Reply With Quotedaki (06-07-10),mal (16-12-09),ssrattus (09-06-09),urban_s0ulja (07-06-09)
Understanding Ground Loops in CCTV
As you all know, the source and destination of a video signal can be at differing AC or DC earth potentials, earth loop currents flow and cause longitudinal hum to be introduced into the video signal this is called Video Hum.
Video hum is low frequency (50 Hz mains frequency or it's harmonics) noise from the ground lines which has influenced the video signal, causing degradation of the displayed signal. Video hum is usually observed as bars rolling vertically through the video image, video hum may also cause video distortion or even tearing of the picture in severe cases.
The most common of Video hum in our industry, will be a problem in any system where video sources and display devices are connected to different A/C power sources with varying grounding potentials.
Typically the humming can be seen as slowly vertically moving horizontal bars in normal TV video signals.
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.
Ground loop isolation in video lines
Isolating video signal is more complicated than isolating audio or antenna signals, because the DC level of the video signal is important and video signals have very high frequency spectrum (normal composite video can have bandwidth from 50 Hz to 6 Mhz).
Isolating video signal needs typically active technology which involves electro-optical isolation or differential amplifier with a floating ground on the input connector. Both technologies are usable in real world situations. Differential input with floating ground works nicely for small ground potential differences and this approach is used in some professional video equipment. Differential inputs are also used in applications where a video signal is transmitted through twisted pair wiring (some CCTV applications which use twisted pair interfacing equipment).
Electro-optical isolation works well in applications where complete electrical isolation is necessary. There are some this type of isolation devices on the market and some special video distribution amplifiers have this kind of option built in.
There are passive hum suppressor transformers which will very effectively remove the hum from the video signal (typically around 40 dB hum level reduction), but do not effect the video signal otherwise. Those special transformers act like a common mode coils, which stop the annoying ground loop currents on the shield of the coaxial cable, but provide a straight path for the signal inside the cable. These kind of devices are capable of passing the signals from DC to tens of MHz without problems. The transformers of this type are usually called "hum bug transformers", "anti-hum video transformers" or "hum suppressor transformers". Generally term hum-bugger refers to any circuit (often a special coil) that introduces a small amount of voltage at power-line frequency into the video path to cancel unwanted AC hum.
There are also special wide band isolation transformers which can isolate video signals. A transformer which can nicely transfer the whole video frequency spectrum without much distortion is very hard to produce so there are not many of them on the market. Some of the isolation transformers are only designed for CCTV applications, where more signal distortion is accepted than in broadcast industry.
The choke (humbugging transformer) is primarily used in Broadcast TV because it passes the DC component of the signal, however, this isolation transformer is primarily used in CCTV.
Hum reduction transformers or common mode coils are constructed with either 75 Ohm twisted pair (made of fine wire) or coaxial cable wrapped around a very high permeability core. Most basic hum isolation transformers are basically just coax cable wound on a toroid-type core. They work by mutual inductance. The coax cable is wound around a transformer core so that both the inner and shield of the cable become inductors. The tight coupling ensures that any voltage in the shield caused by variations in earth potential are transformed into the inner conductor.
The method is an ancient idea and can cope with very large ground loop signals, and has very large bandwidth with very little loss. This type of anti-humming transformer also provides DC continuity between the input and output leads which is a good thing. The transformer does not stop ground loop current flowing (the amount of current is lower because of the added inductance) but the transformer reduces the current, canceling the effect of ground loop current. A Good one can reduce the ground loop effect up to 40-50 dB.
Now that you understand ground loops, where the come from and how they are a pain to get rid of, ensure you pay close attention to the designing of a CCTV system. Do it right the first time to avoid the painful repercussions later on.
The most common form of ground loops in CCTV I have found from personal experience is the use of CHEAP TELEVISIONS as spot monitors. Cheap TV sets only have two-pin mains leads and no reference to ground hence creating a high ground potential difference. In some cases, I have measured voltage upwards of 60VAC between the coax shield and Ground.
daki (06-07-10),mal (16-12-09),ssrattus (09-06-09),urban_s0ulja (07-06-09)
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Good Reading. very in depth.
Most of the time in my experience with CCTV they have this little switch on the DVR or Matrix that is a termination link for balancing the line which sorts out the mismatch in impedence between a monitor and the DVR and difference in potential in both of the power supply grounds.
Infin8, You have been very lucky so far, if all your ground loop problems have been fixed by flicking the termination switch on your DVR... As IntelliGeorge says, Do it right first time to avoid painful repercussions later. Once you have managed to build a ground loop in, it can be hell to get rid of....
Cheers!
Ev.
Lucky for you guys, after talking about ground loops the first service call I get today was for a DVR running slow. When I say slow, I mean 10 minutes is how long it took for the virtual keyboard to come up for me to enter a password, and a further 10 minutes before i saw any configuration menu.
This is because a DVR is designed to record images first and foremost, burning and user interfacing is second priority. So when we introduce a ground loop, the DVR struggles to record images, because the matrix card and capture card are trying to sync with the video signal. So all user activity is put to the side till it sorts itself out hence the 10minute wait for login etc.
Here are a few images for you to see in regards to the ridiculous voltages found on BNC's when using a TV as a spot monitor!
Measuring AC voltage between the BNC shield and Earth(at DVR location)
Results in 106.4VAC Ground Potential Difference aka Ground Loop
So do me a favor and ensure you design CCTV systems properly, use CCTV equipment (CCTV spot monitors) and not TV's. Otherwise, like the installer of this CCTV job, you will lose customers.
So the next time you service a DVR and you get a little shock removing BNC's, 9 out of 10 time that is the result of a ground loop.
I've used Humbugs from Pacom and found them pretty good.
Anybody else found a good line isolator the humbug is a bit exie.
Never needed to use any on my installs as I use VDA's which have built in electrical isolation. But I hear the humbugs are good in most cases.Originally Posted by kalboy
Nice voltage reading but how much current? Volts jolts (and break bones) but mills kill. not so unusual for 2 wire non earthed SOPS in consumer and some pro equipment & also switched mode plugpaks that have appeared.
The little shock can be explained by critical examination of internal circuitry to component level.
Not enough current to kill you, just a jolt when removing the BNC, but definately enough current to screw up DVR's. The DVR that the reading was taken from needed a new power supply and HDD's.
i did a job on the weekend that, i was called to fix. i am not a specialist in this field but know which end of the plug to connect. This to mee was a complex installation, 7 cameras A DVR 8Ch with 7 Ikusi modulators.
The client had lost 3 cameras, this was due to someone removing a power supply. Replaced & all cameras came back on line. I replaced the DVR with one that was upto date and has 500G HDD and USB for media removal. And easy web based interface.
The issue was that the modulated images were heavy on the RF interference, & herringbone pattern appearing on the monitor & on the TV's around the house.
The old DVR had 'T' piece BNC connectors for in and out to the modulators, the new DVR has in & loop connectors. So i removed the "T" BNC connectors & removed the monitor to find the image was perfect and modulated around the house as it has originally been set to do.
The monitor is really only used for set up, so would a TV be sufficient if it is turned off when not needed for viewing?
What got me about this installation was that the installer had used the cheapest crappy RG59 antenna cable for the camera runs. Now this is a house that would have cost $5 -$10 million dollars to build & the owner would not care if they paid top dollar & got top dollar, but this installer wont get another job for them, not even a service call. Because they now know they got shafted.
iam a bogan
what cable would you have used instead f rg59?
RG6 with F-connector terminations. F-BNC connectors are readily available.
It may be a bit heavier to run but it is less susceptible to ingress and other interferences.
iam a bogan
As long as it's RG6 video cable. RG6 Quad TV Cable performs poorly with baseband video signals.
RG59 Quad shield run within the max distance and separated from power is more than enough. Unless you are running over 300m than RG6 is a waste of precious copper.
One other thing George - If for some reason you need to use a BNC-BNC joiner, make sure it's insulated from building metalwork (steel framed buildings etc )
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