Patent GB-794342-A (GB)
Title: Apparatus for determining the transmitting station to which a radio receiver is tuned
IPC: H04H1/00
Filed: 1954-07-23
Abstract: 794,342. Radio receiving apparatus. NIELSEN CO., Ltd., A. C. July 23, 1954 [Aug. 17, 1953], No. 21601/54. Class 40 (5). A system for recording, as a function of time, the frequencies to which a radio receiver is tuned, whereby the particular programmes which have been received may be determined, comprises one or more tuned circuits which may be coupled, preferably inductively, to the aerial leads of the receiver, so that when the frequency of the coupled tuned circuit and the frequency to which the receiver is tuned are the same, energy is absorbed by the tuned circuit and a reduction in amplitude of the received signal ensues, the reduction in amplitude actuating continuously driven recording means to give an indication of the particular tuned circuit which is coupled to the receiver or of the frequency to which the coupled tuned circuit is tuned and thus provide an indication of the frequency to which the receiver is tuned. Fig. 1 illustrates the invention as applied to a television receiver 11. A variable tuned circuit 14 comprising capacitor 16 and inductor 15 is coupled by means of the inductor 15 to the aerial input leads of the receiver 11, the tuning of the variable capacitor 16 being continuously varied by a constant speed motor 18 at a slow rate, e.g. once per minute, and the circuit 14 being such that its range of resonant frequencies covers the band to which the receiver 11 may be tuned. Hence once per cycle of operation of the motor 18, the circuit 14 and the receiver 11 are tuned to the same frequency and energy is abstracted from the input signal to the receiver, thereby causing a reduction in amplitude of the signal applied to its input terminals. To enable this frequency to be determined more readily, a 1300 c/s. signal generated by an oscillator 28 is applied to the rectifier 26 thereby modulating its damping effect upon the tuned circuit 14 and thus modulating the energy extracted by the circuit 14 from the input to the receiver. A signal, including the horizontal synchronizing pulses as shown at A, is taken from the receiver 11 and passed in turn through wide band amplifier 45, limiter 56, tuned amplifier 60 tuned to 1300 c/s., amplifier 65 and detector 67, the output from which comprises a negative D.C. signal so long as the horizontal synchronizing pulses derived from the receiver are modulated in amplitude at 1300 c/s. as indicated at B. This negative-going signal is applied through relay contacts to the grid of valve 68 controlling the operation of relay 40 to switch the relay off. As a result, the energizing circuit of the constant speed motor 18 is interrupted and the motor stops; simultaneously the oscillator 28 is disconnected from the tuned circuit 14 and a constant bias is applied instead to the rectifier 26. Since this results in the negative-going signal from the detector 67 ceasing, the grid of the valve 68 is instead connected to the output from the limiter 56 which consists of negativegoing synchronizing pulses which are sufficient to maintain the valve 68 cut off. Thus so long as the receiver 11 receives a signal comprising synchronizing pulses, the valve 68 remains cut off and the motor 18 remains unenergized in a position which corresponds to the tuning of the receiver 11. The motor 18 is coupled by a shaft 80 to a recording device 76 whereby its position may be recorded thus indicating the frequency to which the receiver 11 I is tuned. When the receiver 11 is retuned the synchronizing signals fail and hence the output from the limiter 56 is insufficient to maintain the valve 68 cut off so that the relay 40 is once again operated and the motor 18 starts continuous searching once more. Fig. 2 illustrates a modification wherein the single tuned circuit tunable over the whole range of the receiver 11 is replaced by a number of tuned circuits each tuned to a spot frequency. These circuits are mounted on a drum 90 which is continuously rotated by the motor 18 so that each tuned circuit is in succession brought into coupled relation with the lead 13 connected to the aerial input terminals of the receiver 11. Each of the tuned circuits resonates at the fre. quency of a corresponding channel to which the receiver 11 may be tuned; thus when one of the tuned circuits is in resonance with the channel to which the receiver is tuned the motor 18 is held at a position which is uniquely representative of the frequency or of the channel which is being received. In a further modification, Fig. 3 (not shown), the oscillator 28 is not used and a reduction in amplitude of the horizontal synchronizing signals derived from the receiver 11 when the coupled tuned circuit is in resonance therewith is used directly as the signal effecting control of the relay which in turn controls the operation of the constant speed motor. It is stated that the invention may also be used in association with sound broadcast receivers. Specification 660,070 is referred to.