[Part 1 briefly reviews the differences between analogue and digital synthesis, and discusses voltage control - "one of the major innovations in the development of the synthesizer." 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. Part 6 considers other methods of analogue synthesis. Part 7 deals with the topology of the modules that make up a typical synthesizer and then looks at categorizing types of synthesizers.]
3.7 Early versus modern implementations
Electronics is always changing. Components, circuits, design techniques, standards and production processes may become obsolete over time. This means that the design and construction of electronic equipment will continuously change as these new criteria are met.
The continuing trend seems to be for smaller packaging, lower power, higher performance and lower cost but at the price of increasing complexity, embedded software, difficulty of repair and rapid obsolescence. Over the last 25 years, the basic technology has changed from valves and transistors towards microprocessors and custom ICs.
3.7.1 Tuning and stability
The analogue synthesizers of the late 1960s and early 1970s are infamous for their tuning problems. But then so are many acoustic instruments!
In fact, it was only the very earliest synthesizers that had major tuning problems. The first Moog VCOs were relatively simple circuits built at the limits of the available knowledge and technology – no one had ever built analogue synthesizers before. The designs were thus refined prototypes which had not been subjected to the rigorous trials of extended serious musical use.
It is worth noting that the process of converting laboratory prototypes into rugged, 'road-worthy' equipment is still very difficult; and at the time, valve amplifiers and electromechanical devices such as tape echo machines were the dominant technology. Modular synthesizers were the first 'all-electronic' devices to become musical instruments that actually left the laboratory.
The oscillators in early synthesizers were affected by temperature changes because they used diodes or transistors to generate the required exponential control law, and these change their characteristics with temperature (diodes or transistors can be used as temperature sensors!). Once the problem was identified, it was quickly realized that there was a need for temperature compensation. A special temperature compensation resistor called a 'Q81' was frequently used – they have a negative temperature coefficient which exactly matches the positive temperature coefficient of the transistor.
Eventually circuit designers devised methods of providing temperature compensation, which did not require esoteric resistors, usually based around differential pairs of matched transistors. Developments of these principles into custom synthesizer chips have effectively removed the need for additional temperature compensation.
Unfortunately, the tuning problems had created a characteristic sound, which is one reason why the 'beating oscillator' sounds heard on vintage analogue synthesizers are emulated in fully digital instruments that have an excellent temperature stability.
Tuning problems fall into four categories:
- overall tuning
- high-frequency tracking
Because of the differences in the response of components to temperature, the tuning of an analogue synthesizer can change as it warms up to the operating temperature. This can be compensated manually by adjusting the frequency CV or automatically using an 'auto-tune' circuit (see later). Some synthesizers used temperature-controlled chips to try and provide elevated but constant temperature conditions for the most critical components: usually the transistors or diodes in the exponential converter circuits. These 'ovens' have been largely replaced in modern designs by careful compensation for temperature changes.
Tuning polyphonic synthesizers requires patience and an understanding of the way that key assignment works (see Section 6.5.3). The tuner needs to know which VCO is making the sound (sometimes indicated by a light emitting diode (LED) or by a custom circuit addon), as well as how to cycle through the remaining VCOs – often by holding one note down with a weight or a little wedge and then pressing and holding additional notes.