Humming bars in CCTV system monitor image can be caused by many different factors and tracing the issue can be a pain. Video Ground Loop Interference for CCTV article at 2M CCTV Security & Surveillance Blog gives some tips how to solve ground loop problems on CCTV systems.
One cause is running your cable over a high power source, so run camera wires away from power lines. Make sure to run camera wires away from power lines (a minimum of 12 inches / 30 centimeters).
Also 24vac power transformer connections can cause similar problems. The type of AC power transformers you use to power your cameras can contribute to Ground Loop problems. A ground can be introduced to your camera “Capacitively” through the power transformer windings depending on the type and construction technique used to build the AC transformer. If you get problems an old trick is to reverse the wire on the 24V AC power supply to get the bad signal to go away. Not that this reversing trick is only for 24V AC system when you know what you are doing; do not try to reverse wires on 12V DC powered systems or you can damage your power supply and/or your camera.
Another issue with ground loops are metal buildings. If you mount cameras to the side of a metal building, remember that the entire building is one large conductor. You should never connect both ends of a video cable to local grounds. To avoid grounding on camera end, some form of electrical insulation between the building steel and camera is a good idea. Video Ground Loop Interference for CCTV recommends to put a piece of wood between the camera and the wall and that will fix the interference caused by video ground loops.
If coaxial cable shields are connected together anywhere in the system, separate them if possible. Similarly remove all but one ground connection on each coaxial cable if possible. The ground is usually at the monitor end of the coaxial cable because the monitor equipment plugs into the main power supply which is grounded.
There are video ground loop isolators (or “isolation transformers”) that can help to solve ground loop problems. My Ground loop problems in video lines article gives more details on them and links to video isolator products. For DIY solutions read my Build video isolator and Build humbugging transformer blog postings.
12 Comments
Tomi says:
Understanding Ground Loops in CCTV
http://www.austech.info/cctv/1608-understanding-ground-loops-cctv.html
Ground loops in the video systems can have following effects:
Hum Bars:
The mains frequency 50 Hz can cause stationary or moving horizontal humming bars to appear on the video signal. If you have light dimmers nearby those humming bars can easily become quite severe and easily visible.
RF Interference:
Herring bone interference on video line is caused by a ground loop (that includes your coax shield) acting as an AM radio antenna. Any large loop of wire makes a good AM antenna. These antennas are especially adept at picking up AM broadcasts if most of the loop is vertical.
Cross-Talk:
Ground loops can cause one signal to interfere with another, because every cable should ideally return through the corresponding shield conductor, but there’s an alternative path through the other shield conductor which causes undesirable voltage differences to nearby cables.
WHAT CAUSES CCTV GROUND LOOP PROBLEMS AND HOW TO FIX THEM
http://www.fmsystems-inc.com/index.cfm?tdc=dsp&page=publication_detail&pid=3
There are several solutions to this problem, for very simple CCTV installations, just making certain that there is only one ground anywhere in the system will work to insure that “60 cycle bars” will not occur. This “one ground” would normally be at the monitoring location. However more complex systems may require lightning arresters at various locations, or perhaps the camera case must be grounded for safely reasons, there may be more than one monitor location, or the camera cables are connected through coaxial “patch panels” which are grounded. All of these systems will necessarily have more than one grounded point in the system. As such they will be subjected to these 60 cycle ground currents with consequent moderate to severe 60 cycle interference.
There are two possible solutions to this type of interference. One is to connect an “Isolation Transformer” at each point in the system where “additional grounds” are attached (all grounded cameras, lightning arresters, patch panels, required coaxial cable grounds, and any other possible source of grounding).
The second possible solution is to isolate all “field grounds” from the ground at the monitor location by installing a GB-60 Ground-Loop Blocker.
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Tomi Engdahl says:
Hum Bars Hampered the TV Screens
http://www.designnews.com/author.asp?section_id=1368&doc_id=268671
Here at XTI Vancouver, we recently completed an audio-video system for a local restaurant/bar that underwent a major renovation. The video portion of the system has 4 LG 60-inch TVs behind the bar, with a Sharp 80-inch and two Sharp 70-inch screens mounted on a faux brick wall perpendicular to the bar.
All sets are fed from a Crestron 16×8 component video matrix switcher. This particular switcher has Cat5 balanced outputs as well as standard 75 ohm outputs, so we used the balanced outs and installed passive Baluns at each set to convert the balanced 110 ohm signal back to the unbalanced 75 ohm coaxial inputs on the sets.
We were rewarded with what appeared to be nasty hum bars all over the picture — bad enough that it was unwatchable
It had to be something to do with ground. Maybe a ground loop. Except these sets don’t have grounded power cords. It can’t have a ground loop. Or can it? I tried to think what had changed since we took down the original set. The mounting plate!
We took down the set and stuck an ohmmeter on the plate. It was grounded!
I plugged the RCA plug into an unused input on the set, and pushed the other end into the ground hole of the U ground outlet the TV was plugged into. Hallelujah! The hum bars vanished!
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Dave says:
If you had to actually purchase a FM systems product to fight ground loops, you will realize that its MUCH cheaper to redesign your system and do it right to begin with.
Tomi Engdahl says:
If redesign your system is feasible option, then it is a good way to fix CCTV systems. I can also recommend that approach.
There is also pöace for isolators in cases where that whole system redesign is not an option for some reason.