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All about Compression Parameters
Compression Ratio The compressor reduces the output level of any signal that exceeds a selected threshold by an amount proportional to how much the input signal goes above threshold. The increase of input signal over threshold needed to cause a 1 dB increase in the output signal of the compressor is called the compression ratio. For example, with a ratio of 4:1, an 8 dB increase of input produces only 2 dB increase in the output. Audio material often varies in loudness, and can be above the threshold at one moment and below it the next. The compressor's action helps to smooth out these large and sometimes sudden changes in level.
Attack (Time) This control sets the compressor's response time, or how fast it will take the compressor to lower the signal level after it goes above the threshold. To use compression most effectively, the attack time should be set so that signals exceed the threshold level long enough to cause an increase in the average level. This helps ensure that gain reduction doesn't decrease the overall volume.
Release (Time) This control sets the amount of time that it takes for the compressor's gain to return to unity (no) gain reduction after the input signal falls below the threshold. Reelase time should be set long enough that if signal levels repeatedly rise above the threshold, they cause gain reduction only once. If the release time is too long, a loud section of the audio material could cause gain reduction that persists into a soft section. Of course, compression has many creative uses that break these rules. The compressor has built-in metering that allows you to monitor the amount of gain reduction taking place. The gain reduction meter usually remains at 0 level when the input signal is below the threshold and falls to show the amount of gain reduction in decibels when the input signal exceeds the threshold.
Phase Invert Inverts the phase (polarity) of the input signal, allowing you to change frequency response characteristics between multi-miked sources or to correct for miss-wired cables.
Gain Provides overall output gain control. You can compensate for heavily compressed signals.
Input Meter Indicates the level of the unprocessed input signal to the compressor.
Output Meter Indicates the output level of the compressor, including any gain compensation added via the Gain parameter.
Reduction Indicates the amount of gain reduction in dB.
Threshold Sets the threshold level. Signals that exceed this level will be compressed. Signals that are below it will be unaffected.
Notes on reading the compressor meters
A reduction reading of 0 dB is equivalent to no compression.
A signal level reading of 0 dB means dB fs (full scale). In the digital world this is the absolute highest undistorted output available. Unlike the analog VU, there is no headroom above 0 dB fs.
Ratio Sets the compression ratio. The range is based on decibels above the threshold. If this is set to 2:1 for example, it will compress changes in signals above the threshold by one half.
Attack Time Sets the compressor's attack time. The smaller the time values the faster the attack. The faster the attack, the more rapidly the compressor applies attenuation to the signal. If you use fast attack times and heavy limiting (10 to 1 ratio or greater), you should generally use a longer release time, particularly with material that contains many peaks in close proximity.
Release Time Controls how long it takes for the compressor to no longer apply gain reduction after the input signal drops below the threshold. In general, with heavy compression, you should use longer release times to avoid 'pumping' which may occur when the compressor is forced to jump back and forth between compressed and uncompressed signal levels. Increasing release time will help smooth changes in level by introducing a lag in the up/down time of attenuation. Use shorter release times on material with fewer peaks that don't occur in close proximity to each other.
Knee Sets the rate at which the compressor reaches full compression once the threshold has been exceeded. This parameter ranges from 0 (hardest response) to 200 (softest response).
Graph Displays the response curve set by the compressor's Threshold, Ratio, and Knee settings. As you adjust these parameters, refer to the graph to see how the shape of this curve changes. This will allow you to see the effect of your settings.
External Key Enables and disables sidechain processing. Sidechain processing allows the compressor to use to the energy of one signal to trigger compression on another signal.
Key Listen When enabled, this allows you to listen to the signal controlling the sidechain input. This is often useful for fine tuning the compressor's settings to the key input.
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