The FM Discriminator

As the FM discriminator is designed to respond to the deviation in the frequency and have very little response to changes in its amplitude, FM is a very effective way to discriminate against most forms of noise - which, generally, amplitude modulate the radio frequency.
With a Frequency Modulated signal, the process of recovering the audio from the output from the IF amplifier is a little more complicated.

The picture shows a Frequency Modulated signal from the IF Amp being fed to a circuit known as a discriminator.

The voltage output from the discriminator is proportional to the radio frequency's deviation from its set centre-frequency. As this deviation is the audio signal, the audio is recovered.

As with the simpler AM detector, the final output is a fully cleaned up, AC audio signal suitable for feeding an audio amplifier.