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Envelope followers< / A >
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< A ID = "sect8.heterodyning" > < / A >
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< P >
Example H06.envelope.follower.pd shows a simple and self-explanatory realization of the
envelope follower described in Section < A HREF = "node153.html#sect8.envelopefollower" > 8.4.2< / A > . An
interesting application of envelope following is shown in Example H07.measure.spectrum.pd (Figure < A HREF = "#fig08.30" > 8.30< / A > , part a). A famous bell sample is looped as a test
sound. Rather than get the overall mean square power of the bell, we would
like to estimate the frequency and power of each of its partials. To do this
we sweep a band-pass filter up and down in frequency, listening to the result
and/or watching the filter's output power using an envelope follower. (We use
two band-pass filters in series for better isolation of the partials; this is
not especially good filter design practice but it will do in this context.)
When the filter is tuned to a partial the envelope follower reports its
strength.
< P >
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< CAPTION ALIGN = "BOTTOM" > < STRONG > Figure 8.30:< / STRONG >
Analyzing the spectrum of a sound: (a) band-pass filtering a sampled
bell sound and envelope-following the result; (b) frequency-shifting a
partial to DC and reading off its real and imaginary part.
< / CAPTION >
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< P >
Example H08.heterodyning.pd (part (b) of the figure) shows an alternative way of finding partial strengths of
an incoming sound; it has the advantage of reporting the phase as well as the
strength. First we modulate the desired partial down to zero frequency. We use
a complex-valued sinusoid as a modulator so that we get only one sideband for
each component of the input. The test frequency is the only frequency that is
modulated to DC; others go elsewhere. We then low-pass the resulting complex
signal. (We can use a real-valued low-pass filter separately on the real
and imaginary parts.) This essentially removes all the partials except for the
DC one, which we then harvest. This technique is the basis of Fourier
analysis, the subject of Chapter 9.
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Miller Puckette
2006-12-30
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