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Discussion Starter · #1 ·
I'm hoping maybe someone can save me a lot of time trying different things by identifying this strange interference on my comcast analog cable feed. It seems to be worse (or at least more evident) with a stronger signal, I also have it on 2 separate comcast cable feeds into my home. It appears on analog cable using a TV, tivo or my home built pvr as well. What it looks like is a rolling line going from bottom to top, it's not entirely horrible but it is distracting enough for me to take some time and see if it can be remedied. Anyways, a picture or video in this case is worth 1000 words, the video can be downloaded at http://www.truetolight.com/post/interference.mpg The file is about 5mb an it's been recorded on my pvr with beyond tv in mpeg2 format.


Any help is greatly appreciated, if anyone needs more information gladly ask and I'll post it here.


Thanks,


Rob
 

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It's known as an AC hum bar. The analog vertical refresh rate is 30 Hz (30 frames a second). AC line frequency is 60 Hz. Since 60 = 2 * 30, there should be no problem. However, the power line frequency is actually slightly different, like 59.97 Hz or something and what you see is a very slow frequency difference, represented as a horizontal bar across your screen that scrolls very slowly up/down your screen.


What does this mean? You need to check for a ground loop. You also need to check that the cable entrance has a properly grounded input. There should be an cable TV adapter at the entrance to your house which is attached to a ground wire (to a copper pipe or a ground rod outside your house).


Also, is all of your equipment on a 3-prong grounded receptacle? CATV boxes are usually 2-prong, but the coax shield is electrically connected to that grounding block outside your house.


This is a common occurrence. I'm surprised you haven't noticed anything like this before. To troubleshoot, you need to break the loop. Disconnect everything from your CATV feeds and try to isolate it to a single feed. Any in-line amplifiers?


(miniz - we posted at the same time
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Discussion Starter · #4 ·
Thanks for the fast replies! While I don't have a cable box, I do have a lot of AV equipment, some of it is 2 prong. like my AV receiver. When I hooked everything up I had a terrible hum (around 60hz) coming out of my system, I wound up putting an isololator on the cable line (which is still on there) and that cut out the hum and also reduced the rising line.

( http://www.partsexpress.com/pe/showd...number=180-075 ) is what I installed. The cable coming into the house is attached to a grounding block the cable company installed which in turn has a green wire going to a clamp on a copper water pipe, however I'm going to recheck all that tomorrow in the light.


Thanks for the direction, at least I know where to look.


Rob
 

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Discussion Starter · #6 ·
Unhooking the ground from the cable's grounding block at the entrance totally eliminated this interference, I just can't imagine this being a safe situation. I wonder if there's another way to safely protect things hooked to the cable and leave the ground off since it solved the problem. Perhaps relocating the cable's ground to the electrical ground may help? If anyone has suggestions in light of this finding that'd be great.


Thanks,


Rob
 

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Quote:
No, the video field rate is 59.94 Hz, AC powerline is 60Hz.

I stand corrected. Thanks.

Quote:
The cable coming into the house is attached to a grounding block the cable company installed which in turn has a green wire going to a clamp on a copper water pipe,

Electrical code compliance is on your side. The cable company has to fix this. Don't mess with it. This is a safety issue.


Going to a copper water pipe? And where does the copper water pipe go?
It had better be the water line that is running outside underground.


I have an old house with a well pump. Before we re-wired, the main electrical ground was connected to a copper water pipe, but on the wrong side of an old rubber hose that went to the well pump. In other words it went to the rest of the house, but the house was not connected to the power company side. It was fixed ASAP.


Here's what you do: Call back the cable company and tell them you have an unsafe hookup. The right way to do this is to sink an 8' (6'?) ground rod at the cable entrance. Connect the grounding block to that. Maybe that's not sufficient, but they should do whatever it takes to bring it up to code. Show them the picture with/without the ground connected and work from there.
 

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Discussion Starter · #10 ·
Thanks for all the replies, I will surely make a call to comcast and let them figure it out. Overall they've been great with customer service in the past few years ( as compared to 10 years ago when they were awful), when our internet was having problems because of something I actually created they fixed it without charge .
 
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