[mythtv-users] Using Cable-tv certain channels fuzzy

[Daniel Leaberry]

On 7/3/06, Brian Wood <beww at beww.org> wrote:
>
>
> On Jul 3, 2006, at 5:58 PM, Daniel Leaberry wrote:
>
> > This is a mystery to me so I figured I'd let others offer help.
> >
> > I receive basic cable (about 20 channels) at my house and use a
> > pvr-500 to record with a dedicated backend in the garage. 1 week
> > ago channels 7-13 began to show moderate amounts of noise. I
> > changed nothing. Today I had the technician come out and check my
> > line. The signals are fine, the filters are fine everything seems
> > fine on their end. The setup has been running flawlessly for over 2
> > months.
> >
> > Things I've tried:
> >
> > 1) replacing the cable between the pvr-500 and the splitter (one
> > end goes to the cable modem)
> > 2) fine tuning the frequencies using ivtv
> > 3) rebooting
> >
> > Things I suspect might have something to do with it:
> >
> > 1) The backend runs in the garage which is un-airconditioned and
> > typically between 80-95 degrees. Maybe the heat affects only
> > certain channels? I would think it would add noise to all the
> > channels.
> >
> > My setup is as follows:
> >
> > *Dedicated backend running dual p3's and pvr-500. lspci -v of the
> > tuner is as follows
> >
> > 02:09.0 Multimedia video controller: Internext Compression Inc
> > iTVC16 (CX23416) MPEG-2 Encoder (rev 01)
> >         Subsystem: Hauppauge computer works Inc. WinTV PVR 500 (2nd
> > unit)
> >         Flags: bus master, medium devsel, latency 64, IRQ 7
> >         Memory at f4000000 (32-bit, prefetchable) [sizedM]
> >         Capabilities: [44] Power Management version 2
> >
> > I'm using ivtv 0.4.5 on kernel 2.6.15 (gentoo).
> >
> > Just fishing for possible issues. I would love to hookup a tv and
> > see if it's just the card but I don't own one (The frontend
> > connects to a 21" monitor). I'll have to ask the neighbor and see
> > if I can borrow one.
> >
>
> Interesting that you mention channels 7 - 13, as that is what's known
> as the "high band" VHF channels. You could not have picked those
> numbers at random unless you knew about frequency allocation, or you
> have a problem directly related to frequency.



I know nothing about frequency allocation (I'm glad you do!)

In fact, if you have only 20 or 22 channels, and are putting them on
> a wire in frequency order, 7 - 13 would be the highest in frequency
> of all.
>
> I know that sounds strange, but the actual order would be :
>
> 2 - 6 (low band)
> 14 - 22 (mid-band)
> 7 - 13 (high-band)



This definitely seems to be it. I didn't mention it but channels 14-22 have
a slight amount of noise. I wasn't sure if it was just me not remembering
what a clear channel looked like but put in this context the whole thing
makes sense.


Coaxial cable attenuates RF energy at a rate proportional to
> frequency. In fact, a piece of cable that has 10db. of loss at
> channel 2 (54 Mhz.) will have 20db. of loss at channel 13 (220 Mhz.).
>
> Cable loss also increases with the temperature of the cable, and
> proportionally with frequency, so as a cable heats up channels 7 - 13
> will be affected the most. Chennel 13 will be affected twice as much
> as channel 2.
>
> So if you were experiencing problems due to either your garage or
> your cable system in general heating up, it would be expected that
> channels 7 - 13 would be affected most noticeably.
>
> Taking the noise floor as a constant (which it is not, but let's make
> things easy) a reduction in signal level would result in a
> degradation of the signal-to-noise ratio, and a noisy picture under
> low or marginal signal conditions.
>
> Thus it would not "add noise to all the channels" equally, but to 7 -
> 13 more so than the others, exactly what you are seeing.
>
> Cable techs are famous for saying "everything's fine at their end",
> when it is not. If you are splitting the signal several times you
> could well be down enough that the increased loss due to high
> temperature would be visible.


This is a local community cable system so I like to think they'd care a
little more. They pulled out something that looked like a fluke (for those
familiar with networking) and plugged the coax in to check it. I only have
one splitter. It's a two way splitter with one run to the cable modem and
one run to the pvr-500. Granted it looks like the cheapest splitter I've
ever seen.

I think you've solved my mystery Brian. I'm going to try a different
splitter, maybe put the box in the living room for a few days to test the
air-conditioning.




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