Handbook for Sound Engineers

940 Chapter 25

each of which is bused up and down the length of

the console; each channel has the option of selecting

one (or more, if one likes danger in one’s life) of

these voltages to be summed into its own VCA. This

is a very convenient manner of grouping related

channels under a control without having to create

real audio subgroups.

4. VCA master. Again, a centralized fader only oper-
   ating as an overall master over all channels contrib-
   uting to the main mix bus. Although seeming to be
   redundant being that there is almost certainly a real
   audio fader on the mix bus output, a VCA Master
   has the advantage that all the levels of sources
   contributing to a mix can be adjusted, rather than the
   output of the mix stage. Helps avoid headroom
   problems in the mix stage.

It should be stressed that the 5 V supply for the 0 to
5 V control signals should be oppressively regulated
and fabulously quiet, squeaky clean. Borrowing some
off the nearest micro and hanging 100 nF across it
doesn’t count—sorry.
In console designs with sophisticated computer
control, all but the local channel dynamics control
signal are manipulated and summed digitally and this
composite result is fed to the channel VCA via a D/A
convertor; this dramatically simplifies the multiplicity
of summed analog control voltages per channel.

25.16.3 Digitally Controlled Amplifiers

VCAs are not the solution for all variable control in
analog circuitry. In order to be driven from a digital
control system a D/A convertor output needs to be used
to derive an analog control voltage for each VCA. This
can get very expensive, very quickly. A gain control-
lable stage that can be more directly connected to the
controlling microcontroller is desirable.

25.16.3.1 Multiplying DACs

A more direct approach, meaning it can be driven
directly off a digital control system, is to use a multi-
plying digital to analog convertor (MDAC) (don’t you
just love that “multiplying” bit?)—In particular, a refer-
enced-input four quadrant multiplier, which implies it
can produce an output both positive and negative in
potential (or current in this particular case) and which is
proportional to a voltage applied to its reference
terminal, Fig. 25-110. Now, bear in mind these devices
were never intended to be used in this way, but luckily
they do so well.

The audio signal is applied to the reference pin; a

digital number, in this case 12 bits wide serially fed into

the device, is applied to the 12 bit R-2R-style ladder

DAC, Fig. 25-127; the audio signal is attenuated in

proportion to the applied digital number with respect to

the 12 bit maximum (1024 steps). The output current is

sensed and converted back to a voltage by the following

virtual-earth input amplifier, using the friendly internal

feedback resistor around the op-amp. The interface is

dead simple, linearity is pretty good, the signal handling

is excellent, and the noise isn’t bad—dancing in the

streets!—except every time the gain is changed (a new

digital word transferred into it) it makes a little tick

noise, which is very audible on high-level signals,

low-frequency signals, and especially the combination.

In fact, as the gain is moved (a la fader), classic zipper

noise is very evident. The only good news about all this

is that for a large part, program material’s spectral

content masks this noise. However, when the device is

used as a frequency or Q-determining element in an

equalizer, the effect becomes comical; depending on

one’s sense of humor. There are two approaches neces-

sary to nail this noise, since it is actually due to two

separate causes.

25.16.3.2 Charge Injection

This is a near unavoidable effect in CMOS and other

electronic switches, where a tiny amount of differenti-

ated charge impinges itself onto the signal path from

transitions of the control port. In a multiplying DAC,

any number of bits may be changing as the gain is

varied, and so the total charge injection varies corre-

spondingly. It is, however, almost completely indepen-

dent of the applied audio signal.

Cancellation works well, with reservations. One

approach is simply to use a second MDAC with its own

inverter that sums into the virtual-earth point of the

main MDAC path, with its reference pin undriven.

However, with only slightly more complexity the

Figure 25-110. MDAC as rudimentary gain control

DAC 8043+15 V

+

0

4

9

Serial data in

SRI

Vref

CLK /LD

1 OUT

R f6

Syncrhonus

serial clock

Data load

strobe

6

8

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