A variety of other FM algorithms have been used to create or simulate sounds.
The use of two carrier oscillators, referred to as “double-carrier FM”, allows control over two frequency “regions” and is effective for simulating formants.
The block diagram shown in Fig. 12 below represents an algorithm used by Dexter Morrill to simulate brass instrument sounds.
Figure 12:
A signal flow diagram for double-carrier FM synthesis (after Morrill).
The second carrier frequency is chosen to be a harmonic of the fundamental frequency that is closest to the desired formant frequency.
The Max/MSP patch brass-double.maxpat, shown below, demonstrates an implementation of Morrill's brass instrument using double-carrier FM.
A common use of multiple-carrier FM is to simulate the singing voice, with the extra carriers set to vocal formants.
The STK instrument FMVoices uses 3 carriers and common modulator:
All oscillators are modulated by vibrato;
The modulating oscillator feeds back on itself through a two-zero digital filter;
The output is given by the sum of the 3 carrier oscillator outputs.