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176 lines
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176 lines
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<H2><A ID="SECTION001342000000000000000">
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Timbre stamping (classical vocoder)</A>
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</H2>
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<P>
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<DIV ALIGN="CENTER"><A ID="fig09.09"></A><A ID="12598"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 9.9:</STRONG>
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Block diagram for timbre stamping (AKA "vocoding'').</CAPTION>
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<TR><TD><IMG
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WIDTH="257" HEIGHT="420" BORDER="0"
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SRC="img1172.png"
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ALT="\begin{figure}\psfig{file=figs/fig09.09.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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A second application of Fourier analysis and resynthesis is a time-varying
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filter capable of making one sound take on the evolving spectral envelope of
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another. This is
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widely known in electronic music circles as
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a
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<A ID="12601"></A><I>vocoder</I>,
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named, not quite accurately, after the original
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Bell Laboratories vocal analysis/synthesis device. The
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technique described here is more accurately called
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<A ID="12603"></A><I>timbre stamping</I>.
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Two input signals are used, one to be filtered, and the other to control the
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filter via its time-varying spectral envelope. The windowed Fourier
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transform is used both on the control signal input to estimate its spectral
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envelope, and on the filter input in order to apply the filter.
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<P>
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A block diagram for timbre stamping is shown in Figure <A HREF="#fig09.09">9.9</A>.
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As in the previous example, the timbre stamp acts by multiplying the
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complex-valued windowed Fourier transform of the filter input by non-negative
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real numbers, hence changing their magnitudes but leaving their phases intact.
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Here the twist is that we want simply to replace the magnitudes of the
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original, <IMG
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WIDTH="61" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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SRC="img1173.png"
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ALT="$\vert S[m, k]\vert$">, with magnitudes obtained from the control input (call
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them <IMG
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WIDTH="62" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
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SRC="img1174.png"
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ALT="$\vert T[m, k]\vert$">, say). The necessary gain would thus be,
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<BR><P></P>
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<DIV ALIGN="CENTER">
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<!-- MATH
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\begin{displaymath}
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g[m, k] = {{|T[m, k]|}\over{|S[m, k]|}}
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\end{displaymath}
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-->
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<IMG
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WIDTH="129" HEIGHT="44" BORDER="0"
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SRC="img1175.png"
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ALT="\begin{displaymath}
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g[m, k] = {{\vert T[m, k]\vert}\over{\vert S[m, k]\vert}}
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\end{displaymath}">
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</DIV>
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<BR CLEAR="ALL">
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<P></P>
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In practice it is best to limit the gain to some maximum value (which might
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depend on frequency) since otherwise channels containing nothing but noise,
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sidelobes, or even truncation error might be raised to audibility. So a
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suitable limiting function is applied to the gain before using it.
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HREF="node173.html">Narrow-band companding</A>
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<ADDRESS>
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Miller Puckette
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2006-12-30
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