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.
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 4dB over the threshold, the gain 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 responds to changes in input level, known as attack, and how quickly the compressor returns to no gain reduction once the input level is below the threshold, known as release. Also, because the compressor is reducing 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.
Some compressors implement sidechaining. This feature uses the dynamic level of another input to control the compression level of the signal. This is used by disk jockies to lower the music volume automatically when speaking, for example (known as ducking). Another use is in music production, to maintain a loud bass track, while still keeping the bass 'out of the way' of the bass drum when the drum hits.
Companding
In telecommunication, this operation and its reverse are together known as companding (for compressing and expanding, also called compansion) so called because 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.
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.