Making Music
  Computer Music Software and Hardware

How's it done?

The analogue synthesis approach to creating sound is pretty simple. It uses three main building blocks - tone generation > tone shaping > volume shaping. In other words, you take a waveform produced by an oscillator, use a filter to change the tone, and shape the volume with an envelope. We've covered all three stages in other Quick Guides so click on the links if you want more information.

Linking modules together is fine but you need a way to control them and to play specific pitches rather than having to twiddle a dial every time you want a new sound. The big breakthrough came with Moog's development of voltage control. This revolutionised analogue synthesisers and lead to the development of modules such as the VCO (Voltage Controlled Oscillator), VCF (Voltage Controlled Filter) and VCA (Voltage Controlled Amplifier).

The pitch of a VCO is determined by voltages produced by the keys on a keyboard. The cutoff frequency of a VCF could be controlled by voltages from other modules and the ADSR envelope generated voltages to control the VCA. The Control Voltage method allowed the output of any module to be patched to the input of any other - an incredibly powerful and versatile system.

If there was a downside it was one of standardisation and this had nothing to do with the CV principle but the way it was implemented. In true music industry style, different manufacturers used different voltage systems so it was difficult to connect one company's synthesiser to another's but eventually, a 1 Volt per octave system dominated.

Gates and triggers

To control pitch, each key on a keyboard generates a voltage. The higher the key, the higher the voltage and the higher the pitch.

In addition, a synth keyboard generates two other signals - a gate pulse and a trigger signal. The gate pulse lasts for as long as a key is held down, telling the synth when a key is pressed and when it's released. This controls the envelope generator - the Attack phase begins when a key is pressed and the gate pulse starts, and the Release phase begins when the key is released and the gate pulse ends.

Some instruments, particularly modular systems, also have a trigger signal. This is simply an on/off signal, a short blip, that's generated as soon as a key is pressed, even if one is already being held down. It can be sent to the VCO to generate a new pitch but without triggering a new envelope, allowing the player to create a slur.

Gates and triggers are not normally a feature on hardwired instruments as their functions are incorporated into the design.

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