In
telecommunicationTelecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
,
signal processingSignal processing is an area of systems engineering, electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time...
, and
thermodynamicsThermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
,
companding (occasionally called
compansion) is a method of mitigating the detrimental effects of a channel with limited
dynamic rangeDynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
. The name is a portmanteau of
compressingDynamic range compression, also called DRC or simply compression reduces the volume of loud sounds or amplifies quiet sounds by narrowing or "compressing" an audio signal's dynamic range...
and ex
panding.
While the compression used in audio recording and the like depends on a
variable-gain amplifierA variable-gain or voltage-controlled amplifier is an electronic amplifier that varies its gain depending on a control voltage .VCAs have many applications, including audio level compression, synthesizers and amplitude modulation....
, and so is a locally linear process (linear for short regions, but not globally), companding is non-linear and takes place in the same way at all points in time. The dynamic range of a signal is compressed before
transmissionTransmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
and is expanded to the original value at the receiver.
The electronic circuit that does this is called a
compandor and works by compressing or expanding the
dynamic rangeDynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
of an analog electronic signal such as sound. 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. Compandors are used in concert audio systems and in some noise reduction schemes such as
dbxdbx is a family of noise reduction systems developed by the company of the same name. The most common implementations are dbx Type I and dbx Type II for analog tape recording and, less commonly, vinyl LPs. A separate implementation, known as dbx-TV, is part of the MTS system used to provide stereo...
and Dolby NR (all versions).
Companding can also refer to the use of compression, where gain is decreased when levels rise above a certain threshold, and its complement, expansion, where gain is increased when levels drop below a certain threshold.
The use of companding allows signals with a large dynamic range to be transmitted over facilities that have a smaller dynamic range capability. For example, it is employed in professional
wireless microphoneA wireless microphone, as the name implies, is a microphone without a physical cable connecting it directly to the sound recording or amplifying equipment with which it is associated...
s since the dynamic range of the
microphoneA microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
audio signal itself is larger than the dynamic range provided by radio transmission. Companding also reduces the noise and crosstalk levels at the receiver.
Companding is used in digital telephony systems , compressing before input to an
analog-to-digital converterAn analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
, and then expanding after a
digital-to-analog converterIn electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...
. This is equivalent to using a non-linear ADC as in a
T-carrierIn telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
telephone system that implements A-law or μ-law companding. This method is also used in digital file formats for better
signal-to-noise ratioSignal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
(SNR) at lower bit rates. For example, a linearly encoded 16-bit PCM signal can be converted to an 8-bit
WAVWaveform Audio File Format , is a Microsoft and IBM audio file format standard for storing an audio bitstream on PCs...
or
AUThe Au file format is a simple audio file format introduced by Sun Microsystems. The format was common on NeXT systems and on early Web pages. Originally it was headerless, being simply 8-bit µ-law-encoded data at an 8000 Hz sample rate. Hardware from other vendors often used sample rates as...
file while maintaining a decent SNR by compressing before the transition to 8-bit and expanding after a conversion back to 16-bit. This is effectively a form of lossy audio data compression.
Many of the music equipment manufacturers (Roland, Yamaha, Korg) used companding for data compression in their digital synthesizers. This dates back to the late 1980s when memory chips would often come as one of the most costly parts in the instrument. Manufacturers usually express the amount of memory as it is in the compressed form. i.e. 24MB waveform ROM in Korg Trinity is actually 48MB of data. Still the fact remains that the unit has a 24MB physical ROM. In the example of Roland SR-JV expansion boards, they usually advertised them as 8MB boards which contain '16MB-equivalent content'. Careless copying of the info and omitting the part that stated "equivalent" can often lead to confusion.
History
The use of companding in an analog picture transmission system was patented by A. B. Clark of
AT&TAT&T Corp., originally American Telephone and Telegraph Company, is an American telecommunications company that provides voice, video, data, and Internet telecommunications and professional services to businesses, consumers, and government agencies. AT&T is the oldest telecommunications company...
in 1928 (filed in 1925):
In 1942, Clark and his team completed the
SIGSALYIn cryptography, SIGSALY was a secure speech system used in World War II for the highest-level Allied communications....
secure voice transmission system that included the first use of companding in a PCM (digital) system.
In 1953, B. Smith showed that a nonlinear
DACIn electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...
could result in the inverse nonlinearity in a successive-approximation
ADCAn analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
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.
See also
- Dolby system
- μ-law
- A-law
An A-law algorithm is a standard companding algorithm, used in European digital communications systems to optimize, i.e., modify, the dynamic range of an analog signal for digitizing.It is similar to the μ-law algorithm used in North America and Japan....
- Signal compression
In telecommunication, the term signal compression has the following meanings:In analog systems, reduction of the dynamic range of a signal by controlling it as a function of the inverse relationship of its instantaneous value relative to a specified reference level.Signal compression is usually...
- Dynamic range compression
External links