Frequency modulation uses the information signal, V_m(t) to vary the carrier frequency within some small range about its original value.

Advantage FM:

• Immune to AM noise riding on it`s carrier frequency. Affected much less by static caused by adverse weather.
• FM radio has a significantly larger bandwidth than AM radio

Disadvantage Fm :

FDM Frequency Division Multiplexing is :-

• Useful bandwidth of medium exceeds required bandwidth of channel

• Each signal is modulated to a different carrier frequency

• Carrier frequencies separated so signals do not overlap (guard bands) e.g. broadcast radio

• Channel allocated even if no data

which states that the highest frequency which can be accurately represented is less than one-half of the sampling rate. So, if we want a full 20 kHz audio bandwidth, we must sample at least twice that fast, i.e. over 40 kHz.The sampling frequency determines the limit of audio frequencies that can be reproduced digitally.Nyquist Theorem is  One of the most important rules of sampling.