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Ground Loop |
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The Ground-loop. This is caused by the camera being grounded to a voltage that is different from that of the head-end. If the [low-voltage] camera is grounded try disconnecting the camera from its ground. Alternately use an active receiver, which has a differential input. If the problem remains, disconnect the UTP wires at one end and check their voltages relative to the local ground. We expect no more than about 200 mV AC.
Are there multiple cameras powered off the same multi-output power supply? As an experiment, try powering a camera off a separate floating supply, or use one or two gel-cell batteries (12 or 24VDC) to power the camera locally. If removing the camera from the multi-output supply solves the problem, then we may have a fault voltage coming from another camera or its wire. It's not uncommon for wire to get scraped as it passes through ends of conduit or T-bar ceilings. Check each camera one-by-one. When isolating, it may be necessary to disconnect both power conductors from the power supply. |
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Split-pair |
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If the Image looks like one video signal with a blurry copy of another image either in one place, or gliding through, then we probably have a split-pair. Video signals must be sent on a twisted-pair of wires, such as white-with-a-blue-stripe and blue-with-a-white-stripe. Do not use un-twisted wire pairs, such as blue-with-a-white-stripe and white-with-an-orange-stripe, as these signals are now coupled into conductors that may be carrying interfering signals. Two video signals wires as split-pairs will show cross talk between them and look like a ghost of the other signal on the monitor. Use caution when installing multi-pair wire that has a solid white wire and a solid blue wire, as the white wires all look the same. If you get them mixed up, do not use an ohmmeter to identify. Rather, disconnect any equipment and use a low-value capacitance meter. You will find that the paired wires have one inter-conductor capacitance value (~19 pF per foot (62 pF/m)) while the un-paired conductors have a lower value (~13pF per foot (43pF/m)).
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Camera Power Considerations |
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CCTV cameras are available with various voltage requirements. These include 12VDC, 24VAC, and 115/230VAC. 115/230V models are rarely used, due to the expense of providing local high-voltage power. 24VAC models are quite common in that they can tolerate greater wire distances than their 12VDC counterparts, and are generally more immune to ground-loops. A significant portion of cameras today are wide-ranging in that they can operate on 12VDC or 24VAC.
Should the camera operate off of 12VDC only, special considerations must be taken to ensure correct operating voltage. These considerations include short wire runs, thick wire gauge, or slightly increasing the power supply voltage to achieve the correct voltage at the camera. Another consideration is that 12VDC cameras often connect the power supply return lead to the camera's ground. The result can be that current from the power supply may flow through the shield of the video path, a recipe for ground-loops. For this reason, it is recommended that 12VDC cameras be powered from a local 12VDC supply that has a floating (not grounded) output. I-View has developed its UTP Camera products that allow 4-pair UTP wire to be used to deliver camera Power and Video. These UTP cameras are "cable integrator" pass-through devices that allow the use of an external power supply and RJ45 connector for in-house wiring. This allows for inter-operability with external low voltage power supplies, including those that deliver 12VDC or 24VDC or 24VAC. |
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Loop resistance measurement |
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Wire distance may be measured with an ohm-meter by shorting the two conductors together at the far end, and measuring the loop-resistance out and back. Please remember to include any coax in the path.
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Ghosts |
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Normally a video signal travels from the source (camera) to the destination (monitor) by way of a transmission path. This path can be coax, UTP, or other media. Normally this is a single path, with no wires going un-expected places.
Now suppose that there is an extra pair of wires (a bridge-tap) tapped into the main pair. Energy travels down the main wire pair, but it hits a fork in the road. Some of the energy continues in the right direction, but some goes down the wrong way into the bridge-tap. Energy cannot just shoot off the far end of the bridge-tap wire, so it reflects back in the direction from which it came. That energy hits the same fork, and some of that travels towards the final destination. But that energy was delayed, so it shows up at the final destination as a "ghost", a copy of the original image shifted to the right. If one takes a ruler or tape measure and measures the right-shift, one can determine the length of the stub. This won't tell where along the main path it is. |
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Interference Immunity |
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The term “balun” is short for “Balance-Unbalance”. It’s typically (but not always) a transformer where one side is connected to a signal and a ground (such as coax), and the other side has both conductors floating relative to ground, with our signal between them. For correct operation one always needs a balun at each end of the wire. |
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Transmission-line Effects |
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Data distribution is accomplished by paralleling the data paths to up to four cameras. Known as star wiring, this fan-out method works very well for camera implementations of RS-485 or outbound RS-422. This is because data is typically traveling at the glacial speed of 4800 baud (confirm the camera's protocol). Signals travel through twisted-pair wire at 575ft (175m) per microsecond. From transmission theory:
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Propagation speed / Frequency = Wavelength |
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575ft/microsecond / 4800 bits / second = 120,000ft |
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175m / microsecond / 4800 bits / second = 37,000m |
Transmission-line effects (reflections, ringing, etc.) start at 1/4 wavelength, so divide by 4 to get 30,000ft (9,000m). Then there are harmonics of the square-edged data pulses. If we take the fifth harmonic (very conservative because edges round off at long distances), we need to divide this number by five 30,000ft / 5 = 6,000ft (1830m). This is longer than our maximum cable length, which is typically spec'd at 3,000 ft (1Km). This distance is much longer than any star-wiring-based stub because the power distribution limits us to 400ft (120m).
This calculation shows that at these speeds, the system acts more like a lumped-element circuit than a transmission line. Indeed, oscilloscope measurements of the data path show clean square pulses under worst-case conditions.
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Shielded or Un-shielded Wire? |
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It is interesting to note that the EIA spec for RS-422/485 does not require shielded wire. When one reads the datasheets for RS-422/485 driver/receiver ICs, they don't require shielded wire either. Most telemetry cameras send data at the glacial speed of 4800 baud, but even if the data rate were higher, UTP wire has less loss than shielded wire. So why do some manufacturers continue to spec it? There are several possible reasons:
Their engineers aren't very experienced with transmission. All they know is that they want to protect their precious data, and shielded wire will have lower interference. This is partially true. Yes, shielded wire will sustain less cross talk, but the interference is down in the mill volts and the RS-422/485 signal is 8 volts peak-to-peak. A few mill volts won't affect such a large signal. What is bad is when a field engineer grounds the drain lead at both ends and large currents flow. The magnetic coupling in this case can be significant.
The equipment has a processor and there's a lot of stray clock-noise inside their box. The engineers don't know how to make their equipment pass the radiated emissions "smog-test", but if they use shielded wire it acts as a distributed filter. There are several ways around this:
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Use three meters of shielded wire, and then convert to UTP. The emissions remaining after the three meters are negligible. |
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Put in a filter at the I/O connector. Place a 1000 pF capacitor from each data conductor to chassis ground. Better yet, use Metal-Oxide Varistors. These MOV devices are around 1000pF too, but they also protect against damaging transients. |
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Design the equipment right from the beginning. |
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Be a stinker. Ignore the emissions. Hey, when was the last time you heard of interference from a data line causing nearby equipment to fail? |
The equipment has a lot of sensitive CMOS that blows out when there's a lightning hit. The solutions in the previous paragraph work here; Use three meters of wire to shunt the fast rise-time energy onto the chassis, or use a filter, capacitor, or MOV. If you're really concerned, use an off-the shelf transient protector, but connect its drain wire to the equipment chassis. It's the way we've always done it. This is the same argument people in our industry use to continue using coax. By the way, does your Ethernet LAN still use coax?
Theory and specifications are no substitute for real-world operation. I-View has been shipping product for few years. There are many installations that employ UTP for their transmission of RS-422/485 data. We know of no installation that ever experienced a problem with it. |
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Transient Protection |
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The electric field density is very high at the top. That makes it a great target for lightning strikes. For the sake of discussion, let's say the voltage at the top of the pole is 50,000 volts. The bottom of the pole is at 50,000 volts too. If the pole is grounded, the top of the ground rod will be at 50,000 volts. And the bottom of the ground rod will also be at 50,000 volts. (Dirt doesn't conduct well.) If you were to sink a new ground rod 5 or 10 feet away, this rod might be at 49,000 volts.
Now, if a camera is powered from a low voltage source, there is no life-safety, electrical code, or UL requirement that it be grounded. (Check the camera's manual, too.) This brings us to an important choice. We can connect this camera to a source of 50,000 volts, or not. If we do ground the camera, then current will flow through our copper back to the head-end, which is at zero volts because it is grounded. If we float the camera, no current will flow. This principle is one-alarm installers know well: ground the head end; float the remote keypads - nothing gets cooked.
Ideally at the receive end, all CCTV equipment, including the I-View UTP camera, should be co-located and share the same building electrical ground. Having the I-View UTP Camera grounded to the telephone-company's ground rod, while the rest is connected to the building ground could allow large voltage differences to damage equipment. Lightning protection is not an exact science. However, customers who have experienced problem installations have successfully employed these practices with excellent results.
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