Compressor

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The Compressor effect reduces the dynamic range of audio. One of the main purposes of reducing dynamic range is to permit the audio to be amplified further (without clipping) than would be otherwise possible. Therefore by default the Compressor amplifies audio as much as possible after compression. The resultant increase in average or RMS level can be useful for audio played in a noisy environment such as in a car, or in speech, to make a distant voice sound as loud as a close one. Because the gain changes relatively slowly, a compressor does not distort the signal in the way that a Limiter or clipping would do.
Accessed by: Effect > Compressor...
Compressor settings window


Graph

The graph shows the input level along the bottom (horizontal axis) and the output level scale on the left (vertical axis) to illustrate the dynamic range compression effect. The graph will change as you adjust the Threshold and Ratio sliders, reflecting those settings. The graph does not reflect changes in any of the other controls, although they all affect how the audio sounds after applying the effect.

Controls

  • Threshold: The level above which compression is applied to the audio.
  • Noise Floor: The compressor adjusts the gain on audio below this background level so as to prevent it being unduly amplified in processing. This is mainly useful when compressing speech, to prevent the gain increasing during pauses and so over-amplifying the background noise.
  • Ratio: The amount of compression applied to the audio once it passes the threshold level. The higher the Ratio the more the loud parts of the audio will be compressed. The Ratio sets the slope of the blue line on the graph above the threshold.
  • Attack Time: How soon the compressor starts to compress the dynamics after the threshold is exceeded. If volume changes are slow, you can push this to a high value. Short attack times will result in a fast response to sudden, loud sounds, but will make the changes in volume much more obvious to listeners.
  • Decay Time: How soon the compressor starts to release the volume level back to normal after the level drops below the threshold. A long time value will tend to lose quiet sounds that come after loud ones, but will avoid the volume being raised too much during short quiet sections like pauses in speech.
  • Make-up gain for 0 dB after compressing: Boosts the resultant audio after compression to a peak level of 0 dB.
  • Compress based on Peaks: Base the threshold and gain adjustment on peak values of the waveform rather than the average (RMS) value (default is unchecked). When using the RMS value, the compressor uses "downward" compression, making louder sounds above the threshold quieter while leaving quieter ones below it untouched. When using peak values, "upwards" compression is applied which makes the entire audio louder, but amplifies the louder sounds above the threshold less than those below it.


The illustration below compares characteristics of compression based on peak (red) and RMS (blue) levels (before make-up gain).

Compression characteristics for peak based and rms based compression

Schematic example

Uncompressed: A simple sine wave that drops off by 6 dB half way through the selection, to demonstrate how some compressors handle signals.

Uncompressed signal

After: The attack part of where the compressor is working is clearly visible at the start of the audio.The release part still affects some audio that is beneath the threshold as the compressor gain change slowly ebbs out and the material fades back to normal level.

Compressed signal
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