Modular Synthesis (Part 11)
More on click-tracks and how to use them, by E&MM's resident literary module, Steve Howell.
Or how a synthesiser's modules can be driven from an audio signal off-tape.
Last month we saw how it is possible to override a sequencer's internal clock and drive it instead from a signal off-tape. This technique means that sequencers can be synchronised during a multitrack recording, for example, saving tracks and enabling complex rhythmic and melodic sequencer patterns to be created with relative ease.
The basic concept behind last month's technique was the conversion of a spikey audio signal off-tape into a voltage that could more easily be 'interpreted' by a synthesiser for triggering purposes. We've already seen that this technique can be used to step through a sequencer's events, but in fact there's no reason why the pulse can't be applied to any synth module that requires a trigger pulse in order to work. Such devices include envelope generators (EGs) and sample and hold units (S/H), both of which can of course be used to shape an incoming signal of some kind.
The first application that springs to mind is the use of a click-track to trigger a synth's EG. This would be shaping a VCF and VCA which, in turn, could be fed from a polyphonic source. The latter could be almost anything - from a string synth or organ to a guitar or the output of your record player, though the last-mentioned option isn't guaranteed to provide musically viable effects...
The patch for all this is shown in Figure 1, and the prime audible result of its application is quite fascinating: chordal rhythm patterns appear as the sound is 'chopped' by the automatically-triggered EGs.
One point to bear in mind here is that the envelope of the polyphonic sound should be kept very 'organish', that is, straight on and straight off with full sustain. The reason for this is that all your envelope shaping is now being derived from the modular synth's EGs and VCFs/VCAs.
There's no reason why you have to use a polyphonic sound source, however. Simply using an internal VCO (or more than one) and routing this into the VCF/VCA in the usual fashion should provide you with some quite sophisticated 'sequencer' effects, because although the synth patch itself is quite standard, the trigger is being derived from the click-track (Figure 2). It must be pointed out though that both the above-mentioned techniques require the player's note and chord changes to be timed with a fair degree of accuracy. The whole object of employing these patches is to obtain metronomic timing, but get it wrong and it'll simply turn out sounding like a mess. Still, practice makes perfect.
In addition to pitched sounds, it's also possible for a noise source to be used for percussive effects, and if you use the patch shown in Figure 3, you'll be able to key in noise effects intermittently. Again, a simple on/off envelope is all that's required of EG3/VCA2, as the actual shaping is accomplished by EGs 1 and 2.
Not enough EG/VCA/VCF options on your synth? Try laying down a whole track full of repeated noises triggered off the click-track and then, during mixdown, 'dub' the effects in and out as required using the channel mute or routing switches on the mixer. This technique can also be useful if you're at all indecisive as to when and where your bursts of noise should be, as it enables such decisions to be left until the very end of the recording process, when all other tracks have been laid down.
Do ensure though that the switches on your mixer are clean and click-free if you want to avoid embarrassing crackles and pops appearing during the process and giving the whole game away.
If your mixer doesn't possess any in/out switches, try using the patch given in Figure 4(a), in which the track containing the continuously triggered noise (or any other sound, for that matter) is routed back into the synth during mixdown and the bursts introduced by initiating the EG/VCA via a remote push-to-make switch conveniently located next to the mixer.
A typical circuit for such a switch is shown in Figure 4(b), and this is suitable for all positive-going 'V' triggers. Korg users will have to reverse the polarity of the battery, while Moog owners are in the fortunate position of being able to do without the battery altogether, since simply closing the switch is enough to trigger the 'Contour Generators', as Moog call them.
Introducing tonal variation is no more difficult than sweeping the VCF with an LFO, but a more interesting - and more original - effect can be obtained by using a sample and hold (yes, it's the good ol' S/H again!) stepped through by the click-track for random tonal changes. The sound source can be anything and the technique employed any of those described above.
The voltage output of the S/H can be routed to some other CV input such as a voltage-controlled phaser or flanger, a DDL set up for similar effects or, for somewhat more unusual results, longer repeat echoes. As explained in last month's column, the CV output of the S/H can be used to trigger the EGs randomly, so that polyphonic sound is heard at completely unpredictable moments, ie. whenever the CV from one sample and hold rises above the EG's trigger threshold level.
Last month's modular chapter also revealed how it was possible to use the envelope follower to convert the audio signal into a voltage suitable for the EGs. A variation on this technique is to use the voltage output from the envelope follower to shape the incoming sound source (polyphonic or otherwise), and the patch for this is represented by Figure 5. You won't, of course, have such an extensive degree of control over envelope shape if you do things this way, but it is a quick and easy way of achieving metronomically-chopped chords.
Feature by Steve Howell
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