Introduction to Computer Music: Volume One

6. Principles of Audio-rate Frequency Modulation | page 7

To compute how the strength of the sideband pairs change over time as the strength of the modulating wave is varied, we divide the peak deviation by the modulating frequency to produce a value called the modulation index or simply I.

If none of the modulating wave is permitted to reach the carrier, the peak deviation and the value of I will be zero, since no modulation will be taking place. As the amplitude of the modulating wave increases, the carrier is pushed farther and farther off its center frequency and the value of I also increases. The effect of an increasing I is different for each sideband pair. In our first formula for predicting sideband frequencies, we used all the integer values of n (Cƒ ± nMƒ). A sideband pair calculated with a particular value of n can be call a pair of the nth order.