Compressor
Purpose
The compressor provides a means of controlling the dynamics in an audio signal.
Inputs
The Compressor has 2 input nodes. The top input is the Audio input, and the bottom is the Side Chain input (note that the standard version of the Compressor has only one node, the side chain input is wired to be the same as the input). The gain computation for the device is based on the RMS level measured at the Side Chain input. Once the gain is computed, it is applied to the Audio input signal.
Controls
Above/Knee/Below Indicators
These indicators show where the input RMS signal level is relative to the Threshold setting. The three indicators are mutually exclusive; only one of them will be lit at any time.
Below Indicator - With Soft Knee disabled the "Below" indicator will light whenever the input signal level is below the Threshold Level. With soft knee enabled, the indicator will light whenever the input signal level is deemed to be below the soft knee region (12dB or more below the threshold level).
Knee Indicator - The "Knee" indicator will never light if Soft Knee is disabled. The soft knee feature is active when the input signal level is in a 24dB region centered about the Threshold Level. The indicator will light whenever the input signal level is in this region.
Above Indicator - With Soft Knee disabled the "Above" indicator will light whenever the input signal level is deemed to be above the Threshold Level. With Soft Knee enabled, the indicator will light whenever the input signal level is deemed to be above the soft knee region (12dB or more above the threshold level).
Note: The Attack and Release controls define the compressor's interpretation of the input signal level. With longer settings of these controls, the compressor will respond slowly to changes in the input signal level. The state of the Above/Knee/Below indicators directly measure the input signal and thus does not take into consideration the effects of the Attack and Release settings.
Attack
Determines how fast the compressor reacts to an increase of the input signal level.
Bypass
The bypass button passes the Audio input directly to the output. The Side Chain input is not affected by Bypass.
Note: When bypass is engaged, the compressor continues to compute a gain reduction factor. And although this computed reduction is not applied to the signal passing through the device, the Gain Reduction Meter and the Above/Knee/Below Indicators continue to function as if Bypass were not engaged. Careful examination of the Schematic should clarify this point.
Gain Reduction Meter
Indicates the amount of gain reduction currently being produced by the compressor.
Note: Although the gain reduction meter cannot graphically indicate a gain reduction greater than -35dB, the compressor is capable of generating greater reductions.
Level Response
Compressor output level (vertical axis) plotted against input level (horizontal axis). Threshold setting is -48dB and Ratio setting is 4 with Soft Knee off (blue) and Soft Knee on (red). The black line shows the unity gain Bypass response.
Ratio
The ratio control adjusts the severity of gain reduction produced by the compressor once the input signal has exceeded the Threshold Level.
Dynamic variations in the input signal will be reduced by a factor equal to the compression ratio.
Release
Determines how fast the compressor reacts to a decrease of the input signal level.
Soft Knee
The soft knee function provides a smooth transition between the non-compressed and compressed modes of operation. When soft knee is enabled the compressor begins introducing gain reduction (at a reduced compression ratio) when the input signal is 12dB below the Threshold Level. The compressor gradually increases the effective compression ration as the input signal level approaches crosses the Threshold Level. The compression ratio specified by the Ratio control is not fully reached until the input signal level exceeds the Threshold Level by 12dB or more. Please see the Amplitude Response Curves for clarification.
Tip: The soft knee feature is most useful with high Ratio settings. A less noticeable limiting response is produced.
Threshold
Specifies the input RMS signal level above which gain reduction will occur in the compressor.
If the level of the signal at the Side Chain input is below the Threshold setting the signal will be passed through the compressor unaffected (assuming Soft Knee is off). If the signal level at the output of the input level control is above the Threshold setting the compressor will act upon the signal level in a manner determined by the other compressor control settings.
Note: When Soft Knee is enabled, the compressor begins introducing gain reduction (at a reduced compression ratio) when the input signal is 12dB below the Threshold Level.
Schematic
Advanced Topic
The Compressor composite device is constructed of a Compressor Algorithm and some additional Level and Meter devices. The Side Chain and Audio inputs are wired internally from the single input on this device.
