Dynamic range compression: Difference between revisions

Revision as of 01:59, 21 January 2005

Revision as of 02:32, 22 January 2005

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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.

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''. The attack of a compressor is always set to at least as slow as the lowest frequency signal in the track, and usualyusually a few times slower. 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.

Compression is commonly used in [[television commercial|TV advertizingadvertising]] to boost the percievedperceived volume of the sound track. [[Television|TV]] broadcasters have limits on the instantaniousinstantaneous peak volume of the audio track in a broadcast rather than its [[root mean square]] volume over the entire 30 second commercial segment. By applying compression followed by gain compensation the overall volume of the track is increased without increacingincreasing its peak volume and the sound is percievedperceived as louder.

Some compressors implement '''sidechainingside-chaining'''. This feature uses the dynamic level of another input to control the compression level of the signal. This is used by [[disk jockey|disk jockeys]] 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 '''comp'''ressing and ex'''panding''', also called '''compansion''') so called because the [[dynamic range]] of signals is compressed before [[transmission (telecommunications)|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.