Sound Definitions

Techniques : audio level compression : controls

A compressor reduces the dynamic range of an audio 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 dB over the threshold, the gain will be reduced so that the output level will only be 1 dB 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. These parameters can be adjusted for different effects.

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.

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.

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