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In telecommunications and signal processing, companding (occasionally called compansion) is a method of mitigating the detrimental effects of a channel with limited dynamic range. The name is a portmanteau of the words compressing and expanding, which are the functions of a compander at the transmitting and receiving ends, respectively. The use of companding allows signals with a large dynamic range to be transmitted over facilities that have a smaller dynamic range capability. Companding is employed in telephony and other audio applications such as professional wireless microphones and analog recording.
Companded quantization is the combination of three functional building blocks – namely, a (continuous-domain) signal dynamic range compressor, a limited-range uniform quantizer, and a (continuous-domain) signal dynamic range expander that inverts the compressor function. This type of quantization is frequently used in telephony systems.W. R. Bennett, " Spectra of Quantized Signals", Bell System Technical Journal, Vol. 27, pp. 446–472, July 1948.Robert M. Gray and David L. Neuhoff, "Quantization", IEEE Transactions on Information Theory, Vol. IT-44, No. 6, pp. 2325–2383, Oct. 1998.
In practice, companders are designed to operate according to relatively simple dynamic range compressor functions that are suitable for implementation as simple analog electronic circuits. The two most popular compander functions used for telecommunications are the A-law and μ-law functions.
Professional wireless microphones do this since the dynamic range of the microphone audio signal itself is larger than the dynamic range provided by radio transmission. Companding also reduces the noise and crosstalk levels at the receiver. A description of companding in wireless microphones
Companders are used in concert audio systems and in some noise reduction schemes.
In 1942, Clark and his team completed the SIGSALY secure voice transmission system that included the first use of companding in a PCM (digital) system.Randall K. Nichols and Panos C. Lekkas (2025). 9780071380386, McGraw-Hill Professional. . ISBN 9780071380386
In 1953, B. Smith showed that a nonlinear DAC could be complemented by the inverse nonlinearity in a successive-approximation ADC configuration, simplifying the design of digital companding systems.B. Smith, "Instantaneous Companding of Quantized Signals," Bell System Technical Journal, Vol. 36, May 1957, pp. 653–709.
In 1970, H. Kaneko developed the uniform description of segment (piecewise linear) companding laws that had by then been adopted in digital telephony.H. Kaneko, "A Unified Formulation of Segment Companding Laws and Synthesis of Codecs and Digital Compandors," Bell System Technical Journal, Vol. 49, September 1970, pp. 1555–1558.
In the 1980s and 1990s, many of the music equipment manufacturers (Roland, Yamaha, Korg) used companding when compressing the library waveform data in their digital synthesizers. However, exact algorithms are unknown, neither if any of the manufacturers ever used the Companding scheme which is described in this article. The only known thing is that manufacturers did use data compression in the mentioned time period and that some people refer to it as companding while in reality it might mean something else, for example data compression and expansion. This dates back to the late '80s when memory chips were often one of the most costly components in the instrument. Manufacturers usually quoted the amount of memory in its compressed form: i.e. 24 MB of physical waveform ROM in a Korg Trinity is actually 48 MB when uncompressed. Similarly, Roland SR-JV expansion boards were usually advertised as 8 MB boards with '16 MB-equivalent content'. Careless copying of this technical information, omitting the equivalence reference, can often cause confusion.
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