THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

18.12 Electrical and Electronic Instruments 559a delayed envelope. A tremolo can be added by a VCA controlled by subsonicperiodic waveform.A VCA can be utilized to multiply two separate audio signals, producing amplitudemodulation, with one signal serving as the carrier and the other servingas a modulator. The output consists of the spectral components of the carrierplus sidebands. More commonly used than amplitude modulation is the balancedmodulation or ring modulation. While the VCA is a two quadrant multiplier (i.e.,the control voltage must be positive for a nonzero output), the balanced modulatoris a four-quadrant multiplier.As the terminology implies, the definitive frequency of a voltage-controlledfilter (VCF) is controlled by a control voltage. In a low-pass VCF, for example, thecutoff frequency increases from a low value to a high value as the control voltageincreases. A scaling of 1 V per octave is the standard. There is a large number ofdifferent filtering circuits available, but predominant design seems to be the statevariablefilter that is constructed from two integrators and a summer. This designbears the advantage of incorporating (a) three simultaneous outputs (low-pass,bandpass, high-pass), (b) a constant value of Q independent of band frequency,(c) adjustability of Q through controlled feedback, and (d) stability of the sineoscillation mode. This type of filter features asymptotic slopes of ±12 dB/octavein the low-pass and high-pass outputs and ±6 dB/octave for the band-pass output.A four-pole low-pass filter with an asymptotic slope of ±24 dB/octave is oftenpreferred by musicians to deal with the more subdued sounds, particularly in thelower registers.Voltage control is applied in filters to generate dynamic effects, particularlymusical attacks. As an example of this application, consider the fact that theharmonics of a brass instrument entering the attack phase are in the order ofascending frequencies (Risset and Mathews, 1965). This effect is electronicallysimulated by raising the cutoff frequency of a low-pass VCF with an envelopegenerator at the onset of the tone. On the other end of the tone, the decay of anenvelope generator is utilized to decrease the cutoff frequency of the low-passVCF to simulate the proclivity of high-frequency modes to become damped morerapidly than the low-frequency modes in a free vibrator (such as a percussioninstrument).A number of other musical effects can be generated. For example, in a delay-andaddapplication for a digital delay line, specifically the generation of reverberation,an input signal is passed through a delay line that is tapped at different delayintervals. Signals at these taps are fed back into the input as a weighted sum.This weight determines the reverberation time of the system. In the flanger use ismade of comb filtering. The flanger uses a single delay-and-add. The delay time isslowly modulated so that the comb filtering changes with time, with typical delaysin the order of 2 ms. Chorusing, which is the attempt to make a single-voice soundlike a group of many, functions in the same manner, except several modulateddelay-and-add circuits are used to transmit to different channels, and the delaysare considerably longer, typically in the order of 10 ms.