Demodulation of FM audio carrier

DETAILED DESCRIPTION What is claimed is: 1.
A method of generating an audio signal from an FM input signal which method comprises: supplying to a gain control circuit said FM input signal comprising a plurality of carrier waves each having a frequency and an amplitude; supplying an output from the gain control circuit to a phase locked loop circuit tuned to a selected one of the frequencies of the plurality of carrier waves; detecting the amplitude of the carrier wave having the frequency to which the phase locked loop is tuned; generating a tuned gain control signal representing the detected amplitude of the carrier wave having the frequency to which the phase locked loop is tuned; detecting a sum of the amplitudes of the plurality of carrier waves in the FM input signal; generating an untuned gain control signal representing the sum of said amplitudes; using said untuned gain control signal to control the gain control circuit prior to locking of the phase locked loop onto the selected frequency; and using said tuned gain control signal to control the gain control circuit after said locking, wherein an output of the phase locked loop provides said audio signal.
2.
A method according to claim 1 in which said phase locked loop includes a voltage controlled oscillator arranged to provide two outputs with a phase difference of 90.
degree.
, one of said outputs being supplied to a phase comparator coupled to receive the output of the gain control circuit and the other of said outputs being coupled to an amplitude detector for detecting the amplitude of the carrier wave having the frequency to which the phase locked loop is tuned and coupled to receive the output of the gain control circuit.
3.
A method according to claim 2 in which said phase comparator operates with a gain which is the same as a gain of said amplitude detector.
4.
A method according to claim 2 in which the voltage controlled oscillator of the phase locked loop is set up with a default power up frequency matching that of the selected carrier frequency