TV on-screen displays and interference

How come TV on-screen displays (I mean the display of volume, channel number, etc generated by the TV itself, not the on-screen channel logos etc generated by the TV station) are affected by interference? If I turn to a channel that is not tuned to anything, the on-screen displays jump around all over the place and break up like the snow that they sit on top of.

Surely if the display is generated by the TV it shouldn’t matter that there is no picture underneath?

Part of the transmitted TV signal is something called the sync pulse. This is transmitted after each pass from top to bottom of the screen (60 times per second in the US). This tells the receiver that it’s time to go back to the top of the screen and start scanning to the bottom again. If the signal is weak or jumbled, the sync pulse is not strong enough for the receiver to lock onto, and everything on the screen is unsynchronized.

The stability of a TV image depends on a signal that is carried as part of the broadcast signal. (I think it is called the vertical sync signal but can’t remember for sure. I’m sure someone will come along and correct me if I am wrong.)

Anyway, without the signal your TV cannot produce a stable picture and so even the internally produced displays are garbled.

If the noise sufficiently disrupts the horizontal and vertical sync signals, then the display will be jittery. Some units generate the OSD completely internally and there is no external video signal present. Those are unaffected.

Thanks guys… thought it was something along those lines. Presumably this is the same reason that once the signal gets weak enough, the whole image “rolls” up or down the screen?

Yep, that’s it exactly.