Making Music
  Computer Music Software and Hardware

Analogue essentials

As well as the VCO, VCF and VCA, there are several other core analogue synthesis modules.

LFO. The Low Frequency Oscillator is undoubtedly the most common additional module. Its low frequency oscillations can be applied in moderation to the VCO, VCF and VCA to add musical nuances such as vibrato and tremolo, or in excess to produce sirens and sounds that change tone over a long period of time. You can, in fact, build a very usable synth from just these four modules - VCO, VCF, VCA and LFO.

Sub-oscillator/Frequency divider. Sub-oscillators were popular in early hardwired synths. They added another  oscillator to the output, usually an octave or two below the original oscillator, to thicken the sound. With more modern multi-oscillator synths you can use a normal oscillator so dedicated sub-oscillators became less important.

Tech terms Vibrato A variation in pitch produced by applying an LFO to the VCO. Tremolo A variation in amplitude produce by applying an LFO to the VCA. Slur The generation of a new pitch without triggering the ADSR envelope. Frequency divider A module that divides the source frequency by 2, 4, 8 or some other amount. This can be used as a sub-oscillator or to generate rhythmic trigger pulses to control other modules. Subtractive synthesis A type of synthesis, like analogue synthesis, that starts with a harmonically-rich sound source such as a square wave, and then removes harmonics through filtering.

However, some modular synths have a Frequency Divider that can be used as a sub-oscillator but which have many more uses such as creating pulses to trigger other sounds, effects or a sequencer.

Ring modulator. This has two inputs and one output. The output is a combination of the sum and difference of the two input frequencies. Without doing any hard sums, this usually produces frequencies that are 'inbetween' the notes in our Western scale. Its most common use is to create metallic sounds.

Glide/Portamento/Slew limiter. Hardware synths have Glide or Portamento controls. Modular synths have a Slew Limiter. Their function is the same - to slide a note from one pitch to another, much the same effect as sliding your finger up a guitar string. Keyboard players cannot produce this effect naturally as each key produces a distinct semitone pitch. This module gives them slide-ability.

Sample & hold. As its name suggests, this samples the input signal and holds it until it's told to take another reading. Let's say you plug a sawtooth into the input and use a square wave for the trigger - when the square wave moves to the top of its cycle it triggers a new sample. Assume the square cycle is four times longer than the sawtooth cycle. Each time the S&H generator is triggered the sawtooth wave will be a little higher in its cycle so if you fed this into a VCO you'd get four rising pitches. Slow down the square wave and you'd get more pitches in the series.

If the two waveforms are not in sync or if you apply a random waveform such as noise to the input, then the output will be unpredictable. Plug this into a VCO for a random series of notes or apply to a filter for a random series of tone changes.

We've looked at the key principles behind analogue synthesis and some of the core modules used in analogue synthesisers. Not only will these help you understand and use any analogue synthesiser, they will also be useful in other forms of synthesis, too.

For more info... The very best primer on sound and synthesis is no longer available but absolutely no apologies for mentioning it. Beg, borrow or steal (er, try begging or borrowing first...) a copy of A Foundation for Electronic Music, published by Roland Corporation, one of a four-book set called The Synthesizer, designed for users of Roland's 100M modular analogue synthesiser. The first book of the set, A Foundation tor Electronic Music, was made available separately at the time and covers all the basic principles of sound and synthesis in detail. May we modestly suggest the Quick Guide to Analogue Synthesis? Sound Synthesis and Sampling (for USA readers) (for UK readers) is very technical but covers the whole gamut of synthesis types. Here's an excellent analogue synthesis resource: http://code404.com/faq/