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Yamaha GC2020C Owner's Manual: What's A Compressor/limiter; Compressor/limiter Functions

Yamaha compressor/limiter owner's manual.
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What's a Compressor/Limiter?

This section explains the main functions of the Compressor/Limiter. Its parameters, setup procedure, and the expander/gate.
Compressor/Limiter Functions
Generally, a compressor is used to fit a large signal into a
small space. Specifically, in a situation where the dynamic
range of the original audio signal is larger than the electronic
reproduction equipment that is to process it can handle, a
compressor can reduce the dynamic range of the signal to fit
neatly within the limits of the recording or reproduction
equipment. Of course, this must be done without adding
distortion to the signal itself.
Compression is expressed in terms of a ratio—the compres-
sion ratio. This ratio describes how much the signal appear-
ing at the output of the compressor changes in relation to a
given change in the level of the original signal applied to the
input. If no compression is applied and the input signal
doubles in level, then the output signal will also double in
level, precisely following the change in the input signal. This
corresponds to a compression ratio of 1:1 (read "one to
one") —a change of 1 at the input produces a change of 1 at
the output—i.e. no compression. Now if we apply some
compression, a smaller change in the level of the output sig-
nal will be observed for the same change in input signal
level. A compression ratio of 2:1, for example, would mean
that the level of the output signal will change only half as
much as the input signal. Expressed in decibels, a compres-
sion ratio of 20:1 would mean that a 20 dB change in the
level of the input signal would result in only a 1 dB change in
the level of the output signal. Thus, a compressor is able to
reduce the dynamic range of an audio signal by any desired
amount. See Figure 1.
Input Signal Level
Figure 1
What's a Compressor/Limiter?
Limiting is basically extreme compression that is set to affect
only signals above a certain level. This is particularly useful
for limiting only peaks that exceed the handling capacity of
the related equipment, without affecting the rest of the sig-
nal. See Figure 2.
Input Signal Level
Figure 2
Suppose we wanted to limit the peak levels in a program to
a maximum of 0 dBm, in order to prevent saturation and
distortion in a tape recorder. First we would set the "thresh-
old" level to 0 dB—the threshold level is the input signal
level at which the limiter will begin to operate. Then we
would set the maximum (or near maximum) compression
available— :1 (infinity compression). Infinity compres-
sion means that absolutely no change in the output level will
occur no matter how much the input signal changes. As a
result, all signal content below the threshold level (0 dB) will
be passed exactly as it appears at the limiters input. Signals
exceeding the threshold level, however, will be output at the
threshold level and will rise no higher. In this case no signal
exceeding 0 dB will appear at the limiter's output. The actual
audio signal remains untouched, just its average (r.m.s.)
level is kept within the defined limits.
GC2020C Compressor/Limiter Owner's Manual


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