TDM Audio 32CL-2 v.2 - Manual (Page 18)

© 1996 TDM Design, Inc. Compressor/Limiter Owner’s Manual Page 18
Uses for Side-Chain Compression
The possible uses for side chain compression are limited only by you imagination, and they are
diverse enough to have entire books written about them. Often side chains are used in recording
studios to create strange and interesting effects. Nonetheless, there are a few common uses for
side chain compression that bear mentioning.
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De-Emphasizing a Frequency Range: If the side chain is fed through an equalizer that
boosts a range of frequencies, the result is that the gain is reduced more when the input signal
contains these frequencies than when other frequencies are present. In other words, the overall
level of the output signal tends to be quieter when a certain range of frequencies is dominant.
The equalizer does not affect the balance of frequencies of present in the signal at any one in-
stant. Instead, the compressor reduces the gain of the signal when a certain range of
frequencies dominates the input signal.
One very practical use for this kind of compression is sibilance control. If the side chain
equalizer boosts the high frequencies in the signal while reducing all other frequencies, then
the gain of the overall signal is reduced when an overabundance of high frequencies is present.
This keeps sibilance under control without reducing the natural high frequency content of the
signal under normal circumstances. It lets you set a threshold, and reduces the overall level of
the signal only when the high-frequency content is too hot. There is a special kind of device
called a de-esser that is designed to do this job, and good de-essers are actually more sophisti-
cated than compressors using side-chain equalization, but a compressor with a side chain will
do the same job quite well under most circumstances.
When compressing a vocalist, it is often desirable to insert some side chain equalization with
low and low-mid frequencies rolled off and higher frequencies boosted. This can help reduce
the undesirable emphasis that compression puts on sibilance and breaths.
There are cases when other frequencies need to be de-emphasized naturally, and for those
cases, side-chain compression is often the best choice. For example, side-chain compression is
sometimes used on an electric bass to de-emphasize the upper midrange frequencies. When
the bass player is plucking the strings with his or her fingers, the level of upper-midrange fre-
quencies is under the threshold and the sound of the instrument is naturally reproduced. When
the bass player is slapping or popping the strings, the compressor makes sure the upper-
midrange attack is not overbearing without reducing the level of the actual tone.
Using side chains to de-emphasize a frequency range is usually done with fast attack and re-
lease times.
•
Emphasizing a Frequency Range: If the side chain is fed through an equalizer that cuts a
range of frequencies, and the threshold is set so that the signal is in compression a good part
of the time, the result is that the range of frequencies that is cut on the side chain equalizer is
emphasized in the output. In other words, the gain of the signal is reduced by the compressor
most of the time, but when the input signal is rich in the range of frequencies that is cut on the
equalizer, the compressor does not reduce the gain as much. The result is that the output sig-
nal tends to be louder when certain frequencies are present. Again, the equalizer does not
directly affect the balance of frequencies in the output signal. Rather, the compressor increases
the overall gain of the output signal when certain frequencies are dominant.