TL Audio C-1 - Manual (Page 11)

Compressor Operation.
A compressor functions by reducing the gain of the signal when it exceeds a certain
level, or threshold. The amount of gain reduction may be fairly gentle through to
limiting, where the signal is clamped at the threshold level. The amount of gain
reduction is determined by the ratio control, which is calibrated as a ratio of the
output to input signals.
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.
Threshold.
The TL Audio 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 variable from 0.5msec to 50msec. 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 variable from 40msec to approximately 4 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” to “slow”.