Oscillators

The machines all built so far were analog synthesisers and the technique they used to create there sounds became known as analog synthesis. We will now look into this important method of sound creation in a little more detail as it contains the foundations of sound construction and creation which is what synthesisers are all about.

When we hear sound we are hearing sound waves vibrate, if we are listening to speech or music from a TV sound system the air is being made to vibrate from loud speakers or if it’s a natural sound source striking a bell will cause the metal of the bell to vibrate and thus cause the air to vibrate around it or plucking a guitar string will cause the string to vibrate and again cause the air around that to vibrate and so we hear the sound.

In an analog synthesiser one of the main sound creators is the oscillator. This produces a continuous tone, the frequency can be altered and as the frequency increases the sound becomes higher, oscillators in analog synthesisers can produce frequencies over the entire audio range. The frequency range is measured in something you’ve probably heard before call hertz, you can hear down to a few hertz as a clicking sound. A click every second is 1 hertz. As you increase the frequency up to about 50 hertz you hear a deep hum, in fact you may of heard of mains hum which is interference on audio systems were the mains supply, which is a 50 hertz oscillation of power, gets picked up by the audio circuits and manifests itself as an unwanted audio buzz!

As you increase the frequency the tone gets higher and higher until it starts to go out of the audio range for human hearing, which depending on the individual is usually between about 16 thousand and 20 thousand hertz, which is usually abbreviated to 16 kHz to 20 kHz.

Oscillators first produced something that was known as a sine wave. It is a very pure sounding wave form so if you took a basic oscillator producing a sine wave output, which would be a continuous tone, meaning it just constantly sounded, and put it into an amplifier you could use the volume control to alter the loudness of the sound and use the frequency control to control the pitch. You would have a very basic electronic instrument, which is in fact what Leo Theremin did.

He made a sine wave oscillator and an amplifier and made the pitch of the oscillator alterable by moving your hand closer to a metal rod or aerial. As you moved your hand closer to it the pitch got higher and similarly you moved your hand further away to make the pitch lower. He did the same with another metal rod doing the same controlling trick with the volume. As you moved your hand closer to the volume aerial the sound got louder. You may have seen this instrument played it was known as a Theremin, named after Leo of course and used for the original Star Trek TV series theme music. You know the bit the main tune with its gliding about melody! They are quite difficult to play but you can still get them in various forms today incidentally.

The sine wave isn’t the most sonically interesting of wave forms so others were soon added to the oscillators output. A sine wave contains no harmonics, that’s why it sounds so pure. Other wave forms contain harmonics which are other oscillations at higher frequencies to the original pitch which is known as the fundamental frequency.

A square wave contains odd harmonics, a triangle wave also only contains odd harmonics but the higher harmonics roll off much quicker than a square wave. The sawtooth wave contains both odd and even harmonics that’s why it sounds so rich and works so well with a filter as there’s more there to filter out to begin with.

A pulse wave can also be a square wave if it’s up time and down time are the same them it would be a 50% – 50% pulse wave but it can be altered to for instance a 20% – 80% wave form where it’s up and down times vary obvious depending on how it is set. An oscillator usually has a sweep control for pulse width as it is known, to vary it over its entire range. If you ever had the chance to use an oscilloscope in you school physics lessons you are probably familiar with or have heard of these waveforms from oscillators. They look pretty much as the name suggests they should.

Voltage Control