Frequency modulation synthesis

A 220 Hz carrier tone modulated by a 440 Hz modulating tone with various choices of modulation index, β. The time domain signals are illustrated above, and the corresponding spectra are shown below (spectrum amplitudes in dB). An audio demonstration of the four synthesized tone timbres is available .

Related Topics:
Modulation index - DB

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Frequency modulation synthesis (or FM synthesis) is a form of audio synthesis where the timbre of a simple waveform is changed by frequency modulating it with a modulating frequency that is also in the audio range, resulting in a more complex waveform and a different-sounding tone.

Related Topics:
Audio synthesis - Timbre - Frequency modulating

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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. Through the 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.

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The technique was invented by John Chowning at Stanford University in the early 1970s, was patented in 1975 and was later licensed to Yamaha.

Related Topics:
Stanford University - 1970s - 1975 - Yamaha

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It should be noted that the implementation commercialized by Yamaha (US Patent 4018121 Apr 1977) is actually based on phase modulation (although some early findings by Chowning, regarding brass instruments, could very well indicate that he himself used FM proper back in the 1960s.)

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FM synthesis is very good at creating 'clang', 'twang' or 'bong' noises. Complex (and proper) FM synthesis using analog oscillators is not generally feasible due to their inherent pitch instability, but FM synthesis (using the frequency stable phase modulation variant) 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 DX7 synthesizer being ubiquitous throughout the 1980s. Casio developed a related form of synthesis called phase distortion synthesis, used in its CZ series of synthesizers. It had a similar (but slightly differently derived) sound quality as the DX series.

Related Topics:
Oscillators - Digital synthesizer - DX7 - 1980s - Casio - Phase distortion synthesis

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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.

Related Topics:
1995 - Additive - Subtractive - Sampling

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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.

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