Formants with Two Carrier Oscillators

In FM synthesis, the peaks in the spectral envelop can be controlled using an additional carrier oscillator.

In the case of a single oscillator, the spectrum is centered around a carrier frequency.

With an additional oscillator, an additional spectrum may be generated that is centered around a formant frequency.

When the two signals are added, their spectra are combined.

If the same oscillator is used to modulate both carriers (though likely using seperate modulation indeces), and the formant frequency is an integer multiple of the fundamental, the spectra of both carriers will combine in such a way the the components will overlap, and a peak will be created at the formant frequency.

Figure 17: The spectrum of individiual and combined FM signals.