A look at using a VCA for pre-programmed sequencer level control.

This month sees the final instalment of sequencer usage and makes use of a synth's VCA module for pre-programmed level control. To re-cap on a patch given in E&MM November, Figure 1 shows how the second CV output of a sequencer can be used to control the synth's volume, and uses a second VCA after the main 'shaping' VCA.

But what happens if you're a bit short on hardware and possess a sequencer that has only one CV output? Referring to Figure 2 should solve your problems. In this example, a sequencer has been laid down on Track 3 in the normal fashion with no dynamic control, and a click-track has been recorded on Track 5 to be used as a master clock. We now feed Track 3 into the VCA on the synthesiser, and by routing the click-track into the 'Step' input of the sequencer and then routing the sequencer's CV output into the control input of the VCA, you can program your dynamics into the sequencer. As the tape rolls and the click-track steps through the sequencer, the volume of each note on Track 3 will vary in accordance with the sequencer's voltage levels. The VCA's output can be fed into another track on the multitrack tape (the original can then be wiped to accommodate another part) or, alternatively, if you keep the click-track until mixdown you can feed the VCA's output direct to the mixer.

Any sound can be processed in this way: a snare voice from a drum machine, a polyphonic synth as discussed last month - the list is endless. It's also possible to create crescendo and diminuendo effects on sustained sounds such as strings.

By inserting a lag-time processor between the sequencer and the VCA, the stepped output can be smoothed out as in Figure 3. Level changes will now be gradual and can be used as an effective single-channel computer mix. In this instance, you don't necessarily have to sync up the sequencer to a click-track: its tempo can be adjusted to suit the speed of your swells and fades.

Dynamics

Another technique well worth employing involves injecting dynamics manually into a sequencer track. Figure 4 gives a patch which utilises the Envelope follower, the CV output of which is routed to the control input of a VCA. By connecting a microphone to the audio input of the envelope follower, you can vary the dynamics of the sequencer part simply by tapping the mic harder and softer. The sound source can come from a patch (as in Figure 1) or off-tape (as in Figure 2) and could therefore be a bass drum, snare drum, or whatever. Crescendo and diminuendo effects can also be achieved by 'AAAHing' into the mic at various levels.

Another method of varying levels is to use a switch to feed the preset bias voltage from a DC voltage source (such as a battery) which allows you to key in accents and dynamics as and when the mood takes you. Again, the sound source can be derived from just about anything, but the initial gain of the VCA must be set to allow sound to pass through when no bias voltage is applied. The greatest advantage of a modular synthesiser is that, because of the high signal levels used within the modules, devices can be used for purposes other than those originally intended for them by their designers. This is especially true of the VCA, as it can be used to control the level of a modulation source.

Figure 5 gives a patch for sequential control of vibrato. In this example, the sine or triangle wave output from an LFO is fed into the VCO(s) via VCA2, and the CV output of the second channel of your sequencer (or the CV output of a suitably synced second sequencer) is fed to the control input of VCA2. By setting the CV levels on the sequencer's second channel high, the LFO will be allowed to pass through the VCA2 and vice versa, so that pre-programmed amounts of vibrato can be introduced on specific notes during a sequence. Of course, a modulation source can be used with and routed to any voltage-controllable device. An envelope generator, for example, can be used for pitch-bend effects, a square wave for trills, and so on.

EG Triggering

Another possibility - along similar lines - employs a technique we looked at quite recently, whereby the sequencer's second channel is used to trigger an EG. By setting the CVs in the second channel above and below the EG's trigger input threshold level, you can trigger an envelope cycle as and where you wish. Figure 5 shows a patch which'll allow you to introduce a delayed vibrato effect on certain notes. The example given in Figure 6 allows you to switch vibrato in and out, but this technique enables the gradual introduction of vibrato to be carried out. Note that the output of EG3 is routed to VCA2 via an inverter. This is to turn the envelope voltage upside-down, as it's the delay/release portion of the cycle that we're going to use. By setting the attack to instant, the decay and release times are adjusted to the required length of the vibrato decay. However, you still have to split the output from the sequencer's CV channel and route that from VCA2, so that when the voltage is low (ie. 0V) no voltage will flow through it. Otherwise, when EG3 is not triggered you'll have permanent vibrato: not a very inspiring state of affairs. By doing this, the VCA will normally be 'closed', so that when the sequencer's output goes high to trigger EG3, the VCA will also be 'opened' - the inverted attack will then 'close' it and it'll be 'opened' gradually by the inverted decay/release portion of the cycle.

Well, I hope all this has given you some food for thought on the creative use of sequencers in the context of a modular synthesiser system. As I hope you can see, the possibilities are almost limitless, and I've only been able to outline them briefly. Suffice to say, experimentation with these patches should yield a whole host of sophisticated effects, and with any luck your music will be more interesting as a result.

As I've pointed out before, the advent of MIDI and computer control doesn't make these techniques obsolete in today's somewhat higher-tech atmosphere. If anything, incorporating these techniques alongside those of newer technology gives you the best of both worlds, and for those readers who simply cannot afford to consider the new-fangled devices currently on offer, these patches should expand your sonic vocabulary considerably without incurring any additional expenditure. It's quite encouraging to remember that many of these patches and the sound they create just can't be done on the most sophisticated digital systems such as the Fairlight and PPG Waveterm...

Well, that concludes our look at modular synthesis for the time being, as most of the less obscure applications have now been covered. Having said that, there are still many possibilities that have not been explored, and seeing as these are also applicable to other, 'conventional' synthesiser systems, the next few issues will see your intrepid reporter covering non-specific subjects such as improving lead-line sounds using vocoders, combining FM synthesis and analogue equipment, and so on.