Handbook for Sound Engineers

Consoles 909

amplified instead. Yes, the degree of attenuation would
become bigger than the corresponding changes in
applied signal, and the VCA’s output would actually
increasingly reduce beyond the threshold. This effect
was brought to stardom by the Eventide Omnipressor.

25.12.5.4 Side-Chain Sources

Figs. 25-84 and 25-85, the block diagram and schematic
respectively, of a feed-forward style dynamics element,
both only consider the side chain as being taken from
the same place as the input to the VCA gain controller.
This need not be the case at all. In fact, with a standard
channel’s signal processing it would be quite a limita-
tion, forcing the dynamics section to be solely post
everything, prefader. The side chain input may be sepa-
rated and instead taken from pretty much anywhere
upstream; the overall effect is (with only some odd side
effects) the same as physically moving the whole
dynamics section to that location. For the most part, the
audio doesn’t care that the control voltage from the
sidechain has no relation to that being passed through
the VCA. The main areas where things might sound
awry are if there is a significant (and audible) deliberate
time delay between sense and activation or extremely
short time constants are invoked. The first disconnect is
obvious, and the second is almost irrelevant since it
wouldn’t be sounding very nice anyway.
Taking this a step forward again, a prime virtue of a
VCA-based system is that many different sources can
operate simultaneously on the one relatively expensive
VCA gain element. If there are multiple side chains

(say, one for keying/gating/expansion, another for

compression) these again need not sense from the same

pick-off points but places more suited. The gate would

likely sense postinput filters, while the compressor

would likely be farther downstream, one side or the

other of the EQ. This situation would work, but would

result in behavior unlike having a discrete gate up front

and a discrete compressor downstream. In the literal

case—i.e., with two separate dynamics elements—the

incoming signal would be gated before it hit the

compressor. In this virtualized case, though, the actual

audio signal hitting the compressor side chain would not

have previously been gated and hence cause the

compressor to act differently than if it had. Subtle,

maybe, but a definite difference.

25.12.5.5 Makeup Gain

The side chain’s thresholded and ratioed control signal

is summed in with a voltage representing the amount of

buildout gain (necessary to compensate for the signal

level reduced by the effect of compression/limiting) and

is also, in the case of a typical console channel, summed

in with the voltage from the automation system repre-

senting the fader position. This summation is scaled to

suit the actual (highly sensitive) VCA control port, to

which it is fed.

25.12.6 A Practical Feed-Forward Design

A highly integrated part, the THAT Corporation’s 4301,

has both an rms detector and a VCA built in, in addition

Figure 25-84. Block diagram—feed-forward VCA Dynamics.

1

2

Input

Precision

rectifier

Log convertor

Averaging

Threshold

Thresholding

precision

rectifier

1

2

Build-out gain

Output

VCA control port

VCA

Summer/scaler

"Fader"

Ratio

Automation system