| 摘要 |
PURPOSE:To receive TV signals, differing in system, with simple circuit constitution, by switching input band characteristics of a sound IF signal automatically by a detection signal obtained by detecting the output amplitude variations of a vertical oscillating circuit due to a difference in field frequency. CONSTITUTION:The output of a vertical synchronism discriminating circuit, which inputs the 1st and 2nd vertical synchronizing signals of the B/PAL and M/NTSC systems and outputs the 1st and 2nd voltage levels synchronizing with those synchronizing signals, is applied to the terminal E of a circuit switching sound IF band characteristics. This switching circuit is provided with a sound IF detector 5, capacitors 7-10, a diode 11, an IF transformer 12, etc. For B/PAL reception, a voltage inputted to the terminal E is held at 0 to turn off the diode 11, and the band characteristics of an IF input resonant circuit are set to the reception frequency of the B/PAL system. For M/NTSC system reception, on the other hand, the voltage inputted to terminal E is held at a prescribed level to turn on the diode 11, and then the capacitor 8 is short-circuited to set the band characteristics to the frequency of the M/NTSC system. |
