Making sounds with analogue electronics - Part 5: Using analogue synthesis

[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.]

3.3.9 Using analogue synthesis
Learning how to make the best use of the available facilities provided by an analogue synthesizer requires time and effort. Although there are a number of 'standard' configurations of VCO, VCF, VCA and envelopes, the key to making the most of an analogue synthesizer is understanding how the separate parts work: both in isolation and in combination. If copies can be located, then Roland (1978, 1979) and De Furia (1986) are excellent references for further reading on this subject.

As a brief introduction to some of the techniques of using an analogue synthesizer, the remainder of this section shows how a subtractive analogue synthesizer can be an excellent learning tool for exploring some of the principles of audio and acoustics. Here are some of the demonstrations which can be carried out using a subtractive synthesizer.

Harmonic content of waveforms
The harmonic content of different waveshapes can be audibly demonstrated by using a low-pass VCF with high resonance (set just below self-oscillation) or a narrow band-pass filter. Each VCO waveform is connected to the filter input, and the filter cut-off frequency is slowly increased from zero to maximum (Figure 3.3.37).

FIGURE 3.3.37 By varying the cut-off frequency of a resonant low-pass filter, the harmonic content of a waveform can be heard. As each of the harmonics which are present in the spectrum pass through the peak of the filter, they will be clearly heard. The frequency of the harmonic can be determined by noting the frequency of the filter when the harmonic is heard.

As the resonant peak passes the fundamental, the filter output will be a sine wave at that frequency. As the cut-off frequency is increased further, the fundamental sine wave will disappear, and the next harmonic will be heard as the cut-off frequency matches the frequency of the harmonic. The audible result is a series of sine waves, whose frequency matches the frequencies of the harmonics.

If noise is passed through the filter, then the output will be sine waves whose frequencies will be within the pass-band of the resonant peak, and whose levels will change randomly. The audible result is rather like whistling.

Harmonic content of pulses
The harmonic content of different pulse widths of pulse waveforms can be demonstrated by listening to the pulse waveform and changing the pulse width manually (Figure 3.3.38). At a pulse width of 50%, the sound will be noticeably hollow in timbre: this is a square wave. The square wave position can be heard because the second harmonic, which is one octave above the fundamental, will disappear.

FIGURE 3.3.38 The harmonic content of a square wave and a rectangular wave is different, especially the even harmonics. The second harmonic is not present in a square wave and yet can be clearly heard in a rectangular waveform. This can be used to produce square waves from a VCO which provides control over the width of the pulse. By adjusting the pulse width control and listening for the disappearance of the second harmonic, a square wave can be produced.

Using the resonant filter technique described in the previous example, individual harmonics can be examined – tuning the filter to the harmonic which disappears for a square wave can be used to emphasize this effect. As the pulse width is reduced, the timbre will then become brighter and brighter, and with very small pulse widths, the sound may disappear entirely. (This is a consequence of the design of the VCO circuitry, and not an acoustic effect!) Conversely, increasing the pulse width from 50% produces the same changes in the timbre and, again at very large pulse widths, may result in the loss of the sound.

Filtering
Many resonance and ringing filter effects can be demonstrated by connecting a percussive envelope to a VCF CV input and turning up the resonance. Just below self-oscillation, the filter can be made to oscillate for a short time by using the envelope to trigger the oscillation (Figure 3.3.39).

FIGURE 3.3.39 If a strongly resonant filter is 'triggered' by a brief pulse of noise or an envelope pulse, then it can 'ring' producing a decaying oscillation at the cut-off or peak frequency.

This 'ringing oscillator' is the basis of the designs for many drum machine sounds in the 1970s (see Section 3.3.5 and Figure 3.3.7).

White noise filtered by a resonant low-pass filter changes from a hiss to a rumble as the cut-off frequency is reduced, because the filter is acting as a narrow bandwidth band-pass filter. With very narrow bandwidths, the noise then begins to produce a sense of pitch; and by connecting the keyboard voltage to the VCF so that it tracks the keyboard, these 'pitched noise' sounds can then be played with the keyboard. Keen experimenters might like to compare this with an alternative approach with audibly similar results: modulating the frequency of a VCO with noise.

Beats
Beats occur when two VCOs or audio signals are detuned relative to each other. The interference between the two signals produces a cyclic variation in the overall level as they combine or cancel each other out repeatedly (Figure 3.3.40). The time between the cancellations is related to the difference in frequency between the two audio signals or VCOs. Using two VCOs with a beat frequency of 1 Hz or less produces a 'lively', 'rich' and interesting sound.

FIGURE 3.3.40 Beats can be demonstrated by mixing together the outputs of two VCOs which have slightly different frequencies. The two waveforms will cyclically add together or subtract, and so produce an output that varies in level. The audible effect is an interesting 'chorus' type of sound for frequency differences of less than 2 Hz and vibrato for 2–20 Hz.

PWM uses an LFO to cyclically change the width of a pulse waveform from a single VCO. The result has many of the audible characteristics of two VCOs beating together.

Vibrato versus tremolo

• Vibrato is FM: The frequency of the audio signal is changed. Using an LFO to modulate the frequency of a VCO produces vibrato.
• Tremolo is AM: The level of the audio signal is changed. Using an LFO to modulate the level of an audio signal using a VCA produces tremolo.

Modulation summary and the cyclic variations of vibrato and tremolo are shown in Table 3.3.2 and Figure 3.3.41, respectively.