Symetrix 528 - Why Dynamic Ranae Processrors Are Necessary; The Threshold Concept

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1.8 Why Dynamic Range Processor s are Necessary
For siirnals to stay below distortion and above noise, their actual dynamic rani'e must be
kept within the specified dynamic rani'e of the circuits through which those siirnals flow.
Unfortunately, the actual dynamic range of real world siirnals often exceeds the available
dynamic range of even the best equipment.
For example, the dynamic range of the best analog tape recorders is around 80d8, while
dii'ital recorders top out at around 96d8. As good as these machines are, there's still
not quite enough room for very wide dynamic range siirnals. In order to maintain a 60dB
siirnal-to-noise ratio (to keep the signals 60dB above the noise floor), the dynamic range
of sipals stored on the analog tape machine would have to be restricted by 20dB, while
the digital recorder would be restricted by 36d8.
A compressor or limiter is often used to reduce dynamic range by setting an upper limit
on the lal"i'er sipals. However, in some cases it's better to put processing to work on
the lower end of the dynamic range than on the upper end. In other words, instead of
reducinir the amount of chani'e at the upper end of the dynamic rani'e with a compressor
or limiter, increasinir the amount of chanire at the lower end of the dynamic range with
an expande r or gate.
1.9 Co mpr esso rs are t o Expan ders as Limiters are t o Gates
Compressors reduce the dynamic range of their output whenever the input signal is above
threshold, while expanders increase the dynamic range of their output whenever the input
sipal is below threshold.
Compressors, limiters, expanders, and gates increase or decrease signal levels by some
ratio. Compressors usually have an adjustable ratio, that is, the ratio of the input level
to the output level can be changed by the user. A compressor operating with a 2:1 ratio
would allow only a ldB increase in output level for every 2dB increase in input level.
Limiters usually have a non-adjustable ratio that is very high (greater than 10:1). At
10:1, the limiter allows only a ldB increase in the output level for every l0dB increase
in the input level. Lim iters can be thought of as high ratio, high threshold compressors .
They are intended to "stay out of the way" until the level goes above threshold.
However, above threshold their action is very definite.
1. 10 The Th re sh ol d Concept
The threshold is the level at which a dynamic range processor's activity begins. In
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operation, the dynamic range processor's sensing circuitry constantly "looks at" the I
incoming siirnal and compares it to a reference level, which is called the threshold
level. In practice that reference level is set by the operator with the threshold control.
Rem ember, co m pressors and limiters respond when siirnals at the input are above
threshold, while expanders and gates respond only when signals at the input are lower I
than the defined threshold.
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