of Microprocessors

510 MUSICAL ApPLICATIONS OF MICROPROCESSORS
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Fig. 14-21. Tapped delay line digital reverberator
A Practical Filter for
Concert Hall Reverberation
In theory, it is possible to exactly duplicate the acoustics of a particular
concert hall by recording its impulse response and then applying the transversal
filter technique to the sound to be reverberated. Typical reverberation
times of 2 sec, however, mean that the filter is 50K to lOOK samples long,
which is clearly impractical. On the other hand just about any assemblage of
delays, summers, and multipliers will produce some kind of reverberation if
it doesn't oscillate instead. Some, of course, are much better at simulating
convincing concert hall reverberation than others.
In order to increase the echo density, it is necessary to use several delays
of unequal length. The structure in Fig. 14-21 is the digital equivalent of
the multiple-head tape reverberation simulator mentioned in Chapter 2. The
placement of the taps and the values of the feedback constants are very
important in determining the sound of the system. Generally, the taps
should be approximately exponentially distributed but placed at prime
number locations. This insures a maximum rate of echo buildup. The feedback
constants strongly interact with each other and in fact there is no easy
way to tell if a particular set will not cause sustained oscillation. Typical
values are around 0.8 with the long delay taps being somewhat more and the
short taps somewhat less. In any case, experimentation with the number,
placement, and gain of the taps is necessary to achieve the type of reverberation
required.
Another approach is based on the concept of cascading simple reverberation
modules. One could use the Fig. 14-20B setup as a module and cascade
two or more of them in order to improve the echo density. One problem that
can arise is that at certain frequencies the peaks and valleys in the individual