Patent GB-1372576-A (GB)
Title: Channel selection devices for television reseivers
IPC: H03J7/28
Filed: 1971-11-04
Abstract: 1372576 Channel selection arrangements MATSUSHITA ELECTRIC INDUSTRIAL CO Ltd 4 Nov 1971 [4 Nov 1970 (15)] 51219/71 Heading H3Q In a channel selection device for a television receiver having two sweep oscillators 21 1 ; 21 2  or 22 1 ; 22 2 , each operable to sweep progressively at least over the same predetermined frequency range, control means control the two sweep oscillators so that they sweep only intermittently with one oscillator sweeping while the other is not sweeping and vice versa, and the control means is operable to change over the sweeping and non-sweeping states of the two oscillators each time the instantaneous difference in the operating frequencies of the two oscillators reaches a value equal to a predetermined frequency interval and at the same time provides an output signal. On selecting a desired channel on a keyboard 1, a binary code representing the desired channel is read into a register counter 5, a counter 7 is reset to zero, a switching circuit 9 is reset to release a main sweep circuit 39 holding circuit and a voltage + B is supplied through a band switching circuit 19 to sweep oscillators 21 1  21 2  so that the local oscillator 24 in a tuner 23 operates in the low frequency band. Only one 21 1  of the sweep oscillators 21 1 , 21 2  starts sweeping, the other 21 2  oscillates at a constant frequency output. When the difference in frequency between the two sweep oscillators 21 1 , 21 1  is 6 MHz. (at A, A<SP>1</SP>, Fig. 2(c), not shown) this difference is detected by a detector 25 and a pulse (Fig. 2(d), not shown) is produced at 26 which is applied to a flip-flop 28. This controls gates 29 1 , 29 2  so as to stop the sweep of the oscillator 21 1  and start the sweep of the other oscillator 21 2 . The pulse produced at 26 is also applied to the counter 7 so that a binary number corresponding to channel 1 is now stored in the counter 7. When the operating frequencies of both the sweep oscillators 21 1 , 21 2  become equal (at B) this is detected by a zero beat detecting circuit 30 which provides a stop pulse (Fig. 2(e), not shown) from a wave shaping circuit 31 to an AND gate 32 which is held disabled by the absence of an output from the comparator 6 provided the binary coded signal stored in the register counter 5 and the counter 7 do not coincide. When the difference in frequency between the oscillators 21 1 , 21 2  is again detected by 25 (at C<SP>1</SP> C) the flip-flop 28 is operated to change the states of the oscillators so that 21 2  stops sweeping and 21 1  starts sweeping. The count in the counter 7 is increased by one. Outputs from a band switching selection circuit 19 are determined by a multiplexer 15 dependent on the output from the counter and the outputs from 19 are fed at 17 1 -17 4  to tuning diodes of VHF or UHF band sweep oscillators 21 1 , 21 2  or 22 1 , 22 2  and to a tuner 23 at 18 1 -18 3  so as to determine lower and higher VHF channels and UHF channels in accordance with the count in the counter 7. The above alternate sweeping of the sweep oscillators 21 1 , 21 2  or 221, 22 2  is repeated until the output from the counter 7 corresponds with the desired channel such as Channel 3, stored in register counter 5. When this occurs the output from the comparator 6 enables AND gate 32 so that when the next stop signal occurs (at F), due to the output frequencies from the sweep oscillators 21 1 , 21 2  corresponding as detected by the zero beat detector 30, the AND gate 32 passes the stop signal to close a gate 27 so that the flip-flop 28 stops the sweeping of the oscillators 21 1 , 21 2  (at F). A tuning voltage (Fig. 2(a), not shown) is applied by superimposed main sweep voltage (I) from 39 and sub-sweep voltage (II) from 43 to a variable capacity diode in the local oscillator 24 of the tuner 23. When the frequency of the local oscillator 24 corresponds to the reference frequency of the sweep oscillators 21 1 , 21 2  zero beats are detected from a mixer 33 by a zero beat detector 34. The outputs from detector 34 and from AND gate 32 control a gate 37 so as to interrupt the power source 38 to the main voltage sweep circuit 39 so as to stop the sweep of the main sweep circuit 39 (at G). Now switching circuit 9 is actuated to open gate 41 so that the subvoltage sweep circuit 43 starts to generate a sawtooth wave (II). This increases the local oscillator frequency and when the required IF is reached a discriminator 12 closes the gate 41 so as to stop the sub sweep circuit 43 so that the receiver is tuned to the selected channel. The last voltage swept by the main voltage sweep circuit 39 is stored by a voltage memory circuit 40 which may comprise a capacitor charged by the main voltage sweep circuit and which controls the gate of a F.E.T. The output voltage from the F.E.T. controls the operating point of the subvoltage sweep circuit 43. A signal from switching circuit 9 via OR gate 35 closes gate 37 when the gate 41 is open so as to allow the main voltage sweep circuit to be held in a non-sweep state even when no zero beats are obtained from 34 in response to the local oscillator frequency slightly increasing. The output from the discriminator 12 is also applied to AND gate 11 so that when a coincidence signal from the comparator 6 also occurs the switch 8 disconnects the supply +B from the sweep oscillators 21 1 , 21 2 . The main sweep voltage may be applied to one end of the variable capacity diode in the tuner 23 while the subvoltage sweep'is applied to the other end of that diode. The power source for the memory 5 can be independent of the power source for the T.V. receiver so that even when the receiver is cut off the memory 5 remains energized so that when the receiver is turned on the channel previously selected can be returned to. The upper limit of the sweep by the two groups of sweep oscillators 21 1 , 21 2 , 22 1  and 22 2  is determined by applying a trigger signal to trigger an SCR in the sweep circuit in response to the band selection signal from the multiplexer 15. Remote control.-By operating a remote control start button 53 the channel selection system is reset and starts to sweep selecting channels in sequence. If there is no output from the discriminator 12 for the channel tuned to by the main subvoltage sweep circuits 39, 43; corresponding to no transmission on that channel, at the end of the sub-voltage sweep a pulse is produced by a wave shaping circuit 56 and is applied via an AND gate 55 to increase the number stored in the counter 5 by one so that the counter 5 is now higher by one than the counter 7. The coincidence output from the comparator 6 disappears and the sweep operation is repeated until the desired channel is reached. An ultrasonic wave remote control device may be used. Channel indication may be obtained from channel indicators 49 1 , 49 2  which display the first and second digits of the channel number. In a modification a read only memory 50 is added which forms a number generator, the output from which is mixed with the video signals in a mixer 51 to display the channel number on a picture tube 52.