of Microprocessors

DIGITAL FILTERING 515
INPUT
(C)
-(.9511 Fc/Fs) -(3.751Fc/FS) WHERE:
A, = e AJ ~ e Fe ~ CENTER FREQUENCY OF BAND
-(10.52Fc/Fs) -(41.50Fe/Fs) Fs = SAMPLE RATE
A, = e
Fig. 14-25. (Cont.). (C) 90 degree phase difference network detail.
The other elements are exceptionally easy to implement in digital form.
The frequency of the sine and cosine waves entering the multipliers determines
the amount of spectrum shift hertz for hertz. As shown, the spectrum
is shifted up, but inversion of the driving oscillator phase will cause a
downshift instead. The oscillator is easily implemented with a sine table and
two pointers one-quarter the table length apart. Smooth transitions from
upshift to downshift are accomplished by letting the table increment go
negative, thereby generating "negative" frequencies.
The entire system is operated much like the simpler chorus synthesizer.
Essentially, all of the variable elements are fed independent, slowly varying
random signals. Again, adjustment is best done by ear, although many
strange effects are possible by misadjustment.
Interpolation
Virtually everyone was taught in junior high school (before the advent
of pocket calculators anyway) how to interpolate in a table of logs or trig
functions in order to get an extra digit or two of precision. However, in
digital music synthesis, interpolation between waveform table entries or