The gain of the signal is reduced by a voltage-controlled circuit. Variable time constants are applied to the
control voltage to adjust the rate at which the gain is reduced, called the attack time, and the rate at which
unity gain is restored after the signal returns to below the threshold, referred to as the release time.
The effect of compression is to limit the dynamic range of a signal. It may be used to keep a variable output
from a bass guitar, for example, at an even level, or to add punch to vocals, drums, guitar or a complete
mix.
Whist the subjective sound quality can be improved by compression, the overall signal level may be
reduced. A gain make-up control at the output of the compressor stage allows the signal level to be brought
back to the same loudness as the uncompressed signal.
Frequency selective compression may be obtained by inserting an equaliser into the sidechain signal, from
which the control voltage is generated. Boosting a range of frequencies will have the effect of lowering the
compressor threshold at those frequencies; i.e. making the compressor more sensitive. Note that the
equalisation is not applied to the audio output, but is used to modify the control sidechain. A common use of
sidechain equalisation is “de-essing” to reduce sibilance on vocals.Suitable equalisers which can be inserted
into the sidechain are the TL Audio Indigo Series 2011 or 2012 valve equalisers, which match the 2021 in
audio levels and styling.
Threshold.
The 2021 compressor has a variable threshold, set by a rotary control calibrated from +20dBu to -20dBu,
resulting in increased compression as the control is rotated clockwise.
Attack and Release.
The attack time is switchable to 0.5msec or 20msec. At 0.5msec attack, the compressor is fast enough to
reduce the gain of a 1KHz signal in less than half a cycle, effectively preventing an overload of any
following equipment which has limited headroom, such as a digital processor, tape machine or transmitter.
The release time is switchable to 40msec or approximately 2 seconds. Adjustment of the attack and release
times allows unobtrusive compression to be applied to virtually any audio signal, but should very short
transients occur the time constants become signal dependant, generally reduced, to prevent a slow release
leaving a “hole” in the signal after the transient. Also, a fast release setting will be extended by a slow attack
setting. Due to this automatic modification of the time constants, the controls are simply calibrated “fast” and
“slow”.
Ratio.
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