Modular Synthesis (Part 8)
Using Sequencers with Modular Systems
In something of a break from tradition, Steve Howell begins a look at how using a modular synth can expand the horizons of the common or garden monophonic sequencer.
This month sees a change of emphasis as Steve Howell turns his attention away from generating specific sounds and begins a look at the possible applications of sequencers in a modular context.
Although the use of sequencers has received a fair amount of coverage in these hallowed pages of late, their application in the context of a modular system involves techniques that are not really practicable on a smaller, prewired synthesiser, and a modular synth offers considerably more in the way of sequencing possibilities, even if its use is still something of a specialist subject.
A modular instrument gives the user control not only of pitch but also of tone, amplitude, pulse width, modulation level, and in some cases, envelope shape, and all these parameters can, if desired, be manipulated by a sequencer. Before we go any further into the world of sequencer application, however, let's first take a look at the various types of unit available.
Analogue sequencers were the first type to be produced, and almost all such models feature a row of potentiometers which are tuned to whichever control voltage levels are necessary to produce a short sequence of programmed events. Most commonly, these events are then applied to one or more VCOs for melodic sequences, though as with any CV, they can also be used in conjunction with any voltage-controllable module.
There are two main problems with these units. First, they are inordinately cumbersome to program, and second, they have only a limited storage capacity - usually in the region of 24 events, depending on the make and model. However, more often than not these can be split into three channels of eight events each, with the result that each channel can be used to control separate VCOs (for melodic counterpart) or routed to a VCF and/or VCA for tonal and amplitude changes. A further alternative is to use one channel to control the tempo of the sequencer's voltage controlled clock, facilitating the construction of more intricate rhythm patterns.
It's also possible to interact with the sequencer as it's running, switching notes in and out and varying the preprogrammed CV levels so that a degree of improvisation can be introduced into what may otherwise be a short and rather repetitive sequence. The normal mode of operation, however, is to feed each channel to a separate set of voice modules (comprising VCO(s), a VCF, a VCA and associated EGs) so that the various layers of sound can be cross-faded to give the effect of longer sequences.
Nowadays, analogue sequencers can seem a bit limited, but they do offer some possibilities not available on the latest microprocessor-based units, and there's no reason why they can't be usefully employed even in today's microcomposed music.
The next stage in technological development was the digital sequencer which had a greater storage capacity but lacked the multiple channels of its analogue predecessor. Hence a number of synthesists (ie. those who couldn't afford to buy more than one digital device) remained unimpressed by the new machines and stuck with their existing instruments. However, it's often possible to split the digital memory into two or four smaller sections which can then be used to store different sequences.
On the other hand, digital sequencers did improve storage matters considerably, a typical memory capacity being about five times as much as that of an analogue counterpart.
Ideally, the next step of evolution would have been a multi-channel device that would have combined the control possibilities of the original analogue sequencers with the greater event storage made possible by the use of digital techniques. Sadly, this never came about, and instead the synth world saw the arrival of the microcomposer, a device of considerably greater flexibility but rather less in the way of operational simplicity. It's for this reason that I still feel 'old-style' analogue and digital sequencers can still be of use to the modern synthesist, because although they can't equal microcomposers' editing facilities and multiple output flexibility, they are at least fairly straightforward to use.
Figure 1 shows the most basic use of a sequencer - to play one or more VCOs for a melodic sequence: of course, the voice modules can be set up for any sound you wish (if you're really stuck, use one of the patches from previous articles in this series!). The keyboard is also connected so that the sequence can be transposed up or down, though if you're using a digital model it'll most likely have this facility as a switchable function.
Figure 2 is essentially the same, except that it uses two control channels to play a two-part sequence. This technique is more suited to use with an analogue sequencer, but if you're lucky enough to own more than one digital device you could doubtless achieve the same result. The number of channels can be multiplied as many times as you wish - assuming you have enough hardware at your disposal - and you can then mix each channel, cross-fading them in and out and adding EQ and effects as desired. This technique lends itself particularly well to the dreamy 'floating' music pioneered by Tangerine Dream and copied (with varying degrees of success) by so many others. Be careful you don't get too carried away with this technique, however: it's an easy mistake to make in these circumstances.
One adaptation of this patch is to use the other channels' CV outputs to control some other voltage-controllable device.
The first one that springs to mind is the VCF, which you can modify so that each note has a different tone colour. This can certainly make an otherwise repetitive sequence sound considerably more interesting, especially if you mess around with the channel controlling the VCF - again, a technique more suited to analogue sequencing. A further option is to use a third CV channel to control the gain of a VCA, thereby giving you dynamic as well as tonal control.
The patches for these alternatives are given in Figures 3a and 3b. Note that in the latter diagram, the third CV channel is routed to a second VCA, as this should give you more control over the final output level than you'd have if the sequencer's CV output was mixed with that of the EG. This is because the EG's CV output will always be at the same level unless you program a higher voltage into the sequencer, and if you do this, you may well find the VCA 'overshoots', causing distortion. So, if you have a second VCA, it's best to use it since the sound will then respond to the slightest voltage change.
If you don't have a second VCA to play with, you may be able to get round the problem by setting the VCA's EG modulation level fairly low so that the sequencer's output determines the final level, though as I say, the degree of control you'll have probably won't be as great.
These patches should provide quite some variation over the simple note-playing sequencer-and-prewired-synthesiser array. If you don't have access to a modular synth but do possess a multi-channel sequencer, you should be able to do some of the things I've outlined if you can get hold of a couple of monophonic synthesisers along the lines of the Pro One, Minimoog, Prodigy, ARP Axxe or Odyssey, which have separate CV inputs for the VCF and, in the case of the Minimoog, a second VCA.
Feature by Steve Howell
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