Cable ground loop issues

[davin]

I thought I'd share my ground loop issue with the community here in hopes that it may help someone out in the future.

I recently moved into a new place and when I set up my Sage box I found a serious ground loop problem. There was an audible hum (not noticeable during loud programming, but it gets annoying otherwise and is definitely audible in menus/desktop when no sound is playing) as well as some visual artifacts... primarily a very slowly rolling darker horizontal bar.

It turned out to be a ground loop between the power line third prong and the cable ground. When I disconnected the cable from my splitter, the hum went away.

I did a little bit of research and discovered that I needed a cable isolation transformer. Basically this isolates the cable and TV/computer grounds while still transmitting the signal.

I found this transformer made by Jensen ( http://www.jensen-transformers.com/datashts/vrd1ff.pdf ) which has a frequency response high enough for cable modems and digital cable. Since I have a digital cable box, this looked like a good choice. However, at $50 I decided to see if a cheaper option would work.

I ordered a $10 isolation transformer ( http://www.partsexpress.com/pe/showd...number=180-075 ) to see if it worked... it had a response up to 1 Ghz as opposed to the Jensen which goes up to 1.3 Ghz. Well, long story short the digital cable really needs that 1 ghz to 1.3 ghz range. I hooked up the cheapo transformer last night and while it got rid of the hum and the analog channels looked fine, the digital channels though the cable box either did not come through at all, or had severe mpeg artifacts to the point where they were unwatchable.

So, if you've got a ground loop and digital cable, be sure to get an isolation transformer that specifically says that it's OK for cable modems and digital cable.

I'll be ordering the Jensen soon... I expect that that will fix my problems nicely.

[waynedunham]

Davin,

Be sure to get back to us on how it works out. I had a similar problem with audio hum once I put my cable TV (analog) into my Home theater setup a few years ago. I never had a problem until I hooked it into my AV Amp, but as soon as I did I got a hum out of the sub-woofer. A friend had a "Mondial Antenna Ground Isolation Circuit" that he wasn't using and I tried that. It worked for the audio hum.

Now my problem is a video hum, especially on the lower channels, that varies in intensity from nothing to quite annoying.
It seems to be introduced through my APC's that I've got connected to my 4 ReplayTV's, and my HTPC.

I've been told I need to get isolation cirucuits. However nobody has been able to tell me if I'll need one on each circuit, or one for every device. If it's one per ciruit then I'm looking at a minimum of 3, probably 4, but if it's every device then I'm looking at 4 ReplayTV's, 2 VCR's, Roku HD 1000 PhotoBridge, Hauppauge Media MVP, 2 TV's, and 2 computers (HTPC, and main computer with a TV/video card) Not a happy thought if I need that many!

[Naylia]

Wayned,

I have exactly the same problem. Can you link to the product you used to solve the issue?

Nevermind, I found it...for $98...arrgggh.

Anyone recommend a slightly cheaper solution? Well I guess there's the $50 one above, hoping to hear soon how that worked for you.

[waynedunham]

Naylia,

Which problem, the audio hum over the sub-woofer, or the video hum bars? If it's the audio hum, then yeah the item wasn't cheap, but I got lucky and my buddy gave me a great deal on it used since it was just sitting in his closet.

If it's the video hum, then I haven't found an economical solution yet.

[Naylia]

The audio hum through the sub woofer...except for mine's not so much a hum but more like the celtics game being broadcast (was a hum until I went from two cheap cables connected together with a little plug for the 30ft run to a high grade 30ft cable).. I live about a mile away from an AM radio station that does directed broadcasting (directly at my house) at the FCC legal limit for power. I got the interference out of my phone lines, now I need to get it out of my cable line.

So this has me sold on the high end cabling, unfortunately it's preserving a signal I don't really want preserved.

[waynedunham]

Ok, so it's an audio problem then. I'm not sure that's exactly what I was getting then because mine matched the exact type of sound when you read up on cable induced audio hum. Your's almost sounds like a combination of that and getting whacked by an outside signal, so I'm not sure the same thing would work for you.

Sadly, with the cost of these things it's not something that's easy to just try out a solution.

Sorry I can't be of more help.

On the subject of the video hum however I did a search on Ebay and there were a couple more cost effective possiblities which I'm going to try. I ordered one of each to see how they work.

$10 & $4 shipping ($.50 shipping each additional unit)

$24.95 & Free shipping in U.S. right now

I'll give a holler back once I receive them (probably 1-2 weeks) and give them a try.

[DFA]

Excellent topic and not discussed or as well known with reasonably priced, tried and proven cures as it should be. I look forward to hearing more feedback on this. I am not suffering any immediate problem but might just put something in on the cable side as a matter of "good practice".

DFA

[waynedunham]

My video hum appeared once I put battery backup onto my ReplayTV's, and my PC's. At the time I didn't know it, but apparently using these is an easy way to get video hum. 90% of the time the hum isn't bad enough to "really" bother me, or at least not enough to make me give up power protection for these devices.

APC is supposed to be coming out with a unit specifically designed with home theater in mind which is supposed to address the problems we're talking about here, but it's not out yet, and I'm kind of scared to think of the price. We'll see once it comes out.

[Gog]

It would be nice in a unit like this...

http://www.apc.com/products/family/i...CountryCode=ww

Gog

[davin]

Wayne, FYI that 24.95 unit in the ebay link you posted looks completely identical to the unit that I received. (See the parts express link in my original post) Physically it looks the same and it also has the same 100MHz-1GHz bandwidth. This DID get rid of my hum problem, but like I said it futzed the digital channels. If you are only using antenna or analog cable, it would work fine if the cable's the source of your hum, but if you're using a digital cable box it probably won't work.

For me, there was an easy way to determine whether or not the cable was the source of my ground loop. I disconnected it from my splitter and the hum went away. Touching the coax screw connector to the metal case of the splitter (without actually plugging it back in) brought the hum back. So you could try this to determine whether or not the cable really is the source of the ground loop.

[waynedunham]

Davin,

The audio hum was definitely introduced via the incoming cable. I verified that in just the manner you described back at that time.

The video hum bars are not cleared up with the unit that I have which does clear up the audio hum. I've tried moving the units location (right now it's at the incoming cable in the basement), to individual devices to see if it cleared up the video hum bars and it had no effect at all.

Thanks for the heads up about the $24.95 unit. If it works for what the video hum bars I'll take it to a friends who has digital cable to be sure it doesn't mess up the digital signal before I buy more of them. I most likely will go digital/HDTV cable at some point down the road.
Sure wish I'd looked more closely at your link vs. the Ebay one before I spent over twice what your link had it for. It sure does look like the same unit.

I'm really hoping (not realistically probably) that the $10 unit will do the job.

[davin]

I just ordered a Jensen VRD-1FF from Markertek (www.markertek.com, $49 plus shipping). I'll let everyone know how it turns out once it arrives.

[davin]

Update... the VRD-1FF shipped yesterday and should be here in a week UPS ground from NY to CA... I'll post an update next Thurs or Fri when I get it.

[waynedunham]

Update, the $10 ($4 shipping) unit I listed in a previous message arrived Saturday. I'm working this weekend so I may not get a chance to try it out until Tuesday. I'll report my findings as soon as I've had a chance to test it out.

Still waiting on the more expensive one. They say t has shipped but no tracking info.

[waynedunham]

Originally Posted by waynedunham

Update, the $10 ($4 shipping) unit I listed in a previous message arrived Saturday. I'm working this weekend so I may not get a chance to try it out until Tuesday. I'll report my findings as soon as I've had a chance to test it out.

Still waiting on the more expensive one. They say t has shipped but no tracking info.

In my application the $10 unit is a no-go. It seems to improve it some, but doesn't eliminate the video hum bars.

Now your mileage may vary if you're running all your equipment with "normal" power. I'm running all my PVR's, VCR's, and HTPC through an APC battery backup so that may be a factor making the filter unable to eliminate the video hum bars. <damn, when is APC going to start shipping their new battery backup AV units?!!>

I still haven't received the $24.95 unit yet. More updates when it arrives and I can test it.

[waynedunham]

Sorry to take so long to get back about the other isolator.

The $24.95 unit showed up and I finally tested it. It does a better job than the $10 unit, but still doesn't completely clear it up. It also doesn't degrade the signal as much as the $10 one, so it definitely is better.

I'm not sure what I'm going to do at this point. I'm going to do some more research.

This coming weekend I'm going to be at the Home Entertainment Show in NYC, so I should be able to pick some brains there. Lots of audio/videophiles there and geeks of every kind.

Now I don't know if buying a bunch of these and putting them on the various machines (ReplayTV's, HTPC, Roku, MVP, etc, etc) would have a positive, or negative effect. I.E. would each one reduce the total ground loop? (do the multiple components "stack" the problem?)

Well hopefully this weekend I can get some help from some of the engineers at the show.

[davin]

Sorry about not replying sooner... I said that I would update on the $50 Jensen high-bandwidth isolator. Unfortunately, it didn't work exactly as advertised. It did get rid of my hum and video bars, and I thought that it was working perfectly... digital channels were coming in fine. But then later that night I switched to something on HBO and the signal was completely corrupted... removing the isolator fixed it. I'm a little teed off here... it was advertised as being compatible with cable modems and digital cable... I suppose my specific box goes above 1300 MHz. I guess I'll see if something better's out there... but I expect price to be outrageous.

Long story short... the Jensen VRD-1FF isn't compatible with all digital cable, even though it says it is.

[waynedunham]

Originally Posted by davin

Sorry about not replying sooner... I said that I would update on the $50 Jensen high-bandwidth isolator. Unfortunately, it didn't work exactly as advertised. It did get rid of my hum and video bars, and I thought that it was working perfectly... digital channels were coming in fine. But then later that night I switched to something on HBO and the signal was completely corrupted... removing the isolator fixed it. I'm a little teed off here... it was advertised as being compatible with cable modems and digital cable... I suppose my specific box goes above 1300 MHz. I guess I'll see if something better's out there... but I expect price to be outrageous.

Long story short... the Jensen VRD-1FF isn't compatible with all digital cable, even though it says it is.

Was the HBO just digital, or HD? When you went back to something in the "standard" digital category (i.e. not a premium channel) did the picture quality return?

[davin]

Standard def. And it was only the HBO channels... this is why I initially thought that it was working. I tried some of the lower digital channels and it worked fine with those. I know that these are digital, since they have pixellation when the channel is changed.

So, I assume that the HBO channels are taking up a higher frequency range of the signal and the transformer isn't letting them through...

[waynedunham]

Bummer, I guess the search goes on.