As suggested in Section 5.2, when considering the result of
modulating a complex harmonic (i.e., periodic) signal by a
sinusoid, an interesting special case is to set the modulating
oscillator to 
the fundamental frequency, which drops
the resulting sound an octave with only a relatively small
deformation of the spectral envelope. Another is to modulate by a
sinusoid at several times the fundamental frequency, which in
effect displaces the spectral envelope without changing the
fundamental frequency of the result. This is demonstrated in
Example E03.octave.divider.pd(Figure 5.10).
The signal we process here is a recorded, spoken voice.
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The subpatches pd looper and pd delay hide details. The first is a looping sampler as introduced in Chapter 2. The second is a delay of 1024 samples, which uses objects that are introduced later in Chapter 7. We will introduce one object class here:
: pitch tracker. The inlet takes a signal to analyze, and
messages to change settings. Depending on its creation arguments
fiddle~may have a variable number of outlets offering
various information about the input signal. As shown here, with
only one creation argument to specify window size, the third outlet
attempts to report the pitch of the input, and the amplitude of
that portion of the input which repeats (at least approximately) at
the reported pitch. These are output as a list of two numbers. The
pitch, which is in MIDI units, is reported as zero if none could be
identified.
In this patch the third outlet is unpacked into its pitch and
amplitude components, and the pitch component is filtered by the
moses object so that only successful pitch estimates
(nonzero ones) are considered. These are converted to units of
frequency by the mtof object. Finally, the frequency
estimates are either reduced by or else multiplied
by 15, depending on the selected multiplier, to provide the
modulation frequency. In the first case we get an octave divider,
and in the second, additional high harmonics that deform the
vowels.