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

3.5 OTHER METHODS OF ANALOGUE SYNTHESIS

3.5.1 Amplitude modulation
AM is a variation on one method used to transmit radio broadcasts. AM radio works by using a high-frequency signal as the 'carrier' of the audio signal as a radio wave. The carrier signal on its own conveys no information – it is the modulation of the carrier by the audio signal that provides the information by changing the level of the carrier.

In the simplest case, a sine wave audio signal is used to change (or modulate) the level of the carrier signal. The resulting output signal contains not only the original carrier frequency but also the sum and the difference of the carrier and audio frequencies; these are called sidebands, because they are on either side of the carrier.

For audio AM, the two frequencies are both in the audio range, but the same principles apply – the output consists of the carrier frequency, and the sum of the two frequencies and difference between the two frequencies (Figure 3.5.1). So with a carrier of 1000 Hz and a modulator of 750 Hz, the output sideband frequencies will be 1000, 1750 and 250 Hz. Note that the modulating frequency is not present in the output. For 100% modulation, the sidebands have half the amplitude of the carrier.

FIGURE 3.5.1 AM with two sine waves produces outputs at the sum and difference of the two input frequencies.

For AM with waveforms other than sine waves, each component frequency is treated separately. So for a sine carrier and a non-sinusoidal wave modulator, there are actually the equivalent of several modulator frequencies: one for each harmonic in the modulator. For a sawtooth modulator wave, this means that there will be integer multiples of the modulator frequency at decreasing levels. Each of these harmonics will produce sidebands around the carrier. The carrier frequency of 1000 Hz will also be present in the output. Again, with 100% modulation, the sidebands will have half the amplitude of the carrier.

With a non-sinusoidal carrier of 1000 Hz and a sine wave modulator of 750 Hz, it is the equivalent of several carrier frequencies, and each carrier produces its own set of sidebands from the modulation frequency. For a sawtooth carrier, this means that there will be the equivalent of a carrier at each integer multiple of the carrier frequency, and each will produce sidebands from the modulator frequency. With 100% modulation the sidebands will have half the amplitude of the carrier. All of the harmonics in the carrier wave will also be present in the output (Figure 3.5.2).

FIGURE 3.5.2 If the modulator is a non-sinusoidal waveform, then each of the harmonics of the modulator produces a pair of sum and difference frequencies in the output.

For the case of two non-sinusoidal waves, AM produces a set of sidebands for each carrier harmonic, using each modulator harmonic. AM is thus a simple way of producing complex sounds with a number of harmonics that are not related to the fundamental (inharmonics) (Figure 3.5.3).

FIGURE 3.5.3 If the carrier is a nonsinusoidal waveform, then each carrier harmonic appears in the output and also produces a pair of sum and difference frequencies.

In an analogue synthesizer, AM is produced by connecting a VCO to the modulation control input of a VCA which is processing the output of another VCO. If the modulating frequency is lower than about 25 Hz, then AM is called tremolo and it is perceived as a rapid cyclic change in the amplitude.