3.2.1 Voltage control
One of the major innovations in the development of the synthesizer was voltage control. Instead of providing mechanical control over many parameters that are used to set the operation of a synthesizer, voltages are used. Since the component parts of the synthesizer produce audio signals which are also voltages, the same signals which are used for audio can also be used for control purposes.
'Mechanical control' here means human-operated switches and knobs.
One example is an oscillator used for tremolo or vibrato modulation when used at a frequency of a few tens of hertz, but the same oscillator becomes a sound source itself if the frequency is a few hundred hertz.
Controlling a synthesizer with voltages requires some way of manipulating the voltages themselves, and for this voltage-controlled amplifiers (VCAs) are used. These use a control voltage (also known as CV) to alter the gain of the amplifier and can be used to control the gain of audio signals or CVs. Using VCAs means that a synthesizer can provide a single common gain control element. Although not all analogue synthesizers contain the same elements, many of the parts are common, and the method of control is the same throughout. Voltage control requires two main parts: sources and destinations.
Voltage control sources include the following:
- Low-frequency oscillators (LFOs): These are required for vibrato, tremolo and other cyclic effects.
- Envelope generators (EGs): These produce multi-segment CVs, where the time and slope of each segment can be controlled independently.
- Pitch control: Typically provided by a pitch wheel or lever, which provides a CV where the amount of pitch-bend is proportional to the voltage.
- Keyboard control: The output from a music keyboard provides a CV where the pitch is proportional to the voltage.
- VCFs: These can self-oscillate and so provide control signals.
- VCOs: These can be used as part of frequency modulation (FM) or ring modulation sounds.
Voltage-controlled destinations include:
- LFOs, where the voltage is used to control the frequency or the waveshape.
- EGs, where the voltages can be used to control the time or slopes of each of the segments.
- VCFs, where the voltage is used to control the cut-off frequency of the filter and perhaps the Q or resonance of the filter.
- VCOs, where the voltage is used to control the frequency of the oscillator, or sometimes the shape or pulse width of the output waveform.
- Voltage-controlled pan, where the voltage is used to control the stereo positioning of the sound.
- VCAs, where the voltage is used to control the gain of the amplifier.
Each of these modules will be explained in more depth in this chapter.
3.2.2 Tape and models
Not all analogue synthesizers have to be voltage controlled. The use of tape manipulation and real physical instruments to synthesize sounds might be regarded as the ultimate in 'analogue' synthesis, since it is actually possible to interact with the actual sounds directly and continuously. Despite this, the word 'analogue' usually implies the use of electronic synthesizers.
The 'source and modifier' model is often applied to analogue synthesizers, where the VCOs are the source of the raw audio, and the VCF, VCA and ADSR (attack decay sustain release) envelopes form the modifiers. But the same model can be applied to sample and synthesis (S&S) synthesizers or even to physical modelling. Even real-world musical instruments tend to have a source (for a violin, you vibrate the string using the bow) and modifier structure (for a violin it is the resonance of the body that gives the final 'tone' of the sound).
The controls of the sound source and the modifier can be split into two parts: performance controls which are altered during the playing of the instrument and fixed parameter controls which tend to remain unchanged whilst the instrument is being played (Figure 3.2.3).
Because it came first, many of the terminology, models and metaphors of analogue synthesis are reused in the more recent digital methods. Although this serves to improve the familiarity for anyone who has used an analogue synthesizer, it does not help a more conventional musician who has never used anything other than a real instrument.
FIGURE 3.2.3 Performance controls are altered during the playing of the instrument, whilst fixed parameter controls normally remain unchanged.
Coming up in Part 2: Subtractive synthesis.
Printed with permission from Focal Press, a division of Elsevier. Copyright 2009. "Sound Synthesis and Sampling" by Martin Russ. For more information about this title and other similar books, please visit www.elsevierdirect.com.
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