of Microprocessors

SOME REAL ApPLICATIONS
713
PRESSURE
r-------------------~
r------,...J .J
MULTIPLEXED
A-TO-D
CONVE RTER
AUDIO
CASSETTE
RECORDER
(USER
SUPPLIEDI
CONTROL
MICROPROCESSOR
CONFIGURATION
REGISTERS
L .J----~ MIXER
STEREO
OUTPUT
Fig. 19-1. Typical microprocessor-controlled hybrid keyboard synthesizer
combination of analog and digital circuitry, applying each where it is most
advantageous. Control-type tasks are best handled by digital circuitry,
particularly if it is microprocessor based. For example, implementing a
multinote polyphonic keyboard would be extremely difficult with analog
circuitry but is a straightforward exercise with digital circuitry as was
demonstrated in Chapter 9. On the other hand, audio-signal-handling tasks
such as filtering are much more economical to perform with analog circuitry.
Thus, at this time, it is generally most cost-effective to use a hybrid approach
to designing a moderate-performance, moderately priced keyboard
synthesizer.
A T.'Ypical Hybrid Synth.esizer
Figure 19-1 shows a generic block diagram of a microprocessorcontrolled
hybrid keyboard synthesizer. Central, of course, is the control
microprocessor, which is typically one of the more powerful 8-bit units,
although increasingly 16-bit architecture processors with 8-bit data buses or
even true 16-bit units are being used. The microprocessor program always
resides in permanent memory, either masked ROM or programmable ROM,
and may be 8-32K or even more. EPROMs are frequently seen because sales
volumes are often not high enough to justify ROM masking charges. Also,