Moog Monday - On Synthesizers: Principles of Voltage Control, Part V

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(This column originally appeared in the November 1978 issue of Contemporary Keyboard magazine.)

MY LAST COLUMN DEALT WITH the exponential control mode of voltage-controlled oscillators. Exponential control gives oscillator pitch changes that are directly proportional to control voltage changes. Thus musical scales, transpositions, and pitch patterns such as trills can be produced by generating and combining corresponding patterns of voltage change.

Linear VCO Control
Fast frequency modulation results in clangorous tones. Sidebands (extra frequencies) are produced, and these alter the quality of the tone. Effects such as steel drum, trumpet, and timpani sounds are often created using fast frequency modulation. However, when the modulating signal is applied to the tone oscillator's exponential control input, the apparent pitch of the modulated tone shifts upward compared to what it would be with no modulation. This is because, in exponential mode modulation, a greater number of vibrations of the tone oscillator are added during the positive half of the modulating waveform than are subtracted during the modulating waveform's negative half, thus making the average modulated frequency greater than that of the unmodulated signal. To the synthesizer musician, this means that the pitch of the tone increases as frequency modulation is injected, and he or she must therefore retune in order to remain in the same key.

On some experimental and modular VCOs, one or more linear frequency control inputs are also provided. A positive control voltage applied to one of these linear inputs will raise the oscillator's frequency by the same number of cycles it would lower it by if the control voltage were negative instead of positive. A graph of VCO frequency versus linear mode control voltage would be a straight line; hence, the name linear. A rapidly alternating control voltage, when applied to a tone oscillator's linear control input, will introduce sidebands and change the tone color without creating an undesirable pitch shift. Linear frequency modulation of this sort is extremely useful in producing dynamically varying tone colors by imparting an envelope to the modulating signal.

A typical patch is shown in Fig.1. The oscillators have linear as well as exponential inputs. The pitch interval between the two oscillators is set to give the desired sidebands. A keyboard control voltage feeds the exponential control inputs of both oscillators. The modulating oscillator output is shaped by a voltage-controlled amplifier, which in turn is being opened and closed by a contour generator. The VCA output modulates the tone oscillator, producing sidebands that increase, then decrease as the contour generator goes up, then down. If the tone oscillator modulation is applied to the linear frequency control input, virtually no pitch change will occur during the course of the contour. On the other hand, if the modulation were applied to the tone oscillator's exponential control input, a pitch shift would be heard with each new contour.

Voltage-Controlled Waveforms
Voltage control of rectangular waveform width is the most common type of voltage-controlled waveshaping in commercial synthesizers. By varying the width of one part of a rectangular wave from, say, 5% to 50% of its complete cycle, you will change the resultant tone color from a thin nasal quality to a full clarinetty timbre. The waveform control mode is almost always linear, which gives a smooth timbral variation for most types of control signals.

Another useful patch for voltage-controlling waveforms is to simply feed the output of a VCO back into its input! Then when the waveform goes positive, it speeds itself up; during the negative half cycle it spreads itself out. Fig. 2a shows what happens to a sine wave output of a VCO when it is fed back into its own control input. The spectrum of this particular waveform is rich in even, low-order harmonics; the timbre is mellow and hornlike. Fig. 2b shows how, with a linear frequency control input, dynamic waveshaping can be achieved with one VCO, one VCA, and a contour generator.

Voltage-Controlled Filters
Most voltage-controlled filters have exponential mode control inputs that exactly match those of the tone oscillators that feed them. This enables a control voltage (say from a keyboard) to move both the oscillator and filter together so that the filter cuts off or emphasizes the same oscillator overtones, regardless of the tone's pitch. This 'tracking filter' arrangement produces a tonal balance in which the tone seems to grow brighter as the pitch goes higher—an important characteristic of additive synthesis instruments. Filters whose control characteristics match those of tone oscillators are also useful in imparting pitch patterns to noisy sounds. This is especially true if the filter is highly resonant and the audio feeding the filter is wideband, with very little pitch of its own.

In other applications it is desirable to impart a tonal balance other than that which is characteristic of additive synthesis instruments. Effective 'brightness balance' in a typical oscillator-filter patch may be achieved simply by inserting an attenuator at the filter control input, as shown in Fig. 3. When the attenuator is turned down a bit, the filter does not open as much when the pitch control voltage raises the oscillator frequency. This mutes the tones at the top end of the keyboard. The adjustment of this attenuator is a subtle but important technique for getting a well-balanced sound during performance.

Next month's column will conclude our series on voltage control with a discussion of control voltage routing schemes

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