[mythtv-users] Horizontal lines with NVidia TVout

SÚrgio Gaspar smmg at nukitel.com
Sun Jun 26 21:45:48 UTC 2005


Try pluging your myth box, TV set and AV system all to the same wall socket.

If this doesn't solve your problem then it's probably a ground loop caused
by your cable provider. I had the same problem a while ago and the
solution was to get a 75ohm/75ohm balun to cut the loop, but as I was
unable to get one of those and my cable provider (TVCabo in Portugal)
refused to admit such a problem existed or was even possible, I solved the
issue by connecting the RF cable shielding directly to the earth terminal
in the wall socket. This way the current which flows because of different
earth potentials goes directly to earth by the shortest route and does not
interfere directly with demodulated signals inside your equipment, and you
get a better picture and possibly eliminate a 50/60Hz buzzing sound you
may hear coming from your AV subwoofer (have you noticed it?).

As a side note, the horizontal stripes move across the screen because the
power line frequency is not exactly the same as the video vertical
frequency.

As a second side note, I got one motherboard destroyed a few years ago
most probably because of a ground loop. Loop currents can be sometimes
large, but motherboard tracks and semiconductors are always weak.

Sergio



> Typically, a slow rolling band comes from a 60 Hz (or 50 depending on
> your localation in the world) AC source.  Check for proper ground
> (ground loops will cause this).
>
> # Endaf
>
> Stuart Morgan wrote:
>
>>On Sun 26 June 2005 21:16, Fedor Pikus wrote:
>>
>>
>>>I've had my Myth box connected to a CRT and a small Toshiba TV (NVidia
>>>composite TV-out), and had excellent picture on both. Now I moved it
>>>to my AV system and connected it to a 32" Toshiba TV (CRT), I tried
>>>SVideo and Component out, and I always get the same problem: light and
>>>dark horizontal lines or bands slowly drifting up the screen, one or
>>>two lines at a time. Very visible when picture is dark, very annoying.
>>>It is not really specific to Myth: I see these lines on Fedora login
>>>screen, on my window manager screen after I log in, and in Myth
>>>everywhere. Interlacing options in Myth don't seem to help at all (and
>>>of course they would have no effect before Myth starts). The other
>>>problem I'm seeing is pretty strong pincussion distortion: left edge
>>>of the screen bends inwards. Increasing overscan helps with the
>>>distortion, to a degree, but the scrolling lines are unaffected. Any
>>>suggestions?
>>>
>>>
>>
>>Sound to me like interference - particularly magnetic. You haven't just
>> stuck
>>two huge speakers next to the TV have you? Or failing that is there
>> anything
>>with a strong EM field nearby (Microwave, electric motor or spacial
>> anomaly)?
>>
>>
>
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