telecommunication Telecommunication is the transmission of information by various types of technologies over wire A wire is a single usually cylindrical A cylinder (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Gr ...

and

signal processing Signal processing is an electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetis ...

, companding (occasionally called compansion) is a method of mitigating the detrimental effects of a channel with limited

dynamic range Dynamic range (abbreviated DR, DNR, or DYR) is the ratio In mathematics, a ratio indicates how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemon ...

. The name is a

portmanteau A portmanteau (, ) or portmanteau word (from "portmanteau (luggage) A portmanteau is a piece of luggage Baggage or luggage consists of bags, cases, and containers which hold a travel Travel is the movement of people between distant ...

of the words compressing and expanding, which are the functions of a compander at the transmitting and receiving end 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 Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...

and other audio applications such as professional

wireless microphone A wireless microphone, or cordless microphone, is a microphone A microphone, colloquially called a mic or mike (), is a device – a transducer – that converts sound In physics Physics (from grc, φυσική (ἐπιστ ...

s and

analog recording Analog or analogue may refer to: Computing and electronics * Analog signal An analog signal is any continuous signal In mathematical dynamics, discrete time and continuous time are two alternative frameworks within which to model variab ...

How it works

The dynamic range of a signal is compressed before

transmission Transmission may refer to: Science and technology * Power transmissionPower transmission is the movement of energy from its place of generation to a location where it is applied to perform useful Mechanical work, work. Power (physics), Power is d ...

and is expanded to the original value at the receiver. The electronic circuit that does this is called a compander and works by compressing or expanding the

of an analog electronic signal such as sound recorded by a microphone. One variety is a triplet of amplifiers: a logarithmic amplifier, followed by a variable-gain linear amplifier and an exponential amplifier. Such a triplet has the property that its output voltage is proportional to the input voltage raised to an adjustable power. 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 The ''Bell Labs Technical Journal'' is the in-house scientific journal In academic publishing Academic publishing is the subfield of publishing Publishing is the activity of making information, literature, music, software and other conte ...

'', Vol. 27, pp. 446–472, July 1948. Robert M. Gray and David L. Neuhoff, "Quantization", ''

IEEE Transactions on Information Theory ''IEEE Transactions on Information Theory'' is a monthly peer-reviewed scientific journal published by the IEEE Information Theory Society. It covers information theory and the mathematics Mathematics (from Ancient Greek, Greek: ) includes ...

'', 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 designed to be suitable for implementation using simple analog electronic circuits. The two most popular compander functions used for telecommunications are the

A-law An A-law algorithm is a standard companding After compressing, before expanding In telecommunication Telecommunication is the transmission of information Information can be thought of as the resolution of uncertainty; it answers t ...

and μ-law functions.

Applications

Companding is used in digital telephony systems, compressing before input to an

analog-to-digital converter In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a Digital signal (signal processing), digit ...

, and then expanding after a

digital-to-analog converter In , a digital-to-analog converter (DAC, D/A, D2A, or D-to-A) is a system that converts a into an . An (ADC) performs the reverse function. There are several DAC ; the suitability of a DAC for a particular application is determined by ...

. This is equivalent to using a non-linear ADC as in a

T-carrier The T-carrier is a member of the series of carrier systems developed by AT&T Bell Labs, Bell Laboratories for digital transmission of multiplexed telephone calls. The first version, the Transmission System 1 (T1), was introduced in 1962 in the ...

telephone system that implements

or μ-law companding. This method is also used in digital file formats for better

signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and ...

(SNR) at lower bit depths. For example, a linearly encoded 16-bit

PCM Pulse-code modulation (PCM) is a method used to Digital signal (signal processing), digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio ap ...

signal can be converted to an 8-bit

WAV Waveform Audio File Format (WAVE, or WAV due to its filename extension A filename extension, file extension or file type is an identifier specified as a suffix to the name A name is a term used for identification by an external observer. T ...

WAV

or AU file while maintaining a decent SNR by compressing before the transition to 8-bit and expanding after conversion back to 16-bit. This is effectively a form of lossy

audio data compression In signal processing Signal processing is an electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electro ...

. Professional

s 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. Companders are used in concert audio systems and in some

noise reduction scheme Noise reduction is the process of removing Noise (signal processing), noise from a signal (information theory), signal. Noise reduction techniques exist for audio and images. Noise reduction algorithms may distort the signal to some degree. All s ...

History

The use of companding in an analog picture transmission system was patented by A. B. Clark of

AT&T AT&T Inc. is an American multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in multiple countries * Multinational force, a military body from multiple countries * Multinational state, a ...

in 1928 (filed in 1925): In 1942, Clark and his team completed the

SIGSALY SIGSALY (also known as the X System, Project X, Ciphony I, and the Green Hornet) was a secure speech system used in World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a that lasted from 1939 to ...

SIGSALY

secure voice transmission system that included the first use of companding in a PCM (digital) system. 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. In 1970, H. Kaneko developed the uniform description of segment (piecewise linear) companding laws that had by then been adopted in digital telephony. In the 1980s (and '90s), many of the music equipment manufacturers (

Roland Roland (; frk, *Hrōþiland; lat-med, Hruodlandus or ''Rotholandus''; it, Orlando or ''Rolando''; died 15 August 778) was a Frankish Frankish may refer to: * Franks The Franks ( la, Franci or ) were a group of Germanic peoples The ...

Yamaha is a Japanese multinational corporation A multinational company (MNC) is a corporate A corporation is an organization—usually a group of people or a company A company, abbreviated as co., is a Legal personality, legal entity repre ...

Korg , founded as Keio Electronic Laboratories, is a Japanese multinational corporation A multinational company (MNC) is a corporate A corporation is an organization—usually a group of people or a company A company, abbreviated as co., ...

) used companding when compressing the library waveform data in their

digital synthesizers performing with a digital synth A digital synthesizer is a synthesizer that uses digital signal processing (DSP) techniques to make musical sounds. This in contrast to older analog synthesizers, which produce music using analog electronics, and Sam ...

. Unfortunately exact algorithms are not known, 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.Dave Polich, sound designer, 13th January 2018 https://www.gearslutz.com/board/showpost.php?p=13068220&postcount=146 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.

References

External links

Companding: Logarithmic Laws, Implementation, and Consequences
{{Compression Methods Lossy compression algorithms Audio engineering

Sound recording technology Recording devices technology Technology ("science of craft", from Ancient Greek, Greek , ''techne'', "art, skill, cunning of hand"; and , ''wikt:-logia, -logia'') is the sum of Art techniques and materials, techniques, skills, Scientific metho ...