Making sounds with analogue electronics - Part 7: Topology of a typical synthesizer

[Part 1 briefly reviews the differences between analogue and digital synthesis, and discusses "one of the major innovations in the development of the synthesizer" - voltage control. Part 2 begins a look at subtractive synthesis with a discussion of VCOs, waveforms, harmonic content, and filters. Part 3 discusses envelopes - the overall 'shape' of the volume of a sound, plotted against time. Part 4 looks at amplifiers as well as other modifiers, including LFOs, envelope followers, waveshapers, and modulation. Part 5 shows how a subtractive analogue synthesizer can be a learning tool for exploring some of the principles of audio and acoustics, and then looks at additive synthesis. Part 6 considers other methods of analogue synthesis, including amplitude modulation, frequency modulation, formant synthesis, ring modulation, ringing circuits and other technologies.]

3.6 Topology
How do the component parts of a synthesizer fit together? This section starts by looking at typical arrangements of VCOs, VCFs, VCAs and EGs. It then looks at categorizing types of synthesizers: the main divisions in type are between monophonic and polyphonic synthesizers, performance and modular synthesizers, and alternative controllers.

This section deals with the topology of the modules that make up a typical synthesizer – how they are arranged and ordered. Although this information is fundamental to the actual construction of analogue synthesizers, the theory behind it is also relevant to some digital instruments, even though digital synthesizers often have no physical realization of the separate modules at all.

3.6.1 Typical arrangements
The most common arrangement of analogue synthesizer modules is based on the 'source and modifier' or 'excitation and filter' model. This uses one or more VCOs plus a noise generator as the sources of the raw timbre. It then uses a VCF and VCA controlled by one or more EGs to shape and refine the final timbre. An LFO is used to provide cyclic modulation: usually of the VCO pitch (Figure 3.6.1).

FIGURE 3.6.1 The basic synthesizer patch uses one or more VCOs and a noise generator as the sound source, with an LFO to provide vibrato modulation. The modifier section comprises a VCF and a VCA, both controlled by one or more EGs.

This basic arrangement of modules is used so often by manufacturers that it has become permanently hard-wired into many designs, even some modular systems! The use of 'normalized' jack sockets allows for this type of preset wiring where the insertion of a plug into the socket opens the switch and removes the hard-wiring and thus allowing it to be overridden and replaced. 'Hard-wiring' is also used in many digital designs where there is no need for a rigid arrangement of modules because they are implemented in software.

One alternative method of subtractive synthesis replaces the single VCF with several. This enables more specific control of portions of the sound spectrum and is often associated with the use of band-pass rather than low-pass filters.

Because having separate filters for the oscillators enables them to be used as components of the final sound, rather than as a single source processed through a single modifier, this paralleling of facilities can be much more flexible in its creative possibilities. It is often used in formant synthesis, where the aim is to emulate the peaks in frequency response which characterize many real-world instruments, and particularly the human voice. Additive synthesis is an extension of this formant synthesis technique, where additional VCOs, VCFs and VCAs are added as required. Ganging of EGs by using voltage control of the EG parameters can make the control easier.

By using one VCO to modulate another, FM synthesis can be used, although the limitations of the VCO tuning stability and scaling accuracy limit its use. By using VCFs to process the outputs of each VCO, the FM can be dynamically changed from using sine waves to using more complex waveshapes by increasing the cut-off frequency of the VCF on the output of the modulation VCO. This is something which most commercial digital FM synthesizers cannot do!

The basic synthesizer patch varies between monophonic and polyphonic synthesizers. It is often simplified for use in polyphonic synthesizers: only one VCO and VCA, and often less controllable parameters. Custom 'synth-ona- chip' ICs are often used to implement polyphonic synthesizer designs, and these chips are based on a minimalist approach to the provision of modules and parameters.