[mythtv-users] Video breaking up after installing new video card

[Stephen Worthington]

On Sat, 3 May 2014 00:22:51 -0400, you wrote:

>On Fri, May 2, 2014 at 9:15 PM, Phill Edwards <philledwards at gmail.com> wrote:
>>> Thanks Daryl, I'm not sure I understood what you said. So I'll describe my
>>> set up (hope this little ASCII diagram works OK!):
>>>
>>> Antenna --
>>>          |
>>>   | --> TV1 (picture quality good)
>>>   |
>>>   | --> TV2 (picture quality good)
>>>          |
>>>   | --> Cheap $5 splitter
>>>               |
>>>               | --> HD Homerun (2 tuners) --> 1Gb switch --> Mythbackend
>>>               |
>>>               | --> PCI Card --> Mythbackend
>>>
>>> Does that make sense? If you still think it's a signal strength issue is
>>> there any way I can verify that? And if I need to get a booster, are there
>>> different types which are better than others? E.g. do some install at the
>>> antenna vs others install at the wall outlet?
>>
>>
>> More info on this... This morning I put 2 programs to record at the same
>> time. All different broadcasters so I knew that all 3 tuners would be used -
>> 2 on the HD Homerun and 1 PCI card. I watched some of the recordings and
>> found two were OK, and one broke up in places. The one that broke up was
>> also highlighted yellow in Watch Recordings which I believe indicates a poor
>> quality recording.
>>
>> I assumed that the bad recording would be the PCI card, and the two from the
>> Homerun would be OK. After running some queries to track down which tuner
>> recorded which program, I found the following:
>>
>> Homerun tuner 0 = BAD recording
>> Homerun tuner 1 = good recording
>> PCI tuner 0 = good recording
>>
>> This surprised me. Surely if one of the Homerun tuners was OK, they would
>> both be OK? Unless it's to do with signal strengths varying by channel,
>> which I presume could mean:
>> - either the poorer channels aren't tuned to the right frequency, or ...
>> - they're more susceptible to a overall weakened signal strength?
>>
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>
>Ok Phill it seems you have a 3-way splitter feeding a two way
>splitter, which indicates to me that your TV is misrepresenting the
>situation to you. One 4-way splitter would be better in your case, but
>may not be logistically possible so an amplifier may be needed. What
>you have done is taken a good signal and reduced it by 3.5db with each
>split, splitters vary, it will be printed on the device "-ndb" where n
>is a numerical value. Splitting the first time gave you a weakened,
>but usable signal to your TV's, splitting again weakened it again,
>which I suspect is giving the results you describe. And you can even
>have some good and some worse recordings from the same source, but
>different channel/multiplex. Other times a bad connector/s can cause
>problems, your cable provider could diagnose this for you.

Just to clarify things about how splitters work - when you split a
signal in a two way splitter, you get less than half the signal on
each output.  How much less is determined by the loss within the
splitter itself and can vary quite a bit between good and bad quality
splitters.  With a three way splitter, you get out less than one third
of the input signal on each output.  And so on - this is basic
physics, you can not get more signal out than you put in without
adding power (an amplifier).  In terms of dB numbers, 3 db means half
the signal, so a perfect two way splitter would be rated at 3 dB loss.
Any more than 3 dB is loss in the splitter due to it being less than
perfect.

This page looks like it has some good explanations:

  http://www.hdtvprimer.com/antennas/basics.html

Your setup has two TVs and your PC splitter off the antenna.  To do
that, there has to be a splitter somewhere that is splitting the
antenna signal to the TVs and PC.  Presumably, that is a three way
splitter.  So your TVs and your PC splitter would all be getting a
little less than one third of the signal.  Previously, you were then
splitting the PC signal in two again for two HDHomerun tuners (it does
have two antenna connections on the back?).  So each HDHomerun tuner
was getting less than one quarter of the antenna signal.  Now, you
have replaced that two way splitter with a three way one.  So each of
the three PC tuners is now getting less than one sixth of the antenna
signal and for some transmitters, significantly less, and that may be
too little signal.  So the recordings will vary depending on the
sensitivity of the PC tuner used for the recording, and the level of
that transmitter as received at the tuner.

For digital recordings, a signal is either enough, or the recording
will not work at all.  The level that works depends on the sensitivity
of the tuner, which can vary with age as component values change. When
the signal is borderline, slight variations in the signal level will
take the signal level below the point where it works for short periods
and you get a dropout which causes corruption in the recording.

Ideally, what you should do is replace the first splitter coming off
the antenna to the TVs and PC with a six way splitter (or
splitter/amplifier) and add two new wires from it to the PC so that
each of the TVs and PC tuners will be getting the same signal level.
That might be too difficult or too expensive.  So the signal level at
the PC tuners can be increased by changing either (or both) the three
way splitters to splitter/amplifiers.  Replacing the first splitter
with a splitter amplifier is the right way to do it if the signal from
the antenna is a bit low in the first place.  But if it is high
enough, then an amplifier at that splitter might be producing an
output level that is too high for a tuner to operate from (outside its
specifications), so if that is the case, it is better to have the
amplified splitter at the second level at the PC.

You should be able to buy 2 to 4 way amplifier/splitters at your local
TV electronics shops or online, but in my experience here in New
Zealand, they are not particularly cheap, so if you can borrow one
from somebody to experiment with, it is worth doing that.  Larger
splitter/amplifiers (6 way and above) tend to only be available from
professional supply shops rather than retail.