Interfacing and Processing 275feed through from the digital control signal. Unlike the previous elements, an M-DAC
has discrete resolution—just like a stepped ( “ detented ” ) pot. At low attenuations, step
size must be no more than l dB for precise control; below –30 dB, larger steps (2 dB)
are usually fi ne enough. To attenuate down to –70 dB in l-dB steps, 12-bit M-DAC is
required.
8.6.2.5 R & R Array
Comprising resistors and relays, this is the mechanical counterpart of the M-DAC,
with relays opening and closing paths in a “ ladder ” or other array of (usually) discrete
attenuator resistors. Only high reliability, ATE-grade, sealed reed relays are suited for
high-performance audio on grounds of both reliability and sonics. Such relays can act in
under l mS and have fast settling, but are still not really suited to dynamics processing!
Getting dB steps to act binarily with a resistor array takes some lateral thinking. Although
the relays required are relatively expensive, by ingenious network adaptation to increment
in binary dB, a mere seven can offer a 60-dB range in I-dB steps. With suitably well-
specifi ed resistors, this type can offer the highest transparency of any gain control device.
8.6.2.6 Summary
Motorized pots, lamp LDRs, and relay/resistor arrays are good for remote- or machine-
controlled gain trim and setting. The latter are the fastest and likely most reliable.
J-FETs and LED LDRs are good for dynamics processing, but attaining accurate,
noninvasive performance takes from the initial simplicity.
VCAs and M-DACs are elements that can do both kinds of jobs well.
8.6.3 Remote Control Considerations
Computers regularly feign precision that is only virtual. Until gain control elements
become self-checking, self-calibrating, and self-aligning, they require careful
specifi cation.
8.6.3.1 Temperature
Pots (particularly conductive plastic), JFET, LDR, and particularly VCA elements are
quite temperature sensitive. Unless designed with very low tempco , then when used in
two or more channel amplifi ers, they must be placed isothermally, that is, cosited to be