Functioning
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.
FM synthesis is a form of "distortion synthesis" or "nonlinear synthesis". It begins with an oscillator generating an audio-frequency "carrier" waveform with a frequency of Fc. An audio-frequency modulating waveform, with a frequency Fm, is then applied to change or "modulate" the frequency of the carrier oscillator.
If the amplitude of the modulator is 0, the output frequency of the carrier oscillator is simply Fc . Otherwise, the amplitude of the modulating signal causes the frequency of the carrier oscillator to swing above and below Fc . This frequency swing is known as "deviation".
In simple terms, the stronger (higher in amplitude) the modulating signal is, the more the carrier frequency changes. For illustration, suppose Fc is 1000 Hz. Modulation amplitude might be applied that causes the carrier to swing between 900 Hz and 1100 Hz, that is, 100 Hz in either direction. This is termed a "deviation" of 100 Hz.
At the same time, the frequency of the modulating signal causes sideband signals to appear at frequencies above and below the carrier frequency. Therefore for each frequency component in the modulating signal, an upper sideband appears above Fc, and a lower sideband appears below Fc. A complex modulating waveform (containing more partials than a simple sinewave) will create sidebands corresponding to each of its sinewave components.
Deviation (d) is partly responsible for the power of each component of the output audio signal. When d=0, all the power is heard at the carrier frequency. The larger the deviation, the more power is shifted to the sidebands.
The ratio of deviation to modulation frequency is called the "index of modulation". ( I = d / Fm ) This ratio controls the spectral richness of the sound. By varying deviation through modulation amplitude, and varying the spectrum of the modulating waveform, the resulting audio can be evolved without further instrument complexity.
Read more about this topic: Frequency Modulation Synthesis
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