of Microprocessors

DIGITAL HARDWARE
611
8-BIT PORT I
MINOR
CYCLE
CLOCK
,
FREQUENCY DATA
~_~~_----1
WAVEFORM
TABLE
DATA
WRITE
ENABLE
FREQUENCY
WRITE
ENABLE
WAVEFORM
Fig. 17-15. Minimal microcomputer interface
changes during the duration ofa si'ngle note, is not really practical. Although
the logic timing is such that a new waveform can be written without interfering
with waveform scanning, it would be very difficult to rewrite a waveform
on the fly and insure that discontinuities due to half-old/half-new waveform
scanning did not occur. One could possibly use two oscillator channels along
with variable-gain amplifiers to alternately interpolate between successive
versions of the waveform. In any case, variable filters would probably be used
for dynamic variation,
What is needed is a Fourier transform or Fourier series tone
generator, which is set up for continuous variation of the harmonic
amplitudes and possibly the phases as well. When one realizes that such a
tone generator is really nothing more than a bank of sine wave oscillators
operating at harmonically related frequencies and having independent
amplitude control, it becomes obvious that a multiplexed oscillator could do
rhe job. Although the previous oscillator could provide up to 16 harmonic
(and inharmonic as well) frequencies, external gain control elements would
be necessary to control relative amplitude. Thus, an optimized Fourier series
tone generator will be briefly described. Note that, although the unit is
multiplexed in order to generate numerous harmonics, it can only generate a
single composite tone, whereas the previous oscillator could generate 16
tones. Many musical situations, however, call for a solo instrument of great
expressiveness with accompaniment in the background. The solo could therefore
be played by the Fourier series generator, while the less critical accompaniment
could be played by the oscillator bank described earlier.
The unit that will be described has the following general specifications:
1. Up to 64 harmonics in an unbroken series from 0 to 63.