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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
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== Controls ==
▲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.
Compressors usually have controls to set how fast the compressor responds to changes in input level, known as ''
▲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. 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.
An engineer wishing to soften the attack of a snare drum might choose a fast attack time and a moderately fast release time. To accentuate the attack of the snare, he might choose a slower attack time, to avoid affecting the initial transient. It is easier to successfully apply these controls if the user has a basic knowledge of musical instrument acoustics.
▲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 usually 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.
Because the compressor is reducing the gain (or level) of the signal, the ability to add a fixed amount of '''"Make-up Gain"''' at the output is provided so that an optimum level can be used.
== Side Chaining ==
Some compressors implement '''side-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. ▼
== Common Uses ==
Compression is commonly used in [[television commercial|TV advertising]] to boost the perceived volume of the sound track. [[Television|TV]] broadcasters have limits on the instantaneous 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 increasing its peak volume and the sound is perceived as louder.
▲Some compressors implement '''side-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.
A different, non-linear type of compressor is used to reduce the dynamic range of a signal for transmission, to be expanded afterwards. This reduces the effects of a channel with limited dynamic range. See [[Companding]].▼
== Underlying Electronics ==
A compressor accomplishes it's task of reducing dynamic range by using a variable gain [[amplifier]] to reduce the [[gain]] of the signal. 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. Optical compressors use a light sensitive diode to detect changes in signal gain, as expressed via a physical light. This vintage technique is believed by some to add smoother characteristics to the signal, because the response times of the light and the diode soften the attack and release.
== Other Uses ==
▲A different
== See also ==
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