Frequency modulation synthesis

FM synthesis using 2 operators

A 220 Hz carrier tone fc modulated by a 440 Hz modulating tone fm, 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).

Waveforms for each β

Spectra for each β

In audio and music 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. The frequency of an oscillator is altered or distorted, "in accordance with the amplitude of a modulating signal." (Dodge & Jerse 1997, p. 115)

FM synthesis can create both harmonic and inharmonic sounds. For synthesizing harmonic sounds, the modulating signal must have a harmonic relationship to the original carrier signal. As the amount of frequency 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.

FM synthesis using analog oscillators may result in pitch instability, but FM synthesis can be implemented digitally, and the latter proved so much more reliable that it became the standard. As a result, digital FM synthesis (using the more frequency-stable phase modulation variant) was the basis of Yamaha's groundbreaking DX7, which brought FM to the forefront of synthesis in the mid-1980s.

History

The technique of the digital implementation of frequency modulation, which was developed by John Chowning (Chowning 1973, cited in Dodge & Jerse 1997, p. 115) at Stanford University in 1967-68, was patented in 1975 and later licensed to Yamaha.

The implementation commercialized by Yamaha (US Patent 4018121 Apr 1977 or U.S. Patent 4,018,121) is actually based on phase modulation, but the results end up being equivalent mathematically, with phase modulation simply making the implementation resilient against undesirable drift in frequency of carrier waves due to self-modulation or due to DC bias in the modulating wave.

As noted earlier, 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 and several other models by Yamaha providing various variations of FM synthesis. The most advanced FM synths produced by Yamaha were the 6-operator keyboard SY99 and the 8-operator module FS1R: each features Yamaha's Advanced FM (AFM) alongside and able to be layered or interfaced with other synthesising technologies, respectively AWM2 (Advanced Wave Memory 2) sample-based synthesis in the SY99 and formant synthesis in the FS1R, neither of which combinations have ever been duplicated, as neither have some of the other advanced FM features of these Yamaha devices.

Yamaha had patented its hardware implementation of FM in the 1980s, allowing it to nearly monopolize the market for that technology until the mid-1990s. Casio developed a related form of synthesis called phase distortion synthesis, used in its CZ range of synthesizers. It had a similar (but slightly differently derived) sound quality to the DX series. Don Buchla implemented FM on his instruments in the mid-1960s, prior to Yamaha's patent. His 158, 258 and 259 dual oscillator modules had a specific FM control voltage input, and the model 208 (Music Easel) had a modulation oscillator hard-wired to allow FM as well as AM of the primary oscillator. These early applications used analog oscillators.

With the expiration of the Stanford University FM patent in 1995, digital FM synthesis can now be implemented freely by other manufacturers. The FM synthesis patent brought Stanford $20 million dollars before it expired, making it (in 1994) "the second most lucrative licensing agreement in Stanford's history". FM today is mostly found in software-based synths such as FM8 by Native Instruments, but it has also been incorporated into the synthesis repertoire of some modern digital synthesizers, usually coexisting as an option alongside other methods of synthesis such as subtractive, sample-based synthesis, additive synthesis, and other techniques. The degree of complexity of the FM in such hardware synths may vary from simple 2-operator FM, to the highly flexible 6-operator engines of the Korg Kronos and Alesis Fusion, to creation of FM in extensively modular engines such as those in the latest synthesisers by Kurzweil Music Systems.

However, hardware synths that were specifically marketed for their FM capabilities have not been seen since the Yamaha SY99: even their later FS1R was marketed chiefly as a device for formant synthesis, despite the fact that it was the only 8-operator FM synth that they, or any other hardware manufacturer, had produced, and had other unique features in its FM engine. Since these machines by Yamaha, no manufacturer has released any hardware synth dedicated solely or mainly to FM synthesis.

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 F_c. An audio-frequency modulating waveform, with a frequency F_m, 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 F_c . Otherwise, the amplitude of the modulating signal causes the frequency of the carrier oscillator to swing above and below F_c . 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 F_c 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 F_c, and a lower sideband appears below F_c. 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 / F_m ) 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.

Spectral analysis

The spectrum generated by FM synthesis with one modulator is expressed as following

$\begin{align} \sin(f_c\cdot t+I\sin(f_m\cdot t)) &= J_0(I)\sin(f_c\cdot t) \\ &+ \sum_{k=1}^{\infty} J_k(I)\left[\sin(f_c+k f_m)t+(-1)^{k}\sin(f_c-k f_m)t\right] \end{align}$

where $f_c\,$, $f_m\,$ are frequencies of carrier and modulator, $I$ is modulation index, and $J_k(...)$ is Bessel function of first kind, respectively.

See also

• Chiptune
• Digital synthesizer
• Electronic music
• Sound card
• Sound chip
• Video game music

References

• Chowning, J. (1973). "The Synthesis of Complex Audio Spectra by Means of Frequency Modulation". Journal of the Audio Engineering Society 21 (7).  (also available in PDF as digital version 2/13/2007)
• Chowning, John; Bristow, David (1986). FM Theory & Applications - By Musicians For Musicians. Tokyo: Yamaha. ISBN 4-636-17482-8. 
• Dodge, Charles; Jerse, Thomas A. (1997). Computer Music: Synthesis, Composition and Performance. New York: Schirmer Books. ISBN 0-02-864682-7. 

External links

• An Introduction To FM, by Bill Schottstaedt
• FM tutorial
• SOUNDSHOCK, an English language discussion forum dedicated exclusively to FM Synthesis
• Synth Secrets, Part 12: An Introduction To Frequency Modulation, by Gordon Reid
• Synth Secrets, Part 13: More On Frequency Modulation, by Gordon Reid
• Paul Wiffens Synth School: Part 3
• F.M. Synthesis including complex operator analysis
• Part 1 of a 2-part YouTube tutorial on FM synthesis with numerous audio examples