Look at the block diagram in Section 7. Notice the sidechain connections that come from the
balanced input stage. They allow access to the control circuit's input signal. The control signal
is derived from, but kept totally separate from, the audio signal path. This means the control
signal can be processed outside the 425 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 425's compressor/limiter/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. Perhaps most importantly,
this happens without actually equalizing the signal being processed through the 425.
To make the 425's processors more sensitive to high frequencies, use an equalizer (graphic or
parametric) to boost the high frequencies in the sidechain signal. This increases the sensitivity
of the control circuits to those particular frequencies so the compressor/limiter/expander
responds more to those frequencies than any others. If the offending frequencies produce a
control signal of greater amplitude than the desired frequencies they will control how the
compressor/limiter/expander behaves with the rest of the signal as well. However, if the
offending signals are of significantly greater amplitude than the rest of the signal, careful
adjustment of the corresponding threshold control (combined with the boost provided by the
EQ in the sidechain) will make the compressor/limiter/expander respond only to the boosted
frequencies.
Keep in mind that the threshold level becomes a function of the amount of overall gain
through the equalizer, including the boost. This technique can be used with any frequency
that can be controlled by the equalizer.
Cutting a frequency creates the inverse effect, making the 425 less sensitive to the frequencies
that were removed from the control signal.
Since the expander only discriminates between different levels (not different sounds), it can be
fooled by signals whose levels are nearly the same, even if the frequency content of those
signals is fundamentally different. When the 425's expander is used to shut out unwanted
sounds, any signal exceeding the threshold setting triggers the expander. When this happens,
it's often possible to eliminate the false triggering by equalizing the control signal.
For example, if low frequency signals transmitted through a desk or podium are triggering the
425's expander unnecessarily:
❒ Use an equalizer in the sidechain to remove the low frequencies from the control
signal.
and/or
❒ Use the equalizer to boost the voice-range frequencies in the control signal.
When the offending frequencies are removed or minimized, the relative level of the desired
frequencies increases and the expander can now tell the difference between the wanted and
unwanted signals. Use this technique in any situation where levels are nearly the same, but
the fundamental frequencies involved are different.
Rev 2.0, 10/29/93
2-4
To Next Page
Loading...
