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Advanced Music Synthesis

Sample and Hold

The last workshop dealt with aspects of oscillator modulation, and next I'm going to look at Sample and Hold units (which will be, for convenience, abbreviated to S/H). To understand what possibilities are open to us we must first find out how they operate.

The S/H has two inputs and one output. One input is for the voltage that is to be sampled and the other is for the clock trigger. What happens in a S/H is that when it detects a voltage signal at its sampling input, it holds it for the duration of the clock pulse. It doesn't matter if the sampled voltage fluctuates at all during one pulse's cycle - the output will remain constant until a new clock pulse is received, whereupon the output will change in accordance with the sampled voltage. In most small synths the output from the Noise Generator is connected to the sample input and the LFO square wave supplies the trigger pulse. Figure 1 shows this and Figure 2a demonstrates what happens electronically. As you can see the output from the sample and hold is a totally random stepped voltage. It is this that allows us to create the random, tinkling computer effects that the S/H is famous for.

Figure 1. Sample and Hold patch in small synthesiser.

Figure 2. The result of sampling a sawtooth waveform.

If, however, a sawtooth wave is applied to the sample input instead of the noise generator, the result will be a staircase waveform (see Figure 2b). If this is applied to the CV input of a VCO the result will be an atonal arpeggio whose rate is controlled by the S/H's clock. It is possible to connect any varying voltage to the sampling input in order to get a stepped output voltage - a favourite of mine is to apply the output of an ADSR that has a long attack and release. I then connect the S/H output to the VCOs so that every time I play a note I get a rising glissando which falls as the note dies away.

Figure 3. Modulation of VCF cutoff frequency with random S/H output.

It is also possible, of course, to modulate the cutoff frequency of a voltage controlled filter (VCF) with the output of a S/H for many interesting tonal variations. The most basic patch for this is shown in Figure 3. The harmonic structure of the VCO will be emphasised and cut in a totally random fashion - this is again useful for computer type effects but can also be put to many musical uses.

Figure 4. Using KBD gate to trigger S/H.

Perhaps the most interesting use of a S/H is where you replace the clock input with an external pulse of some form. If we patch up as in Figure 4, we replace the clock pulse with the gate output of the keyboard. The output of the S/H is connected to the CV input of the VCF so that every time a new note is played there will be a tonal change. If we connect the S/H output to the VCF via a lag time integrator, (simply a small circuit that takes a finite time to charge up or fall from one CV level to another, usually via a resistor/capacitor network), the resultant change in tone will be smooth instead of abrupt. This technique is useful for creating vocal effects.

Figure 5. Using a sequencer clock pulse to trigger S/H.

By referring to Figure 5 you can see that the clock pulse is now being derived from a sequencer. This will enable you to create sequencer patterns that have different tonal characteristics for every note. This technique can be effective as it takes away the repetitiveness of some sequencer patterns. It also creates randomly occurring accents as the S/H 'opens' the VCF. By multitracking sequencers in this way many interesting polyrhythms can be created where pitch and tempo are constant but the dynamics and tonal qualities are constantly shifting. Also, instead of feeding a VCO into the VCF you might like to connect the noise generator. When multitracked with a pitched sequencer many varied percussion effects can be obtained - Depeche Mode use this technique very effectively in some of their songs.

Figure 6. Using the random S/H voltage output to randomly trigger the EG.

An extension of this 'random accents' technique is where you connect the output of the S/H to the trigger input of an EG. If your EG will only trigger with, say, 4 volts or more then it will not fire until the S/H voltage is at that level. The patch for this is shown in Figure 6 - again, many interesting polyrhythms can be created if this technique is multitracked.

In the same way, you might like to connect the S/H output to the step input of a sequencer so that it will only advance a note when the S/H output generates a voltage high enough to trigger it. The same technique could be applied to an arpeggiator such as on a Roland Juno 60, Jupiter 8, Memorymoog or whatever.

Figure 7. Aleatoric self-compositional S/H patch.

Finally, if the rate of the S/H's clock is voltage controllable, it is possible to generate some very interesting random effects where the tempo of the effect varies with the pitch. By patching up as in Figure 7 where the S/H output is controlling the speed of its clock as well as the pitch of the VCO, whenever the output of the S/H is high the clock will move towards the next pulse faster than when the S/H puts out a low voltage. Therefore, the higher the pitch the shorter the note and vice-versa. With careful adjustment of levels and frequency controls some amazing Webern-like, aleatoric melodic and rhythmic structures can be created; it is also useful (if an EG is also routed to the CV input of the VCO) for creating birdsong effects that are automatic but non-repeating.

As I hope you can see, the S/H is a far more useful device than it might first appear. I hope it's given you some further ideas for using your synthesiser.

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Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Jul 1983

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