US 4,383,136 A · Muting circuit for AM stereophonic receiver

Abstract

A muting circuit for an AM stereophonic receiver of the type in which a received signal is phase modulated with a sub signal and amplitude modulated with a main signal and in which noise generated under certain adverse conditions is eliminated. A first detection signal is generated when the level of a received AM stereophonic signal is lower than a predetermined value, a second detection signal is generated upon detection of a non-locked state of a phase-locked loop circuit used to detect the sub signal, a third detection signal is generated when a frequency deviation detection senses is that a free-running frequency of a voltage controlled oscillator in the phase-locked loop circuit and a frequency of the AM stereophonic signal are different in frequency by more than a predetermined value; and a fourth detection signal is generated when a stereo pilot signal is absent. A muting circuit is activated in response to any of first through fourth detection signals. The frequency deviation detector is preferably implemented as a window detector which has an input-output characteristic including hysteresis.

Claims8

Dependent claims

A muting circuit for an AM stereophonic receiver adapted for detecting a sub signal which is a difference signal between a first channel signal and a second channel signal with a phase-locked loop circuit upon reception of an AM stereophonic signal which is obtained by amplitude modulating a carrier, which is phase modulated with said sub signal, with a sum signal of said first and second channel signals, comprising:

means for generating a first detection signal when the level of the AM stereophonic signal is lower than a predetermined value;
means for generating a second detection signal upon detection of a non-locked state of said phase-locked loop circuit;
frequency deviation detection means for generating a third detection signal when a free-running frequency of a voltage controlled oscillator in said phase-locked loop circuit and a frequency of the AM stereophonic signal are different in frequency by more than a predetermined value;
means for generating a fourth detection signal when a stereo pilot signal for said AM stereophonic signal is not present;
and muting means for muting a detected output signal in response to at least one of said first to fourth detection signals.

2 more elements

A muting circuit for an AM stereophonic receiver adapted for receiving a sterephonic signal composed of a carrier signal phase modulated by a sub signal and amplitude modulated by a main signal, comprising:

a level detector having an input coupled to receive a IF signal produced in response to a received AM stereophonic signal;
a limiter having an input coupled to receive said IF signal;
an envelope detector having an input coupled to receive said IF signal;
a phase-locked loop comprising a phase comparator having a first input coupled to an output of said limiter, a loop filter having an input coupled to an output of said phase comparator, a DC amplifier having an input coupled to an output of said loop filter and a voltage-controlled oscillator having an input coupled to an output of said DC amplifier and an output coupled to a second input of said phase comparator;
a pilot signal level detecting circuit having an input coupled to said output of said phase comparator;
a lock deviation detector having an input coupled to said output of said phase comparator;
a frequency deviation detector having an input coupled to said output of said loop filter;
an OR gate having inputs coupled to outputs of said level detector, said lock deviation detector, said pilot signal level detector, and said frequency deviation detector;
a muting circuit having a signal input coupled to said output of said phase comparator and a muting input coupled to an output of said OR gate;
and a matrix circuit having inputs coupled to outputs of said envelope detector and said muting circuit, left and right channel outputs being produced by said matrix circuit.

7 more elements

Dependencies are inferred from the claim text.

Muting circuit for AM stereophonic receiver

Life of this patent

Muting circuit for AM stereophonic receiver

Life of this patent

Abstract

Claims8