of Microprocessors

SOME REAL ApPLICATIONS
721
~~ILT
VCA
~A A
roscL .~rmTl fVCAl t
~~
[A)
+
TWO INDEPENDENT CHA,~NELS
(B I INDEPENDENT CHANNELS WITH
RING MODULATION
Ici FILTER FM
(D) PARALLEL
FILTERS
lEI PARALLEL FILTERS WITH
RING MODULATIOI~
IF I PARALLEL FILTERS WITH
FILTER FM
IGI SERIES FILTERS
(HI SERIES FILTERS WITH
RING MODULATION
fQSCL. VCA
L!.J' B
I I I SERIES FILTERS WITH
FILTER FM
Fig. 19-6. Possible configurations of voice board. (A) Two independent
channels. (8) Independent channels with ring modulation. (e)
Filter FM. (0) Parallel filters. (E) Parallel filters with ring modulation.
(F) Parallel filters with filter FM. (G) Series filters. (H) Series
filters with ring modulation. (I) Series filters with filter FM.
pole RC low-pass-filtered with a time constant of about 1.3 msec. This
prevents an unnatural "tic" sound when an instantaneous attack is applied to
low-frequency sounds.
As can be seen, signal routing switches are scattered throughout the
block diagram. Most of the switches are type 4052 dual 1-of-4 analog
multiplexors. The first level of switching selects one of four possible sources
for each oscillator output. These are rectangle, a mix of sawtooth and
rectangle, white noise, and pink noise. Ir is interesting to note that an equal
mix of a rectangle and a sawtooth waveform yields a result equivalent to
mixing two sawtooth waves! The phasing between the two virtual sawtooth
waveforms is determined by the rectangle width. Thus, varying the rectangle
width will change the phasing as if the two waveforms were of slightly
different frequency. In fact, periodic variation of the width sounds exactly
like periodic beating between two oscillators and random variation gives a
remarkably good chorus effect. Most Chroma patches make use of this
surprising result to greatly increase the apparent richness of otherwise simple
synthesized rones. The two noise sources are generated externally on the
synthesizer bus motherboard and feed to all of the voice boards.