Handbook for Sound Engineers

Consoles 939

tation to be more casual (and cheaper) with control stuff
that should be avoided.
A less-than-obvious concern springs from the fact
that the audio path and the control path are not only
cross-disciplinary, but are architecturally dissimilar.
Audio paths are (assuming mixer channels) following
the signal flow, as are their grounds, while the control
voltages for a number of, if not all, channels are being
handled en masse and distributed star fashion. If ever
there was an inadvertent recipe for a ground-induced
noise problem, this is it. If the CV is referenced to a
ground that is moving in any fashion at all in relation to
the audio ground at the VCA, then that difference is
effectively added to the CV as far as the VCA is
concerned creating noise modulation.

25.16.2.6 VCA Channel Application

Fig. 25-109 shows a typical implementation of a
high-end integrated VCA.
The THAT Corporation (“son of dBx”) VCA-type
2180 is a current-in, current-out device for audio, hence,
a standard current-to-voltage convertor using a good
bipolar op-amp following. Note also a seeming overkill
op-amp on the control-voltage summer. The control port

is where things get interesting. The control feed to the

VCA can be a summed combination of several different

sources (a quick point here—since VCA control volt-

ages are logarithmic, adding voltages results in multipli-

cation in the VCA, or in other words, the dBs

represented by the voltages add or subtract):

1. The channel fader, only it isn’t, really. It’s actually
   the output of a D/A convertor that is either
   reflecting the fader position as sensed by an A/D
   convertor, or replaying a prior fader position from
   the automation system. But, for now, we’ll call it the
   fader.


2. Gain-reduction control from the channel dynamics.
   It is common to use the high-quality fader VCA as
   the gain-control element for on-channel dynamics. It
   presupposes the dynamics have deterministic
   feed-forward detectors and conditioners. Obvi-
   ously, a feedback-style compressor could not use
   this VCA.


3. VCA subgroups. A common feature on sound-rein-
   forcement consoles, these are controlled by a central
   set of a number of VCA group master faders
   (usually eight). These generate a control voltage,

Figure 25-109. Simplified channel-style VCA using commercial IC.

10 MF

10 MF

+15 V

15 V

+5V

100 nF

20 k

5 k1

N.C.

THAT 2180

Audio in

+

++

22 pF

10 MF

100 nF

AD 797

+

7

2

3

6

5

4

1 8

+

Out

+

0

Audio out

20 k

IN

AD 797

Gain offset voltage

Control voltage

from local dynamics

Fader

control

voltage

Down

Up

'Fader' A/D Micro D/A

+

0

Sym

G