of Microprocessors

72 MUSICAL ApPLICATIONS OF MICROPROCESSORS
Spectrum Tracking
Generalized speerrum tracking can be done in several ways. For definitely
pitched sounds, the amplitude envelope of each individual harmonic
can be determined with computer-processing techniques. A somewhat simpler
technique involves a bank of 10 to 30 bandpass filters and an amplitude
envelope follower connected to the output of each filter. Any sound can be
passed through such afilterbank analyzer, and its rough spectrum shape as a
function of time can be determined.
Another method that also involves a computer is formant tracking. The
spectrum of many interesting sounds such as speech can be fairly well described
by the frequency, height,and width of peaks in their spectrum
shape. Frequently, even the height and width parameters are ignored. The
resulting small number of parameters describing the time varying spectrum
shape is quite attractive for analysis-synthesis purposes. For example, tracking
of just the frequency of the two lowest formants of voiced speech sounds
is adequate for intelligible speech reconstruction.
Unfortunately, accurate formant following is as difficult as accurate
pitch tracking. For example, there need not necessarily be a constant number
of formants in the sound being analyzed. As a result, the analyzer must
recognize when two formants merge into one or when one just decreases in
amplitude and disappears. If the overall spectrum shape contains a long
downward slope, the ear will often recognize a local flattening in that trend
as a subdued formant, even though no real peak occurs.
Use ofAnalysis Results
It should be obvious from the foregoing that really accurate analysis of
sound into the simple parameters that have been discussed is not always
possible. However, the accuracy that is attainable is gen~rally quite acceptable
for analysis-synthesis experiments. Occasionally, some hand editing of
the data obtained is necessary to correct gross errors or fill in missing data
caused by nearly inaudible "defects" in the prototype sound.
Generally, the data obtained from the analysis can be represented as a
number of curves showing how the parameters vary with time. Such a set is
shown in Fig. 2-11. On a computer-based system, these curves would
actually be available to the user to study and modify as desired. In a real-time
system, these parameters would really be just varying voltages generated by
the analysis equipment. In either case, some, if not all, of the parameters
would be processed and then they would pass to synthesis equipment, which
generates the modified sound.
The analysis-synthesis technique can be applied in a number of ways.
One intriguing possibility is transferral of certain characteristics from one
type ofsound to another. For example, let's assume that a short trumpet solo