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<p>In the second case we just add together the two simple FM spectra, but in the first case 
we get a more complex mixture involving all the sums and differences of the modulating frequencies. 
These sum and difference tones ("intermodulation products") are not limited to FM.  Any
nonlinear synthesis technique produces them.  Being non-linear, it must have something that involves
a power of its input other than 0 or 1; if we feed in sin a + sin b, f