

title: Wave impedance


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Wide Area Telephone Service

Wide Area Telephone Service (WATS) was a flat-rate long-distance service for customer dial-type telecommunications in the service areas of the North American Numbering Plan (NANP).

The service was between a given customer phone (also known as a "station") and stations within specified geographic rate areas, employing a single telephone line between the customer location and the serving central office. Each access line could be arranged for outward (OUT-WATS) or inward (IN-WATS) service, or both.

WATS was introduced by the Bell System in 1961 as a long-distance flat-rate plan by which a business could obtain a special line with an included number of hours ('measured time' or 'full-time') of long-distance calling to a specified area. These lines were most often connected to private branch exchanges in large businesses. WATS lines were the basis for the first direct-dial toll-free 1-800-numbers (intrastate in 1966, interstate in 1967); by 1976, WATS brought AT&T Corporation a billion dollars in annual revenue ($ in dollars)

For outbound calls, the 1984 AT&T divestiture brought multiple competitors offering similar services using standard business telephone lines; the special WATS line was ultimately supplanted by other flat-rate offerings. The requirement that an inbound toll-free number terminate at a special WATS line or fixed-rate service was also rendered obsolete by the 1980s due to intelligent network capability and technological improvement in the 800-service. A toll-free number may now terminate at a T-carrier line, at any standard local telephone number or at one of multiple destinations based on time of day, call origin, cost or other factors.

Outbound WATS
For Outbound WATS, the United States was divided into geographical Bands 0 through 5. Band zero was intrastate calling and bands 1 through 5 (or 6) were interstate calls that were progressively further from the originating number. Historically, the higher band number carried a higher price per month or per minute. These lines could be used for outbound long-distance only; not local. In the U.S., interstate WATS lines could not be used for intrastate calls, and vice versa. With wider availability of inexpensive long distance using regular business lines, OutWATS service became obsolete late in the 20th century.

InWATS
A form of toll-free telephone service in North America was the Zenith number, published in distant cities from where a company expected or desired frequent customer calls. Published as "Zenith" and a four- or five-digit number, these collect calls required operator assistance. The subscriber of the service was charged for the call.

With "inward WATS", introduced for interstate calls by the American Telephone and Telegraph Company (AT&T) in 1967, subscribers were issued a toll-free telephone number in a designated toll-free area code. Unlike a standard collect call or a call to a Zenith number, 1800 normally may be dialed directly with no live operator. Callers within a designated area could call without incurring a toll charge as the recipient paid for the calls at a fixed rate.

The introduction of InWATS fortuitously fell around the same time as the early centralized, automated national airline and hotel reservation systems, including Sabre (American Airlines, 1963), Holidex (Holiday Inn, 1965) and Reservatron (Sheraton, 1969). Hundreds of local reservation numbers for a major chain could be replaced with one central number, backed by a national computerized reservation system.

InWATS exchanges were assigned to Canada and other North American Numbering Plan countries, but the original InWATS in each country accepted domestic calls only. Initially 1800NN2XXXX numbers were U.S. intrastate and specific prefixes (such as 1800387 Toronto and 1800267 Ottawa) were assigned to Canada. In the 1970s, AT&T's internal routing guides included separate U.S. and Canadian 1-800 exchange maps which looked much like area code maps as each geographic area code had one or more specific freephone exchange prefixes. Sheraton's 8003253535, one of the notable early adopters in late 1969, was hard-wired into St. Louis area code 314; 1800HOLIDAY at that time could not be a U.S. number if the 1800465 prefix was hard-wired to Thunder Bay's area code 807. Any attempt to call a foreign 1800 gave a pre-recorded error, "the number you have dialed is not available from your calling area."

Like the OutWATS service, AT&T's InWATS was divided into intrastate and interstate, with interstate calls priced into five or six "bands" of calling. This favored placement of US national call centers in low-population Midwestern states such as Nebraska, whose central location meant a carefully situated "band 3" number reaching halfway across the US in every direction could potentially reach 47 states. A San Diego call center would be less fortunate; even with "band 6" (the most expensive lines), its national number would be unreachable to millions as California is a populous state and intrastate calls needed a separate toll-free number.

The original InWATS system was supplanted by "Advanced 800 Service" in the 1980s. Modern systems eliminated requirements tying toll-free numbers to dedicated flat-rate inbound WATS lines. Direct inward dial, introduced in 1983, allowed one trunk to carry calls for multiple numbers. AT&T's monopoly on U.S. toll-free number routing ended in 1986, encouraging flexibility in order to match rivals Sprint and MCI. By 1989, fixed "bands" of coverage area had been largely replaced by distance-based billing, a growing number of 1800 numbers were being terminated at standard local business or residence lines and one number could be sent to multiple locations based on call origin, least-cost routing or time of day routing. RespOrgs were established in the U.S. in 1993 and Canada in 1994 to provide toll free number portability using the Service Management System (SMS/800) database. Calls from Canada and the U.S., intrastate and interstate, could terminate at the same 1800 number, even via different carriers. Vanity numbers became easier to obtain as a toll-free exchange prefix was no longer tied to a geographic location. By the 21st century, Voice over IP placed toll-free and foreign exchange numbers into the hands of even the smallest users, to whom dedicated inbound lines under the original InWATS model would have been prohibitively expensive.

Civil Rights Movement
During the Civil Rights Movement in the U.S., activist organizations such as SNCC used WATS as a convenient way for eyewitnesses on the ground to convey information quickly. Notes from these phone calls were compiled into "WATS Line Reports" and mailed to civil rights leaders, the media, the Justice Department, and others involved in the events. WATS was also how organizations communicated with local leaders across the country. A "Bay Area Friends of SNCC" newsletter in 1965 described WATS: "The WATS (Wide Area Telephone Service) line is the heart of all SNCC security and communications. For a flat monthly rate, an unlimited number of calls can be dialed directly to any place in the country — or the state — depending on what line one uses. The Jackson office has a state-wide line, the Atlanta office has the national WATS line. Both run on a 24-hour basis. A project worker can call in news of any incident, threat or major activity to the Jackson office. The WATS operator there takes down the details and relays it to Atlanta if the event is of national importance. In the case of a threat or incident involving Federal laws, Jackson will notify the FBI and the Justice Department. Atlanta uses its national WATS line to notify SNCC groups around the country."

title: Wide Area Telephone Service


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Wideband modem

In telecommunication, the term wideband modem has the following meanings:

 * 1) A modem whose modulated output signal can have an essential frequency spectrum that is broader than that which can be wholly contained within, and faithfully transmitted through, a voice channel with a nominal 4 kHz bandwidth.
 * 2) A modem whose bandwidth capability is greater than that of a narrowband modem.

title: Wideband modem


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Wildcard character

In software, a wildcard character is a kind of placeholder represented by a single character, such as an asterisk (*), which can be interpreted as a number of literal characters or an empty string. It is often used in file searches so the full name need not be typed.

Telecommunication
In telecommunications, a wildcard is a character that may be substituted for any of a defined subset of all possible characters.

 * In high-frequency (HF) radio automatic link establishment, the wildcard character ? may be substituted for any one of the 36 upper-case alphanumeric characters.
 * Whether the wildcard character represents a single character or a string of characters must be specified.

Computing
In computer (software) technology, a wildcard is a symbol used to replace or represent zero or more characters. Algorithms for matching wildcards have been developed in a number of recursive and non-recursive varieties.

File and directory patterns
When specifying file names (or paths) in CP/M, DOS, Microsoft Windows, and Unix-like operating systems, the asterisk character (*, also called "star") matches zero or more characters. For example, doc* matches doc and document but not dodo. If files are named with a date stamp, wildcards can be used to match date ranges, such as  to select video recordings from , to facilitate file operations such as copying and moving.

In Unix-like and DOS operating systems, the question mark ? matches exactly one character. In DOS, if the question mark is placed at the end of the word, it will also match missing (zero) trailing characters; for example, the pattern 123? will match 123 and 1234, but not 12345.

In Unix shells and Windows PowerShell, ranges of characters enclosed in square brackets ([ and ]) match a single character within the set; for example, [A-Za-z] matches any single uppercase or lowercase letter. In Unix shells, a leading exclamation mark ! negates the set and matches only a character not within the list. In shells that interpret ! as a history substitution, a leading caret ^ can be used instead.

The operation of matching of wildcard patterns to multiple file or path names is referred to as globbing.

Databases
In SQL, wildcard characters can be used in LIKE expressions; the percent sign % matches zero or more characters, and underscore _ a single character. Transact-SQL also supports square brackets ([ and ]) to list sets and ranges of characters to match, a leading caret ^ negates the set and matches only a character not within the list. In Microsoft Access, the asterisk sign * matches zero or more characters, the question mark ? matches a single character, the number sign # matches a single digit (0–9), and square brackets can be used for sets or ranges of characters to match.

Regular expressions
In regular expressions, the period (., also called "dot") is the wildcard pattern which matches any single character. Combined with the asterisk operator .* it will match any number of any characters.

In this case, the asterisk is also known as the Kleene star.

title: Wildcard character


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Wink pulsing

Winking is a telephony signaling technique used both in connection with DC signaling on a trunk, and with indicator lamps on a key telephone.

In telephone switching systems, wink pulsing is recurring pulsing in which the off condition is relatively short compared to the on condition. In wink start trunks, the exchange at the originating end sends an off-hook to alert to a call. The terminating end indicates readiness to receive the dialed telephone number by sending an off-hook of approximately half a second duration, or "wink". Upon receiving this go ahead signal, the originating end uses multi-frequency or other address signalling to send the phone number.

On 1A2 key systems or similar key-operated telephone instruments, the hold position, i.e., the hold condition, of a line is often indicated by winking the associated lamp at 120 impulses per minute. During 6% of the pulse period the lamp is off and 94% of the period the lamp is on, i.e., 30 ms (milliseconds) off and 470 ms on.

title: Wink pulsing


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Wireless mobility management

Wireless mobility management in Personal Communications Service (PCS) is the assigning and controlling of wireless links for terminal network connections. Wireless mobility management provides an "alerting" function for call completion to a wireless terminal, monitors wireless link performance to determine when an automatic link transfer is required, and coordinates link transfers between wireless access interfaces.

One use of this is wireless push technology, by pushing data across wireless networks, this coordinates the link transfers and pushes data between the backend and wireless device only when an established connection is found.

title: Wireless mobility management


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Work station

Work station may refer to:

 * Workstation, a high-performance desktop computer
 * Music workstation, an electronic musical instrument
 * Cubicle, a work space usually for one employee
 * Computer desk, a piece of furniture

title: Work station


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X-dimension of recorded spot

In fax systems, the X-dimension of recorded spot is the effective recorded spot dimension measured in the direction of the recorded line. The effective recorded spot dimension" is the largest center-to-center spacing between recorded spots, which gives minimum peak-to-peak variation of density of the recorded line. X-dimension of recorded spot implies that the facsimile equipment response to a constant density in the object (original) is a succession of discrete recorded spots.

title: X-dimension of recorded spot


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Zero-dispersion wavelength

In a single-mode optical fiber, the zero-dispersion wavelength is the wavelength or wavelengths at which material dispersion and waveguide dispersion cancel one another. In all silica-based optical fibers, minimum material dispersion occurs naturally at a wavelength of approximately 1300 nm. Single-mode fibers may be made of silica-based glasses containing dopants that shift the material-dispersion wavelength, and thus, the zero-dispersion wavelength, toward the minimum-loss window at approximately 1550 nm. The engineering tradeoff is a slight increase in the minimum attenuation coefficient. Such fiber is called dispersion-shifted fiber.

Another way to alter the dispersion is changing the core size and the refractive indices of the material of core and cladding. Because fiber optic materials are already highly optimized for low scattering and high transparency alternative ways to change the refractive index were investigated. As a straightforward solution tapered fibers and holey fibers or photonic crystal fibers (PCF) were produced. Essentially they replace the cladding by air. This improves the contrast of refractive indices by a factor of 10. Therefore, the effective index is changed, especially for longer wavelengths. This type of refractive index change versus wavelength due to different geometry is called waveguide dispersion. As these narrow waveguides (~1-3 μm core diameter) are combined with ultrashort pulses at the zero-dispersion wavelength pulses are not instantly destroyed by dispersion. After reaching a certain peak power within the pulse the non-linear refractive index starts to play an important role leading to frequency generation processes like self-phase modulation (SPM), modulational instability, soliton generation and soliton fission, cross phase modulation (XPM) and others. All these processes generate new frequency components, meaning that input light with narrow bandwidth expands into a wide range of new colours, through a process called supercontinuum generation.

The term is also used, more loosely, in multi-mode optical fiber. There, it refers to the wavelength at which the material dispersion is minimum, i.e. essentially zero. This is more accurately called the minimum-dispersion wavelength.

Zero-dispersion slope
The rate of change of dispersion with respect to wavelength at the zero-dispersion point is called the zero-dispersion slope. Doubly and quadruply clad single-mode fibers have two zero-dispersion points.

title: Zero-dispersion wavelength


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Zip-cord

Zip-cord is a type of electrical cable with two or more conductors held together by an insulating jacket that can be easily separated simply by pulling apart. In Australia it is known as 'figure-8' cable. The zip-cord term is also used with optical fiber cables consisting of two optical fibers joined in a similar manner. The design of zip-cord makes it easy to keep conductors that carry related electrical or optical signals together and helps avoid tangling of cables. Typical uses include lamp cord and speaker wire. Conductors may be identified by a color tracer on the insulation, or by a ridge molded into the insulation of one wire, or by a colored tracer thread inside the insulation. Zip cords are intended for use on portable equipment, and the US and Canadian electrical codes do not permit their use for permanently installed wiring of line-voltage circuits.

title: Zip-cord


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Absolute pitch

Absolute pitch (AP), often called perfect pitch, is the ability to identify or re-create a given musical note without the benefit of a reference tone. AP may be demonstrated using linguistic labelling ("naming" a note), associating mental imagery with the note, or sensorimotor responses. For example, an AP possessor can accurately reproduce a heard tone on a musical instrument without "hunting" for the correct pitch.

About
The frequency of AP in the general population is not known. A proportion of 1 in 10,000 is widely reported, but not supported by evidence; a 2019 review indicated a prevalence of at least 4% amongst music students.

Generally, absolute pitch implies some or all of these abilities, achieved without a reference tone:
 * Identify by name individual pitches played on various instruments.
 * Name the key of a given piece of tonal music.
 * Identify and name all the tones of a given chord or other tonal mass.
 * Name the pitches of common everyday sounds such as car horns and alarms.

Absolute pitch is distinct from relative pitch. While the ability to name specific pitches can be used to infer intervals, relative pitch identifies an interval directly by its sound. Absolute pitch may complement relative pitch in musical listening and practice, but it may also influence its development.

Adults who possess relative pitch but do not already have absolute pitch can learn "pseudo-absolute pitch" and become able to identify notes in a way that superficially resembles absolute pitch. Some people have been able to develop accurate pitch identification in adulthood through training.

History of study and terminologies
Scientific studies of absolute pitch commenced in the 19th century, focusing on the phenomenon of musical pitch and methods of measuring it. It would have been difficult for the notion of absolute pitch to have formed earlier because pitch references were not consistent. For example, the note known as 'A' varied in different local or national musical traditions between what is considered as G sharp and B flat before the standardisation of the late 19th century. While the term absolute pitch, or absolute ear, was in use by the late 19th century by both British and German researchers, its application was not universal; other terms such as musical ear, absolute tone consciousness, or positive pitch referred to the same ability. The skill is not exclusively musical.

Difference in cognition, not elementary sensation
Physically and functionally, the auditory system of an absolute listener evidently does not differ from that of a non-absolute listener. Rather, "it reflects a particular ability to analyze frequency information, presumably involving high-level cortical processing." Absolute pitch is an act of cognition, needing memory of the frequency, a label for the frequency (such as "B-flat"), and exposure to the range of sound encompassed by that categorical label. Absolute pitch may be directly analogous to recognizing colors, phonemes (speech sounds), or other categorical perception of sensory stimuli. For example, most people have learned to recognize and name the color blue by the range of frequencies of the electromagnetic radiation that are perceived as light; those who have been exposed to musical notes together with their names early in life may be more likely to identify the note C. Although it was once thought that it "might be nothing more than a general human capacity whose expression is strongly biased by the level and type of exposure to music that people experience in a given culture", absolute pitch may be influenced by genetic variation, possibly an autosomal dominant genetic trait.

Influence by music experience
Evidence suggests that absolute pitch sense is influenced by cultural exposure to music, especially in the familiarization of the equal-tempered C-major scale. Most of the absolute listeners that were tested in this respect identified the C-major tones more reliably and, except for B, more quickly than the five "black key" tones, which corresponds to the higher prevalence of these tones in ordinary musical experiences. One study of Dutch non-musicians also demonstrated a bias toward using C-major tones in ordinary speech, especially on syllables related to emphasis.

Linguistics
Absolute pitch is more common among speakers of tonal languages, such as most dialects of Chinese or Vietnamese, which depend on pitch variation to distinguish words that otherwise sound the same—e.g., Mandarin with four possible tonal variations, Cantonese with nine, Southern Min with seven or eight (depending on dialect), and Vietnamese with six. Speakers of Sino-Tibetan languages have been reported to speak a word in the same absolute pitch (within a quarter-tone) on different days; it has therefore been suggested that absolute pitch may be acquired by infants when they learn to speak a tonal language (and possibly also by infants when they learn to speak a pitch-accent language). However, the brains of tonal-language speakers do not naturally process musical sound as language; such speakers may be more likely to acquire absolute pitch for musical tones when they later receive musical training. Many native speakers of a tone language, even those with little musical training, are observed to sing a given song with consistent pitch. Among music students of East Asian ethnic heritage, those who speak a tone language fluently have a higher prevalence of absolute pitch than those who do not speak a tone language.

African level-tone languages—such as Yoruba, with three pitch levels, and Mambila, with four—may be better suited to study the role of absolute pitch in speech than the pitch and contour tone languages of East Asia.

Speakers of European languages make subconscious use of an absolute pitch memory when speaking.

Perception
Absolute pitch is the ability to perceive pitch class and to mentally categorize sounds according to perceived pitch class. A pitch class is the set of all pitches that are a whole number of octaves apart. While the boundaries of musical pitch categories vary among human cultures, the recognition of octave relationships is a natural characteristic of the mammalian auditory system. Accordingly, absolute pitch is not the ability to estimate a pitch value from the dimension of pitch-evoking frequency (30–5000 Hz), but to identify a pitch class category within the dimension of pitch class (e.g., C-C♯-D ... B-C).

An absolute listener's sense of hearing is typically no keener than that of a non-absolute ("normal") listener. Absolute pitch does not depend upon a refined ability to perceive and discriminate gradations of sound frequencies, but upon detecting and categorizing a subjective perceptual quality typically referred to as "chroma". The two tasks— of identification (recognizing and naming a pitch) and discrimination (detecting changes or differences in rate of vibration)— are accomplished with different brain mechanisms.

Special populations
The prevalence of absolute pitch is higher among those who are blind from birth as a result of optic nerve hypoplasia.

Absolute pitch is considerably more common among those whose early childhood was spent in East Asia. This might seem to be a genetic difference; however, people of East Asian ancestry who are reared in North America are significantly less likely to develop absolute pitch than those raised in East Asia, so the difference is more probably explained by experience. The language that is spoken may be an important factor; many East Asians speak tonal languages such as Mandarin, Cantonese, and Thai, while others (such as those in Japan and certain provinces of Korea) speak pitch-accent languages, and the prevalence of absolute pitch may be partly explained by exposure to pitches together with meaningful musical labels very early in life.

Absolute pitch ability has higher prevalence among those with Williams syndrome and those with an autism spectrum disorder, with claims estimating that up to 30% of autistic people have absolute pitch. A non-verbal piano-matching method resulted in a correlation of 97% between autism and absolute pitch, with a 53% correlation in non-autistic observers. However, the converse is not indicated by research which found no difference between those with absolute pitch and those without on measures of social and communication skills, which are core deficits in autistic spectrum disorders. Additionally, the absolute pitch group's autism-spectrum quotient was "way below clinical thresholds".

Nature vs. nurture
Absolute pitch might be achievable by any human being during a critical period of auditory development, after which period cognitive strategies favor global and relational processing. Proponents of the critical-period theory agree that the presence of absolute pitch ability is dependent on learning, but there is disagreement about whether training causes absolute skills to occur   or lack of training causes absolute perception to be overwhelmed and obliterated by relative perception of musical intervals.

One or more genetic loci could affect absolute pitch ability, a predisposition for learning the ability or signal the likelihood of its spontaneous occurrence.

Researchers have been trying to teach absolute pitch ability in laboratory settings for more than a century, and various commercial absolute-pitch training courses have been offered to the public since the early 1900s. In 2013, experimenters reported that adult men who took the antiseizure drug valproate (VPA) "learned to identify pitch significantly better than those taking placebo—evidence that VPA facilitated critical-period learning in the adult human brain". However, no adult has ever been documented to have acquired absolute listening ability, because all adults who have been formally tested after AP training have failed to demonstrate "an unqualified level of accuracy... comparable to that of AP possessors".

Pitch memory related to musical context
While very few people have the ability to name a pitch with no external reference, pitch memory can be activated by repeated exposure. People who are not skilled singers will often sing popular songs in the correct key, and can usually recognize when TV themes have been shifted into the wrong key. Members of the Venda culture in South Africa also sing familiar children's songs in the key in which the songs were learned.

This phenomenon is apparently unrelated to musical training. The skill may be associated more closely with vocal production. Violin students learning the Suzuki method are required to memorize each composition in a fixed key and play it from memory on their instrument, but they are not required to sing. When tested, these students did not succeed in singing the memorized Suzuki songs in the original, fixed key.

Possible problems
Musicians with absolute perception may experience difficulties which do not exist for other musicians. Because absolute listeners are capable of recognizing that a musical composition has been transposed from its original key, or that a pitch is being produced at a nonstandard frequency (either sharp or flat), a musician with absolute pitch may become confused upon perceiving tones believed to be "wrong" or hearing a piece of music "in the wrong key". The relative pitch of the notes may be in tune to each other, but out of tune to the standard pitch or pitches the musician is familiar with or perceives as correct. This can especially apply to Baroque music, as many Baroque orchestras tune to A = 415 Hz as opposed to 440 Hz (i.e. roughly one standard semitone lower than the ISO standard for concert A), while other recordings of Baroque pieces (especially those of French Baroque music) are performed at 392 Hz. Historically, tuning forks for concert A used on keyboard instruments (which ensembles tune to when present), have varied widely in frequency, often between 415 Hz to 456.7 Hz.

Variances in the sizes of intervals for different keys and the method of tuning instruments also can affect musicians in their perception of correct pitch, especially with music synthesized digitally using alternative tunings (e.g. unequal well temperaments and alternative meantone tunings such as 19-tone equal temperament and 31-tone equal temperament) as opposed to 12-tone equal temperament. An absolute listener may also use absolute strategies for tasks which are more efficiently accomplished with relative strategies, such as transposition or producing harmony that is microtonal or whose frequencies do not match standard 12-tone equal temperament. It is also possible for some musicians to have displaced absolute pitch, where all notes are slightly flat or slightly sharp of their respective pitch as defined by a given convention. This may arise from learning the pitch names from an instrument that was tuned to a concert pitch convention other than the one in use (e.g. A = 435 Hz, the Paris Opera convention of the late 19th and early 20th centuries, as opposed to the modern Euro-American convention for concert A = 442 Hz). Concert pitches have shifted higher for a brighter sound. When playing in groups with other musicians, this may lead to playing in a tonality that is slightly different from that of the rest of the group, such as when soloists tune slightly sharp of the rest of the ensemble to stand out or to compensate for loosening strings during longer performances.

Synesthesia
Absolute pitch shows a genetic overlap with music-related and non-music-related synesthesia/ideasthesia. They may associate certain notes or keys with different colors, enabling them to tell what any note or key is. In this study, about 20% of people with absolute pitch are also synesthetes.

Correlations
There is evidence of a higher rate of absolute pitch in the autistic population. Many studies have examined pitch abilities in autism, but not rigidly perfect pitch, which makes them controversial. It is unclear just how many people with autism have perfect pitch because of this. In a 2009 study, researchers studied 72 teenagers with autism and found that 20 percent of the teenagers had a significant ability to detect pitches. Children with autism are especially sensitive to changes in pitch.

Correlation with musical talent
Absolute pitch is not a prerequisite for skilled musical performance or composition. However, there is evidence that musicians with absolute pitch tend to perform better on musical transcription tasks (controlling for age of onset and amount of musical training) compared to those without absolute pitch. It was previously argued that musicians with absolute pitch perform worse than those without absolute pitch on recognition of musical intervals; however, experiments on which this conclusion was based contained an artifact and, when this artifact was removed, absolute pitch possessors were found to perform better than nonpossessors on recognition of musical intervals.

title: Absolute pitch