YMU757B Datasheet, PDF (41/43 Pages) YAMAHA CORPORATION – MA-1C

YMU757B
General description of FM sound generator
"FM" stands for Frequency Modulation.
The FM sound generator utilizes the higher harmonic wave produced by the frequency modulation for synthesis of the
musical sounds
As use of this FM system enables a comparatively simple circuit to produce such waveform that has a harmonic wave
including disharmonious sounds, it is possible to create a wide range of sounds from the synthesized sounds of the
natural musical instruments to the electronic sounds.
The diagram below shows the most basic configuration of the FM system.
β
ωm
Phase
Generator
Sin Wave
Table
I
Envelope
ωc
Generator
Operator 1 : Modulator
Sin Wave
Table
f
ωc
A
Phase
Generator
Envelope
Generator
Operator 2 : Carrier
The "Operator" refers to the section where a sine wave is generated and the combination of the operators is called
"algorithm". The operator in the front stage is called "modulator" and that in the rear stage "carrier".
Each operator is capable of setting the frequency and the envelope waveform.
The configuration in the above diagram can be expressed in the formula as follows.
FM(t) = A sin(w ct + B sin w mt )
A : Amplitude of the carrier. B : Amplitude of the modulator. w : Angle frequency of the carrier
wm : Angle frequency of the modulator
In addition, a system called "feedback FM" is available to create a wider range of sounds. In this system, the frequency
modulation is fed back as shown in the diagram in the following page.
B is called "feed-back ratio". Using the feed-back FM function, it is possible to produce the strings type sounds.
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