printscreen
join:2003-11-01
Juana Diaz, PR
 ·Choice Cable TV
 ·Coqui/PRTC
| Cable TV causing trouble with audio?
I recently setup a home theater in one room in the house. My computer desk is in the same room and I just got the needed cables to hook up my 32-inch HDTV to the computer as a secondary monitor using a DVI/HDMI cable. I also hooked up the audio output to the home theater and I immediately got this horrible ground hum noise. This was either on the computer speakers or on the home theater speakers, depending on the connections I made. After some experimenting I narrowed down the source to the cable TV drop coming from outside. If I disconnect the cable coming from the outside at any point (splitter inside house or the cable between the converter box and the TV) the hum will stop completely.
I find it odd that even if the cable is "disconnected" via a switch I have to allow switching between cable TV and the rooftop antenna the hum will be there. Even if the switch is set to "Antenna". The switch is hooked up as labeled.
When I was setting up the home theater I wanted to connect the audio output from the cable box into the receiver and had the same problem. I blamed it to a faulty cable box and decided to get the audio from the TV audio output instead. Now I get this problem in a seemingly unrelated component (the computer) and it appears to be the cable TV drop.
I do know that the drop does not have a grounding block anywhere. Could this be the reason? Should I call the cable guy to have this fixed? |
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printscreen
join:2003-11-01
Juana Diaz, PR | Anyone? |
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Unknown_P
@verizon.net
| reply to printscreen
said by printscreen  :After some experimenting I narrowed down the source to the cable TV drop coming from outside. If I disconnect the cable coming from the outside at any point (splitter inside house or the cable between the converter box and the TV) the hum will stop completely. I do know that the drop does not have a grounding block anywhere. Could this be the reason? Yes. |
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ArthurS
Watch Those Blinking Lights
Premium
join:2000-10-28
Hamilton, ON
| reply to printscreen
Seems like you did a good job in narrowing down all the possibilities! 
Yes, call your cable company and have them fix *their* problem. They should do this for you at no charge. It is a very common problem with them. |
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printscreen
join:2003-11-01
Juana Diaz, PR
·Choice Cable TV
·Coqui/PRTC
| I have done some more research and learned about ground loops and multiple ground paths. Multiple ground paths can cause this hum and certainly there are at least two ground paths here. The computer and home theater are both in the same electric circuit, so they are on the same ground path in this part. But the cable TV line provides another ground path (even if the drop is not grounded but the tap at the pole should be). The phone line plugged to the computer provides another gound path.
I also learned about isolator transformers that supposedly fix this and are placed in the coax cable to prevent this problem.
Any opinions or comments? |
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ArthurS
Watch Those Blinking Lights
Premium
join:2000-10-28
Hamilton, ON
| said by printscreen :I also learned about isolator transformers that supposedly fix this and are placed in the coax cable to prevent this problem. Any opinions or comments? The good quality (not cheap Asian junk) isolation transformers cost a bit of money. Why spend money for a bandaid on the problem that your cable provider should fix for free? |
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printscreen
join:2003-11-01
Juana Diaz, PR
·Choice Cable TV
·Coqui/PRTC
| I understand but I also did some experimenting myself. There is one cable splitter outside very close to the electric drop. I know this is probably against some codes or something but tried it just to see what would happen. I attached a wire between the body of the splitter and the galvanized piping that carries the power cables to the electric meter. Those pipes must be grounded to the rod at the meter box so it should make a good ground. Nothing changed. The hum is still there.
I will call the cable company to have this fixed.
Also, after reading some reviews for an isolator transformer sold by Radio Shack, I noticed that most people reviewing had the same problem I have: a computer connected as an audio source to an audio receiver or a home theater. I found that quite odd. |
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drjim
Premium,MVM
join:2000-06-13
Long Beach, CA
clubs: | reply to printscreen
We called them "Hum Buckers" at the broadcast facility I worked at. They were a large toroid core with lots of turns of coax on them.
And they were not cheap!
--
One man's Magic is another man's Engineering. |
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printscreen
join:2003-11-01
Juana Diaz, PR | Actually the isolator I found goes in the audio cables, not the coax. It is relatively cheap and it will be plan B if fixing the cable TV drop does not solve the problem. |
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drjim
Premium,MVM
join:2000-06-13
Long Beach, CA
clubs: | coax = shielded cable
While these were designed for video, they also worked well for audio.
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One man's Magic is another man's Engineering. |
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ArthurS
Watch Those Blinking Lights
Premium
join:2000-10-28
Hamilton, ON
| reply to printscreen
said by printscreen :Actually the isolator I found goes in the audio cables, not the coax. It is relatively cheap and it will be plan B if fixing the cable TV drop does not solve the problem. Yeah, the cheap audio isolators (such as those you get at Radio Shack) have a problem with reproducing a flat frequency response and in particular, saturate at lower frequencies. You get what you pay for, there's no substitute for having an iron core of decent weight inside!
Examples of one manufacturer that does it right, and has won numerous awards doing so:
»www.jensentransformers.com/
Read their white papers. And yeah, they are pricey, but pay for themselves with an hour or two of my time wasted chasing after ground problems. |
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ArthurS
Watch Those Blinking Lights
Premium
join:2000-10-28
Hamilton, ON
| reply to printscreen
said by printscreen :I understand but I also did some experimenting myself. There is one cable splitter outside very close to the electric drop. I know this is probably against some codes or something but tried it just to see what would happen. I attached a wire between the body of the splitter and the galvanized piping that carries the power cables to the electric meter. Those pipes must be grounded to the rod at the meter box so it should make a good ground. Nothing changed. The hum is still there. I will call the cable company to have this fixed. Not only is it a grounding problem, but many cable companies inject power over the trunklines to supply power to remote amplifiers in the system. Unfortunately some of this power leaks on the distribution side, I'm sure if you stuck a meter between ground and the coax, you'll read some voltage there! My coax at one point had so much voltage, it was an unpleasant shock I got when disconnecting the main line from my in-house splitter! I could even see the sparks! They fixed the problem pretty quick after an irate phone call from me! |
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gallowsroad
join:2004-08-09
Tulsa, OK
| reply to printscreen
said by printscreen :Also, after reading some reviews for an isolator transformer sold by Radio Shack, I noticed that most people reviewing had the same problem I have: a computer connected as an audio source to an audio receiver or a home theater. I found that quite odd. This is very common.
Ground loops can occur in this situation even if your cable line is properly grounded.
Try this: plug everything temporarily not only into the same circuit, but the same physical outlet (use strips) and see if it stops.
I have my computer as a source to my audio system, and had to plug the power amp (which is across the room) via a heavy duty extension into the same physical outlet as the computer in order to defeat a 60Hz hum. Everything else I tried did nothing to alleviate it.
--
Ha ha haaaaaaa....ever get the feeling you've been cheated?
- John Lydon, last Sex Pistols show |
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printscreen
join:2003-11-01
Juana Diaz, PR
·Choice Cable TV
·Coqui/PRTC
| Actually they are ALL in the same outlet. This an old house with just one outlet in the room. Well, not physically in the same outlet as in everything is plugged on it but I wired a second outlet from the only outlet in the room using a good three-conductor cable. The UPS is plugged into the outlet and in turn the computer, monitor and DSL modem are on the UPS. Other things on the desk (printer and speakers) are plugged into the surge protector side of the UPS. Then in the second outlet there is a power strip with all audio/video equipment plugged on it. But again, they are all sourced from the same point. |
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gallowsroad
join:2004-08-09
Tulsa, OK
| And the grounds are correct?
That's the reason I asked about the same outlet, as opposed to the same circuit.
In my case, both outlets are on the same circuit, but I still get a nasty hum if I use anything in the comp/audio system on the second outlet.
So the UPS, with a pass through to a power strip deals with the comp and the front end audio gear, the second plug on the same outlet serves the power amp across the room. Any other combo results in the ground hum.
If you can wing it just for testing purposes, it might be worth doing. I was surprised that two grounded outlets on the same circuits would still cause a ground loop, but they did. Even lifting the ground on the power amp when attached to the second outlet did nothing.
--
Ha ha haaaaaaa....ever get the feeling you've been cheated?
- John Lydon, last Sex Pistols show |
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printscreen
join:2003-11-01
Juana Diaz, PR
·Choice Cable TV
·Coqui/PRTC
| The UPS has a wiring fault indicator that is not on, so I understand this means wiring is ok. As I said in a previous message, this is an old house and there is no ground wire at any outlet, just the hot and neutral. Any ground path comes through the pipe. This UPS has shown a wiring fault at other locations in the house but not in this particular outlet.
I tried what you suggested, plugging the power strip for the home theater into the surge protector side of the UPS and nothing changed. Hum is still there.
Until this is resolved I am leaving the cables from the computer unplugged and when I want to use the tv as monitor I disconnect the cable from the back of the cable box and reconnect the computer audio and video cables. |
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gallowsroad
join:2004-08-09
Tulsa, OK | Sorry my suggestion didn't help.
Let us know how it turns out. Ground loops are a pain, so every solution posted might be of help to someone else further down the line. |
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russotto
join:2000-10-05
Collegeville, PA
| reply to printscreen
Your best bet would be to install proper grounding to the outlets. That might not solve the problem but it will almost certainly greatly reduce it. If that doesn't solve it, a cable TV isolator (on the coax, not the audio line) should work. They don't have to be big and expensive like the ones at broadcast facilities, because they don't handle as much power.
What you have now is that any current that would be going to ground via the outlet ground wire is instead going over the signal path to ground via the cable TV shield. |
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printscreen
join:2003-11-01
Juana Diaz, PR
·Choice Cable TV
·Coqui/PRTC
| reply to printscreen
An update on this thing. Grounding didn't work. And the problem was later compounded when I switched from DSL to cable Internet, providing yet another path for the cable TV network to cause the annoying hum. I decided to go with the ground isolator and got this one. It works perfectly. No more hum or bars in the screen. Cable TV reception and cable modem are unaffected by the isolator!! |
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