[mythtv-users] Picture Bars - Was: Electrical power in my cable line? Is that bad?

Brian Wood beww at beww.org
Tue Sep 11 14:22:01 UTC 2007

A JM wrote:
> Since this thread seems to be cable grounding oriented with regard to
> bars and lines in the picture can someone tell me what the term is for
> vertical bars in the picture that repeat across the screen?
> Is it called banding? is that caused by a bad ground or some other
> issue? similar to the problem Keith is having?

Almost certainly not.

Vertical bars can be caused by a lot of things. One common cause is a
"ghost" (reflection) of the horizontal blanking.

Think of this as a vertical bar to the left of the picture, ordinarily
out of sight. A flaw in the transmission path can cause this image to be
reflected at a time later than the original, thus showing up in the picture.

Such reflections can be caused just about anywhere: In the RF path
(atmosphere) between the transmitter and the receiving antenna (often
caused by tall buildings), in the RF path from the antenna to the TV
set, by flaws in the cable of a CATV system, even problems (like bad or
missing terminations) in the receiving device (TV set or capture device).

The problem can also be in the video path (as opposed to the RF path).

Questions to start asking are: Is the problem on every channel? Are you
using an antenna or a cable/satellite feed?  Is all the cable of good
quality and is everything properly terminated (unused ports on splitters
etc. should be terminated properly). Connectors tight?

One quick test is to try another TV set, if it looks OK then your
problem is in the receiver. You could also try feeding the output of a
VCR or DVD player directly into the set and see if that is OK.

This problem has nothing to do with grounding, which will pretty much
always produce horizontal bars moving up through the screen.


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