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Aura8ip / Jan 2012
A u r a 8 i p P r o G U I
AGC and Compressor Controls - Part 2
To the right of the AGC Master controls in column #1 are additional adjust-
ments for each of the three AGC/Compressor bands. As pointed out earlier,
the number of AGC/Compressor control columns depends on the number
of processing bands chosen. We have chosen to color code the controls in
each column to make navigation easier. Please note that column #2 and #3
are the only two AGC/Compressor columns serving two roles:
Column #2: Processing adjustments for the lowest frequency band when
in 2 or 3 Band mode, or all of the controls when in Broadband mode. The
control values in this column are always orange.
Column #3: Processing adjustments for the “Mid” band when in 3 Band
mode, or the “High” band when in 2 Band mode. The control values in this
column are always yellow.
Column #4 is only visible when the AGC/Compressors are operating in
3 Band mode and it hosts the processing adjustments for the “High” band
when in 3 Band mode. The control values in this column are always blue.
What follows is an explanation of what each processing control is called,
what it does, and if applicable, what its audible effect is.
Threshold
The first adjustment in each column is the Threshold (Thresh) control. This control
sets the level at which the AGC will start working (or processing) the audio. If the AGC
is defeated, then this control will govern the behavior of the compressor.
Like many other controls, the Threshold control is calibrated in dFBS. And, like
the Gate Threshold discussed previously, the Threshold control operates with a peak
program reference level of -20dBFS. What this means is that if the onset of AGC gain
control action should begin 20dB below average program level (to achieve 20dB of
average compression), this control should be set 20dB below the internal reference
level, or at -40dBFS.
AGC Attack/Release
The next two adjustments in the column control the speed of the AGC. The AGC
Attack setting controls how fast the AGC adapts to increases in audio levels. The range
of the AGC Attack control is 50-500ms.
Conversely, the AGC Release control setting controls how fast the AGC responds
to decreases in audio level. The range of the Release control is 1000-7000ms (1 second
to 7 seconds).
Recommending “perfect” settings for the AGC Attack and Release controls is difficult
because of how highly subjective the resulting processing will be. What this means is
that the settings are highly dependent on many factors including the desired density of
processing and its audibility. In general, slower settings in both controls create less no-
ticeable AGC action while faster settings cause the audibility of processing to increase.
If we were to recommend starting points for the AGC Attack and Release we would
specify around 300 ms for Attack and 4000ms (4 seconds) for Release. While the range
