<br><br><div><span class="gmail_quote">On 7/3/06, <b class="gmail_sendername">Brian Wood</b> <<a href="mailto:beww@beww.org">beww@beww.org</a>> wrote:</span><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<br>On Jul 3, 2006, at 5:58 PM, Daniel Leaberry wrote:<br><br>> This is a mystery to me so I figured I'd let others offer help.<br>><br>> I receive basic cable (about 20 channels) at my house and use a<br>> pvr-500 to record with a dedicated backend in the garage. 1 week
<br>> ago channels 7-13 began to show moderate amounts of noise. I<br>> changed nothing. Today I had the technician come out and check my<br>> line. The signals are fine, the filters are fine everything seems<br>
> fine on their end. The setup has been running flawlessly for over 2<br>> months.<br>><br>> Things I've tried:<br>><br>> 1) replacing the cable between the pvr-500 and the splitter (one<br>> end goes to the cable modem)
<br>> 2) fine tuning the frequencies using ivtv<br>> 3) rebooting<br>><br>> Things I suspect might have something to do with it:<br>><br>> 1) The backend runs in the garage which is un-airconditioned and
<br>> typically between 80-95 degrees. Maybe the heat affects only<br>> certain channels? I would think it would add noise to all the<br>> channels.<br>><br>> My setup is as follows:<br>><br>> *Dedicated backend running dual p3's and pvr-500. lspci -v of the
<br>> tuner is as follows<br>><br>> 02:09.0 Multimedia video controller: Internext Compression Inc<br>> iTVC16 (CX23416) MPEG-2 Encoder (rev 01)<br>> Subsystem: Hauppauge computer works Inc. WinTV PVR 500 (2nd
<br>> unit)<br>> Flags: bus master, medium devsel, latency 64, IRQ 7<br>> Memory at f4000000 (32-bit, prefetchable) [size=64M]<br>> Capabilities: [44] Power Management version 2<br>>
<br>> I'm using ivtv 0.4.5 on kernel 2.6.15 (gentoo).<br>><br>> Just fishing for possible issues. I would love to hookup a tv and<br>> see if it's just the card but I don't own one (The frontend<br>> connects to a 21" monitor). I'll have to ask the neighbor and see
<br>> if I can borrow one.<br>><br><br>Interesting that you mention channels 7 - 13, as that is what's known<br>as the "high band" VHF channels. You could not have picked those<br>numbers at random unless you knew about frequency allocation, or you
<br>have a problem directly related to frequency.</blockquote><div><br><br>I know nothing about frequency allocation (I'm glad you do!)<br></div><br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
In fact, if you have only 20 or 22 channels, and are putting them on<br>a wire in frequency order, 7 - 13 would be the highest in frequency<br>of all.<br><br>I know that sounds strange, but the actual order would be :<br>
<br>2 - 6 (low band)<br>14 - 22 (mid-band)<br>7 - 13 (high-band)</blockquote><div><br><br>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.
<br> </div><br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Coaxial cable attenuates RF energy at a rate proportional to<br>frequency. In fact, a piece of cable that has 10db. of loss at
<br>channel 2 (54 Mhz.) will have 20db. of loss at channel 13 (220 Mhz.).<br><br>Cable loss also increases with the temperature of the cable, and<br>proportionally with frequency, so as a cable heats up channels 7 - 13<br>
will be affected the most. Chennel 13 will be affected twice as much<br>as channel 2.<br><br>So if you were experiencing problems due to either your garage or<br>your cable system in general heating up, it would be expected that
<br>channels 7 - 13 would be affected most noticeably.<br><br>Taking the noise floor as a constant (which it is not, but let's make<br>things easy) a reduction in signal level would result in a<br>degradation of the signal-to-noise ratio, and a noisy picture under
<br>low or marginal signal conditions.<br><br>Thus it would not "add noise to all the channels" equally, but to 7 -<br>13 more so than the others, exactly what you are seeing.<br><br>Cable techs are famous for saying "everything's fine at their end",
<br>when it is not. If you are splitting the signal several times you<br>could well be down enough that the increased loss due to high<br>temperature would be visible.</blockquote><div><br>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.
<br><br>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.<br><br><br> </div><br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
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