index is seen to be the ratio of the peak frequency deviation of the carrier wave to the frequency of the modulating sine wave.

If , the modulation is called narrowband FM (NFM), and its bandwidth is approximately . Sometimes modulation index is considered NFM and other modulation indices are considered wideband FM (WFM or FM).

For digital modulation systems, for example, binary frequency shift keying (BFSK), where a binary signal modulates the carrier, the modulation index is given by:

where is

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