565E
5
A gate is similar to an expander except that its ratio is much higher; thus the action is more like a
switch once the signal falls below threshold. Some expander applications for the 565E may be quite
similar to a gate, like tending a lone announce microphone, but downward expanders generally are
not well-suited to typical gate applications such as removing leakage from drum microphones,
where very tight control is desired.
Ratio
The compression ratio of the 565E determines how much the output changes for a change in the
input. A linear amplifier (like a simple preamp) has a ratio of 1:1 because a change of 1 dB at its
input results in a 1 dB change at its output. A compressor alters the input/output relationship by
its compression ratio. Thus a 20:1 ratio means that a 20 dB above-threshold change at the input
results in a 1 dB change at the output. In other words, a very audible change at the input (20 dB)
turns into a barely discernible change at the output (1 dB).
Compressors are not the only devices to have an input/output ratio. Any device that is capable of
changing the input/output relationship can be said to have a ratio. Thus expanders, gates, com-
pressors and limiters all fit this category.
An expander magnifies output changes for a given input change. Thus, once the input signal falls
below threshold, the expander changes the output by the amount of the ratio. The 565E’s down-
ward expander has an expansion ratio of 1:1.5, which means that a below-threshold input signal that
gets 10 dB quieter turns into an output signal that gets 15 dB quieter.
A gate can be looked at as an expander with an infinite expansion ratio. Thus the slightest peak in
the input signal from below-threshold to above-threshold switches the gate from closed to open.
Gain versus Output Level
The GAIN control compensates for signal level lost to compression. As an example, set the THRESH-
OLD control counter-clockwise and set the RATIO control of the 565E for a 4:1 ratio. Now adjust the
THRESHOLD control for 10 dB of gain reduction as viewed on the compressor’s gain reduction
display. The output level should be significantly lower than it was when the Threshold control was
counter-clockwise (i.e. when no compression was being applied). You supply the additional gain
(make-up gain) by adjusting the GAIN control until the input and output signal levels match.
Release Time
Most dynamics processing equipment has a control marked RELEASE on the front panel. This
control refers to release time, and it affects the length of time required for the gain to recover to the
no-signal state (when no signal is applied to the input). The release time control allows tailoring
the expander’s or compressor’s recovery time to the program material.
For compressors, the no-signal state is unity gain (passing the signal straight through the compressor,
with no change in gain). Compressors pass any signal whose level is below threshold at unity gain.
The release time control determines how long it will take for the compressor to return to unity gain, once
the input signal has fallen below the compression threshold. Generally, peak limiting is associated with
short release times and compression or leveling associated with longer release times.
For expanders, the no-signal state is determined by the amount of gain reduction applied when no
signal is present at the expander’s input. The amount of gain reduction is set by the threshold
control and by the ratio of the expander, and this applies to any signal whose level is below-
threshold. In this case, the release time control governs how long it takes for the expander to
reduce the gain when the signal disappears. Typically, smooth expansion of speech or music is
associated with longer release times, and gating-type functions, such as feedback suppression, are
associated with short release times.
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