Wideband FM

Wideband FM (Frequency Modulation) is a kind of FM in which the maximum frequency offset (the maximum change in frequency in the modulated signal from the carrier frequency when the baseband signal is at full amplitude) is much larger than the bandwidth of the baseband signal. Conversely, narrowband FM means that the maximum frequency offset is smaller than the bandwidth of the baseband signal.

For example, in broadcast FM audio (88-108MHz) the maximum frequency offset is something like 75kHz (may differ in different countries), much larger than the bandwidth of the baseband audio signal which is 15kHz, therefore it is wideband FM.

Wideband FM signals take more bandwidth than AM or narrowband FM signals. AM signal's bandwidth is twice the baseband bandwidth, while WFM signal's bandwidth is approximately twice the sum of the baseband bandwidth and the maximum frequency offset. The extra bandwidth pays off in quality, though: with the same SNR (signal-to-noise ratio) of the modulated signal, WFM's corresponding baseband signal after demodulation has significantly higher SNR than AM's, and WFM's baseband SNR continues to increase as the bandwidth of the modulated signal is increased. This is why broadcast FM audio sounds so much less noisy than AM.