Patent GB-1377100-A (GB)
Title: Channel selecting circuit arrangements for television receivers
IPC: H03J7/28
Filed: 1971-12-29
Abstract: 1377100 Channel selecting circuits; transistor oscillator and switching circuits MATSUSHITA ELECTRIC INDUSTRIAL CO Ltd 29 Dec 1971 [28 Dec 1970 (8) 3 March 1971 (6)] 60286/71 Headings H3Q and H3T In a channel selecting circuit arrangement for a television receiver first and second sweep means supply a main variable frequency local oscillator 24 having a variable capacity tuning diode and an auxiliary variable sweep oscillator 21, 22, switching means 31 stops the sweep of one of the two oscillators which is sweeping and starts the sweep of the other oscillator which is not sweeping on each occasion that the difference in frequencies of the two oscillators reaches a predetermined value, digital counting means 7 counts the number of occasions and a memory means 5 digitally stores the number of a selected channel. A comparator means 6 compares the number stored in the memory means 5 with the number counted by the counting means 7 and when these numbers are coincident generates a coincidence signal (at 9) and control means 39, 40, 34 causes the switching means to stop the repetitive sweep operations in response to this coincidence signal so that when the sweeping is halted the voltage supplied to the variable capacitor tuning diode causes the local oscillator 24 to oscillate at a frequency corresponding to that of the selected channel. In the tuning arrangement shown for a TV receiver the required channel is read into a register counter 5 by a keyboard 1 which also resets a counter 7, a switching circuit 8 and a bistable flip-flop 31. A band switch 19 operates the auxiliary sweep oscillator 21 and the local oscillator 24 in the tuner 23 in the lower band of the VHF band. The auxiliary oscillator 21 starts immediately sweeping and the local oscillator 24 operates at a frequency of the IF plus the frequency of the channel 0. The oscillation frequency of the local oscillator 24 is determined by a voltage from a voltage memory 26 and an auxiliary voltage sweep circuit 27 applied to a variable capacitance diode. When the oscillation frequency of the auxiliary sweep oscillator 21 becomes 3 MHz, i.e. half the channel width of 6 MHz, higher than the local oscillator 24 as detected by a detecting circuit 32 a channel stepping-up signal is applied to the flip-flop 31. The output from the flip-flop 31 stops the auxiliary sweep oscillator 21 and starts the main voltage sweep circuit 25 of the local oscillator 24. When the frequency of the local oscillator 24 becomes 3 MHz higher than the auxiliary sweep oscillator 21 a channel stepping pulse is obtained from the detecting circuit 32 to operate the flipflop 31. The output from the flip-flop 31 is applied to the counter 7 and if the contents of the counter 7 coincides with that stored in the register counter 5 the comparator 6 provides an output, which if applied simultaneously with an output from 31 to gate 39 deenergizes the flipflop 31. This stops the sweep of the main sweep circuit, the sweep voltage of which is stored in the memory circuit 26. The coincidence signal from the comparator 6 also starts the auxiliary voltage sweep circuit 27 which increases the frequency of the local oscillator 24 and an output from a frequency discriminator 11 halts further charging of a capacitor in the auxiliary sweep circuit 27 so as to automatically control the local oscillator frequency for the desired channel selected. The discriminator 11 may be replaced by a synchronizing separator or an AGC voltage circuit. Band switching.-When channel stepping pulses (4, 13 and 0, Fig. 2, not shown) are generated a band switching pulse is obtained from a band switch circuit 19 to reset the auxiliary sweep oscillations 21 and 22 and the main sweep circuit 25.  Channel indicator.-The binary coded signals stored in the register counter 5 are applied via decoders 48 1 , 48 2  to channel indicators 49 1 , 49 2 . Alternatively the binary coded signals are applied to a memory 50 the output of which is mixed with the video signals in a mixer 51 to be displayed on a picture tube 52. Remote control channel selection may be obtained by operating start button 53 which resets the circuits and starts the signal sweep described above. If after the auxiliary sweep circuit 27 has completed its sweep no output has been obtained from the discriminator 11 the flyback line pulse derived from 27 is applied via 58, 55, 57 to increase the content of the register counter 5 by one to become greater by one than the content in counter 7 so that there is a non coincidence signal from the comparator 6. The above sweeping action is continued until a selected TV channel is received. The remote control may be a switch connected by a cable to the TV receiver or may be by ultrasonic control. Tuning memory.-The power supply to the register memory 5 can be maintained even when the TV receiver is switched off so that when the receiver is switched on again it receives the same channel as when it was switched off. In a transistor circuit (Figs. 3A, 3B, 3C, not shown) which shows part of a modification of Fig. 1 and which excludes the auxiliary voltage sweep circuit 27, the flip-flop 31 (62) alternately switches a main voltage sweep circuit 25 (60) and voltage memory circuit 26 (60) and an auxiliary voltage sweep circuit (61) so as to alternately sweep the local oscillator 24 (63) and the auxiliary sweep oscillator 21 (64). A 3 MHz difference between these oscillators is detected by the detecting circuit 32 in the form of a narrow band amplifier including a piezoelectric resonator (82) and transistor (80, 83). The output from the detecting circuit operates via the wave shaping circuit 33 (84), gate 34 (85) and a monostable circuit (86) the flip-flop 31 (62). The local oscillator (63) and the auxiliary sweep oscillator (64) are tuned by variable capacity diodes (65, 66, 106) and waveband switching diodes (70, 71) are included and are controlled by a transistor circuit (75), e.g., to switch between higher and lower VHF bands. A clamping circuit (93) determines the lowest channel frequency. The main and auxiliary voltage sweep circuits (60, 61) are reset by discharging capacitors (108, 110) by firing SCR's (91, 92) The output of the flip-flop (62) is counted through a differentiating circuit (107) and when a desired channel is selected a signal is applied to a terminal (100) so as to stop the sweep.