The Threshold control sets the level that an input signal must exceed to trigger compression or limiting. Signals that exceed this level will be compressed. Signals that are below it will be unaffected.
The Attack control sets the attack time, or the rate at which gain is reduced after the input signal crosses the threshold.
The smaller the value, the faster the attack. The faster the attack, the more rapidly the Compressor/Limiter applies attenuation to the signal. If you use fast attack times, you should generally use a proportionally longer release time, particularly with material that contains many peaks in close proximity.
The Ratio control sets the compression ratio, or the amount of compression applied as the input signal exceeds the threshold. For example, a 2:1 compression ratio means that a 2 dB increase of level above the threshold produces a 1 db increase in output. The compression ratio ranges from 1:0:1 to 20:0:1.
Once the Ratio control passes 20:0:1 the Compressor/Limiter effect functions as a limiter rather than a compressor.
At the limiter setting (LMTR), for every decibel that the incoming signal goes over the set Threshold, 1 dB of gain reduction is applied.
Once the Ratio control passes the LMTR setting, it provides negative ratio settings from –20:0:1 to 0:1.
With these settings, for every decibel that the incoming signal goes over the set Threshold, more than 1 dB of gain reduction is applied according to the negative Ratio setting. For example, at the setting of –1.0:1, for each decibel over the set threshold, 2 db of gain reduction is allied. Consequently, the output signal is both compressed and made softer. You can use this as an creative effect, or as a kind of ducking effect when used with an external key input.
The Depth control sets the amount of gain reduction that is applied regardless of the input signal. For example, if the Limiter is set at a Threshold of –20 dB and Depth is set at 0 dB, up to 20 dB of gain reduction is applied to the incoming signal (at 0 dB). If you set Depth to –10 dB, no more than 10 dB of gain reduction is applied to the incoming signal.
The Release control sets the length of time it takes for the Compressor/Limiter to be fully deactivated after the input signal drops below the threshold.
Release times should be set long enough that if signal levels repeatedly rise above the threshold, the gain reduction “recovers” smoothly. If the release time is too short, the gain can rapidly fluctuate as the compressor repeatedly tries to recover from the gain reduction. If the release time is too long, a loud section of the audio material could cause gain reduction that continues through soft sections of program material without recovering.
The Knee control sets the rate at which the compressor reaches full compression once the threshold has been exceeded.
As you increase this control, it goes from applying “hard-knee” compression to “soft-knee” compression:
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With hard-knee compression, compression begins when the input signal exceeds the threshold. This can sound abrupt and is ideal for limiting.
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With soft-knee compression, gentle compression begins and increases gradually as the input signal approaches the threshold, and reaches full compression after exceeding the threshold. This creates smoother compression.
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The Gain control lets you boost overall output gain to compensate for heavily compressed or limited signals.