Abstract: In a radio receiver including a demodulating circuit with monaural and stereophonic modes for respectively demodulating a monaural broadcast signal having a carrier amplitude modulated with a monaural signal and demodulating an AM stereophonic broadcast signal having a carrier modulated with the sum and difference of right- and left-channel stereophonic signals and with a relatively low frequency pilot signal, and a tuning section operable for tuning the receiver to receive a broadcast signal whose carrier has a selected frequency; a filter is effective to extract the pilot signal from the received broadcast signal when the latter is an AM stereophonic broadcast signal and the stereophonic mode of the demodulating circuit is established in response to the extracted pilot signal, a disturbance signal generated upon operation of the tuning section or in response to external noise and which is in a frequency range near to the frequency of the pilot signal so as to be possibly extracted with the latter, and there

Abstract: An AM stereophonic signal transmitter includes a matrix circuit which generates a sum signal corresponding to the sum of left and right channel stereophonic signals and a difference signal corresponding to the difference between the left and right channel stereophonic signals; a first phase shifting circuit which phase shifts the sum signal by -45.degree.; a second phase shifting circuit which phase shifts the difference signal by +45.degree.; a sub-modulator which modulates the phase-shifted difference signal by the phase-shifted sum signal to produce a first modulated output signal having the form Y.sub.+ (1+m.sub.t X.sub.-), where X.sub.- represents the phase-shifted sum signal, m.sub.t represents the modulation factor of the sub-modulator and 1 represents a carrier component; a modifying circuit for modifying the phase-shifted sum signal to produce a modified signal having the form .sqroot.(1+X.sub.-).sup.2 -Y.sub.+.sup.2 (1+m.sub.t X.sub.-).sup.2, where Y.sub.

Abstract: A stereo demodulating circuit for an AM stereo receiver includes an IF stage for producing an IF signal from a received AM stereo signal of the type having a carrier amplitude-modulated with the sum of left and right channel signals and the carrier phase-modulated with the difference of the left and right channel signals; an envelope detector for producing a sum signal corresponding to the sum of the left and right channel signals in response to the IF signal, and having level information, amplitude-modulation information and a DC component; a negative peak limiter for limiting the minimum level of the sum signal from the envelope detector; a divider for dividing the IF signal by the sum signal with its minimum level limited to produce a phase-modulation signal; a phase-locked loop for producing a non-modulation signal from the IF signal; a low-pass filter for removing the amplitude-modulation information from the sum signal from the envelope detector to produce a level information signal having only the leve

Abstract: A dividing circuit for dividing a received AM stereo signal, which has been multiplied by a distortion correcting signal cos .phi., by the distortion correcting signal, including a first differential amplifier comprised of first and second transistors, the bases of the transistors being supplied with the received AM stereo signal, at least one transistor for supplying the distortion correcting signal to either the emitters or collectors of the first and second transistors, at least one diode connected between the collector of the first transistor and a reference voltage source and between the collector of the second transistor and the reference voltage source, a current source connected in parallel with each at least one diode for maintaining the emitter resistances of the transistors at a small constant value, and a second differential amplifier connected in cascade with the first differential amplifier for producing the divided output signal of the dividing circuit.

Abstract: Apparatus for receiving an AM stereophonic signal of the type having monaural information represented by amplitude modulations of a signal carrier and stereophonic information represented by phase modulations of the signal carrier. A reference signal generator generates a reference signal that is synchronized with the signal carrier of the received AM stereophonic signal. In one embodiment, the reference signal generator is a phase-locked loop which generates a reference carrier whose frequency is substantially equal to the signal carrier frequency and whose phase is locked to a predetermined signal carrier phase. An amplitude limiter limits the AM stereophonic signal to remove amplitude modulations of the signal carrier and thereby produce an amplitude-limited signal carrier. A demodulator demodulates the information represented by the AM stereophonic signal, and a detector is provided to detect if the stereophonic information will be properly demodulated.