The operational theory behind FM was not new in 1984. FM radio modulates one very high frequency wave, called the carrier with a second wave called a modulator.
What was new was that Dr Chowning at Stanford University discovered what happened when the carrier frequency and the modulator were both in the audio range.
It turned out that if you modulate one sine wave with another of the same frequency the result was a sawtooth wave whose number of harmonics increased with the volume of the modulator – a function almost identical to raising the cutoff on an analog sawtooth wave. If you modulate a carrier with a modulator double that of the carrier the result was a square wave whose number of harmonics depended on the volume of the carrier.
If the material of the source of oscillation has a high degree of stiffness the harmonics of the resulting waste at pushed sharp. So the harmonics on a violin string are in tune with each other but the harmonics of a struck metal bar( xylophone vibes etc). The resulting tone is characteristic of bells, until the invention of the DX7 it was impossible to make such sounds with a synth, however it turned out that if the ratio of the pitch of the modulator was not an integer ( like 1.7 :1 instead of 2:1) the result was beautiful inharmonic sound which became the signature sound of FM,
Yamaha took FM and ran with it. I heard that when Dr Chowning saw what the DX7 could do he nearly cried with joy. Instead of two operators Yamaha had developed six sine wave operators each with its own pitch, EG, volume and noise content arranged in the following algorithms (patterns).
The competitive advantage of the DX was not only the sound, it offered the engineers a HUGE acoustic space for a tiny computer load. This gave it 16 Note Polyphony and brought the price of a digital synth down to $1,995
From a musician’s point of view the biggest feature was that it replaced two tons of gear with a single 75 pound device that fit in the trunk of a car. It had a great electric piano sound which became it’s signature sound. It also covered for brass and organ which was adequate.
What Dr Chowning had discovered was that if the carrier wave and the modulator wave were both in the audio range increasing the strength of the modulator created sidebands or harmonics on both sides of the carrier wave, Further more when he folded the side bands below the carrier to a corresponding frequency above the result was a musical harmonic series.
Please review the original paper by clicking Here
If the modulator is at the same frequency as the carrier increasing the strength of the modulation produces a sawtooth wave. If the frequency is double that of the carrier wave the result is a square wave. No integer relationships produce bell tone and inharmonc sounds. This was a first for synthesizers.