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[[Image:Waveformcompanded.png|right|thumb||After compressing, before expanding]]
 
The name is a [[portmanteau]] of [[Audio level compression|'''com'''pressing]] and ex'''panding'''. In [[telecommunication]], [[signal processing]], and [[thermodynamics]], '''companding''' (occasionally called '''compansion''') is a method of mitigating the detrimental effects of a channel with limited [[dynamic range]]. 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 microphones 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.
 
While the compression used in audio recording and the like depends on a [[variable-gain amplifier]], 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 [[transmission (telecommunications)|transmission]] and is expanded to the original value at the receiver.
 
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Companding is used in digital telephony systems, compressing before input to an [[analog-to-digital converter]], and then expanding after a [[digital-to-analog converter]]. This is equivalent to using a non-linear ADC as in a [[T-carrier]] telephone system that implements [[A-law]] or [[μ-law]] companding. This method is also used in digital file formats for better [[signal-to-noise ratio]] (SNR) at lower bit rates. For example, a linearly encoded 16-bit [[PCM]] signal can be converted to an 8-bit [[WAV]] or [[au file format|AU]] 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==
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