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HREF="node180.html">Narrow-band companding: noise suppression</A>
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<B> <A ID="tex2html3272"
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<H2><A ID="SECTION001373000000000000000">
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Timbre stamp ("vocoder")</A>
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</H2>
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<P>
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<DIV ALIGN="CENTER"><A ID="fig09.17"></A><A ID="12742"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 9.17:</STRONG>
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Timbre stamp.</CAPTION>
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<TR><TD><IMG
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WIDTH="596" HEIGHT="523" BORDER="0"
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SRC="img1236.png"
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ALT="\begin{figure}\psfig{file=figs/fig09.17.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 I05.compressor.pd (Figure <A HREF="#fig09.17">9.17</A>)
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is another channel compander which is presented in preparation
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for Example I06.timbre.stamp.pd, which we will examine next. This is a realization of
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the timbre stamp of Figure <A HREF="node174.html#fig09.09">9.9</A>, slightly modified.
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<P>
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There are two inputs, one at left to be filtered (and whose Fourier transform
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is used for resynthesis after modifying the magnitudes), and one at right which
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acts as a control source. Roughly speaking, if the two magnitudes
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are <IMG
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WIDTH="30" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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SRC="img1167.png"
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ALT="$f[k]$"> for the filter input and <IMG
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WIDTH="28" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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SRC="img1237.png"
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ALT="$c[k]$"> for the control source, we just
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"whiten" the filter input, multiplying by <IMG
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WIDTH="46" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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SRC="img1238.png"
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ALT="$1/f[k]$">, and then stamp the control
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magnitudes onto the result by further multiplying by <IMG
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WIDTH="28" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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SRC="img1237.png"
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ALT="$c[k]$">. In practice, we
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must limit the gain to some reasonable maximum value. In this patch this is
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done by limiting the whitening factor <IMG
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WIDTH="46" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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SRC="img1238.png"
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ALT="$1/f[k]$"> to a specified maximum value
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using the <TT>clip~</TT> object. The limit is controlled by the
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"squelch" parameter, which is squared and divided by 100 to map values
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from 0 to 100 to a useful range.
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<P>
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Another possible scheme is to limit the gain after forming the quotient
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<IMG
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WIDTH="63" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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SRC="img1239.png"
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ALT="$c[k]/f[k]$">. The gain limitation may in either case be frequency dependent.
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It is also sometimes useful to raise the gain to a power <IMG
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WIDTH="11" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
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SRC="img57.png"
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ALT="$p$"> between 0
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and 1; if 1, this is a timbre stamp and if 0, it passes the filter input through
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unchanged, and values in between give a smooth interpolation between the two.
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