[mythtv-users] Picture Bars - Was: Electrical power in my cable line? Is that bad?
beww at beww.org
Tue Sep 11 15:58:07 UTC 2007
A JM wrote:
> So, horizontal bars are more associated with grounding issues and
> vertical bars are more reception oriented and hardware oriented?
Yes, with a LOT of possible exceptions.
> I'll try getting back to the cable source (entry point) after trying a
> different set, thanks.
That's what I'd do.
> How does terminating ports impact the picture quality?
Assuming you are using standard TV cable, the characteristic impedance
should be 75 ohms throughout the entire path. Any deviation from that
will result in a "mismatch" which, in extreme cases, results in visible
The final "destination" of the signal should be terminated at 75 ohms.
If it isn't, some of the signal will be reflected back into the system
from the improperly-terminated end.
A splitter "splits" (obviously) the signal, and each individual path (or
output port) needs to be terminated properly, just as a single cable
needs to be.
It actually more complicated than that, as an improperly terminated
splitter will present a non-75 ohm impedance on all of its ports if all
of the ports are not properly terminated.
As opposed to "hybrid splitters", which are most commonly used in homes,
"directional couplers" or other "back matched devices" are less
dependent on proper terminations for correct operation, but these
devices are more expensive, and have higher insertion losses
(essentially they are self-terminating and you are "throwing away" some
of the signal in the internal termination components).
Directional couplers are actually rated as to how well they resist
changes in the main "through path" when the "tap leg" termination varies
from a short to an open circuit.
The tap-off devices of CATV systems are directional couplers, not
splitters, so as to minimize problems on the system due to individual
subscriber drop faults.
So, if everything isn't properly terminated you may see reflections in
the picture. Also, the frequency response will no longer be as "flat" as
it should be, resulting in potentially large variations of signal
strength from channel to channel. If two adjacent channels are of very
different levels that can cause picture degradation as well.
Sorry you asked ?? :-)
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