for the voltage-controlled amplifier. There is also a
difference in the attack time constant (RaC) and release
time constant (RrC) of the RC filter in the control loop.
The attack time for both compressors and limiters is
fast (< 1 msec), but a compressor has a longer release time
than a limiter. The release time in a compressor is usually of
the order of a few hundred milliseconds, but for a limiter it
can be shorter than 10 milliseconds.
The operation of a compressor is shown in Figure 2. At
signal levels below a threshold, the input and output levels
track each other and the audio is unchanged. Above the
threshold, increasing the input level causes a smaller
change in output level. The ratio of input signal change to
output signal change is called the compression ratio. A 5:1
ratio means that a five dB increase in signal amplitude gives
just a one dB increase in output. In this way, loud signals
A large compression ratio will effectively suppress loud
signals, but there are problems with too high a ratio. First,
you'll have no dynamic range for your audio, and
everything will sound too loud. There's also too much
amplification of low level signals that leads to what's
poetically described as "whoosh." Whoosh is the sudden
insult of system noise you'll hear during pauses in the audio
signal; for example, between sentences in speech or
between songs. Aside from the whoosh problem, when the
input signal reappears the attack time might be slow
enough to let a short over-amplitude burst pass through.
A high compression ratio also leads to waveform
distortion at low frequencies. What happens is that the
compressor tries to level the peaks and valleys of the audio
waveform itself. For such reasons, smaller compression
ratios are used. Since this isn't wholly effective in preventing
over-modulation of transmitters, the compressor is followed
by a limiter that absolutely prevents any signals from going
higher than a predetermined level.
The way a limiter processes audio is shown in Figure 3.
The action is just like a compressor below the threshold —
there's no change in the signal. Above the threshold, a
limiter acts like a compressor with a very high compression
ratio, essentially infinity. A limiter is designed to absolutely
block any signals above the threshold. Since limiters follow
compressors in an audio chain, the compressor does most
of the work and the limiter acts as a safety valve. In proper
operation, a limiter rarely goes above its threshold, and it
doesn't contribute to whoosh.
Platform Gain Principle
A compressor followed by a limiter was the usual signal
processing chain for most broadcast stations until the
1960s. At that time, a new type of audio controller hit the
market and started to dominate. This was the platform gain
controller, conceived by CBS Labs. CBS — a major
television and radio network with many corporate-owned
stations — had a vested interest in advancing broadcast
technology. CBS Labs was started in 1936, and it operated
■ FIGURE 2. The
operation of a
above a threshold
according to a
■ FIGURE 3.
of a limiter.
in one form or another for 50 years. A CBS Labs employee,
Dennis Gabor, even received a Nobel Prize in Physics,
although it was for work in holography not related to CBS
In 1959, CBS Labs introduced the first platform gain
controller — the Audimax Audio Gain Controller — that was
designed to replace compressors. CBS Labs also introduced
a Volumax line of audio limiters. These were still the
vacuum tube days, and the patents on the platform gain
principle — now long expired — show that (B.B. Bauer and
Arthur Kaiser, Gain Control Apparatus Providing Constant
Gain Interval, US Patent No. 3,187,268, June 1, 1965;
Arthur Kaiser and Emil Torick, "Compensated Platform Gain
Control Apparatus, US Patent No. 3,260,957, July 12, 1966;
Emil Torick and Arthur Kaiser, Control Circuit for Restricting
Instantaneous Peak Levels in Audio Signals, US Patent No.
3,398,381, August 20, 1968).
The platform gain principle is shown in Figure 4 which
has some similarity to the limiter transfer function of Figure
3, with a strange twist. Just as for a limiter, signals below a
threshold are passed without modification, and any signals
above a threshold are blocked. That's when the signal is
increasing, as from point B to point C. When the signal
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