Synthesizer for organ voices

Abstract

In an electronic musical instrument, such as an electronic organ, certain organ voices are synthesized by applying a tone signal to a voltage controlled low-pass sharp cutoff filter, the pass characteristic of which has a relatively sharp knee and a rapid rate of rolloff, thereby to sharply attenuate the harmonics contained in the tone signal which have frequencies above the cutoff frequency. The filter works on the principle that by switching one or more frequency-determining resistors in and out of the filter network at a rapid rate and varying the time on versus the time off, the effective value of the resistance varies to thereby alter the cutoff frequency and the character of the resulting sound signal. In accordance with the present invention, precise control over the switching duty cycle is achieved by establishing a reference voltage to which is added a voltage increment according to a binary weighting determined by selection of a particular organ voice. The sum of these voltages is compared with a sawtooth voltage in a comparator to produce a rectangular pulse the width of which is a function of the intersection of the ramp voltage with the sum voltage. By varying the sum voltage in digital increments in accordance with a selected voice, the width of the pulses is varied and these pulses, which are supplied to the filter, thus vary the filter characteristic to produce a sound signal simulative of the selected voice.