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Synthesizer Design (Part 2)

Technical details from Jezz


Jezz Woodroffe flies off to Dallas but leaves the second part of his exclusive synth series behind...

Once again I'm writing my article in a hotel room. This time I'm waiting for a plane to Dallas, Texas, for the final rehearsals for the Robert Plant tour. The album 'Principle of Moments' is soaring up the American charts, the single 'Big Log' is in the British Top Twenty, and what with Top of the Pops last week things are getting a bit hectic.

Anyway, last month I tried to explain the basic outline of a synthesizer's main principles, and this month I shall go into more detail. Last time we broke the synth up into three main sections, VCO, VCF and VCA, which of course misses out some important pieces such as the Envelope Generator and the Low Frequency Oscillator. Really these are the five basic sections on the common synth.

VCO



Fig. 1 Typical VCO Waveshapes.

Last month I explained the function of the VCO, which is basically to act as a sound source at a particular pitch. Most VCOs can produce a variety of different waveforms, that is, their output current varies with time in different ways. The common waveforms are shown in Fig. 1, and it is with these waveforms or combinations of them that the original sound is created.

A sawtooth, for instance, can be used to create horn-type sounds and piano sounds. A square wave can create sharper sounds and most warm sounds, and sine waves produce very pure sounds like a whistle. Variable Pulse sounds are good for string chorus effects. Variable Pulse is not in fact a waveshape in its own right but a modified form of the Square wave, as we'll see later.

Sawtooth



The sawtooth wave is made up of many harmonics at a fixed ratio. Being rich in higher harmonics this waveform, when processed by the filter, changes sound very rapidly and dramatically; rich brass sounds and harsh punchy bass can easily be created.

Square Wave



Square waves contain only odd harmonics at a fixed ratio, in other words only half the number of harmonics of a sawtooth. So when you process a square wave through a filter the accuracy of sound changes is not as great as a sawtooth, but the relative lack of harmonics is ideal for creating hollow, woody effects.

Sine Waves



Sine waves contain hardly any harmonics at all, and their use as a sound source tends to be fairly limited. Very pure high whistling sounds can be quite effectively used though. On the other hand, sine waves are very commonly used for low frequency modulation as we'll see later.

Variable Pulse



A normal square wave is said to have a 50% pulse, or a 1:1 Mark/Space Ratio, because the time that the oscillator is sounding in each cycle is equal to the time that it is not. If that time is decreased, for instance to a thin pulse (25% or 1:3 Mark/Space Ratio) or to a very thin pulse (1 % or 1:99 Mark/Space Ratio) the sound becomes gradually more nasal and thin. The 50% pulse sounds much thicker, and twisting the Pulse Width control on some synths by hand produces an interesting sweeping effect.

The best thing about the variable width pulse wave is that the width can usually (but not always, for instance on the Moog Prodigy) be controlled by a slow oscillator. The regular sweeping produces a chorus effect which thickens up any synth square wave sound considerably and is ideal for string section or honky-tonk piano effects.

Still concentrating on that box on the synth labelled VCO, what else can be seen? Probably a range control knob for octave change, each position decreasing the voltage to the oscillators by one volt on a conventional synth. Octaves are still commonly marked in feet, which refers to the length of organ pipes — a 64 foot pipe sounding an octave deeper than a 32 foot pipe, which is in turn an octave deeper than a 16 foot pipe and so on. On a lot of modern microprocessor-controlled synths, such as the Jupiter 6/8 and the Prophet 600, octave switching is replaced by a continuous rotary control which gives only semitones rather than a smooth slide of notes. If you're lucky the manufacturers will have included a small neutral band to mark the divisions between octaves.

There'll also probably be manual or LFO control of Pulse Width and LFO control of pitch, which is very important indeed, so let's now look at what a Low Frequency Oscillator does (Fig. 2)

Low Frequency Oscillator



Fig. 2 LFO Modulation

We know what a VCO is, and in fact an LFO is a VCO except that it has the ability to oscillate so slowly that its output is no longer audible as a particular pitch, only as a clicking sound. But that doesn't matter because we don't want it to act as the source of a musical note, but of a slow regular variation of the setting of the VCO, VCF and VCA (modulation).

The LFO on most small synths produces a sine wave, though some also offer square wave or a sawtooth. Some synths produce a more regular sine wave than others, and in fact some are closer to Triangle waves with a straight rather than a curved edge. Although this difference isn't great enough to be obvious, it's enough to give the Moog vibrato, for instance, a distinctive quality. Vibrato speed is controlled by the LFO's frequency (pitch in fact, although as we've seen it isn't a pitch that's high enough to hear as a musical note) and vibrato depth (the distance the VCO's pitch is bent up and down) is controlled by the Mod or modulation depth control. Often this takes the form of a wheel to the left-hand side of the keyboard as on the Minimoog, Prodigy or Korg.

Square wave modulation to the VCO gives pitch changes in small steps not unlike the effect of playing from one note to another and back over and over again. Increase the modulation depth and you can get variation in pitch of well over an octave, and careful use of this can create effects very similar to a simple sequencer.

Sawtooth modulation is not very common on low budget synths; it gives you the effect of a note sliding up to a pitch and then dropping straight down to the original note.

Modulating The VCF



In the same way that the LFO affects pitch, it can be used to modulate the filter sound. Sine wave modulation produces repeated Wah type sounds and square wave modulation produces helicopter type sounds. The speed is very important and can be used effectively, again to create simple sequencer-like effects.

Modulating The VCA



This isn't possible on most synths, but does appear on the Jupiter 6 and 8. The most frequent use is to produce a tremolo, a rapid variation in volume, not to be confused with a vibrato which is a rapid variation in pitch. Rapid rise and fall of volume can produce very interesting effects, for instance on a piano-type sound you can get a Fender Rhodes/Wurlitzer effect, and on a flute sound you can get that 'shaky' movement so characteristic of wind instruments.

You can of course send LFO modulation to the VCO, VCF and VCA all at the same time if you want to; creating sounds which alter in pitch, tone, volume and pulse width at the same time, particularly if you have more than one LFO and can do these at different speeds, can be very interesting.

At this point I was going to dive into explaining that an Envelope Generator is not the dynamo on a postman's bike, but I've run out of time as my plane leaves in just over an hour. I've got my camera with me and my little Yamaha synth and breath controller are packed in my bag. See you next time!


Series

Read the next part in this series:
Synthesizer Design (Part 3)



Previous Article in this issue

Record Talkback

Next article in this issue

Analogue Echo


Electronic Soundmaker & Computer Music - Copyright: Cover Publications Ltd, Northern & Shell Ltd.

 

Electronic Soundmaker - Oct 1983

Donated & scanned by: Mike Gorman

Topic:

Synthesis & Sound Design


Series:

Synthesizer Design

Part 1 | Part 2 (Viewing) | Part 3


Feature by Jezz Woodroffe

Previous article in this issue:

> Record Talkback

Next article in this issue:

> Analogue Echo


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