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''Note: This article is about audio level compression, which reduces the dynamic range of audio signals. This should not be confused with [[audio data compression]], which reduces the data rate of digital audio signals.''
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The term "audio compression", meaning '''audio level compression''' is used in the sound recording and live sound reinforcement fields. In this context, it refers to a process whereby the [[dynamic range]] of an audio signal is reduced.
An audio level compressor reduces the dynamic range of an audio signal by using a variable gain amplifier to reduce the gain of the signal if it passes a set threshold. The amount of gain reduction is usually determined by a ratio control. That is, with a ratio of 4:1, if the input level is 4[[decibel|dB]] over the threshold, the gain will be reduced so that the output level will only be 1dB over the threshold.
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In [[telecommunication]], this operation and its reverse are together known as '''companding''' (for '''comp'''ressing and ex'''panding''')
See also: [[Dolby system]], [[mu-law]], [[A-law]]▼
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▲In [[telecommunication]], '''companding''' (for '''comp'''ressing and ex'''panding''') is an [[operation]] in which the [[dynamic range]] of signals is compressed before [[transmission]] and is expanded to the original value at the receiver.
This is typically carried out in analog systems by using a [[voltage controlled amplifier]] which has its gain reduced as the power of the input signal increases.
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The use of companding allows signals with a large dynamic range to be transmitted over facilities that have a smaller dynamic range capability. Companding reduces the [[noise]] and [[crosstalk]] levels at the receiver.
▲See also: [[Dolby system]], [[mu-law]], [[A-law]], [[squelch]], [[automatic gain control]]
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