906 Chapter 25
25.12.2.2 Subtractive Feedback Gate or Expandor
An alternative gating method cunningly uses a limiter,
albeit with a very low threshold, to subtract from or
cancel the straight signal, Fig 25-83. The signal gains
through both the straight path and the limiter (below its
threshold) are arranged to be the same but out of phase;
they cancel out. Above the limiter’s threshold the
limiter output remains fixed but the straight signal is left
unhindered, so the two no longer cancel, leaving the
straight signal predominating. Time constants of the
effective gate are determined by those of the limiter,
threshold by the gain of the amplifier within the limiter
loop, and depth by contriving a mismatch between
unlimited level and straight path level to produce less
than total cancellation and some residual.
25.12.2.3 Keying
Keying is the triggering of a gate from an external
source that is not from the signal that is actually passing
through the gate. Perhaps the best and most commonly
heard examples are keyed snare and kick drum
sounds—all the rage in the dark ages of disco. In
circumstances where a new drum sound was needed or
alternatively the existing drum sound was not fit for
human consumption (very common on live stages), the
existing drum sound is used to key a gate that is
carrying in its signal path something which can be
convincingly shaped into a better drum noise. Favorites
are white, or EQ’ed white, noise to emulate a snare
sound; similarly, some tone around 20–60 Hz (some-
times even ac line hum) when shaped by the attack,
hold, and decay times of the gate can make for a good
kick drum!25.12.3 CompressionAs briefly outlined at the beginning of this section,
compression is where the output signal from the
processor does not increase as much as the input signal
is increasing. If an input signal jumps in level by 10 dB,
a compressor with a ratio of 4:1 would only allow the
output to rise 2.5 dB. Correspondingly, a drop in input
level of 16 dB into the same compressor would result in
4 dB output level change. A compressor reduces the
dynamic range of an input signal by the amount of its
ratio.
A true compressor acts on all signals, regardless of
actual signal level, in the same manner. No matter if the
input signal is way down at 60 dBu or up at +20 dBu,
a change in input signal level of a given amount will
cause a similar, reduced, change in output signal. Practi-
cally speaking, there is no such thing as a true
compressor; things that come close and work down to
very low signal levels are used in noise reduction
systems for telephone lines, tape recorders, and wireless
microphones, where they are used with a complemen-
tary expander (see later) to reinstitute the original
dynamic range.
Most compressors have a threshold below which
they leave a signal unscathed (a 1:1 ratio) and above
which they proceed to compress the dynamic range,
much as a limiter does. The family resemblance
becomes all the more striking as compressors with high
ratios are considered. A 10:1 compressor above its
threshold reduces a 10 dB input level jump to just 1 dB.
Infinity-to-1 reduces anything above the threshold to the
same output level. Looks like a limiter, smells like a
limiter. Generally, compressors are used at far gentler
ratios (between 1.5:1 and 4:1) to bring up lower level
program material in a less take-it-or-leave-it manner
than a limiter while leaving some sense—albeit
reduced—of light and shade, louder and quieter. They
are also used to subtly make sounds chunkier—a degree
of compression tends to accentuate lower frequencies,Figure 25-82. Gating feed forward sidechain processor.
Figure 25-83. A subtractive gate, canceling a limiter from
inverted input.
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