565E
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Dynamics Processing Tutorial and 565E Functional Basics
Audio signals possess several basic properties: amplitude or volume (measured in volts or dB),
frequency or pitch (measured in Hertz), duration (measured in hours:minutes:seconds) and
waveform (described graphically, like sine, square, triangle, or pulse). Complex signals like musical
sounds are made up of simpler waveforms such as sine waves, mixed in the proper proportions.
Signal processors allow you to manipulate various parameters of an audio signal. Equalizers change the
amount of amplification given to different frequencies (a perfect amplifier increases the gain of all
frequencies by the same amount). Dynamics processors change the dynamic range of audio signals.
The dynamic range of an audio signal is the difference between its loudest and softest moments. For
audio equipment, this is the difference between the noise floor (residual noise output, with no input
signal) and peak clipping (the point at which the output clips or distorts). A hypothetical black-box
having a noise floor of -90 dBu and a maximum peak output level of +24 dBu would have a dynamic
range of 114 dB (+24 minus -90). Audio storage devices like tape machines have a much narrower
dynamic range; a typical professional analog two-track tape machine may have a dynamic range of 65 to
70 dB.
If you’ve used an analog tape recorder before, then you are already familiar with the problem of
setting recording level. Record too hot and you get distortion; record too cold and get noise in
return. Many musical instruments have dynamic ranges that exceed that of most tape recorders. So,
how do we squeeze a 80 or 90 dB signal into a 60 or 70 dB window?
The answer lies in a common audio signal processor: the compressor.
Compressors and Limiters
A compressor or limiter monitors the level (or amplitude) of a signal and reduces the amplitude
according to a rule whenever the signal level exceeds a predetermined level. The predetermined
level is known as the threshold level and is usually set by a front-panel control. The amount by
which the compressor lowers the level is the compression ratio, and this parameter is usually set
via a front-panel control as well.
Compression ratio refers to the ratio of a change at the input versus the change at the output of
the device. Thus, if we apply an above-threshold signal that changes 10 dB to the input of a
hypothetical compressor, and measure a 2 dB change in the output signal, that compressor would
have a compression ratio of 10:2, or 5:1 (reduce the fraction). Different compression ratios have
different uses. Use lower ratios (6:1 or less) for level control, intermediate ratios (8:1 to 12:1) for
leveling (making the signal level more or less constant), and higher ratios for limiting (putting an
absolute ceiling on the signal level).
Limiters are nothing more than compressors, but are possessed of much higher compression ratios
(20:1 or higher). Limiters are typically used to stop occasional peaks which would otherwise cause
overload or distortion. Typically a limiter is set (via its threshold control) so that it “stays out of
the way” until a peak comes along.
Expanders and Gates
While a compressor or limiter reduces the dynamic range of a signal by reducing its level once it
has exceeded a threshold level, an expander does the opposite (well, almost). The easiest way to
visualize an expander is to think of it making loud signals louder. This is fine, except that in the real
world, the device following the expander (such as a processor or an amplifier) would go into
terminal overload.
The solution is to make soft signals softer, a.k.a. downward expansion. This is what the 565E’s down-
ward expander does. When the signal level falls below the level set by the threshold control, the
expander reduces its gain by the amount dictated by its expansion ratio. Thus, for a below-threshold
signal, a 10 dB output change results from a 5 dB change in the input signal, if the expansion ratio is 1:2.
Tutorial Chapter 4
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