would have otherwise caused overload or distortion. Typically you set a limiter (via its
threshold control) so that it "stays out of the way" until a peak comes along.
2.1.2 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, you run into the limitations of processors and amplifiers after the
expander going into terminal overload.
The solution is to make soft signals softer, or downward expansion. This is what the 425's
downward expander does. When the signal level falls below the level set by the threshold
control, the expander reduces the 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.
A gate is similar to an expander except that its ratio is much higher; thus the action is more
like a switch once the signal falls below threshold. Some expander applications for the 425
may be quite similar to a gate, like tending a lone announce microphone, but the 425's
expander isn't well suited to typical gate applications like removing leakage from drum
microphones.
2.1.3 Ratio
The compression ratio of the 425 tells how much the output changes for a change in the
input. A linear amplifier (like a simple preamp) has a ratio of 1:1 because a change of 1 dB at
its input results in a 1 dB change at its output. A compressor alters the input/output
relationship by its compression ratio. Thus a 20:1 ratio means that a 20 dB change at the
input results in a 1 dB change at the output. In other words, a very audible change at the
input (20 dB) turns into a barely discernible change at the output (1 dB).
Compressors are not the only devices to have an input/output ratio. Any device that is
capable of changing the input/output relationship can be said to have a ratio. Thus
expanders, gates, compressors and limiters all fit this category.
An expander maginifies output changes for a given input change. Thus, once the input signal
falls below threshold, the expander changes the output by the amount of the ratio. The 425's
downward expander has an expansion ratio of 1:2, which means that an input signal that gets
5 dB quieter turns into a 10 dB quieter output change.
A gate can be looked at as an expander with an infinite expansion ratio. Thus the slightest
change in the input signal, above threshold, turns the gate full-on.
2.1.4 Gain vs Output
The gain control allows compensating for signal level lost to compression. As an example, try
setting the 425 for a 4:1 ratio. Now adjust the
THRESHOLD control for 10 dB of gain reduction
as read on the compressor's gain reduction display. The output level should be significantly
lower than what it was. You supply the additional gain (make-up gain) by adjusting the
GAIN
control until the input and output signal levels match.
2.1.5 Attack Time
The attack time represents the amount of time that a compressor (or limiter, or expander, or
gate) needs to react to an input change. You might ask, why not just make it lightning fast?
Because you may sometimes want to let occasional peaks through, which helps maintain the
dynamic characteristics of the input signal. Usually, what we're after is control of the input
signal, not total homogenization.
Rev 2.0, 10/29/93
2-2
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