Side chain (sound)

Dynamic range compression (DRC) or simply compression is a signal processing operation that reduces the volume of loud sounds or amplifies quiet sounds by narrowing or compressing an audio signal's dynamic range.

Compression is commonly used in sound recording and reproduction, broadcasting,live sound reinforcement and in some instrument amplifiers.

A dedicated electronic hardware unit or audio software that applies compression is called a compressor. In the 2000s, compressors became available as software plugins that run in digital audio workstation software. In recorded and live music, compression parameters may be adjusted to change the way they affect sounds. Compression and limiting are identical in process but different in degree and perceived effect.

A limiter is a compressor with a high ratio and, generally, a fast attack time.

Downward compression reduces loud sounds over a certain threshold while quiet sounds remain unaffected. Upward compression increases the loudness of sounds below a certain threshold while leaving louder sounds unaffected. Both downward and upward compression reduce the dynamic range of an audio signal.

An expander performs the opposite function, increasing the dynamic range of the audio signal. Expanders are generally used to make quiet sounds even quieter by reducing the level of an audio signal that falls below a set threshold level. A noise gate is a type of expander.

The signal entering a compressor is split, one copy sent to a variable-gain amplifier and the other to a side-chain where the signal level is measured and a circuit controlled by the signal level applies the required gain to an amplifier stage. This design, known as a "feed-forward" type, is used today in most compressors. Earlier designs were based on a "feedback" layout where the signal level was measured after the amplifier.

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