6.15 Compressor
The Compressor reduces, or compresses, the dynamic range of the audio signal. This makes the quieter and louder sections closer to the same volume level by progressively reducing the gain of louder signals. When subsequently amplified, this has the effect of making the quieter sections louder while keeping the louder sections from clipping. This allows listening to the quiet sections of dynamic material in noisy environments while preventing sudden loud sections from being overbearing.
There are several settings associated with the compressor. The first, and most important, is the Threshold. The threshold is the audio input level at which the compressor begins to act. Any level louder than the threshold will be compressed to some extent. The maximum amount of compression, or the quietest level at which the compressor will operate, is -24 dB. The default of Off disables the compressor.
The Makeup Gain setting has two options: Off and Auto. Off means that the compressed audio will not be amplified after compression. The default of Auto will amplify the signal so that the loudest possible signal after compression will be just under the clipping limit. This is desirable because the compressed signal without makeup gain is quieter than the input signal. Makeup Gain in Auto restores the signal to the maximum possible level and brings the quieter audio up with it. This is what makes
it possible to hear the quieter audio in noisy environments.
The Ratio setting determines how aggressively the compressor reduces gain above the threshold. For example, the 2:1 setting means that for each two decibels of input signal above the threshold, the compressor will only allow the output to appear as one decibel. The higher the ratio, the harder the signal is compressed. The ratio setting of Limit means essentially a ratio of infinity to one. In this case, the output signal is not allowed to exceed the threshold at all.
The Knee setting determines how abrupt the transition is from a non-compressed signal to a compressed signal. Hard Knee means that the transition occurs precisely at the threshold. The Soft Knee setting smoothes the transition from plus or minus three decibels around the threshold.
The Attack Time setting sets the delay in milliseconds between the input signal exceeding the activation threshold and acting upon it.
The Release Time setting sets the recovery time after the signal is compressed. Once the compressor determines that compression is necessary, the input signal is reduced appropriately, but the gain isn’t allowed to immediately return to normal levels. This is necessary to reduce artifacts such as “pumping.” Instead, the gain is allowed to return to normal at the chosen rate. Release Time is the time for the gain to recover by 10 dB.
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