Symetrix 258E, User Manual, Page: 29 / 46

528E
27
USING THE SIDECHAIN
The sidechain is a patch point in the control circuit of a dynamic range processor, which provides
access to the part of the circuitry that tells the VCA what to do. The 528E’s sidechain routes
through a rear panel TRS jack that allows the control signal to be processed outside the unit (see
Figure 7-6 for specific hookup information).
Refer to the block diagram at the beginning of this chapter. Notice the sidechain connections that
come from the compressor/downward expander section. These connections allow access to the
audio signal at the input to the control circuit that drives the dynamics processor. This control
signal is derived from, but kept totally separate from, the audio signal path, which means that the
control signal can be processed outside the 528E without actually processing the signal that’s going
through the VCA (the audio signal itself). This presents some very interesting possibilities for
changing or improving the operation of the dynamic range processor.
The best use of the sidechain is to make the action of the 528E’s compressor/downward expander
frequency dependent, that is, to make it respond more (or less) to certain frequencies. Because the
audio signal and the control signal remain completely separate (even while the control circuit tells
the VCA whether to turn the gain up or down), you can equalize the sidechain without changing the
EQ in the main audio path. Removing unwanted frequencies from the control signal before it
actually reaches the VCA prevents those frequencies from being used to create gain changes.
Applications utilizing the sidechain may be found in the next chapter of this manual.
THE VOICE SYMMETRY SWITCH
Human speech, especially male human speech, contains a great deal of asymmetry. In broadcast
(especially AM broadcast), this wastes transmitter power because the asymmetrical waveforms do
not utilize the full power of the modulator. The bottom line is that you risk negative
overmodulation if you don’t correct speech asymmetry before the modulator. The V
OICE
S
YMMETRY
switch corrects asymmetric speech waveforms before they get to your board; an added bonus is
that you can apply the correction only to the announce mics without affecting the music.
In recording applications, this switch may help give slightly higher overall levels by improving the
symmetry of speech signals which may allow lightening up on the compression. Non-speech
signals may be adversely affected by the V
OICE
S
YMMETRY
switch. Let your ears be your guide.