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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

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