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<p class=MsoNormal><b><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma;font-weight:bold'>From:</span></font></b><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'>
mythtv-users-bounces@mythtv.org [mailto:mythtv-users-bounces@mythtv.org] <b><span
style='font-weight:bold'>On Behalf Of </span></b>Brian Wood<br>
<b><span style='font-weight:bold'>Sent:</span></b> Monday, July 03, 2006 8:29
PM<br>
<b><span style='font-weight:bold'>To:</span></b> Discussion about mythtv<br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: [mythtv-users] Using
Cable-tv certain channels fuzzy</span></font><o:p></o:p></p>
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12.0pt'><o:p> </o:p></span></font></p>
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style='font-size:9.0pt;font-family:Helvetica;color:black'><br>
<br>
</span></font><o:p></o:p></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>On Jul 3, 2006, at 7:01 PM, Daniel Leaberry wrote:<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
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<br>
<o:p></o:p></span></font></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><span class=gmailquote><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>On 7/3/06, <b><span style='font-weight:bold'>Brian
Wood</span></b> <<a href="mailto:beww@beww.org">beww@beww.org</a>> wrote:</span></font></span><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><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.<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><br>
<br>
I know nothing about frequency allocation (I'm glad you do!)<o:p></o:p></span></font></p>
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margin-left:4.8pt;margin-right:0in'>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>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)<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><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>
<o:p></o:p></span></font></p>
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margin-left:4.8pt;margin-right:0in'>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>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.<o:p></o:p></span></font></p>
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<p class=MsoNormal style='margin-bottom:12.0pt'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'><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.<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>The device that "looked like a Fluke" was a Signal Level
Meter (sometimes wrongly called a "field strength meter"). It
measures the level of an RF TV signal in Dbmv., which is decibels relative to
one milli-volt across 75-ohms. This used to be known as a "dbj" (J
for Jerrold) in the 1950s before the standard was adopted officially. dbj or
dbmv was initially measured with the Jerrold model 704 meter, a beast that
contained lead-acid batteries and vacuum tubes and was great fun to haul up a
telephone pole. There was great rejoicing when it was replaced by the model
727, which was transistorized, but still weighed a ton.<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Jerrold, if you're interested, was a company involved in the early
construction and supplying of CATV systems, and was named for its founder
Milton Jerrold Schapp, who later became the Governor of Pennsylvania. Jerrold
was bought out by General Instrument in the 1980s.<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Interestingly, Fluke now makes signal level meters.<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Without giving the entire standards for signal strength (which involve
variation over a 24-hour period, and adjacent channels being allowed less delta
than non-adjacent ones and other specs) I will say that nominally a cable
system should deliver at least 0 dbmv., (that's a "zero") although
most TV sets and capture cards are happier with something closer to 7 - 10
dbmv.<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Systems also have problems with "tilt", the difference in
level between the lowest and the highest frequency channel, caused mainly by differential
attenuation of the cable. This problem is dealt with by the cable companies by
using equalizers and tilt-control amplifiers of various designs, often using
"dual pilot" designs with the AGC responding to signals at both ends
of the spectrum.<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>A two-way splitter will have a loss of 3.5 db. to each leg, so it would
be nice if you could put 10 -14 dbmv. into the splitter input.<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>There is a very good chance that the device you think is a splitter is
not. It is more likely to be a "directional coupler", as they are
often used to get more signal to the cable modem than if you were using a
two-way splitter, and isolate the modem from problems. DCs have very little
loss on the "through" leg, less than a db. in some cases, and from 8
to 35 db. loss on the "tap" leg. They provide much better isolation
between devices than hybrid-splitters and thus more protection to the modem
from problems caused by troubles in other places on your system. They are also
less dependent on proper termination of the ports to provide adequate
isolation.<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>All such devices are made extremely cheaply as they are bought by the
thousands by cable companies.<o:p></o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>So much for CATV 101, quiz tomorrow morning :-)<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 color=navy face=Arial><span style='font-size:
12.0pt;font-family:Arial;color:navy'>I hate to state the obvious but have you
tried to connect a regular TV to the cable that is connected to your PVR card?
This way you will be able to tell if the signal going into the PVR card is
actually any good. I had a cable guy tell me for two months that the
cable was fine even though I had a snowy picture. I switched to satellite
and kept the same TV and now my picture is crystal clear.<o:p></o:p></span></font></p>
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