2. Dynamics Processing Tutorial and Functional Basics
The 425 brings together three widely-used signal processors: a downward expander, a
compressor, and a peak-limiter. It's important to note that although the 425 is three
processors in one box, it contains one-third less circuitry than there would be if each
processor was in its own box. The reason is simple: there is only one VCA (voltage-controlled
amplifier) in each channel of the 425. Smart analog computer signal processing circuitry (the
sidechain by any other name) combines the three control signals from the downward
expander, compressor, and limiter to control the one VCA. There is no compromise involved in
doing this and the signal passes through two fewer VCAs.
This part of the 425 tells how to use your 425. If you are new to signal processors, read on; the
remainder of this section is a primer on the three basic parts of the 425.
2.1 Dynamics Processing 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, 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 amplifies
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 worse limitations, 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.
2.1.1 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 rule by which the compressor lowers the level is the compressor's compression ratio and
this parameter is also usually set via a front panel control.
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 a 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 being possessed of much higher
compression ratios (20:1 or higher). Limiters are typically used to stop occasional peaks which
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