| 摘要 |
PURPOSE:To improve the noise margin of a character broadcast receiver without giving any effect on the normal TV display, by eliminating the sound trap of 4.5HMz of the base band section only at a VBL period where a character signal is superimposed. CONSTITUTION:A digital signal corresponding to the superimposed position of a character signal is generated from a digital signal generating circuit 9. A switch 11 is closed only when a signal from the circuit 9 goes to ''1''. A buffer 10 is a circuit not giving noise to a signal for sound detection because of on/ off of the switch 11. In this constitution, the switch 11 is closed only during the character signal superimposing period and no sound trap 6 is operated. That is, the sound trap at 4.5MHz is only due to the sound trap at a video intermediate frequency filter as shown in broken lines in Figure, and the gentle shoulder characteristic of the sound trap 6 at the base band section compensates the relative amplitude at 2.86MHz being attenuated conventionally by 2dB into 0dB. This operation operates only during the superimposing period of the character signal, then no effect is given on the normal picture. |
