Demodulation of FM audio carrier
AssigneeSGS-Thomson Microelectronics S.A.
InventorsWayne L. Horsfall, Gary Shipton
Life of this patent
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Abstract
A method of demodulating an FM carrier wave and a demodulating circuit are described which utilize a gain control circuit. An output is supplied from the gain control circuit to a phase locked loop circuit tuned to a selected carrier wave frequency. A first output of the phase locked loop circuit is used to generate a tuned gain control signal dependent on the amplitude of the input FM carrier wave to which the phase locked loop is tuned and the tuned gain control signal is used to control the gain of the gain control circuit.
Claims19
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.
A method of generating a stereo audio signal from an FM input signal which method comprises demodulating two channels of the stereo signal by use of respective phase locked loop circuits each operating according to the method comprising:
- 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 the 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.
A demodulating circuit for generating an audio signal from an FM input signal, which circuit comprises:
- a gain control circuit for receiving said FM input signal comprising a plurality of carrier waves each having a frequency and an amplitude;
- a phase locked loop circuit coupled to the gain control circuit for locking onto a selected one of the frequencies of the plurality of carrier waves, said phase locked loop circuit having a phase detector for tracking frequency variation in the carrier wave having said selected frequency;
- a tuned amplitude detecting circuit for detecting amplitude variation in said carrier wave having said selected frequency, said tuned amplitude detecting circuit further providing a tuned gain control signal;
- and
- an untuned amplitude detecting circuit coupled to receive the FM input signal to detect the sum of the amplitudes of the plurality of carrier waves in the FM input signal and generate an untuned gain control signal dependent upon the sum of said amplitudes, said tuned amplitude detecting circuit being coupled to said gain control circuit for said tuned gain control signal to control the gain of said gain control circuit when the phase locked loop circuit is locked onto the selected carrier wave and said untuned amplitude detecting circuit being coupled to said gain control circuit for said untuned gain control signal to control the gain control circuit prior to said phase locked loop being locked.
Dependencies are inferred from the claim text.