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Advanced Music Synthesis

Oscillator Modulation

This music workshop is aimed at those people who want more out of their synthesiser systems than filter sweeps, assorted spacey sounds or attempts at impersonating acoustic instruments. That is not to say that this column is meant only for the eyes of those people who synthesise with slide rules — its intention is to stimulate musicians to understand their synthesisers more fully and use them more creatively.

It could be argued that Voltage Controlled Oscillators (VCOs) are the most important part of a synthesiser for without them there would be no sound — in the early days of electronic music oscillators were all they had to play with but nowadays, of course, these devices are far more sophisticated and allow a greater variety of sounds and textures to be obtained. The very fact that they are voltage controllable means that their pitch can be manipulated both automatically and manually. This manipulation, known as modulation, could be anything from simple vibrato to computer control. In between these two extremes, however, lie many exciting possibilities, some of which will be outlined here.

Figure 1. Basic patch for mixing two LFOs.

One of the simplest forms of modulation uses an oscillator to repeatedly vary the pitch of another VCO. In most commercially available synths this modulating oscillator usually operates in the low frequency range for vibrato and trill effects, filter sweeps and tremolo, but when the frequency of this oscillator is increased so that it is in the audio range, many interesting effects can be obtained as the modulated oscillators output has many of the modulating oscillator's tonal qualities imparted into it. I call this effect 'scrunging' because of the drastic tonal changes that can take place using this technique, especially if the modulating waveform is a pulse or square wave. I won't dwell on this particular use of modulation just now as it has already been covered in E&MM in the March '81 and '82 editions so for further information I would suggest you refer to that.

Let us return to modulation using Low Frequency Oscillators (LFOs). As I just mentioned, using an LFO for vibrato is common practice with nearly every synthesiser, but many more interesting effects can be obtained by mixing two (or more) LFOs together and applying them to the control voltage (CV) input of a VCO. Dependant on the relative levels of the waveforms many varied results can be obtained. Figures 1a and 1b give some examples of the various waveforms that can be produced with this technique. As you can see there are a lot of possibilities for special modulation effects to be had and the permutations (as with all synthesis) are endless.

Figure 1a. Mixing a sawtooth and inverted sawtooth waveform to produce staircase-type waveform.

Figure 1b. Mixing a triangle and square wave to produce a rising and falling trill effect.

One interesting effect in particular uses two square waves to give a sequencer or arpeggiator effect. By setting the depth of modulation of LFO1 so that the VCO is made to jump, say, an octave and the output of LFO2 is adjusted to give an interval jump of, say, a fifth, by varying the rate of the two LFOs many interesting rhythmic and melodic effects can be created. I hasten to add that you will have to spend some time experimenting to get the best results but it's worth it.

If the rate of your LFOs is voltage controllable one has even more possibilities available. One could, for example, route the output of an envelope generator to the CV input of an LFO that is set up to provide vibrato so that as the note dies away the vibrato will slow down. If tastefully used this can be an extremely useful method of modulation.

I think it's worth pointing out, though, that the validity of some of these effects in the strictly musical sense is questionable but they do serve as interesting exercises for you to become more familiar with your synthesiser system. Having said that, however, there are occasions when you might be required to produce an effect such as I have outlined above; by being totally familiar with your synthesiser this should be no problem. Also, effects such as these are useful for a tonal Eno-type music as a subtle rhythmic backing. As with all cyclic modulation effects, though, care must be taken not to make the results monotonous because of their inherent repetitiveness.

Figure 2. Basic patch for modulation of VCO by ADSR envelope generator.

Another form of modulation is by an envelope generator (EG). We are all familiar with the 'pew-pew' syndrum effect — this is simple modulation of a VCOs pitch with an ADSR-type EG whose attack is almost instant and whose decay/release portion is variable. This causes the pitch to rise immediately and fall as the sound dies away. EGs can be used far more usefully than that, however. Patch an EG to the CV input of a VCO as in Figure 2 and put the sustain level on the EG to maximum (for the time being). Use an attack time of about 500ms and a release of about the same time. Adjust the modulation depth control carefully so that when a note is played it will 'swoop' up an octave in accordance with the attack time and sustain at a constant pitch while a note is held. By using another VCO as in Figure 3 and tuning that VCO an octave up from VCO1's initial frequency, whenever a note is played VCO1 will fly up to meet VCO2 — this is very useful for brass and vocal sounds as well as many synthesiser effects. For those amongst you with fairly elaborate synths you might like to try the patches in Figures 4 and 5. In Figure 4, because the EG is connected to VCO3 via an inverter, that oscillator will move down to meet VCO2 while VCO1 will still 'fly' up to meet VCO2. In Figure 5 where the EG is connected via a lag time integrator (a device that introduces a portamentolike slur to any voltage fed into it) it is possible to have, effectively, different rise times for each VCO.

Figure 3. Bending one VCO against another with an ADSR.

Figure 4. Two VCO's bending against a third with +ve and -ve ADSR envelopes.

Figure 5. Producing different rise times with one ADSR.

You might like to try lowering the sustain control on the EG so that a note will rise and fall in accordance with the attack and decay times to a level set by the sustain control (you will have to adjust the modulation depth control to compensate for this drop in sustain level). Dependant on the length of the decay portion, different sounds can be obtained — with very short decay times various 'squags' and 'squeaks' can be introduced to a sound which can be particularly effective for blurting fanfare brass sounds.

Try also experimenting with various release times — many useful automatic pitch bend effects can be produced with long release times.

Finally, it is also possible to use a noise generator as the modulation source. This is handy for synthesising pipe organs and 'breathy' sounds. Try experimenting with different noise 'colours' for varied effects. If you want to hear good examples of this particular technique have a listen to Wendy Carlos — she uses it to great effect in the Brandenburg Concertos (Switched On Bach LP), and her latest contribution, the film music for 'Tron', has some amazing sound effects in it.

Well, I hope that has given you some ideas to play around with until next time when we will be dealing with the application of Sample and Hold devices. Until then... happy patching!

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


Electronics & Music Maker - Feb 1983

Feature by Steve Howell

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