A ground loop is simply an added path to ground, which creates an antenna loop, and you get noise generated from any electrical signal in the vicinity, mostly the AC mains. You probably already know this.
Get a bucket of those old adapters that take a 3 prong grounded plug and allow it to be plugged into a 2 prong plug. Put those on all your gear, except for a primary component, and they all share the same ground, through the wires connecting the equipment. (better than just cutting off all the ground plugs on your power cords)
Used to have to chase this in audio studios when they were all analog. Just put ground lift plugs on everything except a primary piece of gear, like the Switcher or Audio console. I use the ones that have polarity blades, so that I don't end up pumping full power down to ground....You'll know when that happens when the lights go out, and things go spark!
You also have to make sure that nobody made bum cables (Audio XLR mostly) which swap the ground and hot leads.
As an alternative, I used to use hum-buckers... Basically transformers that allow you to electronically isolate the systems from each other. They roll off the high frequencies, but for basic sync this shouldn't be a problem.
You can also have one of the bays on an opposite leg of your AC coming into the building. If it's 220 with a center tap at the mains, all gear that is interconnected needs to be on the same leg of mains. Usually lighting and AC go on one side, and all the tech gear on the other, in the ideal situation.
Only other thing that can cause an issue is if your have a shield loose on a video cable. Check continuity on the shields.
Good Luck
Dave Hogan
Burbank, CA
On Friday, April 24, 2015 4:30 PM, "John Moore bigfish@pacbell.net [Avid-L2]" <Avid-L2@yahoogroups.com> wrote:
Chasing a sync issue in one bay out of 10 or more. We have it localized to some sort of ground loop that is distorting the ntsc ref black and tri-leve sync signals. If the problematic bay is the only thing connected to the AJA gen-10 all is well. As soon as any of the other sync connections shield in the machine room touch the shield of any connector on the Gen-10 the noisy kicks in and Avid loses sync lock. A tek rasterizer scope wfr-7000 will still accept the sync signal as ntsc ref and lock to it but the Nitris won't. The NitrisDX has been swapped along with the HBP card and interconnect cable. This is a bay that has worked for years and in fact after replacing a broken off bnc connector on the sync line from MCR last year there was nothing like the noise I see in 2 field mode on a Tek 1750. All the power is tech power with isolated grounds. I can only guess something changed in the machine room or the edit bay that is causing the hum/distortion on the black signal to be to much for NitrisDX to lock to.
I'm adding ground loops to a close number two to the things I hate most right after fonts. Could additional equipment added in MCR somehow unbalance the isolated tech power that comes from several 20 amp isolated ground circuits to the racks and raise the isolated ground to create an issue. I know this is a lot like herding cats but figure this group might have some of their own experiences that might shed some light. I have seen at my regular gig an intermittent hum pop on and off on my online bay sync signal, that I post about here, come and go and now it's gone after I replaced the batteries in a UPS and the engineer replaced some in a few other ups units. I'm assuming that had something to do with it because those aging battery units would kick into battery mode when the power hadn't even gone out for some reason and I think that was contributing to the sync issue I was having. I have been told some of the ups units in this machine room have been serviced or replaced in the recent past so maybe there's something there. I'm thinking of trying my video humbucking isolation transformer if I can find it, but that didn't resolve anything at my regular gig so I'm not holding my breath. Next maybe 3 to 2 ground lifters on the power cords to tech power in the edit bay. Then maybe start turning off equipment in the bay and or machine room to see if there is any effect on the noise. The one thing I haven't had a chance to do is look at the black ref signal in the other bays that are not experiencing the sync issue.
So someone be my Ground Loop Fairy. If I leave a tweaker under my pillow will you come by and make this problem go away? I won't even expect a quarter. ;-)
John Moore
Barking Trout Productions
Studio City, CA
bigfish@pacbell.net
Barking Trout Productions
Studio City, CA
bigfish@pacbell.net
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Posted by: Dave Hogan <mactvman@yahoo.com>
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