Method and integrated circuit for the flexible combination of four operators in sound synthesis

A four-operator sound synthesis integrated circuit comprises a first through a fourth sound synthesis operator, a first programmable multiplier connecting the output of the first operator to the input of the second operator, a second programmable multiplier connecting the output of the second operator to the input of the third operator, a third programmable multiplier connecting the output of the third operator to the input of the fourth operator, a fourth programmable multiplier connecting the output of the first operator a first input of a four-input adder, a fifth programmable multiplier connecting the output of the second operator a second input of the four-input adder, a sixth programmable multiplier connecting the output of the third operator a third input of the four-input adder, and a seventh programmable multiplier connecting the output of the fourth operator a fourth input of the four-input adder. The product of the combination is taken from the output of the four-input adder. Each programmable multiplier can be programmed to multiply from zero to one, e.g., zero, one-half, and one.

RELATED APPLICATION 
A copending application, Ser. No. 08/305,087 filed Sep. 13, 1994, entitled, 
METHOD AND INTEGRATED CIRCUIT FOR ELECTRONIC WAVEFORM GENERATION OF VOICED 
AUDIO TONES, is incorporated herein by reference. 
1. Field of the Invention 
The present invention relates generally to electronics devices, and more 
particularly to waveform synthesizers for the generation of voiced musical 
and electronic tones. 
2. Description of the Prior Art 
Musical instruments each have characteristic voices that relate to the 
sound amplitude envelope attack rate, decay rate, sustain level and 
release rate. For example, the envelope of piano sounds has a sharp attack 
rate when the keys are pressed and then gradually diminishes as the key is 
held down. By pressing a piano's sustain pedal, the sound will fade away 
slower. A piano also has a large number of harmonic overtones during the 
attack. These harmonic overtones decrease over time until a near constant 
harmonic content results. 
Frequency modulation (FM) techniques are conventionally employed in audio 
sound synthesis digital integrated circuits to generate complex waveforms 
that contain high harmonic and non-harmonic sounds. The correspondence 
between the modulation index and spectrum distribution of higher harmonics 
is-natural and makes possible a wide range of sound generation. FM sound 
generation uses effects obtained from modulating a carrier. The pitch, 
tone and level of a musical sound can be obtained by skillful manipulation 
of basic FM parameters, e.g., carrier output level, modulator output 
level, feedback level of the modulator, frequency of the carrier and 
frequency of the modulator. 
Conventional approaches to tone synthesis are described by Nishimoto in 
U.S. Pat. Nos. 4,297,933, ELECTRONIC MUSICAL INSTRUMENT FOR TONE FORMATION 
BY SELECTABLE TONE SYNTHESIS COMPUTATIONS, and 4,554,857, ELECTRONIC 
MUSICAL INSTRUMENT CAPABLE OF VARYING A TONE SYNTHESIS OPERATION 
ALGORITHM, and by Hiyoshi in U.S. Pat. No. 4,253,367, MUSICAL TONE FORMING 
DEVICE BY FM TECHNOLOGY. 
John Chowning, U.S. Pat. No. 4,018,121, issued Apr. 19, 1977, describes 
basic FM synthesis for musical sounds, e.g., a modulating sine wave, a 
time varying envelope function for the modulating waveform, a carrier sine 
wave, and a time varying envelope function for the amplitude of the 
frequency modulated wave. The amplitude of the modulating wave, or the 
envelope function for the modulating wave, varies with time so that the 
frequency spectrum of the resulting frequency modulated waveform varies 
during the attack, sustain, and decay of the sound. It is conventional to 
use a envelope function to vary the amplitude of the final sound, but 
Chowning added an envelope function to the modulating waveform to time 
vary the frequency spectrum of the generated sound. 
The Yamaha OPLII allows two operators to be combined such that one 
modulates the sine wave output of the other and, alternatively for the 
sine wave outputs of two operators to be combined in an adder. The Yamaha 
OPLIII model YM262 allows four operators to be combined in any of four 
ways. 
In the first case, the four operators are connected in a string of four, 
wherein a first (e.g., "A") modulates the sine wave of a second (e.g., 
"B"), which modulates the sine wave of a third (e.g., "C"), which 
modulates the sine wave of a fourth (e.g., "D"), which is then output. The 
output ("Q(t)") of this first combination can be expressed mathematically 
as shown in equation (1), 
EQU I.sub.3 sin(w.sub.3 t+I.sub.2 sin(w.sub.2 t+I.sub.1 sin(w.sub.1 t+I.sub.0 
sin(w.sub.0 t)))). (1) 
In a second case, two operators (e.g., "A" and "C") are respectively 
connected to modulate the sine waves of the other two operators (e.g., "B" 
and "D") and the two strings are then combined in an adder. The output 
("Q(t)") of this second combination can be expressed mathematically as 
shown in equation (2), 
EQU I.sub.3 sin(w.sub.3 t+I.sub.2 sin(w.sub.2 t) )+I.sub.1 sin(w.sub.1 
t+I.sub.0 sin(w.sub.0 t)). (2) 
In a third case, a first operator (e.g., "A") is added to the output of a 
string of three other operators (e.g., "B", "C" and "D"). The output 
("Q(t)") of this third combination can be expressed mathematically as 
shown in equation (3), 
EQU I.sub.3 sin(w.sub.3 t+I.sub.2 sin(w.sub.2 t+I.sub.1 sin(w.sub.1 
t)))+I.sub.0 sin(w.sub.0 t). (3) 
In a fourth case, a first operator (e.g., "A") is added to both the output 
of a string of two other operators (e.g., "B" and "C") and a fourth 
operator (e.g., "D"). The output ("Q(t)") of this third combination can be 
expressed mathematically as shown in equation (4), 
EQU I.sub.3 sin(w.sub.3 t)+I.sub.2 sin(w.sub.2 t+I.sub.1 sin(w.sub.1 
t))+I.sub.0 sin(w.sub.0 t). (4) 
The conventional method of combining such operators includes using 
multiplexers to switch some of the inputs and outputs of the four 
operators into the four configurations represented by formulas (1)-(4). A 
more flexible method of combining four operators is needed to better 
utilize the sound synthesis capabilities of four operators. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a method of 
utilizing four operators for sound synthesis. 
It is a further object of the present invention to provide an electronic 
circuit that implements a single master algorithm with sufficient input 
parameters to represent substantially all desirable tone formulas. 
Briefly, a four-operator circuit embodiment of the present invention 
comprises a first through a fourth sound synthesis operator, a first 
programmable multiplier connecting the output of the first operator to the 
input of the second operator, a second programmable multiplier connecting 
the output of the second operator to the input of the third operator, a 
third programmable multiplier connecting the output of the third operator 
to the input of the fourth operator, a fourth programmable multiplier 
connecting the output of the first operator a first input of a four-input 
adder, a fifth programmable multiplier connecting the output of the second 
operator a second input of the four-input adder, a sixth programmable 
multiplier connecting the output of the third operator a third input of 
the four-input adder, and a seventh programmable multiplier connecting the 
output of the fourth operator a fourth input of the four-input adder. The 
product of the combination is taken from the output of the four-input 
adder. Each programmable multiplier can be programmed to multiply from 
zero to one, e.g., zero, one-half, and one. 
An advantage of the present invention is that a tone generator is provided 
that generates substantially all desired tone formulas with a single 
circuit. 
Another advantage of the present invention is that a method is provided for 
time-varying multiplication factors which permit the tone color to be 
modified during the tone generation. 
These and other objects and advantages of the present invention will no 
doubt become obvious to those of ordinary skill An the art after having 
read the following detailed description of the preferred embodiment which 
is illustrated in the various drawing figures.