Patent GB-1475512-A (GB)
Title: Television signal detecting circuits
IPC: H04N5/44
Filed: 1974-06-18
Abstract: 1475512 Detecting end of pulse train; muting; radio receiving circuits SONY CORP 18 June 1974 [20 June 1973] 27006/74 Headings H3T and H3Q In a television signal detecting circuit having a resistor 23, Fig. 1 and a capacitor 24 connected in series and a switching device Q2, Q12 connected in parallel with the capacitor and in use being switched on and off in dependence on a synchronizing signal of a television signal, a staircase-wave generator 22 is coupled to the junction of 23 and 24 and a voltage detector Q13, Q14 is coupled to the junction of 23 and 24 and to the output of the staircase-wave generator so that in use the output of the detector indicates the presence or absence of the television signal. As disclosed line flyback pulses applied from terminal 3 to transistor Q12 render it conductive, so that transistor Q2 can then be rendered conductive by separated sync pulses applied to its base from terminal 2. The generator 22 derives a staircase voltage (Fig. 3D, not shown) which ultimately turns transistor Q13 conductive, but this does not occur unless pulses are also passed regularly to the capacitor 24 so discharging it, as when capacitor 24 is charged transistor Q14 is conductive and clamps the base of transistor Q13 to earth potential. Channel skip circuit.-If a remote control channel-change pulse is applied to terminal 4 transistor Q6 becomes conductive and transistor Q7 non-conductive. As a result a multivibrator comprising transistors Q8, Q9 goes into oscillation and its output periodically energizes coil 6 of tuning drive means. The tuner is then stepped round; if it tunes to an unoccupied channel, transistor Q13 remains non-conductive and diode 7 is biased off. As a result, even when the positive remote control signal is absent, transistor Q6 is held conductive and rotation of the tuner continues until the presence of a signal in the selected channel causes transistor Q13 to become conductive; transistor Q6 is thereby biased non-conductive and the multivibrator Q8, Q9 stops oscillation. When a vacant channel is selected, i.e. transistor Q13 is non-conductive, the time-constant of the multivibrator Q8, Q9 is changed so as to increase the oscillation frequency. Switch-off circuit.-If switch 8 is closed, the receiver is switched off when transmission ceases. At cessation of the signal, transistor Q13 becomes non-conductive and after a period determined by capacitor 10 and resistor 9, biases transistor Q5 conductive. Transistor Q10 of flip-flop 11 then becomes conductive and transistor Q11 non-conductive so that relay 12 is de-energized and the power source of the receiver switched off. Diode 13 serves to discharge capacitor 10 through transistor Q13 when the switch 8 is closed so as to make constant the period elapsing between transistor Q13 becoming non-conductive and transistor Q5 conductive. In addition diode 14 serves to earth the base of transistor Q5 so as to prevent reversal of flip-flop 11 by noise after the receiver has been switched off. A manual push-button switch 15 when operated reverses flip-flop 11 to close or interrupt the power supply.