One application area requiring careful thought about the control stream/audio signal boundary is sampling. Until now our samplers have skirted these issues by looping perpetually. This allows for a rich variety of sound that can be accessed by making continuous changes in parameters such as loop size and envelope shape. However, many uses of sampling require the internal features of a sample to emerge at predictable, synchronizable moments in time. For example, percussion samples are usually played from the beginning, are not often looped, and are usually played in some kind of determined time relationship with the rest of the music.
In this situation, control streams are better adapted than audio signals as triggers. Example patch C05.sampler.oneshot.pd (Figure 3.14) shows one possible way to accomplish this. The four tilde objects at bottom left form the signal processing network for playback. One object generates a phase signal (actually just a table lookup index) to the object; this replaces the of patch B02.wavetable.FM.pd(page ) and its derivatives.
The amplitude of the output of is controlled by a second object. This is in order to prevent discontinuities in the output in case a new event is started while the previous event is still playing. The ``cutoff" ramps the output down to zero (whether or not it is playing) so that, once the output is zero, the index of the wavetable may be changed discontinuously.
The sequence of events for starting a new ``note" is, first, that the ``cutoff" is ramped to zero; then, after a delay of 5 msec (at which point has reached zero) the phase is reset. This is done with two messages: first, the phase is set to 1 (with no time value so that it jumps to 1 with no ramping.) This is the first readable point of the wavetable. Second, in the same message box, the phase is sent to 441,000,000 over a time period of 10,000,000 msec. This corresponds to 44.1 units per millisecond and thus to a transposition of one. The upper (which generates the phase) receives these messages via the object above it.
The example assumes that the wavetable is ramped smoothly to zero at either end, and the bottom right portion of the patch shows how to record a sample (in this case four seconds long) which is ramped smoothly to zero at either end. Here a regular (and computationally cheaper) object suffices. Although the wavetable should be at least 4 seconds long for this to work, you may record shorter wavetables simply by cutting the object off earlier. The only caveat is that, if you are simultaneously reading and writing from the same sample, you may have to avoid situations where read and write operations attack the same portion of the wavetable at once.
The objects surrounding the were chosen over because the latter's rounding of breakpoints to the nearest block boundary (typically 1.45 msec) can make for audible aperiodicities in the sound if the sample is repeated more than 10 or 20 times per second, and would prevent you from getting a nice, periodic sound at higher rates of repetition.
We will return to -based sampling in the next chapter, to add transposition, envelopes, and polyphony.