Whereas additive synthesis has its corresponding analysis technique that lets you resynthesize (and modify) any sound, the situation is more difficult with FM. A popular extension is Spectral Modelling Synthesis (SMS) which incorporates noise. This technique is known as the tracking phase vocoder. From that representation you would usually pick the strongest spectral peaks and synthesize the sound from a sum of sinusoids. By doing a Fourier analysis of short windows of the sound you get a series of time frames of the spectrum at that moment.
The advantage of additive synthesis is that any sound whatsoever can be represented. For a very easy introduction to FM, try to find a copy of FM Theory & Applications by Chowning and Bristow.Īdditive synthesis builds up sounds from sums of sinusoids with given frequencies and amplitudes that may vary over time. There are extensions to more than one modulator or carrier, cascaded FM and feedback FM, all of which extend the possibilities in various ways. The partials' amplitudes depend on the modulation index, and cannot be controlled independently. Simple integer ratios such as 1:2 or 3:2 produce harmonic spectra, irrational ratios yield inharmonic spectra. Depending on the ratio of the modulator to carrier frequency and the modulation index (strength of modulation), you get different sounds. In the simplest form of two oscillator FM, a sinusoid modulates the frequency of the second sinusoidal oscillator.
There are several ways of doing FM as well as a few different approaches to additive synthesis.
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