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To determine which peaks are acceptable and which are too loud, the
limiter establishes a reference level known as the threshold, and then
responds to those peaks which exceed this point. In the Ashly CL-SERIES,
~
this threshold is fixed at OdBV. When the input level reaches 0dBV, the
limiter will begin to react.
If the source feeding the limiter is increased in overall level, then more of
the peaks will now be above OdBV; those peaks which were previously just
below threshold level will now be at or above threshold, in addition to the
volume peaks which had already been above threshold. Thus, varying the
input level to the limiter is inversely proportional to changing the threshold
level. On the Ashly CL-SERIES, this effect is easily accomplished by
changing the input GAIN control. Increasing the GAIN control is equivalent
to lowering the threshold point. A gain control range of plus or minus 30 dB
is provided. What the limiter does once a peak above threshold is detected is
determined by the RATIO, ATTACK TIME, and RELEASE TIME controls.
Compression
There is a good deal of disagreement and confusion in the audio industry
over the definitions of limiting and compression, and the terms are
frequently and incorrectly swapped back and forth. The fact is, compression
and limiting are closely related effects, and the differences between them are
largely
a matter of degree. Indeed, it could be said that the difference
between compression and limiting is, more often than not, a matter of what
you intend the device to accomplish.
Returning
to our original definition of limiting, we stated that gain
reduction occurred when input signals rose above a predetermined threshold
level of 0 dBV, and that the effective threshold could be varied by changing
the GAIN control.
A very significant difference in dynamic range is achieved simply by
changing the relationship between nominal signal level and threshold, as a
result of varying the GAIN control. The most interesting effect to be noted,
however, is seen by comparing the original Input signal with the Output
signal. The quietest portions of the original signal will be effectively
+
increased
in volume while
the volume of the loudest portions of the original
l
signal will be decreased. In effect, both ends of the dynamic spectrum will be
pushed toward the "middle". This is quite different from simple limiting,
where only loud peaks are subjected to gain reduction. More than
anything else, it is this double-ended effect which distinguishes compression
Remember, in all of these examples, the only control which was
changed was the GAIN control. The RATIO, ATTACK TIME, and
RELEASE TIME were assumed to be identical in all cases. The only factor
which turned limiting into compression was a change in the relationship
between input level and threshold. Compression is further differentiated
from limiting by careful selection of Attack and Release times.
When limiting is employed to protect an audio system against
transient volume peaks and possible overload, Attack time is usually set as
fast as possible, consistent with distortion-free performance. Release time
would also be relatively short, so that the output signal would be restored
to normal as quickly as possible after the transient.
Compression is frequently used to keep overall signal level within a
specific dynamic range, and for this application, slower Attack and Release
times are usually chosen. This approach is analogous to our manual gain
riding example, where our operator is fading the music up and down to
keep it fairly constant, but is doing it slowly enough so that the listener is
unaware that the gain is being altered.
It should now be clear that the Ashly CL-50E, CL-100, and CL52 can
function as either limiters or compressors, due to the wide range of control
parameters which are available to the user.
Compressor/Limiter Circuitry
Volt
Controlled
ifiers
Early VCA's were based on vacuum tubes with a "remote cutoff"
characteristic. The tube would simply change its gain in response to a
changing bias voltage. Tubes developed for this purpose did an excellent
job, in fact they could exceed the noise and distortion performance of
today's best solid state VCA's. Unfortunately, they also had some serious