of Microprocessors

54 MUSICAL ApPLICATIONS OF MICROPROCESSORS
different frequency are fed in, the output spectfum contains not only harmonic
frequencies of each tone, but every possible combination of sum and
difference frequencies between the tones and their harmonics. For musical
instrument use, the resulting sound is close to garbage unless the input
frequencies are simply related such as a 3:2 ratio. For more abstract goals,
however I anything is potentially llseful even if it is not predictable.
Filters
While nonlinear circuits are relatively simple spectrum modifiers, theif
action is indirect. A different class of device calledfilters acts directly on the
spectrum changing the amplitude and phase of each sine wave component of
the input signal by a predictable amount. Furthermore, theif action on the
spectrum is unaffected by the actual spectrum of the input signal.
Filters can be completely specified by giving a plot of their amplitude
response and phase response as a function of frequency. Nearly always, the
amplitude response is termed, incorrecdy, the frequency response but the
former term will be used exclusively in this book. The test setup shown in
Fig. 2-3 can be used to make such plots. Here we have a variable-frequency
sine wave signal generaror, a calibrated oscilloscope with dual-trace capability,
and the filter under test. The gain of the fil ter at a parricular frequency
may be determined by measuring the amplitude of the signal at the filter
VARIABLE -FREQUENCY
SINE WAVE
GENERATOR
FILTER
UNDER
TEST
A
B
DUAL- TRACE
OSCILLOSCOPE
INPUT
OSCILLOSCOPE PRESENTATION
Fig. 2-3. Experimental setup for characterizing a filter