US 4,300,435 AGranted

Synthesizer for organ voices

AssigneeCBS Inc.

InventorGeorge F. Schmoll, III

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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.

Claims8

Dependent claims

In an electronic organ which includes:

a tone signal generating system for generating tone signals corresponding to notes in a musical scale;
an output system for translating tone signals into audible musical tones;
a keyboard having a plurality of keys, each identified with a particular note of the musical scale;
and at least four stop tablets, each identified with a different organ voice;
a circuit for synthesizing a selectable one of a plurality of different organ voices, comprising:
voltage controlled sharp cutoff low-pass filter circuit means connected to couple player-selectable tone signals from said tone signal generating system to said output system, said filter circuit means having a cutoff frequency controllable over a range between a first frequency and a second higher frequency in response to variations in the duty cycle of a control signal comprising a sequence of pulse width modulated pulses, and a rapid rate of roll-off above the cutoff frequency,
means including said at least four stop tablets for generating responsively to player actuation of a selected one or more of said at least four stop tablets a selected one of a plurality of different actuating signals each of which is associated with one and only one of said plurality of organ voices and each of which is uniquely identified by one of a plurality of different 4-bit binary words selectable in accordance with which one or more of said at least four stop tablets are actuated, and
control signal generator means for applying a sequence of pulse width modulated pulses to said filter circuit means, said control signal generator means including means responsive to a selected actuating signal for controlling the width of said pulses to cause said filter circuit means to have a cutoff frequency appropriate to the tone quality of the organ voice determined by the stop tablets actuated by the player.

5 more elements

In an electronic organ which includes tone signal generating means for generating tone signals corresponding to notes in a musical scale;

an output system for translating tone signals into audible musical tones;
a keyboard having a plurality of keys, each identified with a particular note of the musical scale;
and at least four stop tablets, each identified with a different organ voice;
a system for synthesizing a selectable one of a plurality of different organ voices, comprising:
voltage controlled sharp cutoff low-pass filter circuit means connected to couple player-selectable tone signals from said tone signal generating means to said output system, said filter circuit means including cascade-connected state variable filters each having two integrators connected in a loop with a summing network, the integrating time constant of each of said integrators being substantially the same and determined by a capacitor and a resistive network including a bilateral switch having a much higher resistance when off than when on, said filter circuit means having a cutoff frequency controllable over a range between a first frequency and a second higher frequency in response to variations in the duty cycle of a sequence of pulse width modulated pulses supplied to said bilateral switches for turning them on and off in unison at a high rate, and having a rapid rate of roll-off above the cutoff frequency,
means including said at least four stop tablets for generating responsively to player actuation of a selected one or more of said at least four stop tablets a selected one of a plurality of different actuating signals each of which is associated with one and only one of said plurality of organ voices and each of which is uniquely identified by one of a plurality of different 4-bit binary words selectable in accordance with which one or more of said at least four stop tablets are actuated, and
control signal generator means for generating and applying to the said bilateral switches included in said filter circuit means a sequence of pulse width modulated pulses the width of which pulses are controlled in accordance with the 4-bit binary word selected by player actuation of said at least four stop tablets for causing said filter circuit means to have a cutoff frequency appropriate to the tone quality of the selected organ voice.

4 more elements

Dependencies are inferred from the claim text.