[mythtv-users] Changing Physical Locations (zip code and cable provider)

Brian Wood beww at beww.org
Sun Dec 31 20:17:16 UTC 2006

On Dec 31, 2006, at 11:27 AM, mailing list wrote:

> Brian, I followed your instructions and everything seemed to work  
> out on that front.  Big thanks!
> Now here's the thing, At my old location I was on broadcast (rabbit  
> ears) and recording and playback seemed fine.  But at the new  
> location using cable, Playback is choppy and seems to stutter every  
> other second.
> Is there something different about processing cable input??  I  
> didn't change any settings except for broadcast to cable somewhere  
> and the refilling of the database.

Well RF is RF is RF..., but cable signals differ in a few ways:

The frequency assignments are different, and can be "standard", "HRC"  
or "IRC". You seem to have that set correctly though.

The aural carriers on a cable system are run from 15-17 db. below the  
visual ones, as opposed to 10db. down for a normal broadcast signal.  
This is unlikely to cause the problem you describe, normally it  
causes an increased tendency for buzzing in the audio, especially  
with a lot of white or characters on the screen.

You will never have an upper adjacent channel from an off-air  
antenna, unless you are between two markets. This is the reason for  
the lower level of the cable aural carriers. This problem usually  
results only in diagonal lines in the picture though.

Signal levels from a cable system tend to vary a lot more than  
antenna signals. This is because the attenuation of coax cable varies  
with temperature. This is especially true on the coasts where they  
use black pvc-jacketed cable to prevent salt corrosion - sun shining  
on black plastic gets a lot hotter than on bare aluminum.


So the mere fact you are using cable instead of the antenna shouldn't  
really cause your problem, though it is possible that if the cable  
signals are particularly noisy they might be causing larger mpeg  
files. Noise compresses extremely poorly.

You are somewhat on the low end of the CPU requirements for SD (don't  
even think about HD on that CPU), and the PCI video card is perhaps a  
tad marginal as well.

But if it worked before it should work now. I would first make sure  
you have as clean a signal as possible going into the card, then  
perhaps try reducing the max bitrate a bit and see if that helps.

If your cable system is using so-called "baseband" converters the  
output can be pretty non-standard. I am assuming that you are putting  
the cable RF directly into the PVR card. BB converters are pretty  
rare anymore, they cost too much :-)

Since you're running Mythdora you might post the problem on their  
forum, if you haven't already.

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