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188 lines
7.2 KiB
188 lines
7.2 KiB
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HREF="node54.html">Signals to controls</A>
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HREF="node52.html">Converting controls to signals</A>
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<H2><A ID="SECTION007103000000000000000">
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Non-looping wavetable player</A>
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</H2>
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<A ID="sect3.noloopsampler"></A>
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<P>
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One application area requiring careful attention to the control stream/audio
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signal boundary is sampling. Until now our samplers have skirted the issue
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by looping perpetually. This allows for a rich variety of sound that can be
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accessed by making continuous changes in parameters such as loop size and
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envelope shape. However, many uses of sampling require the internal features
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of a wavetable to emerge at predictable, synchronizable moments in time. For
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example, recorded percussion sounds are usually played from the beginning, are not
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often looped, and are usually played in a determined time
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relationship with the rest of the music.
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<P>
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In this situation, control streams are better adapted than audio signals as
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triggers. Example C05.sampler.oneshot.pd (Figure <A HREF="#fig03.14">3.14</A>) shows one possible
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way to accomplish this. The four tilde objects at bottom left form the
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signal processing network for playback. One <TT>vline~</TT> object
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generates a phase signal (actually just a table lookup index) to the
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<TT>tabread4~</TT> object; this replaces the <TT>phasor~</TT> of
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Example B03.tabread4.pd (Page <A HREF="node34.html#fig02.13"><IMG ALIGN="BOTTOM" BORDER="1" ALT="[*]"
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SRC="crossref.png"></A>) and its derivatives.
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<P>
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<DIV ALIGN="CENTER"><A ID="fig03.14"></A><A ID="3762"></A>
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<TABLE>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 3.14:</STRONG>
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Non-looping sampler.</CAPTION>
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<TR><TD><IMG
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WIDTH="528" HEIGHT="422" BORDER="0"
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SRC="img347.png"
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ALT="\begin{figure}\psfig{file=figs/fig03.14.ps}\end{figure}"></TD></TR>
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</TABLE>
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</DIV>
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<P>
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The amplitude of the output of <TT>tabread4~</TT> is controlled by a
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second <TT>vline~</TT> object, in order to prevent discontinuities
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in the output in case a new event is started while the previous event is still
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playing. The "cutoff" <TT>vline~</TT> object ramps the output down to zero
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(whether or not it is playing) so that, once the output is zero, the index
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of the wavetable may be changed discontinuously.
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<P>
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In order to start a new "note", first, the "cutoff" <TT>vline~</TT> object is
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ramped to zero; then, after a delay of 5 msec (at which point <TT>vline~</TT> has reached zero) the phase is reset. This is done with two messages: first,
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the phase is set to 1 (with no time value so that it jumps to 1 with no
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ramping). The value "1" specifies the first readable point of the wavetable,
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since we are using 4-point interpolation. Second, in the same message box,
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the phase is ramped to 441,000,000 over a time period of 10,000,000 msec. (In
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Pd, large numbers are shown using exponential notation; these two appear as
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4.41e+08 and 1e+07.) The quotient is 44.1 (in units per millisecond) giving
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a transposition of one. The upper <TT>vline~</TT> object (which generates the
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phase) receives these messages via the "r phase" object above it.
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<P>
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The example assumes that the wavetable is ramped smoothly to zero at either
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end, and the bottom right portion of the patch shows how to record such a
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wavetable (in this case four seconds long). Here a regular (and
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computationally cheaper) <TT>line~</TT> object suffices. Although the wavetable
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should be at least 4 seconds long for this to work, you may record shorter
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wavetables simply by cutting the <TT>line~</TT> object off earlier. The only
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caveat is that, if you are simultaneously reading and writing from the same
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wavetable, you should avoid situations where read and write operations attack
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the same portion of the wavetable at once.
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<P>
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The <TT>vline~</TT> objects surrounding the <TT>tabread4~</TT> were chosen over <TT>line~</TT> because the latter's rounding of
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breakpoints to the nearest block boundary (typically 1.45 msec) can make
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for audible aperiodicities in the sound if the wavetable is repeated more than
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10 or 20 times per second, and would prevent you from getting a nice, periodic
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sound at higher rates of repetition.
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<P>
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We will return to <TT>vline~</TT>-based sampling in the next chapter, to
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add transposition, envelopes, and polyphony.
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<P>
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<ADDRESS>
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Miller Puckette
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2006-12-30
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