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Prefabricated time-varying band-pass filter< / A >
< / H2 >
< P >
Time-varying band-pass filtering, as often used in classical
subtractive synthesis (Section < A HREF = "node152.html#sect08.subtractive" > 8.4.1< / A > ), can be done
using the < TT > vcf~< / TT > object, introduced here:
< P >
< BR > <!-- MATH
$\fbox{ \texttt{vcf\~}}$
-->
< IMG
WIDTH="53" HEIGHT="39" ALIGN="MIDDLE" BORDER="0"
SRC="img1022.png"
ALT="\fbox{ \texttt{vcf\~}}">:
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< A ID = "10846" > < / A > a "voltage controlled" band-pass filter,
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similar to < TT > bp~< / TT > , but with a signal inlet to control center frequency.
Both < TT > bp~< / TT > and < TT > vcf~< / TT > are one-pole resonant filters as
developed in Section < A HREF = "node143.html#sect8.twopolebandpass" > 8.3.4< / A > ; < TT > bp~< / TT > outputs only
the real part of the resulting signal, while < TT > vcf~< / TT > outputs the
real and imaginary parts separately.
< P >
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< DIV ALIGN = "CENTER" > < A ID = "fig08.29" > < / A > < A ID = "10612" > < / A >
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< CAPTION ALIGN = "BOTTOM" > < STRONG > Figure 8.29:< / STRONG >
The < TT > vcf~< / TT > band-pass filter, with its center frequency
controlled by an audio signal (as compared to
< TT > bp~< / TT > which takes only control messages to set its center frequency. < / CAPTION >
< TR > < TD > < IMG
WIDTH="303" HEIGHT="355" BORDER="0"
SRC="img1024.png"
ALT="\begin{figure}\psfig{file=figs/fig08.29.ps}\end{figure}">< / TD > < / TR >
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< P >
Example H04.filter.sweep.pd (Figure < A HREF = "#fig08.29" > 8.29< / A > ) demonstrates using the < TT > vcf~< / TT > object for a simple and characteristic subtractive synthesis task. A
< TT > phasor~< / TT > object (at top) creates a sawtooth wave to filter. (This is
not especially good practice as we are not controlling the possibility of
foldover; a better sawtooth generator for this purpose will be developed in
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Chapter 10.) The second < TT > phasor~< / TT > object (labeled "LFO for sweep")
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controls the time-varying center frequency. After adjusting to set the depth
and a base center frequency (given in MIDI units), the result is converted into
Hertz (using the < TT > tabread4~< / TT > object) and passed to < TT > vcf~< / TT > to set
its center frequency. Another example of using a < TT > vcf~< / TT > object for
subtractive synthesis is demonstrated in example H05.filter.floyd.pd.
< P >
< HR >
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