of Microprocessors

626 MUSICAL ApPLICATIONS OF MICROPROCESSORS
OUTPUT
WAVESHAPE
DEFINITION
FILTER
PARAMETERS
AMPLITUDE
PARAMETER
I I I I I
CONTROL
COMPUTER
\
\
ENVELOPE
PARAMETERS
ENVELOPE
GENERATOR
Fig. 17-22. General voice module
the amplitude and dynamic waveshaper. A very wide variety of new and
existing musical instruments can be simulated by suitable setting of
parameters.
We have seen how digital circuitry does an excellent job implementing
the oscillator and static waveshaper and, when multiplexed sufficiently, does
so at low cost. The dynamic waveshaper can be implemented via Fourier
series (in which case the static waveshaper is not needed) or with a digital
filter. The amplitude-control element is simply a multiplier, whereas envelope
generation can be done by the using system or a dedicated microprocessor.
When cost is an overriding factor, the Fourier series approach is out
(remember that the entire unit diagrammed in Fig. 17-18 is just one voice).
A digital filter and amplitude multiplier would only be practical in a highly
multiplexed voice module and even then the hardware for such "simple"
signal processing would dominate the module's cost, mainly because of the
multiplications required. At this time, analog-variable filters and controlled
amplifiers can do the job cheaper, while their inaccuracies are not significant
in this application. Thus, in a voice module, digital techniques are most
effective for frequency- and static-waveform generation, while analog techniques
are best for economical processing of the tone.
When hybrid techniques are used in a voice module, multiplexing of
the frequency generator and waveshaper becomes less attractive because the
analog portion of the module cannot be multiplexed. In fact, much current
work in this area is with nonmultiplexed "voice-per-board" modules that can
be built, or purchased, one at a time as need (or budget) dictates. This does
not mean that some logic functions cannot be profitably shared. The crystal
frequency standard, for example, can be shared among modules as can the
interface to the control computer.