D - 8 G U I
page 30
D-8 / Feb 2009
The compressor is “soft-knee,” meaning the compression ratio
increases slowly with increasing applied level, greatly easing the sonic transition
into full compression. This helps to avoid the “snatching” and “pumping” at
threshold that many “hard-knee” dynamics units exhibit.
A full range of controls is available over the compressor’s behavior:
CMP IN — A switch that allows the compressor to be enabled and disabled.
THRESHOLD (-30.0dB - +10.0dB) — Threshold, the level above which gain
reduction is applied.
RATIO (1:1.00 - 1:20.0) — The proportion by which a signal exceeding the
threshold is reduced in level. “1:3” means that a change in signal level above the
threshold by 15dB will be reduced to only a 5dB change in level.
ATTACK (0.10mS - 330.0Ms) — How quickly the compressor responds to a
signal exceeding the threshold.
RELEASE (50.0mS - 3.000S) — The speed with which the compressor
recovers as the exciting input signal reduces or disappears.
MAKEUP (0.0dB - 20.0dB) — When fairly deep compression is invoked
(large gain reduction) it can be necessary to increase the compressor’s output level
back up to nominal system signal level. Up to 20dB of output gain is available to
allow this.
AGC
The AGC is an additional sidechain operating in conjunction with the
compressor. Generally the AGC has much slower integration times for attack and
release, so that it responds more to the medium or long-term energy of the program
material in a given band, unlike the more typically energetic actions of the
compressor.
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, so that 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,
but if the intention is wild effect, deriving all the gain-reduction from the
compressor alone is probably better.
