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:Note: This usage should not be confused with [[audio data compression]].
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 gian will be reduced so that the output level will only be 1dB over the threshold.
Compressors usually have controls to set how fast the compressor respondes to changes in input level, known as ''attack'', and how quickly the compressor returns to no gian reduction once the input level is below the threshold, known as ''release''. Also, because the compressor is reduceing the gain (or level) of the signal, the ability to add a fixed amount of gain at the output is provided so that an optimum level can be used.
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