Compatible AM stereophonic receivers involving sideband separation at IF frequency

A system for receiving a compatible single sideband radio frequency carrier wave with stereo related intelligence appearing as upper and lower sidebands of the carrier wave and with an infrasonic tone also modulated on the carrier wave to indicate stereo presence and to control receiver audio output mode, comprising:

• a. means separately detecting the stereo related audio signals,
• b. means comprising the DC components of the detected stereo signals,
• c. means for demodulating out the infrasonic frequency tone,
• d. means controlling the audio output channels of the stereo receiver responsive to the presence of such tone to apply stereo related audio signal inputs thereto when the tone is present, and
• e. means passing the isolated infrasonic frequency tone to visual indicator means when the DC components of the compared stereo signals are substantially equal, such visual indicator means thereby providing an indication of stereo presence.

A stereophonic radio receiver for receiving a carrier wave modulated by two stereo related audio signals with at least most of the stereophonically distinguishable components of one stereophonic signal appearing as one first order single sideband of the carrier wave and at least most of the stereophonically distinguishable components of the other stereophonic signal appearing as the other first order single sideband of the carrier wave, and with said carrier wave further modulated at low level with an infrasonic frequency tone utilizable in the receiver to indicate the presence of stereo related signals, said receiver comprising:

• a. superheterodyne means developing an IF signal and including IF bandpass means passing both the USB and LSB of the received signal,
• b. means developing from the output of said IF bandpass means separate USB, LSB and DSB outputs at IF frequency,
• c. separate amplitude modulation detector means separately responsive to each of the USB, LSB and DSB outputs,
• d. means receiving an input from one of the detector outputs and isolating therefrom the said infrasonic frequency tone,
• e. gate means receiving as control inputs portions of the detected USB and LSB outputs including the DC components thereof, said gate means being operable to pass the controlled input thereto only when said DC components are approximately equal,
• f. means applying the isolated infrasonic frequency tone as the controlled input to said gate means, and
• g. means applying the output from said gate means to electronic switching means connected in one position to apply the respective USB and LSB detector outputs to respective stereo audio output channels and in the other position to apply the DSB detector output to both said audio output channels.

A stereophonic radio receiver for receiving a carrier wave modulated by two stereo related audio signals with at least most of the stereophonically distinguishable components of one stereophonic signal appearing as one first order single sideband of the carrier wave and at least most of the stereophonically distinguishable components of the other stereophonic signal appearing as the other first order single sideband of the carrier wave, and with said carrier wave further modulated at low level with an infrasonic frequency tone utilizable in the receiver to indicate the presence of stereo related signals, said receiver comprising:

• a. superheterodyne means developing an IF signal and including IF bandpass means passing both the USB and LSB of the received signal,
• b. bandpass filter means developing from the output of said IF bandpass means separate USB, LSB and DSB output at IF frequency,
• c. separate amplitude modulation detector means separately responsive to each of the USB, LSB and DSB outputs,
• d. gate means receiving as control inputs the DC components of the detected USB and LSB outputs and controlled thereby to pass the controlled input thereto only when said DC components are approximately equal,
• e. low pass filter means receiving an input from one of the detector outputs and isolating therefrom the said infrasonic frequency tone,
• f. means applying the isolated infrasonic frequency tone as the controlled input to said gate means,
• g. means applying the output from said gate means to stereo presence visual indicator means, and
• h. means applying the output from said gate means to electronic switching means connected in one position to apply the respective USB and LSB detector outputs to respective stereo audio output channels and in the other position to apply the DSB detector output to both said audio output channels.