Making sounds with analogue electronics - Part 6: Other methods of analogue synthesis

Formant synthesis is intended to emulate the strong resonant structure of many real instruments, where the spectrum of the output sound is dominated by one or more formants. Some analogue synthesizers have a simple high-pass filter after the low-pass filter to give some additional control over the bandwidth of sounds, and thus a simple type of formant.

In a formant synthesizer, this extra filtering is extended further: a graphic equalizer or complex filter is used to provide control over the bandwidth of the sound in addition to a VCF and VCA. Several parallel sections may be used to enable more detailed control over the individual formant areas of the sound (Figure 3.5.6).

FIGURE 3.5.6 A formant synth is intended to emulate the resonance found in real instruments. This can be achieved by using formant filters in addition to VCFs and VCAs.

3.5.5 Damped oscillators and ringing filters (drum sounds)
Circuits that have a strong resonance at a specific frequency can be made to oscillate if a sudden input causes them to self-oscillate. This 'ringing' is usually a sine wave and it dies away at a rate which is dependent on how close to self-oscillation the circuit is. The nearer it is to oscillating, the longer the ringing will last.

Some VCFs can be made to self-oscillate if their Q or resonance is high enough, and at Q values just below this, they will ring. Conversely, an oscillator can be 'damped' so that it does not self-oscillate, but it will then ring. Filters and oscillators are just different applications of resonant circuits.

Decaying sine waves are very useful for producing percussive sounds, and many of the drum sounds produced by rhythm machines in the 1970s and early 1980s were produced by using ringing circuits (Figure 3.5.7).

FIGURE 3.5.7 (i) A resonant circuit can produce some ringing when a trigger pulse is applied. (ii) When a resonant circuit is placed in the feedback loop of an amplifier with a gain of less than one, then the ringing of the resonant circuit is enhanced. (iii) If the gain of the amplifier is greater than one, then the circuit will oscillate at the frequency of the least attenuation in the resonant circuit.

3.5.6 Organ technologies
Most traditional organs are based around additive synthesis techniques, where a large number of sine waves are produced from a master oscillator, and then individual notes select mixes of sine waves through drawbar or other controls for the harmonic content (Figure 3.5.8).

FIGURE 3.5.8 Organs typically produce sounds by the addition of sine waves. The methods of producing the sine waves can be mechanical, electromechanical and electronic.

Unlike additive synthesizers, until the middle of the 1980s, organs tended not to have envelope control over the individual harmonics which make up the sounds. The advent of digital technology and sampling has made organs much more closely related to sample and synthesis synthesizers. Chapter 3 gives further details of digital master oscillators, whilst Chapter 4 describes sample and synthesis in more detail.

3.5.7 Piano technologies
Before digital sampling technology, piano-type sounds were produced by taking square or rectangular waveforms, often derived from a master oscillator by a divider technique, and then applying a percussive envelope and filtering. This produces a completely polyphonic instrument, although the sound suffers from the same lack of dynamic individual harmonic control as organs of the same time period.

By using narrow pulse waveforms and different envelopes, the same techniques can be used to produce string-like sounds, and this was used in many 1970s 'string machines'. Section 4.5.3 describes 'beehive noise', a side effect of this sound generation technique.

By the mid-1980s, separate 'stand-alone' dedicated string machines had been replaced by polyphonic synthesizers, with the typical electronic piano becoming a specialized sample-replay device by the end of the 1980s (Figure 3.5.9).

FIGURE 3.5.9 Simple 'piano' and 'string' type sounds can be produced by gating and filtering pulse waveforms which are derived from a master oscillator.