Last month we took a look at understanding the synthesiser and the way in which sound is produced. The next technique involved in Electronic Music is to understand the modules of the synthesiser, being able to know exactly what sound you are trying to obtain and what modules are used to create various sound effects.

The terminology used on synthesisers today does vary from manufacturer to manufacturer, from old design synths to new and from analogue to digital. I shall try to explain all.

Voltage Controlled Oscillator (VCO)

The VCO or Signal Generator (SG) commonly found on most music synthesisers consists of a exponential amplifier, a resettable integrator, and waveshaping circuitry. 'Voltage Controlled Oscillator' means an oscillator whose operating frequency or pitch can be controlled by an external control voltage.

As seen previously the VCO is responsible for the pitch and the basic timbre. On most VCO modules you will find a waveform selector that selects tone colours (sine, triangle, sawtooth, square and pulse wave). If within the waveform selector there is a facility of Pulse Width Modulation (PWM) then there will be a control which adjusts the intensity of PWM (ie. the amount of variation in the pulse width from a square wave to a narrow pulse wave). Some synthesisers also combine a square wave and pulse wave together with a separate control voltage adjusting the pulse width. The VCO will also contain an octave selector which in most cases is marked in 'feet', akin to the footages of an organ.

The graphic notation of a VCO is shown in Figure 1(a).

Voltage Controlled Filter (VCF)

The VCF module receives an audio signal from the VCO and also accepts CV signals from other modules. The Voltage Controlled Filter is a filter whose cut-off frequency can be controlled by an external control voltage. The filter processes the waveform from the VCOs and Noise Generator. If on your synthesiser there is only one filter, in most cases it will be Low-Pass, ie. the harmonic elements of the waveform which are higher than the cut-off will be removed, producing a rounder tone. Remember a filter takes away harmonics. The VCF module will contain a cut-off frequency (Fc) control, and usually a 'resonance' control which emphasises harmonics at the cut-off point. On some synthesisers there is a 'keyboard follower' control which adjusts the Fc over the scale of the keyboard, preserving the same timbre for different pitches.

There are various types of filters available as you go further into sound synthesis. A Band-Pass filter allows one frequency band to pass, a Band Reject Filter or Notch stops a particular band passing. High-Pass filters allow high frequencies through and cut out low frequencies. Low-Pass filters cut out high frequencies and allow low frequencies to pervade. This can be shown in graphic notation (see Figure 1(b)).

Voltage Controlled Amplifier (VCA)

The VCA determines the volume of signal passing through it in accordance with the total control voltage from the envelope generator (EG) and from any external controller connected. A Voltage Controlled Amplifier is an amplifier whose gain can be controlled by an external control voltage. The VCA module might contain a gain control which allows any signal passing through the other modules to reach the output of the synthesiser, the other control will be a selector switch which tells the VCA which type of envelope is controlling it. On some synthesisers you will find extra facilities, including a VCA CV 'Intensity' which adjusts the VCA's input sensitivity to the control signal being used and an 'Initial Level' Control which allows the sound through when the EG is 'off'.

Envelope Generator (EG)

The envelope shaper consists of a combination of VCA and EG. The VCA varies the volume of the sound in accordance with the envelope signal. The envelope signal is initiated by a gate signal whenever a key is depressed. As in the VCF there are different forms that the envelope can take. The most typical are ADSR and AR. A = Attack, D = Decay, S = Sustain, R = Release. This may seem complicated but if we remember what we said regarding sound and its change in volume over time you will obtain a clearer picture.

In an ADSR envelope module you will find four controls. The attack time adjusts the rate of rise of the envelope to the peak value. The decay time controls the time for the volume to fall from its peak to its sustain level. The sustain level adjusts the level at which the volume will be sustained from the end of the decay time until the end of the gate signal (when the key is released) and the release time will denote how long the sound will take to fade away after the end of the gate signal. If your synthesiser has a hold switch or control this will extend the trigger signal by a variable specified amount of time.

When you find a synthesiser with more than one EG, the second will most probably be an AR or AD envelope. This envelope is used mainly for VCF control.

Understand the effects you are trying to obtain when using two EGs: the ADSR will shape your sound and AR will then control what happens within that sound by introducing another modifier.

Low Frequency Oscillator (LFO)

The LFO module allows a control voltage in the shape of a waveform to effect and change the characteristics of the audio signal. It is basically an oscillator which is designed to generate a cyclic voltage signal at subaudio frequencies. The oscillator can normally produce many waveshapes that can be used specifically for modulation purposes. On most synthesisers the LFO section is situated near or with the particular module that will be affected. The LFO module will contain a waveform selector and a frequency control. Whether you will be modulating the VCO, VCF or amplitude these controls will be the basic system, there will also be a control which allows the intensity of the effect to be altered. Modulation of the VCO will allow vibrato, trills and siren type effects and when modulating the VCF you will obtain cut-off frequency modulation (FcM), for growling sounds.

There are synthesisers on the market which include an additional VCA: this is usually controlled by the LFO but will have a specific control of the intensity of amplitude modulation. The extra VCA is a very useful addition to the synthesiser, allowing effects from tremolo through to ring modulation.

If your synthesiser has two or more LFO's then the capability is greatly enhanced. In some cases the LFO can be directed through to the filters allowing timbre modulation of the audio signal independent of other modulations.

There are of course many more modifiers in a synthesiser. In synthesising any sound you use two types of voltage. An audio signal voltage is the first type which provides the fundamental base for the sound that you will hear. Whether it be a square wave or sine wave, the audio signal is the raw material. And secondly, when creating synthesised sounds you will be using control voltages. Control voltages are not heard although you will hear their effect on the audio signal, eg. vibrato, trills etc.

Most synthesisers work in the same way to produce sounds. To delve further into electronic music and sound synthesis you must be aware of what your synthesiser is capable of, and this is a technique in itself.