Interfacing and Processing 273Usually a motor connects to the same shaft as a knob, but the latter via a slipping clutch.
Either may override. This keeps the simplicity but shares the wide setting tolerance,
sonic, and some of the mechanical limitations, of ordinary pots, for example, fragile shaft,
relatively low setting speed. Which overrides the other depends on which way confi dence
most leans—toward human fi ngers or computers! Control circuitry is needed to decode
remote command signals, which may be a variety of formats. Special driver ICs (e.g., BA
series made byRohm in Japan) make design and manufacture easy but might pose major
replacement headaches to some owners in the future.
8.6.2.1 Voltage-Controlled Amplifi ers
Commonly called a voltage-controlled amplifi er, most are used as VC attenuators ,
usually as a solid state and always an analogue circuit. Most are ICs based around one
of a limited number of proprietary schemes, which are made (or licensed, e.g.,That
Corp. in U.S. licenses, National in Japan) by one of three main patent holders, all in the
United States.^14 Otherwise they are based on a discrete circuit or on a consumer grade
‘ OTA ’ IC. Gain is accurately settable to within a fraction a dB, down to at least –70 d B
and even into positive gain with some parts. Gain is always defi ned by an analogue
control voltage (or current) that may be derived locally after decoding from a digital line
or buss. Refi ned VCAs introduce considerable added circuitry into the signal path, which
may defeat its own purpose. The simplest parts add two stages. They may boast low
noise but it is at the expense of exposing the unnatural distortion patterns they create. The
best performers add as many as fi ve sequential stages and more than 5 op-amps may be
required. If part quality is not to be compromised, the added cost seems high. Operating
speed with most types can be very high, under 1 ItS. In this way, VCAs and all the
following contrivances are applicable to dynamic functions, up to the fastest meaningful
audiopeak limiting.
8.6.2.2 LED LDRs
With this method, the control signal drives an LED so that full brightness is defi ned as
either maximum level or full attenuation. An adjacent light-dependent resistor (LDR)
acts as the upper or lower arm of a passive attenuator. The intrinsic circuit isolation
and physical separation that is possible makes LED/LDRs attractive in systems where
isolation (of both grounds and common-mode voltages to 2.5 kV or more) is important for
safety or EMC. These parts provide remote control connections analogous to connecting
digital feeds via opto-isolators.