TDM Audio 32CL-2 v.2 - Manual (Page 17)

© 1996 TDM Design, Inc. Compressor/Limiter Owner’s Manual Page 17
Using Side Chains
How Side Chains Work
o understand the concept of side chains, you must first understand the basics of compressor
operation. The signal that you feed into a compressor channel is split and goes in two differ-
ent directions. On the one side of this split feeds a level detector circuit that senses how loud the
signal is. The other side goes through a voltage-controlled amplifier (VCA). A VCA is an ampli-
fier circuit whose gain is determined by a control voltage. When the input level is below the
threshold, the VCA just passes the signal through normally. When the input level rises above the
threshold, the level detecting circuitry determines how far above the threshold the level is, and
feeds a signal to the VCA to adjust its gain accordingly. The important concept here is that there
are two signal paths, the one that gets compressed and output, and the one used to determine the
level of the input signal.
Side chains are paths by which the level-detecting part of the signal is processed before the level
detecting circuitry. For example, sometimes the signal is equalized before the level detector. Here
is a picture illustrating this kind of side chain processing.
Figure 1 - Side Chain Equalization
As you can see in Figure 1, the path from the signal to the output does not pass through the
equalizer. The equalizer only changes the signal that the level detector uses. This results in a
change in the way the signal is compressed, but the signal itself is not altered by the side chain
processing.
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