| 摘要 |
<p>1379698 Stereophonic radio systems L R KAHN 5 April 1973 [10 May 1972] 16277/73 Heading H4L In a system of the type disclosed in Specification 970,051 an infrasonic frequency control signal is transmitted to switch automatically a co-operating receiver into the stereo mode of reception. Transmitter.-The L and R signals are added at 12, Fig. 11, delayed at 16 and supplied to the amplitude modulator of the transmitter. A 15 Hz oscillator 52 may be switched in for stereophonic transmission in one arrangement. The difference between the L and R signals is derived at 10 and supplied via a low-pass filter 24 to a phase shifter 40 providing a 90 degree phase relationship with the output of phase shifter 16. The L-R signal from 10 is frequencydoubled at 32 and added at 38 to the output of phase shifter 26 and the resultant is supplied via adjustable delay 44 to a phase-modulator 46 receiving a carrier wave from oscillator 45. In an alternative method of transmitting the control signal the output of a 15 Hz oscillator 58 is supplied to frequency or phase modulate the carrier from 45. The output of phase-modulator 46 is frequency-multiplied at 48 and the resultant signal is amplitude-modulated by the output 18 to provide a signal in which the two first-order sidebands carry the L and R signals respectively. Receiver.-The output of a conventional superheterodyne receiver 62, 64, 66, 68, Fig. 2, is supplied to upper and lower sideband filters 72, 74 followed by detectors 94, 96, respectively, and to a double sideband detector 76. The output of detector 76 is used for A.G.C. and also as the monophonic signal supplied via electronic switches 82, 84 to the output. In the case of a stereo signal, filter 74 also acts as a slope detector for the control signal which is supplied to a voltage comparator 102 receiving the two D.C. voltages produced in envelope detectors 94, 96. When the receiver is properly tuned and a control signal is present, the two D.C. voltages are approximately equal and the control signal is supplied from gate 102 via amplifier 108 to energize a stereo indicator lamp SL which may also be used as a tuning indicator. If the two voltages are unequal, the control signal is blocked. The output of gate 102 is also supplied via diode 114 to control the switches 82, 84 to connect the respective stereo signals to the outputs 86, 90 and 88, 92, respectively. In a modification, the outputs of the upper and lower sideband filters are added and supplied to a double sideband detector to provide the monophonic signal and A.G.C.</p> |
