A receiver for stereophonic program signals which employs a narrow-band intermediate-frequency signal extracting circuit and a noise signal detecting circuit for producing a gate signal. The receiver further includes a change-over switch which is controlled by the gate signal to select the monaural or stereophonic mode of operation so that the receiver may achieve monaural reproduction when noise in the received signal exceeds a predetermined level.
A noise limiting circuit for an FM stereo receiver comprises a filter circuit 13 for grounding high frequency components in the detector output signal through a transistor switch 14 in response to a low field strength signal supplied by the IF amplifier. The filter circuit, when activated, also grounds the 19 KHz pilot signal to automatically switch from a stereo to monaural mode, which further improves the S/N ratio. The pilot signal grounding effect may be enhanced by including an inductor in the filter circuit which, in combination with the filter capacitor, is tuned to the pilot signal frequency. The transistor 14 is switched in a rapid on -- slow off mode by providing a storage capacitor 20 and discharge resistors 17, 19 in the base control circuit of the transistor, and the filter switching or squelch level may be adjusted by a variable resistor 22.
Stereo receiver circuit for processing a multiplex signal comprising a pilot signal by means of a pilot signal detector which detects frequency components in the multiplex signal at the frequency of the pilot signal and which controls a Schmitt trigger for influencing the mono/stereo operation, said circuit also comprising a control circuit capturing the noise components in the multiplex signal and acting on the Schmitt trigger. To prevent the Schmitt trigger from switching between the state for mono operation and the state for stereo operation in the case of weak reception field strengths, the control circuit is only active in mono operation and influences the input signal of the Schmitt trigger in the case of reception of sufficiently strong noise components to such an extent that the Schmitt trigger remains in its switching state for mono operation.
A radio frequency (RF) modulation apparatus for modulating input video and audio signals into a television broadcast signal is provided. The apparatus includes a buffer amplifier, a decoder, a switching device, and an RF modulator. The buffer amplifier amplifies and stores the input audio signal as a stored audio signal. The decoder inputs and decodes the stored audio signal to produce a decoded audio signal. The switching device inputs the stored audio signal from the buffer amplifier and the decoded audio signal from the decoder and selectively outputs either the stored audio signal or the decoded audio signal as a selected audio signal. Specifically, the stored audio signal is output as the selected audio signal if the input audio signal corresponds to a stereophonic audio signal or an audio multiplexing signal. On the other hand, the decoded audio signal is output as the selected audio signal if the input audio signal corresponds to a mono audio signal. Finally, the RF modulator modulates the selected audio signal and outputs the corresponding television broadcast signal.
A stereo inhibit circuit adaptable for use in an FM stereo radio having a stereo indicating light is disclosed. A level detector and a mute circuit develop a mute signal which is related to the amount of signal noise at the output of an FM detector. This mute signal is used to inhibit a stereo switch that controls the stereo indicating light while permitting the creation of audible signals by a stereo demodulator and a pair of speakers. Thus the stereo inhibit circuit permits the creation of an audible hiss and positively prevents any flickering of the stereo indicating light when the FM radio is not tuned to a station.
A noise-cancelling apparatus for use in an FM stereo receiver. This apparatus blends the signals of the left and the right channels of the receiver with each other substantially evenly throughout the entire audio frequency range of such signals so that noise components are cancelled with each other and S/N ratio is improved. This blending is accomplished either by connecting the output terminal of the left channel to the output terminal of the right channel by a common resistor, or by causing a decay, through a filter, of those frequency components higher than the pilot signal frequency among the input signals of the multiplex demodulator provided in the FM stereo receiver.
