TDM Audio 32CL-2 v.2 - Manual (Page 4)

© 1996 TDM Design, Inc. Compressor/Limiter Owner’s Manual Page 4
Introduction
hank you for purchasing the TDM 32CL series compressor/limiter. These units are made from
the finest components and engineered to exacting standards. Precision components are used
in all critical circuitry for the finest sonic quality and performance. To get the most out of your
new TDM compressor/limiter, please take a few minutes to review this manual and familiarize
yourself with the proper operation of the unit.
The remainder of this section provides background information about the theory of compressor
and limiter operation. Individuals that do not need this information may skip directly to the next
section: Mounting the Unit in a Rack.
Compressor Fundamentals
Compressors are used to reduce the dynamic range of a sound signal. A signal’s dynamic range is
the difference between the loudest part of the signal and the quietest part. In theory, compressing
the dynamic range of a signal results in the quietest parts getting a little louder and the loudest
parts getting a little quieter. The way most compressors actually work, however, is by reducing
the volume of the loudest parts. If desired, the overall volume of the output of the compressor
may then be raised to bring the quietest parts up in volume without the loud parts being too loud.
The main use of a compressor is to even out the volume of a signal. For example, in a radio sta-
tion, sometimes the announcer speaks softly while other times he or she might shout. This
presents a problem because there is a limited modulation range that is acceptable for broadcasting.
Radio stations use compressors so the signal is strong enough when the announcer is speaking
quietly without causing overmodulation during the loud parts. The quality of the radio an-
nouncer’s voice changes, but the actual volume of the signal is relatively constant. Even without
the modulation limits, it is desirable while you are listening to the radio in your car to be able to
hear the announcer over the road noise without being blown away when they get excited.
Compressors are used extensively in broadcasting, music recording, film-making, live sound
reinforcement, and just about anywhere else sound equipment is found. In order to understand
and operate a compressor, you need to be familiar with a few terms.
The first term you need to understand is compression ratio. The compression ratio is the mathe-
matical expression of how much compression is taking place. For example, for a 2:1 compression
ration (read “two to one”), the dynamic range is halved. The first number represents the dynamic
range of the sound before compressing, and the second represents the dynamic range after com-
pressing. Two to one means that if the dynamic range were two before compressing it would be
one after compressing. A more realistic example would be a symphonic program which might
have a dynamic range of around 70 dB before compressing. This same program would have a
dynamic range of 35 dB after compressing at 2:1. If you flip the numbers over and place a line
between them you get the fraction of the original dynamic range that describes the result. For
example the result of 2:1 compression is ½ the original dynamic range. Although this is the theory,
the reality is a little more complicated, so read on.
The problem with achieving a pure compression ratio comes from the way compressors work.
Without knowing what the loudest and softest parts of the program are going to be, how can a
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