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

[Brian Wood]

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) [size=64M]
>         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.

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)

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.