Telex Integrated Signal Processor ISP-100 - Manual (Page 118)

8-48
10 July 1998
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