FM synthesis

FM synthesis is a method of sound synthesis used in synthesizers for generating sounds. It is based on frequency modulation, in which sidebands are generated around the carrier frequency.

In FM synthesis, the sidebands do not have the same spacing from the carrier frequency as in amplitude modulation; instead, complex sideband spectra are created, which are determined using Bessel functions. The position of the sidebands depends on the ratio of the carrier frequency to the modulation frequency.

If a carrier frequency is frequency modulated with a sinusoidal signal, then the first sideband frequency has a frequency spacing that corresponds to the ratio of carrier frequency to modulation frequency. For a carrier frequency of 5 kHz and a modulation frequency of 4 kHz, the ratio of carrier frequency to modulation frequency is 1.25, so the first lower sideband is at 3.75 kHz and the first upper sideband is at 6.25 kHz, the following sideband is at 2.5 kHz (3.75 kHz - 1.25 kHz) and 7.5 kHz (6.25 + 1.25), etc. The level and the number of sidebands are determined by the modulation index.

FM synthesis can be used to create complex sound structures with a carrier signal and a modulation signal. In contrast, additive sound synthesis works with two carrier signals.