Patent GB-1376208-A (GB)
Title: Remote control system
IPC: H03J9/04
Filed: 1972-04-18
Abstract: 1376208 Controlling and actuating by sounds PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 18 April 1972 [21 April 1971] 17865/72 Heading G4F In a television receiver remote control system employing ultrasonic signalling between a transmitter and a receiver of the system, the transmitter embodies means for transmitting successively first and second ultrasonic signals of different frequencies as a means of conveying pulse duration information to the receiver through the time difference between the two signals.  In the transmitter (Fig. 1) one of a number of function selection members 29, 31, 33 is depressed as a result of which an associated one of a group 57 of switches is closed to allow a generator circuit 1 to oscillate. Since the potential of the junction between a capacitor 43, resistor 41 and resistor 47 suddently decreases by a voltage step, thyristor 49 is out-off, transistors 53 and 23 conduct and the resonant frequency of circuit 9, 11 decreases so that the generator circuit 1 provides its first frequency, f 1 . As capacitor 43 charges, thyristor 49 is brought to conduction and switches off capacitor 21 by blocking transistors 53 and 23. Hence, the resonant frequency of circuit 9, 11 is increased and the generator circuit 1 provides its second frequency, f 2 , for as long as one of the function selection members 29, 31, 33 remains depressed. The period of time which elapses between occurrence of the two frequencies depends on which function selection member is depressed due to their different effects in the time-delay circuit 27.  In the receiver (Fig. 2) circuits 71, 73 are tuned, respectively, to the frequencies f 1 ,f 2 . On the assumption that frequency f 1  extends from time t 0  to t 1  and frequency f 2  from t 1  to t 2 , then a signal of frequency f 1  will occur at t 0  at the output of circuit 71, there being no simultaneous output from circuit 73. As a result, the base of transistor 77 receives an alternating voltage through circuit 71, and hence, due to that transistor's rectifying action, its collector voltage decreases. At t 1 , when frequency f 2  is received, diode 83 starts to conduct so that transistor 77 can no longer conduct. Thus a pulse, whose duration T 1  is determined by the time difference between the commencement of the frequencies f 1  and f 2 , is produced at the collector of transistor 77.  The pulse results in a counter 107 receiving pulses from a clock pulse generator 113 and hence one of three gates 97, 99, 101 is prepared dependent upon the number of pulses counted. At the instant that counter 107 ceases receiving pulses, the appropriate gate is enabled since the output of transistor 93 is applied thereto. With the gate enabled, pulses from a clock pulse generator 119 pass to one of the counters 121, 123 until the receiver no longer receives a signal. The latter counters provide for adjustment of volume control, tuning, luminance, contrast &c. in the TV receiver. Hence, by determining function selection from the time difference between commencement of the two frequencies f 1 , f 2 , echoes can have no adverse effects on the accuracy of such selection.