In
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
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 ...
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
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 ...
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 ...

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
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 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 ...
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 ...

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
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 ...