of Microprocessors

722 MUSICAL ApPLICATIONS OF MICROPROCESSORS
Following the source selection switch for oscillator A is another switch
that selects between the oscillator output and a "ring-modulated" combination
of the oscillator A and B rectangle outputs. The ring modulator is really
just an exclusive-or digital gate (using CMOS logic that provides exact 5-V
amplitude outputs) and generates true ring modulation only when the input
waveforms are square. Nevertheless, the audible effect of ring modulation is
adequately produced even when the signals are nonsquare.
The switches immediately before and immediately after the filters
perform most of the patching function. Since all are controlled by the same
two patch bits, there are acrually only fout possible configurations. Following
the B side VCA, another four-way switch also participates in the
configuration function. The switch afrer the A side VCA controls the
disposition of the board's final output onto one of four signal output buses.
The output signal is actually a current and the buses are current-sensitive as
described in Chapter 8.
Of the possible configurations, the Chroma's software allows the user to
select any of the 16 most useful ones by a number between 0 and 15. Nine of
these are shown in simplified form in Fig. 19-6. In addition to these basic
patches, the A and B waveforms, B oscillator sync, and filter modes can be
selected to yield several dozen effective patches. By using the two filters in
the parallel mode, a band-reject response is possible as well as dual-humped
resonances. The series filter configuration allows a bandpass response as well
as four-pole low-pass and high-pass shapes. It is interesting to note that
frequency modulation of the oscillators is not provided for. Conversely, the
filter center frequencies can be modulated. Since the oscillators cannot
produce sine waves but the filters can