AM Stereophonic signal receiver with electric field strength detection

Claim 2

An AM stereophonic signal receiver comprising: detecting means operating in response to a received AM stereophonic signal for producing a sum signal representative of the sum of first and second channel signals and a difference signal representative of the difference between said first and second channel signals; electric field strength detecting means for producing a detection signal when an electric field strength of said received AM stereophonic signal is smaller than a first predetermined value; a low-pass filter having an input coupled to receive said difference signal, said low-pass filter operating in response to said detection signal to reduce high frequency components in said difference signal; and matrix circuit means operating in response to said sum signal and an output of said low-pass filter for producing said first and second channel signals.

Claim 3

The AM stereophonic signal receiver as claimed in claim 1 wherein said electric field strength detecting means comprises control signal generating means for generating a control signal corresponding to said electric field strength of said received AM stereophonic signal, and said low-pass filter comprises a variable control type low-pass filter having a treble cut-off characteristic controlled according to said control signal.

Claim 4

The AM stereophonic signal receiver as claimed in claim 2 wherein said control signal generating means comprises a plurality of comparators, each of said comparators having a first input coupled to a reference voltage source with the voltage applied to said first inputs being different for each of said comparators and said comparators having a second input coupled in common and wherein said control signal generating means further comprises a level detector having an input coupled to said received AM stereophonic signal and an output coupled to said second inputs of said comparators.

Claim 5

The AM stereophonic signal receiver as claimed in claim 1 further comprising switch means bypassing said low-pass filter, said switching means operating in response to an output of said electric field strength detecting means for bypassing said low-pass filter when said electric field strength is greater than a second predetermined value.

Claim 6

The AM stereophonic signal receiver as claimed in claim 4 further comprising second switch means coupled between said output of said low-pass filter and said matrix circuit means and means for setting said second switch means in an ON state when a pilot signal is received and in an OFF state when no pilot signal is received.

Claim 7

The AM stereophonic signal receiver as claimed in claim 4 further comprising second switch means coupled between said output of said low-pass filter and said matrix circuit means, pilot signal detecting means operating in response to said different signal for producing a signal indicative of the presence of a stereophonic pilot signal; a comparator having a first input coupled to a fixed voltage source and a second input coupled to an output of said electric field strength detecting means; and an AND gate having a first input coupled to said signal indicative of the presence of a stereophonic pilot signal, a second input coupled to an output of said comparator and an output coupled to a control input of said second switch means wherein said second switch means is in an ON state for received stereophonic signals and in an OFF state for monophonic signals.

Claim 8

An AM stereophonic signal receiver comprising: a main signal detector for producing a sum signal representative of the sum of first and second channel signals; a sub signal detector for producing a difference signal representative of the difference between said first and second channel signals, said main signal detector and said sub signal detector having inputs coupled to a IF output of said receiver; a variable control type low-pass filter having an input coupled to said difference signal; matrix circuit means for producing first and second channel signals, said sum signal being coupled to a first input of said matrix circuit means; monophonic/stereophonic switch means coupled between an output of said low-pass filter and a second input of said matrix circuit means; bypass switch means coupled to bypass said low-pass filter; level detecting circuit means having an input coupled to said IF signal for producing a signal corresponding to the electric field strength of a received signal; a plurality of first comparators, said comparators each having a first input coupled to said signal corresponding to the electric field strength of said received signal at an output of said level detecting means and each of said comparators having a second input coupled to a reference voltage source with the voltage applied to said second inputs of each of said comparators being different, outputs of said comparators being coupled to control inputs of said low-pass filter; pilot signal detecting means having an input coupled to said difference signal for producing a signal indicative of the presence of a received stereophonic signal; a second comparator having a first input coupled to an output of said level detecting means and a second input coupled to a reference voltage source; and an AND gate having a first input coupled to said signal indicative of the presence of a stereophonic signal and a second input coupled to an output of said second comparator, an output of said AND gate being coupled to a control input of said monophonic/stereophonic switch means.

Claim 9

An AM stereophonic signal receiver comprising: a main signal detector for producing a sum signal representative of the sum of first and second channel signals; a sub signal detector for producing a difference signal representative of the difference between said first and second channel signals, said main signal detector and said sub signal detector having inputs coupled to a IF output of said receiver; a variable control type low-pass filter having an input coupled to said difference signal; matrix circuit means for producing first and second channel signals, said sum signal being coupled to a first input of said matrix circuit means; monophonic/stereophonic switch means coupled between an output of said low-pass filter and a second input of said matrix circuit means; bypass switch means coupled to bypass said low-pass filter; level detecting circuit means having an input coupled to said IF signal for producing a signal corresponding to the electric field strength of a received signal; a plurality of first comparators, said comparators each having a first input coupled to said signal corresponding to the electric field strength of said received signal at an output of said level detecting means and each of said comparators having a second input coupled to a reference voltage source with the voltage applied to said second inputs of each of said comparators being different, outputs of said comparators being coupled to control inputs of said low-pass filter; pilot signal detecting means having an input coupled to said difference signal for producing a signal indicative of the presence of a received stereophonic signal; a second comparator having a first input coupled to an output of said level detecting means and a second input coupled to a reference voltage source; and an AND gate having a first input coupled to said signal indicative of the presence of a stereophonic signal and a second input coupled to an output of said second comparator, an output of said AND gate being coupled to a control input of said monophonic/stereophonic switch means.