Telex ISP-100 manual Unfamiliar Controls

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Unfamiliar Controls

There are two controls on the ISP-100 limiter that are not available in traditional dynamic range limiters. These controls provide much greater flexibility and utility to this limiter than can be achieved with traditional limiters.

Detection Window

The detection window is the period of time over which the limiter computes the average signal level. Every dynamic range limiter that operates upon average signal level uses a detection window. In traditional limiters this window is of fixed length, and the operator may not even be aware of that length.

Some traditional limiters provide switching between average and peak detection. This is actually just a selection between a relatively long detection window of generally a few tens or hundreds of milliseconds, and a very short detection window of generally a few tens or hundreds of microseconds.

The ISP-100 limiter provides for operator control of the length of the detection window. Very dynamic signals and signals containing a lot of high frequency energy require shorter detection windows. Non-dynamic signals and signals containing mostly low frequency energy require longer detection windows.

Detection Window is adjustable in the range {20 uSec (one sample) to 5 Sec}.

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 limiter 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 limiter 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 — always a number between zero and

10 July 1998