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Synthesizer Design (Part 1)

Jezz Woodroffe

Article from Electronic Soundmaker & Computer Music, September 1983

Jezz Woodroffe explains

We asked Jezz Woodroffe, ex-Black Sabbath and now keyboard wizard alongside Robert Plant, for a few hints on synth design — he came up with a series.

May I firstly say how pleased I am to be able to contribute to this magazine, which I believe should help many budding electronic musicians on their way. I feel that there is not nearly enough space allocated to people interested in electronic music in most of the papers and books available today, and I'm glad to see something being done to put this right.

As some of you will already know, my main job is playing keyboards in the Robert Plant band, which takes up most of my time. In fact I'm writing this article in my hotel room having just finished another day's hectic rehearsals, getting the band ready for an American tour. Robert's second solo album "The Principle of Moments" is just being released: if anyone who has heard either that album or "Pictures at Eleven" would like to know how I achieved some of the keyboard sounds, just drop me a line and, time permitting, I will write back and explain how it was done.

One of the tracks on the new album has no fewer than 16 layers of keyboards on it, so you can imagine that the rehearsal necessary to reproduce the same sound live is a pretty demanding task. Bob Mayo, the keyboard player from Foreigner, has been drafted in to help me out, so with four Roland Jupiter 8's between us it doesn't sound too bad!

Apart from working with Robert I also make soundtracks for nature films on independent television and for the occasional commercial, but only if I find the visual image stimulating enough to act as a creative incentive. I find moving pictures a great medium to put sounds to; rather than produce a definitive piece of constructed music I prefer to make sounds that capture the image. When I was commissioned to produce the soundtrack for "Wonders of the Underwater World" I set my keyboards up directly underneath the film screen, played whatever came into my head at the time and recorded it immediately. At a later date I added whatever overdubs I thought were needed, the whole thing being a very satisfying experience.

But having met and spoken to many people who own synthesizers or have access to one, I've been amazed at the number who have little idea of why all those knobs and sliders and words are stuck on the top of their keyboards. And that takes us back to why I'm writing for this magazine. I believe that you ought to know the purpose of every single one, and though on the surface it can look very complicated, when broken down into sections it's not really that bad. Over a period I will attempt to explain how everything works and why it is there, but if your brain is anything like mine it will only take in so much and then start to wander on to something else; so I am going to try to keep this as simple as possible.

Getting Keyed Up

The most important thing of all is to grasp the main principle of exactly what a synthesizer is and what it does. A synthesizer is not just a musical instrument, in fact making music is just one of its many functions. Basically what we have is a machine which can produce a variety of pitches, tones and volumes. In synthesizer language these are produced respectively by a Voltage Controlled Oscillator, Voltage Controlled Filter and Voltage Controlled Amplifier. This is a very basic way of looking at things, but very helpful indeed if you want to get the best out of your machine. Most synths have some kind of block diagram printed on the control panels, so before going into detail on the individual controls it is necessary to understand the overall job of each section or block.


The best way to understand the VCO's function is to imagine that you're holding a tuning fork in your hand. A normal tuning fork vibrates, when it's struck, at 440 times per second to produce a concert pitch 'A'. If you could increase its speed to 880 cycles per second this would increase the pitch by exactly one octave, or if you could reduce it to 220 cycles this would decrease the pitch by one octave. If you could do this electronically by varying a voltage you would have a form of voltage controlled oscillator! In practice there are no moving parts in a VCO, the sound being produced by a "vibrating" or regularly varying voltage in a circuit which is eventually fed to a speaker and transformed into sound waves.

The keyboard on a synthesizer is normally designed to produce a voltage which increases by one volt for every octave you play up the keyboard, so the pitch of the oscillator increases in accordance with a musical scale. Although EMS synths used to work to a ⅓V/octave scale and Korg synths to a curving Logarithmic scale, the 1V/octave synthesizer is by far the most common nowadays.

Filters And Reservoirs

First of all it is important to understand the difference between pitch and sound. A steam engine for instance makes sounds, but not at any specific pitch or note. We already know that pitch is taken care of by the VCO, but the quality of a sound is taken care of by the filter, so what does it do and how does it work?

I find the best way to explain the action of a filter is to imagine a reservoir and a dam. The water behind the dam represents every different type and pitch of sound you could possibly imagine, and the dam itself represents the filter. As the sluice gates open, more and more of the possible sounds can get out. The greater the opening, the greater the sound; it contains a greater mixture of pitches and will generally be louder.

The most important control on a filter is the Frequency control, which sets the point below which sounds can get past. If every possible pitch was being produced and allowed to get past you would get a rushing wind sound known as "White Noise". If simpler more specific sounds are being produced, a filter allows you very accurate control over their exact nature and tone. The other main control on a filter is the resonance control. This produces increased volume just below the point where the filter cuts off, and so can give the impression that one particular pitch is being favoured by the filter.

Potential Amplifiers

Basically all the VCA does is to control loudness. On any guitar amplifier or hi-fi amplifier there is a volume control knob: by turning this control you increase the AC voltage used and increase the volume correspondingly. To obtain accurate control of volume for a synth you also need an Envelope Generator which controls, for each note played, the speed at which the volume builds up, the length of time a certain volume remains, the length of time it takes to die away and so on. But to go into that any deeper we need to apply a little more brain power, and I think that for this first attempt to explain things we've gone far enough.

So to round things up, a synthesizer is a Pitch, Sound and Volume machine, looking at it in the most simple way possible. I haven't mentioned square waves, low frequency oscillators, pulse width modulation, reversed envelopes and many other things, but they'll come in the future. If anyone has not grasped what I've been saying, try reading this article again possibly with a synth in front of you. In the meantime, if anyone feels they need to do some homework, may I suggest listening to the new Yazoo album "You and Me Both" on Mute: there are some very interesting sounds there. And it's back to the rehearsals tomorrow for me!

Series - "Synthesizer Design"

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

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Electronic Soundmaker - Sep 1983

Donated & scanned by: Mike Gorman


Synthesis & Sound Design


Jezz Woodroffe


Keyboard Player


Synthesizer Design

Part 1 (Viewing) | Part 2 | Part 3

Feature by Jezz Woodroffe

Previous article in this issue:

> A Selective Survey of Lead L...

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> Innovators

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