For synthesizing harmonic sounds, the modulating signal must have a harmonic relationship to the original carrier signal. As the amount of FM modulation increases, the sound grows progressively more complex. Use of modulators with frequencies that are non-integer multiples of the carrier signal (i.e., non harmonic), bell-like dissonant and percussive sounds can easily be created.
FM synthesis is very good at creating 'clang', 'twang' or 'bong' noises. Complex FM synthesis using analog oscillators is not generally feasible due to their inherent pitch instability, but FM synthesis is easy to implement digitally. As a result, FM synthesis was the basis of some of the early generations of digital synthesizers from Yamaha, with Yamaha's flagship DX-7 synthesizer being ubiquitous throughout the 1980s. Casio, in order to avoid the FM patent, developed a related form of synthesis called phase distortion synthesis, used in its CZ101 line of synthesizers.
With the expiration of the Stanford University FM patent in 1995, FM synthesis is now part of the synthesis repertoire of most modern synthesizers, usually in conjunction with additive, subtractive and sometimes sampling techniques.
The harmonic distribution of a simple sine wave signal modulated by another sine wave signal can be represented with Bessel functions - this provides a basis for a simple mathematical understanding of FM synthesis.