Chapter Four: Synthesis
6. Principles of Audio-rate Frequency Modulation | page 11
Some variations on FM
Many things can be done to create more complex spectra with FM. The DX-7 was built around the idea of both double-carrier FM, in which a single modulator controls two carriers, tuned differently. This allows the creation of formant areas not possible with single FM. Also, stacks of modulators, where a modulator was itself modulated, could either produce wildly complex spectra if tuned inharmonically or produce weighted spectra, which could create a more realistic bass. This helped with one of FM's greatest drawbacks--the strength of the upper and lower sidebands are equal, but our human hearing requirings more energy in the lower frequencies to be considered as equally loud as the higher frequencies. Therefore, single FM always seemed weighted to the treble, particularly at higher values of I. Another interesting idea is to modulate the modulation index itself, providing a rapid timbral shift. or to low-frequency modulate the modulator or carrier, changing the C:M ratio and therefore the frequencies of the sidebands for some very nice effects.
Suggested listening examples
To hear audio-rate FM used with a high level of artistry, there can be no better source than the works of John Chowning himself. Highly recommended are Stria (1976), Turenas (1972) and Phoné (1981). Barry Truax was another pioneering FM composer with Arras, Androgyny, Wave Edge, Solar Ellipse, and Sonic Landscape No. 3.
J. Chowning, "The Synthesis of Complex Audio Spectra by Means of Frequency Modulation," Journal of the Audio Engineering Society 21(7), 1973; reprinted in Computer Music
Journal 1(2), 1977; reprinted in Foundations of Computer Music, C. Roads and J. Strawn (eds.). MIT Press, 1985.
B. Truax, "Organizational Techniques for C:M Ratios in Frequency Modulation", Computer Music Journal, 1(4), 1978, pp. 39-45; reprinted in Foundations of Computer Music, C. Roads and J. Strawn (eds.). MIT Press, 1985.
C. Dodge and T. Jerse, Computer Music, 2nd Ed., Schirmer Books, 1997.
F. Richard Moore, Elements of Computer Music, Prentice Hall, 1990.