THE AFFECTS OF COMMON MODE INTERFERENCE ON IP VIDEO SIGNALS
THE AFFECTS OF COMMON MODE INTERFERENCE ON IP VIDEO SIGNALS
BY: Frank McClatchie
The transmission of a digital IP video image involves the generation of “differential digital signals” on the two pairs of wires that constitute the wire transmission path. The term differential means that whenever there is a negative pulse on one wire, there must be an exactly equal and opposite positive pulse on the other wire. Any difference between the two pulses other than the reversal of polarity constitutes a COMMON MODE distortion product or a cross-talk from another source.
Thus, the DIFFERENTIAL DIGITAL SIGNAL is being transmitted to the other end, while any COMMON MODE SIGNAL being common to both wires is not seen by the receiving end of the cable, and is radiated from the cable. Also because there is no shielding on the wires in the Network Cable this unbalanced signal become a source of CROSS-TALK into any other nearby wire.
The Network Cable itself is also capable of creating COMMON MODE signals out of perfectly good DIFFERENTIAL MODE signals by simply delaying the signal on one wire more than it does on the other wire as the signal travels down the Network Cable. This difference in the delay can be caused by changes in wire diameter or insulation thickness at any point along the cable length.
Imperfections in the “Rate of Twist” of the wire pairs, excess un-twisting of wire pairs at the 8P8C connector, and any installation process that alters the arrangement of the twists of the internal wires is perfectly capable of generating COMMON MODE SIGNALS.
Once a Common Mode signal is generated either due to unbalance within the signal originating equipment or by inequity in the Network Cable, that signal will be radiated out of the cable or rejected by the receiver equipment and will be a source of the Excess Loss beyond the expected Transmission Loss of the cable.
Thus any signal that is converted from Differential Transmission to Common Mode Transmission can be considered excess loss, over and above the expected loss created by the cable manufacturer. The main difference in loss between the various grades of Network Cable is controlled by the precise nature of its construction and conformity to the specifications, uniformity is the key to a low loss cable.
Any amount of Common Mode Signal that creeps into the transmission will increase the probability of reaching the “Digital Cliff”, the point at which your receiver equipment fails to recognize the incoming signal and shuts down reception or becomes intermittent.
So what are the sources of this COMMON MODE signal that can wipe my picture completely out?
Every transmission system has two ends. One end is the originating site, and the other end is the terminating site. In the case of the I P Picture Transmission system, there are not only two ends, but two directions of transmission, since the camera and Recorder both talk to each other. For our purposes when using test equipment, we consider the end that we are measuring at (whether the Camera end or the Recorder end) as the NEAR END, and the opposite end of the cable as the FAR END. So, depending on where we are doing the measuring, a Camera may be a Near end, if that is where the measurement is taking place, or a FAR END if we are measuring from the NVR end of the cable. In this way it is possible to measure the loss of a Network cable from one end, and then prove an identical loss in the other direction by going to the other end for dual measurements.
You would think at first glance that the reverse set of measurements would always be perfect copies, since they are from opposite ends of the same cable, but this is not necessarily true. Different pairs inside the same cable can have different losses from opposite ends of the cable depending on twist errors or cable damage that may be evident from one end but not from the other end of the same cable. It is best to measure from both ends of the cable then compare the results to make sure that the reverse measurements are in fact reasonably identical. In this way you can identify a faulty cable.
There is another aspect of the network that is often overlooked that can cost the installer many hours of troubleshooting time and effort. Just like the old video installations, IP video cables require terminations to work correctly. These terminations are built into the terminal equipment and the termination value is 100 Ohms for each end. The exact terminating resistance on any given Network cable may vary substantially from the correct value, causing, not only excess loss, but also echoes to reverberate from one end of the Network cable to the other, either intermittently blocking the transmission of the picture, or permanently stopping all such transmissions. New equipment will have this termination correct and you should not worry about new equipment.
However there is an invisible force present in many installations that works to destroy this termination and render your IP Camera system useless over time. With analog cameras this force is easy to see in the video image, it is called a Ground Loop voltage. With analog a ground loop could be seen on the video image as a set of two or more horizontal bars that usually scrolled slowly up the screen. Now with the IP video cameras this ground loop voltage is still there but it cannot be seen on the screen. If you have a large enough ground loop in your system it slowly burns out the terminating resistance at the end of the cable over time and will cause reflections that cancel parts of the digital signal and can cause permanent failure of the IP signal. Ground Loops are caused by both the camera and the NVR equipment being grounded to local grounds that are not always at the same voltage potential due to power spikes, bad grounding of the building electrical system or even lightning.
A nearby lightning strike is capable of damaging a termination enough to change the resistance of a termination without any visual signs. The resistive coatings on the terminating resistors can be blown off raising the resistance without even burning the paint on the component.
So the next question is what can I do about any of this?
The answer is to measure the Level, Balance, and Common Mode interference when you install an IP camera or are troubleshooting an existing job. With these three measurements taken at each end of the system you can determine if the terminal equipment is outputting the correct level and if the terminations are correct.
Then you can check for unbalance in the system for both the wire and terminal equipment. Next you can check for Common Mode interference, both at the near end and the far end. All of these measurements can be made with the IPM-4 IP video test meter.
This meter is hand held and battery powered portable for checking your IP video installations before you leave to make sure that all the equipment and cables are in working order and that there are no unseen problems that will bring you back to the site. If you are troubleshooting a system, you don’t want to waist time swapping out equipment to find a problem. Use this meter to measure the IP signals and cables to locate the source of the problem fast.
Call 800-235-6960 to order an IPM-4 and have the confidence of knowing your IP video levels and cables are correct and working inside specifications.
