of Microprocessors

SOME REAL ApPLICATIONS 715
Fig. 19-2. Rhodes "Chroma" keyboard synthesizer
needed control voltages are produced by a centralized, multiplexed D-to-A
converter, usually 12 bits, and the analog configuration switches are
controlled by external registers. Generally six to eight voice modules are
included, although some synthesizers may be expandable. A final analog
switch array selects the module outputs and mixes them into a stereo audio
output.
A multiplexed A-to-D converter is also included to read the pitch and
modulation wheels, variable footpedal, and other proportional operator
controls. If the keyboard includes pressure sensing, this too is connected to
the ADC. There may be either a single sensor under rhe entire keyboard or
individual sensors under each key. Often a signal from the voice module
output multiplexor is fed back to the ADC for diagnostic usage and automatic
tuning, which will be described in the next section.
The Rhodes Chroma
Perhaps the best way to become familiar with the specifics of hybrid
synthesizer technology is to briefly study a well-designed example unit. The
Rhodes Chroma, pictured in Fig. 19-2, is one such synthesizer that sells in
the $3,000-5,000 range. It is packaged into a wooden tabletop cabinet that
weighs about 70 pounds and has a five-octave, 61-key keyboard, also made of
wood. The control panel is mounted above the keyboard and is largely a
printed plastic overlay sheet with an array of 50 membrane switches, several
slide pots, several individual LEDs, a large two-digit display, and a smaller
eight-digit display. One interesting feature is a solenoid "tapper" that
literally thumps the control panel when any of the membrane switches is