A second carrier oscillator sharing the same modulator adds a formant region at any spectral position

Chowning’s extended FM instrument (Fig. 17) adds a second carrier oscillator whose frequency is independent of the first, but which shares the same modulating oscillator. The second carrier’s deviation can be independently scaled, allowing a different effective index for it. By placing the second carrier at a frequency that is a high harmonic multiple of the modulating frequency (e.g., 7× the modulator), the second carrier’s sidebands cluster around a high spectral region, creating a ‘formant peak’ — a concentration of energy at a chosen part of the spectrum. This allows simulation of vocal or resonant formants using pure FM without filters. The proportion of the two modulated carriers determines the formant’s strength relative to the fundamental.

Examples

Two-carrier patch: first carrier at 300Hz (c/m=1/1), second carrier at 2100Hz (c2/m=7/1). The second carrier adds sideband energy centered around the 7th harmonic, boosting that spectral region and producing a formant-like peak.

Assessment

Explain how to produce a vocal ‘ah’ vowel formant at approximately 800Hz using a two-carrier FM instrument with m=100Hz.