V O R S I S H D P 3 G U I
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HD P3 / Aug 2006
AGC
The AGC is an additional
sidechain operating in conjunction
with each of the three compressors;
generally the AGC has much slower
integration times for attack and re-
lease, so that it responds more to the
medium or long-term energy of the
program material in a given band,
rather than more typically energetic
actions of the compressor. Its time-
constants are controlled by “AGC Attack” and “AGC Release”, in the
middle of the top row of controls on the GUI’s compressor screen. A single
set of time-constants for all three bands was felt more than adequate, and
has proven so. This single set of parameters does not imply that the three
bands’ AGCs are coupled in any way; all three operate independently.
The AGC may be used alone (by enabling it and turning off the
compressor) for gentle control of inconsistent input signals. It can also be
used in conjunction with the compressor; in this case, the AGC action rides
“underneath” the compressor; depending on the time-constant settings and
character of the program material, the AGC can follow the average long
term level changes and create a gain-reduction “bed” some 12dB to 6dB
under the compression gain-reduction peaks. The AGC rides the general
level, the compressor processing for effect takes place on a consistently
controlled signal.
On sudden application of an input signal, the faster compressor’s attack
captures the onslaught, with the AGC eventually catching up. On release,
the effect is identical to the much-vaunted “two-slope release” of classic
compressor units such as the Audio and Design F760xrs and Joemeek
SC2. On departure or reduction of the input signal the usually faster
compressor release predominates until its gain-reduction contribution
falls below that of the AGC, whose much slower release rate takes over.
A big advantage of the compounded processes is that on normal program
material, the compressor does not have to “move as far” to capture signal
peaks, so reducing the “snatching” which can occur at deep compression
onset.
For most purposes the combination AGC/compression is the most
transparent; if the intention is wild effect, deriving all the gain-reduction
from the compressor alone is probably better.
