Telex ISP-100 manual Crest Factor Sensitivity

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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

1.00the 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

Processing Components - Compressor