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153 lines
5.4 KiB
153 lines
5.4 KiB
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HREF="node157.html">Prefabricated time-varying band-pass filter</A>
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<H2><A ID="SECTION001253000000000000000">
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Envelope followers</A>
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</H2>
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<A ID="sect8.heterodyning"></A>
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<P>
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Example H06.envelope.follower.pd shows a simple and self-explanatory realization of the
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envelope follower described in Section <A HREF="node153.html#sect8.envelopefollower">8.4.2</A>. An
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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
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sound. Rather than get the overall mean square power of the bell, we would
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like to estimate the frequency and power of each of its partials. To do this
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we sweep a band-pass filter up and down in frequency, listening to the result
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and/or watching the filter's output power using an envelope follower. (We use
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two band-pass filters in series for better isolation of the partials; this is
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not especially good filter design practice but it will do in this context.)
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When the filter is tuned to a partial the envelope follower reports its
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strength.
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<P>
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<DIV ALIGN="CENTER"><A ID="fig08.30"></A><A ID="10622"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 8.30:</STRONG>
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Analyzing the spectrum of a sound: (a) band-pass filtering a sampled
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bell sound and envelope-following the result; (b) frequency-shifting a
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partial to DC and reading off its real and imaginary part.
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</CAPTION>
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<TR><TD><IMG
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WIDTH="524" HEIGHT="435" BORDER="0"
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ALT="\begin{figure}\psfig{file=figs/fig08.30.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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Example H08.heterodyning.pd (part (b) of the figure) shows an alternative way of finding partial strengths of
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an incoming sound; it has the advantage of reporting the phase as well as the
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strength. First we modulate the desired partial down to zero frequency. We use
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a complex-valued sinusoid as a modulator so that we get only one sideband for
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each component of the input. The test frequency is the only frequency that is
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modulated to DC; others go elsewhere. We then low-pass the resulting complex
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signal. (We can use a real-valued low-pass filter separately on the real
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and imaginary parts.) This essentially removes all the partials except for the
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DC one, which we then harvest. This technique is the basis of Fourier
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analysis, the subject of Chapter 9.
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HREF="node157.html">Prefabricated time-varying band-pass filter</A>
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<ADDRESS>
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Miller Puckette
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2006-12-30
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