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Compressor (Single Band)

The audio compressor reduces the dynamic range of the audio, not the amount of data required to store the audio. In CINELERRA-GG the compressor actually performs the function of an expander and compressor of the signal's dynamic range. A third and more sophisticated use serves to highlight the voice with respect to the sound background. It is a multitrack effect and can also be applied as a Shared Effect. (figure 10.13).

Figure 10.13: GUI of configuration for Compressor plugin

The compressor works by calculating the maximum sound level within a certain time period of the current position. The maximum sound level is taken as the input sound level. For every input sound level there is an output sound level specified by the user. The gain at the current position is adjusted so the maximum sound level in the time range is the user specified value. The compressor has a graph which correlates every input sound level to an output level. The horizontal direction is the input sound level in dB. The vertical direction is the output sound level in dB. The user specifies output sound levels by creating points on the graph. Click in the graph to create a point. If two points exist, drag one point across another point to delete it. Moving the point horizontally is equivalent to the makeup gain parameter. Note that it is impossible to create a vertical curve; points would be deleted. Put a part of the curve in horizontal means clamping the signal to the same, unique value (as Limiter). The most recent point selected has its values displayed in textboxes (Output and Input) for more precise adjustment. To have the compressor reduce the dynamic range of the audio, make all the output values greater than the input values except 0 dB. To make the compressor expand the dynamic range of the audio, make all the output values except 0 dB less than the input values. The algorithm currently limits all sound levels above 0 dB to 0 dB, so to get an overloaded effect put a gain effect before the compressor to reduce all the levels and follow it with another gain effect to amplify all the levels back over 0 dB. The volume (in Db) of the input signal is shown in the In meter on the left. Next to it is the Gain meter which indicates the gain added (green) or subtracted (red) to the original signal according to our settings.

Attack secs
  : determines where in relation to the current position the maximum sound level is taken and how fast the gain is adjusted to reach that peak. It is in seconds. If the reaction time is negative the compressor reads ahead of the current position to get the future peak. The gain is ramped to that peak over one reaction time. This allows it to hit the desired output level exactly when the input peak occurs at the current position. If the reaction time is positive the compressor scans only the current position for the gain and ramps gain over one reaction time to hit the desired output level. It hits the output level exactly one reaction time after detecting the input peak.
Release secs
  : if the peak is higher than the current level, the compressor ramps the gain up to the peak value. Then if a future peak is less than the current peak it ramps the gain down. The time taken to ramp the gain down can be greater than the time taken to ramp the gain up. This ramping down time is the decay seconds.
Trigger type
  : the compressor is a multi-channel effect. Several tracks can share one compressor. How the signal from many tracks is interpreted is determined by the trigger type. The Trigger type uses the value supplied in the Trigger textbox as the number of the track to use as input for the compressor. This allows a track which is not even heard to determine the loudness of the other tracks. The maximum trigger takes the loudest track and uses it as the input for the compressor. The Total trigger type adds the signals from all the tracks and uses the total as the input for the compressor. This is the most natural sounding compression and is ideal when multiple tracks are averaged into single speakers.
Trigger
  : This parameter is used in conjunction with trigger type as described previously. Normally only one track is scanned for the input peak. This track is specified by the Trigger. By sharing several tracks and playing with the trigger value, you can make a sine wave on one track follow the amplitude of a drum on another track, for example.
Smooth only
  : for visualizing what the compressor is doing to the sound-level, this option causes it to replace the sound wave with just the current peak value. It makes it very easy to see how reaction secs affects the detected peak values.
Gain
  : moves the curve in the vertical direction only, to change the gain of the Output.
Clear
  and Reset: Reset the parameters to the default values (no compression).