Telex Integrated Signal Processor ISP-100 - Manual (Page 77)

8-7
Processing Components - Compressor
Crest Factor Sensitivity
A signal’s crest factor is the ratio of a its peak level to its average
level. The signal having the lowest possible crest factor is the
square wave, whose peak and average levels are equal and
whose crest factor is therefore 1.0. The peak level of a pure
sinewave tone is 1.414 times its average level; a sinewave
therefore has a crest factor of 1.414. Most music and speech
signals contain peaks that are very much higher than their
average levels; therefore their crest factors are very much
greater than one. The important points to remember are that a
signal’s peak value is always greater than or equal to its average
value, and that the crest factor of any signal is therefore always
greater than or equal to 1.0.
The Crest Factor Sensitivity adjustment on the ISP-100
compressor function controls its sensitivity to the crest factor of
the input signal. It works like this:
The signal level detection algorithm used in the ISP-100
compressor consists of two separate detectors operating in
parallel. One of the detectors computes the average level over
the selected detection window; the other computes the peak
level of each input sample.
The peak level is multiplied by the setting of the Crest Factor
Sensitivity control, which is always a number between zero and
one. The two quantities are then compared and the larger is
returned as the signal level.
When the Crest Factor Sensitivity is set to 0.00 the ISP-100
compressor operates as a traditional compressor responding to
average signal levels; when the Crest Factor Sensitivity is set to
1.00 the ISP-100 compressor operates as a traditional
compressor responding to peak signal levels. Values in-between
represent various hybrids of the two. If the compressor is to
ignore peaks and respond only to average levels, then the Crest
Factor Sensitivity may be set near zero. If the compressor is to
respond only to peaks, then the Crest Factor Sensitivity may be
set near one.
Good results may be obtained by following these steps:
1. Set the Crest Factor Sensitivity to 0.00. This will cause the
ISP-100 compressor to behave like a traditional compressor
responding to average signal levels.
2. Set all of the other compressor controls as appropriate for the
signal being compressed, just as they would be set in a
traditional compressor.
3. Adjust the Crest Factor Sensitivity control so that the
compressor responds appropriately to short signal peaks. If it
were possible to know, in advance, what the crest factor of the