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Modular Synthesis (Part 10)

Using Sequencers with Modular Systems

Steve Howell looks at different methods of triggering modular sequencers from click-tracks on tape.


As a foretaste to the start of our 'Everything But the Kitchen... series on musical instrument syncing, this month's column looks at running sequencers from a click-track. Steve Howell

Over the past two months we've been looking at the use of sequencers with a modular synthesiser. As I hope you've seen, the applications are considerably more varied than they would be using a smaller, 'off-the-shelf' synth and sequencer and I hope that some of the advantages of the simpler and more humble analogue sequencer have been made more apparent. Some of the techniques we've covered are just as useful today as they were when these sequencers were all you could get hold of, and I hope none of you have been tempted to think that, because it is not the very latest in technology, this hardware and the possibilities it offers are an anachronism in today's world of microcomposers and computer-based controllers. Indeed, that way of thinking is as Luddite and as peurile as the Musicians Union's attempted ban on synthesisers. There are still a lot of avenues of analogue and modular synthesis that have yet to be explored, and I'd like to think that some of the techniques we've looked at will inspire you to experiment because, let's face it, experimentation is what synthesis is all about.

Having said that, this month's column may also be of use to owners of every type of synthesiser, be it an SH101, a giant Moog Modular or even a more computer-based instrument, because the topic is syncing sequencers to tape. This has been something of a problem for many people in the past, but it's actually a fairly easy procedure that needn't cause too much of a headache.


As every schoolboy knows, before a sequencer can do a great deal it needs a trigger or a gate pulse, and normally this pulse is provided by the units own internal clock. It is possible, however, to override this internal clock so that pulses from an external unit can step through it. Figure 1 shows a patch in which a drum machine's clock output has replaced that of a sequencer so that it steps the sequencer through its pattern which, in turn, 'plays' the synthesiser.


What happens if you want to add more pre-programmed parts but only have the one sequencer? There are a few options, the first of which uses an audio output from the drum machine. Take a separate voice out from a spikey percussive sound (such as claves or rimshot), program it in on every beat and then feed that to a separate track on your tape machine so that on, shall we say, track 3 you have the percussive sound (the click track), on 4 and 5 you have the stereo mix of the drum machine and on 6 your first sequencer part (See Figure 2). Having recorded your basic track, you can rewind the tape, reprogram your sequencer and then route the output from track 3 to the step input of the sequencer so that when the tape starts, so will your sequencer, routed to, say, track 7. This can be done as many times as you have spare tracks available - well, that's the theory anyway. In practice you may well find that the output level of the percussive sound off-tape is not high enough to have any effect on your sequencer and must therefore be amplified in some way. This can be done by routing track 3 through a channel on the mixer and setting the gain very high so that the track is boosted to a level more suited to triggering. You can then route that channel to the input of the sequencer via either the auxiliary sends or the direct channel output. Should this fail, try EQing the channel to see if that affects its performance - you may well find that a boost in treble will be the answer.

If you're still experiencing difficulties, you could employ the following technique. In fairness, this method is more suited to owners of a modular system as it uses modules not normally found outside of them. If your synthesiser possesses a simple preamp (with a gain of anything up to x1000) for boosting external audio signals for processing through the instrument, you should try using that card, and in the event of this not being suitable, you could route the output of the preamp to an envelope follower if your system possesses such a thing. This will turn the audio signal into an envelope voltage that follows the 'shape' of the input signal, and the level of this voltage will probably be sufficient to step through your sequencer and should solve most of your problems. In fact, this is the method I've been using myself and it's given me no problems that are really worth mentioning.

Voltage Mixing



If your synthesiser doesn't possess either of these you could use voltage processors to achieve much the same result. You might, for instance, have a voltage multiplier that can be used in much the same way as the preamp. Another option is to use a voltage mixer patched up as in Figure 3. Here, a DC bias voltage is fed into one channel of the mixer, which is then mixed with the click-track on the other channel. The principle behind this patch is based on the theory that your sequencer requires an input of, say, four volts. Anything below that voltage will not be sufficient to trigger the sequencer, but as soon as the voltage rises above the four-volt threshold the sequencer will advance a step. So, if your sound on tape is only about 500mV or so, by setting the bias voltage to 3½Volts (or perhaps slightly more) the arrival of the 500mV from the click-track will push the output of the voltage mixer to over four volts and this will trigger your sequencer. To set this patch up, push the control handling the level of the bias voltage up so that it triggers the sequencer of its own accord, back it off slightly and run the tape - you should find that the sequencer will now run in time with the click-track successfully.


Many of these problems have been sorted out with the arrival of the excellent MPC Sync Track which is a godsend to would-be sync-to-tapers. This innocent box of tricks encodes a trigger signal onto tape and then decodes it into something the sequencer can understand. Its real beauty lies in the fact that it caters for the new breed of digital sequencers and drum machines that require 12, 24, 48 or 96 pulses for one event as well as a start/stop pulse, and as a result allows such equipment to be synced to tape, something none of the above techniques will do. However, if you're not using any of this newer equipment, the methods outlined above should enable you to multitrack sequences quite easily.

Clock Outputs



As I have said, there are a number of ways you can lay down a click-track, and another technique involves recording an actual clock output. There are a variety of sources for such a thing, such as the clock output of a drum machine or sequencer or, alternatively, the square wave output of a low frequency oscillator. To do this, you simply record the output of your clock or LFO onto tape as if it were an audio signal. You'll hear a series of clicks as the voltage swings up and down abruptly, and it's these audible clicks that trigger your sequencer. Figure 4 shows the signal going onto tape in relation to the signal coming off-tape (ie. the recorded version). Note that off-tape there is a spike as the square wave rises and falls. This means that the pulses off-tape will be twice as fast as the LFO output, so you'll have to compensate accordingly, either by running the LFO or clock at half the speed of the sequence when recording or by programming rests into the sequencer so that it runs at the right tempo.


Recording



The art of laying down a click-track is not a particularly complex one and I imagine that each of you will find a solution to the specific problems of your system. Top artists use various techniques to generate their click-tracks: Larry Fast used to use a good, old-fashioned metronome with a pickup on it to generate his, whilst Wendy Carlos generates hers manually using a spikey sound. There are no hard and fast rules, but then again, there are a few things you should bear in mind if you want to achieve the best results:

1 Try to avoid noise reduction systems as their action on a sound can confuse things: this is especially true of companding types.

2 Try to avoid recording the click on an edge track, as dropout is more likely on outside tracks than on internal ones. Also, keep your tape recorder heads clean if you want to avoid signal dropout generally.

3 Record the click-track at as high a level as possible, but be careful not to let it spill onto other tracks, otherwise you'll hear it breaking through the music. One way round this is to record the click-track and then rewind and clean the other tracks by recording over them.

4 Be careful when recording sequencer parts onto the track adjacent to the click-track, as crosstalk may be enough to spill onto the click-track which will, in turn, throw the sequencer out of sync.

If you bear these points in mind and spend some time experimenting with any (or all!) of the methods we've looked at for 'translating' the off-tape signal into something more suitable for the sequencer, you should succeed in syncing your sequencer(s) to tape. I've used most of these techniques in various different situations using a variety of tape machines including humble little Portastudios, Teac A3440s and 80-8s, with little or no problem, so don't fall into the trap of assuming that these methods will only work with top grade 16- or 24-track machines in mega recording studios.



Previous Article in this issue

Understanding the DX7

Next article in this issue

Computer Musician


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

 

Electronics & Music Maker - Oct 1984

Donated & scanned by: Stewart Lawler

Topic:

Sequencing

Syncronisation

Synthesis & Sound Design


Series:

Modular Synthesis

Part 1 | Part 3 | Part 10 (Viewing)


Feature by Steve Howell

Previous article in this issue:

> Understanding the DX7

Next article in this issue:

> Computer Musician


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