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 Crimson Series
3011 or 3012 equalisers, which match the 3051 in audio levels and styling.
Threshold.
The 3051 compressor section 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.
The ratio may be varied from 1:1.5 (very gentle compression) to 1:30 (near limiting).
The compressor normally operates with a “soft-knee”, i.e. the compression is
gradually introduced as the signal passes the threshold, in which case the ratio refers
to the compression eventually obtained.
Gain Make-Up.
Up to 20dB of gain make-up may be applied, to retain the subjective loudness of the
signal.
