Patent GB-1395768-A (GB)
Title: Automatic tuning apparatus
IPC: H03J7/28
Filed: 1972-05-05
Abstract: 1395768 Channel selecting circuits; AFC MATSUSHITA ELECTRIC INDUSTRIAL CO Ltd 5 May 1972 [8 May 1971 (5)] 21168/72 Headings H3Q and H3A In an automatic tuning apparatus for TV receivers having first and second frequency sweeping means 2, 3 and 4, 5 which sweep their frequencies in steps from respective first and second starting frequencies (#(S1), #(S2), Fig. 3, not shown), and a control means that stops the sweeping of one of the sweeping means and starts the sweeping of the other sweeping means on detection that the difference in frequency of the sweeping means reaches a first predetermined frequency and vice versa on detection of a frequency difference of a second predetermined value, the control means stops sweeping of both the sweeping means on detection that the frequencies of the first and second frequency sweeping means reach respective predetermined frequencies corresponding to a selected broadcasting channel, the first and second starting frequencies are lower than the lowest frequency of a broadcasting channel to which the apparatus is to be tuned and the first frequency sweeping means sweeps to enable the apparatus to tune to the selected channel in a number of successive sweeps each covering a frequency range which is less than the frequency difference between neighbouring broadcast channels. This frequency difference can be 1/n where n is an integer greater than one of the frequency difference between neighbouring broadcasting channels and the frequency range of each sweep can be set to be smaller than the frequency difference between a video signal carrier and an audio signal carrier of the broadcasting channel. Oscillators 2 and 4 contain varactors and alternately sweep under control of voltage sweep circuits 3 and 5. The output frequency of the oscillator 2 or 4 which is sweeping is compared with the other oscillator 4 or 2 and when the frequency difference reaches a predetermined value as set by standard frequency oscillators 8, 9 a frequency comparator 10 provides an output to a pulse shaper 11 which operates via a gate 12, a switching circuit 13 which stops one sweep oscillator and starts the other. The frequency difference between the first and the second standard frequencies can be set to be ¢ of the frequency difference between the video carrier frequencies of adjacent channels. The output from the switching circuit 13 is counted in a counter 26 and is compared in 27 with a preselected number set in a memory 25 by operating a key channel setting arrangement 24. When the count in the counter 26 reaches the preselected channel number the comparator 27 operates to disable the gate 12 so as to stop the sweep of a main voltage sweep circuit 32, Fig. 4, of sweep circuit 3. This makes the frequency of the oscillator 2 slightly below the local oscillation frequency for the selected channel. Fine frequency sweeping of the local oscillator 2 is then obtained by an auxiliary voltage sweep circuit 43. If a video signal of the selected broadcast channel is detected by the IF detector 28 the frequency sweep of the auxiliary voltaga sweep circuit 43 is stopped on this signal. If the charge on the capacitor 33 in the main sweep circuit 32 decreases the charge on the capacitor 44 of the auxiliary voltage sweep circuit then increases to maintain the tuning voltage to the local oscillator 2 constant. The main voltage sweep circuit 32, local oscillator 2, mixer 6, IF amplifier 7, IF detector 28 and the auxiliary voltage sweep circuit 43 act as an automatic control loop so as to maintain the local oscillator tuned to the selected received signal. If the voltage across the capacitor 44 exceeds a certain value a Schmitt circuit 51 operates to recharge the capacitor 33 in the main voltage sweep circuit 32. The output from the auxiliary voltage sweep circuit 43 is then reduced by the automatic control loop so as to retain the resultant output voltage of the sweep circuits at 58 to the local oscillator 2 constant. When the TV signal is interrupted after being tuned to the IF detector 28 output fails and the auxiliary voltage sweep circuit 43 starts to sweep. However, a clamping circuit 1<SP>1</SP> operates to prevent the output of the Schmitt circuit 51 from operating the main sweep circuit 32. When the TV signal is restored the auxiliary sweep circuit 43 is controlled to maintain the tuning to the received signal. A detailed circuit of the switching circuit 13 is disclosed (Fig. 6, not shown). By counting the output of the switching circuit 13 the channel tuned to can be digitally indicated. Channel selection by remote control may be obtained. Specifications 1,377,100 and 1,389,071 are referred to.