of Microprocessors

DIGITAL FILTERING
513
INPUT
VARIABLE
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OUTPUT
Fig. 14-24. Direct chorus simulator
tance but tends to emphasize momentary enhancements and cancellations of
the multiple sources.
The random control signals are white noise that has been low-pass
filtered with a cutoff in the range of 10 Hz to 20 Hz. It is important that the
noise used for each channel be uncorrelated (taken from a separate generator),
which is easily done with a digital noise generator (to be discussed in Chapter
15). If naturalness is desired, it is important that the peak frequency
modulation caused by changing delay times be held to 1 Hz or less for
midfrequencies. Because of the numerous variables, adjustment is best done
by ear.
The system shown with a half-dozen delays is remarkably effective in
simulating a small chorus. Although a fair amount of computer memory is
necessary for the delay lines (much of it can be shared by programming a
single long line and variable taps), very little computation is required relative
to the results obtained. Because of the numerous parameters available for
experimentation, many weird and whacky effects should also be possible.
The other method of chorus simulation is considerably more involved
but much simpler than hundreds of the sections just discussed. Basically, the
idea is to use a collage of randomly varying delays, phase shifters, frequency
shifters, and frequency selective filters. Figure 14-25 is a block diagram! of
the system that was originally implemented with purely analog circuitry.
Because of its complexity, it is a good candidate for digital implementation.
This system differs from the one described earlier in that the input
spectrum is split into several bands and that differences in intonation are
simulated primarily with spectrum (frequency) shifters rather than phase or
delay shifters (although these too are used extensively). As was mentioned in
Chapter 3, a spectrum shift destroys harmonic relationships among components
of the shifted spectrum, whereas a delay shift does not.
IThe block diagram was raken from nores gathered from a technical paper delivered by
Robert Orban at the 55th convention of the Audio Engineering Society.