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HREF="node40.html">Audio and control computations</A>
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HREF="node46.html">Event detection</A>
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<H1><A NAME="SECTION00770000000000000000"></A>
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<A NAME="sect3.analog"></A>
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<BR>
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Audio signals as control
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</H1>
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<P>
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From the tradition of analog synthesis comes an elegant, old-fashioned approach
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to control problems that can be used as an alternative to the control
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streams we have been concerned with so far in this chapter. Instead, or
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in addition to using control streams, we can use audio signals themselves to
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control the production of other audio signals. Two specific techniques from
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analog synthesis lend themselves well to this treatment: analog sequencing
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and sample-and-hold.
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<P>
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The analog sequencer [<A
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HREF="node202.html#r-strange72">Str95</A>, pp. 70-79]
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[<A
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HREF="node202.html#r-chamberlin80">Cha80</A>, pp. 93,304-308]
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was often used to set off a regularly or semi-regularly
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repeating sequence of sounds. The sequencer itself typically put out a
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repeating sequence of voltages, along with a trigger signal which pulsed at
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each transition between voltages. One used the voltages for pitches or
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timbral parameters, and the trigger to control one or more envelope generators.
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Getting looped sequences of predetermined values in digital audio practice is
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as simple as sending a <TT>phasor~</TT> object into a non-interpolating table
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lookup. If you want, say, four values in the sequence, scale the
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<TT>phasor~</TT> output to take values from 0 to <IMG
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WIDTH="60" HEIGHT="13" ALIGN="BOTTOM" BORDER="0"
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SRC="img332.png"
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ALT="$3.999\ldots$"> so that the
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first fourth of the cycle reads point 0 of the table and so on.
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<P>
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<DIV ALIGN="CENTER"><A NAME="fig03.08"></A><A NAME="3656"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 3.8:</STRONG>
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Multiplying and wrapping a sawtooth wave to generate a higher
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frequency.</CAPTION>
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<TR><TD><IMG
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WIDTH="585" HEIGHT="515" BORDER="0"
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SRC="img333.png"
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ALT="\begin{figure}\psfig{file=figs/fig03.08.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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To get repeated triggering, the first step is to synthesize another sawtooth
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that runs in synchrony with the <TT>phasor~</TT> output but four times
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as fast. This is done using a variant of the technique of Figure
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<A HREF="node29.html#fig02.08">2.8</A>, in which we used an adder and a wraparound operator to get
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a desired phase shift. Figure <A HREF="#fig03.08">3.8</A> shows the effect of multiplying
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a sawtooth wave by an integer, then wrapping around to get a sawtooth at
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a multiple of the original frequency.
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<P>
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From there is is easy to get to a repeated envelope shape by wavetable lookup
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for example (using an interpolating table lookup this time, unlike the
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sequence voltages). All the waveform generation and altering techniques
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used for making pitched sounds can also be brought to use here.
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<P>
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The other standard control technique from analog synthesizer control is the
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sample and hold unit [<A
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HREF="node202.html#r-strange72">Str95</A>, pp. 80-83]
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[<A
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HREF="node202.html#r-chamberlin80">Cha80</A>, p. 92]. This takes an incoming signal, picks out certain
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instantaneous values from it, and "freezes" those values for its output. The
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particular values to pick out are selected by a secondary, "trigger" input.
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At points in time specified by the trigger input a new, single value is taken
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from the primary input and is output continuously until the next time point,
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when it is replaced by a new value of the primary input.
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<P>
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<DIV ALIGN="CENTER"><A NAME="fig03.09"></A><A NAME="3665"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 3.9:</STRONG>
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Sample and hold ("S/H"), using falling edges of the trigger signal.</CAPTION>
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<TR><TD><IMG
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WIDTH="535" HEIGHT="416" BORDER="0"
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SRC="img334.png"
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ALT="\begin{figure}\psfig{file=figs/fig03.09.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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In digital audio it is often useful to sample a new value on falling edges
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of the trigger signal, i.e., whenever the current value
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of the trigger signal is smaller than its previous value, as shown in
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Figure <A HREF="#fig03.09">3.9</A>. This is especially convenient for
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use with a sawtooth trigger, when we wish to sample signals in
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synchrony with an oscillator-driven process. Pd's sample and hold object
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was previously introduced in the context of sampling
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(Example B08.sampler.nodoppler.pd, Page <A HREF="node36.html#fig02.15"><IMG ALIGN="BOTTOM" BORDER="1" ALT="[*]"
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SRC="crossref.png"></A>).
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