In FM synthesis, the ratio of the modulator to the carrier determines your spectrum by setting the relationship between partials. In general, integer ratios produce harmonic sounds, where pitch is perceivable due to the partials being in an integer relationship to the fundamental. Non-integer relationships produce inharmonic sounds, with partials not having an integer relationship to the fundamental.
It is important to note that the strength (amplitude) of the modulator directly relates to the number of partials present in the modulated sound. By varying the amplitude of the modulator (with an envelope generator or LFO) over time, you can control the “brightness” of the output signal.
Some basic M:C ratios:
- 1:1, produces a sawtooth-like wave. The higher the amplitude of the modulator, the brighter (and more brass-like) the resulting output.
- 2:1, produces a wave with odd harmonics only, like that of a clarinet.
- 1.4:1, produces a bell-like sound.
Other non-harmonic ratios, such as 3.3:1, can produce other percussive types of sounds. Many acoustic instruments have non-harmonic attacks (think guitar, brass instruments, vibraphone). Using multi-modulator (and multi-carrier) instruments, you can program different attack and sustain C:M ratios. For example, a guitar has a very noisy attack, but it only lasts a tiny fraction of a second. The sustain is more of a sawtooth wave, but with limited harmonics that decay much quicker than the fundamental.
Other programming tips that can help are to use ratios that are slightly detuned (1.1:1), or multiple modulators and carriers that are slightly detuned. You can also use ratios that produce recognizable timbres, like a bell, but use amplitude envelopes in a way that is unnatural. For example, a bell timbre with an attack that fades in, or a sort of reverse envelope on a clarinet type tone, where the higher partials fade in to full right before an abrupt stopping of the tone.
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