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

558 18. Music and Musical Instrumentspedal keyboard and incorporate a system of stops and couplers. Loudspeakersmay be housed in a separate cabinet; and the driving electronics housed in theconsole.The electronic organ has been displaced in the 1980s by the growing popularityof synthesizers. Modular synthesizers made their appearance in the1960s in both analog and digital versions. The analog synthesizer consists ofa group of signal generating and signal processing modules operating on electricalvoltages. The modular approach to analog synthesis is embodied in threefundamental voltage-controlled modules: (1) the voltage-controlled oscillator(VCO), (2) voltage-controlled amplifier (VCA), and (3) voltage-controlled filter(VCF).The VCO is really a function generator that produces a periodic voltage signaland is the initial source of pitched sounds. The frequency of the periodic signalis determined by a control voltage. Usually when the control voltage increases byone volt, the frequency doubles (i.e., it goes up one octave). The control mode istherefore exponential, which fits beautifully into the scheme of things in music andin electronics. Controllers can be fabricated in the form of keyboards, and musicaleffects of modulation (trill, vibrato, or glissando) are independent of the DC valueof the control voltage. Switching keys of a melody require only the addition of aconstant to the series of control voltages representing the notes of a melody. Theexponential feature inherent in the voltage control is appropriate electronically,given the fact that the collector current in a bipolar transistor is an exponentialfunction of the base-emitter voltage. In addition to being stable and accurate, theVCO needs a subsequent filter to shape the waveforms for the desired tone color. Ahigh-purity sine wave output is normally needed, particularly if the VCO output isprocessed by a nonlinear waveform shaper, distorter, or a ring modulator or an FMsynthesizer. Otherwise an excessively dense spectrum that is downright unmusicalcan be created.The voltage-controlled amplifier is a voltage amplifier with a gain that dependslinearly upon a control voltage. An exponential control is generally available, butit is more usual for the amplifier gain to grow linearly with the control voltage witha slope of unity gain for a 5-V control input and a maximum gain of 2. Ideallythe gain should be zero when the control voltage is zero or negative. It is the taskof the VCA to turn on tones. Since the oscillators run continuously, the VCAbears the responsibility of shaping the amplitude envelope of the tone. Here thecontrol voltage comes from an envelope generator. When a note is begun froma keyboard (or sequencer), the envelope generator begins a transient phase. Theenvelope generator sustains an appropriate level for the duration of the tone andends the tone with an exponential decay. As with the tone control parameters ona synthesizer, the performer can set the envelope transients, the sustain level, anddecay time by adjusting individual potentiometers on a modular analog instrumentand through programming on a digitally controlled keyboard. Special timbres canbe created by adding two sounds, one of them with a delayed onset. To achievethis the output of two VCAs can be added, where one of them is controlled by