Frequency modulation synthesis (or FM synthesis) is a form of sound synthesis where the frequency of a waveform, called the carrier, is changed by modulating its frequency with a modulator. The frequency of an oscillator is altered "in accordance with the amplitude of a modulating signal." (Dodge & Jerse 1997, p. 115) FM synthesis can create both harmonic and inharmonic sounds. For synthesizing harmonic sounds, the modulating signal must have a harmonic relationship to the original carrier signal. As the amount of frequency modulation increases, the sound grows progressively more complex. Through the use of modulators with frequencies that are non-integer multiples of the carrier signal (i.e. inharmonic), inharmonic bell-like and percussive spectra can be created. FM synthesis using analog oscillators may result in pitch instability. However, FM synthesis can also be implemented digitally, the latter proving to be more stable and is currently seen as standard practice. Digital FM synthesis (implemented as phase modulation) was the basis of several musical instruments beginning as early as 1974. Yamaha built the first prototype digital synthesizer in 1974, based on FM synthesis, before commercially releasing the Yamaha GS-1 in 1980. The Synclavier I, manufactured by New England Digital Corporation beginning in 1978, included a digital FM synthesizer, using an FM synthesis algorithm licensed from Yamaha. Yamaha's groundbreaking DX7, released in 1983, brought FM to the forefront of synthesis in the mid-1980s. FM synthesis had also become the usual setting for games and software until the mid-nineties. Through sound cards like the AdLib and Sound Blaster, IBM PCs popularized Yamaha chips like OPL2 and OPL3. The related OPN2 was used in the Fujitsu FM Towns Marty and Sega Genesis as one of its sound generator chips. Similarly cases, Sharp X68000 and MSX (Yamaha computer unit) also use FM-based soundchip, OPM.
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