Getting It There
Getting It There

Jul 1, 2004 12:00 PM
By Michael Fickes

Closed circuit television (CCTV) systems have typically used copper coaxial cable to transmit video signals from cameras to control devices and monitors. But times are changing. As video applications have proliferated, transmission methods have kept pace. Today, video signals are traveling here, there, and everywhere, along copper wires, fiber-optic cables, computer networks and through the air.

Familiarity with various video transmission techniques enables users to choose the most efficient way possible to move signals from security cameras to virtually any destination: to a security station, to the desktop in an office, or to the laptop in a car. Here's an overview of the major methods of transmitting surveillance video.

The most common video transmission medium is coaxial cable. The electrical transmission characteristics of coaxial cable are ideal for video. Video coax is designed for 75-ohm impedance systems, an electrical resistance value that works well with a video signal.

Coax comes in several thicknesses. The thicker the coax, the farther it will transmit video. RG-59, the most common coax size, is recommended for distances up to 700 or 750 feet. In practical applications, integrators often limit RG-59 runs to 500 feet or less. Going beyond the 500-foot limit to 1,200 feet requires a step up to thicker RG-6 coax. For runs longer than 1,200 feet, manufacturers recommend RG-11, an older cable television solution.

For CCTV systems that do not need to send video signals beyond 500 feet, RG-59 coaxial cable represents the simplest and least expensive transmission choice. According to industry observers, 90 percent or more of the world's CCTV systems use this transmission medium.

But video doesn't have to travel through coaxial cable. Today, more and more cameras are sending video along copper telephone cables and through copper computer network cables.

These copper cables are made of two strands of twisted wire, called twisted pair. The twists are designed to reduce interference. By adding more twists per inch to twisted pair cables, electrical engineers have substantially improved the performance of twisted pair cables. Today's Category 5 cables, with five twists per inch, and Category 6 cables, with six twists per inch, provide excellent video transmission performance.

Twisted pair cabling can carry video signals for much greater distances than coaxial cable ! as far as 6,000 to 8,000 feet.

Moving a video signal from a camera into a twisted pair cable requires electrical manipulation. When video comes out of a camera, it looks for an unbalanced 75-ohm cable ! otherwise known as coaxial cable. Twisted pair is a balanced 50-ohm transmission cable.

There is a $40 device that transforms a video signal from its unbalanced 75-ohm origins into a balanced signal that will run along the 50-ohm pair of wires in a twisted pair. Called a balun (short for balanced to unbalanced and vice versa), this device accepts the unbalanced coaxial cable output of a camera, transforms the signal to run through a balanced twisted pair. Back at the receiving end, another balun re-converts the signal to an unbalanced 75 ohms for use by head-end video equipment. Sometimes this conversion is carried out in a receiver hub or distribution amplifier that accepts video from a number of cameras, converts the signals to unbalanced 75-ohm impulses, and duplicates the unbalanced signal for use by the monitor, switcher, DVR, and other head-end devices.

Twisted pair cables will carry a video signal up to a mile without requiring amplification. At that point, the signal must be amplified before sending it any further. Manufacturers recommend only one stage of amplification, which can get a signal from a camera to a destination as far away as 8,000 feet.

The cost of twisted pair compares favorably with coax. One thousand feet of twisted pair costs about $50 ! not counting the cost of the baluns ! compared to $200 for a similar length of coax. A twisted pair run will require a $20 to $50 balun at each end, which reduces the cost differential. In either case ! coax or twisted pair ! users have to install wires to carry power and pan/tilt/zoom(PTZ) controls at a cost of about $220 for a 1,000-foot run.

Structured cabling extends the utility of copper twisted pair. A single twisted pair cable will move video from one camera to the head-end of a system. If you have 50 cameras, you will need 50 twisted pairs and 100 baluns, a relatively expensive undertaking in terms of purchase prices and installation labor.

Structured cabling addresses this problem by enclosing 25 or 50 twisted pair cables in a single jacket. For a security director adding cameras to an existing building, consolidated structured cabling cuts both cabling and installation costs.

More important, a security director can work with an electrical engineer to specify structured cabling runs for CCTV installations in new buildings. Here, cabling would be placed in trays used by the building's network cabling system. This cabling can transmit analog CCTV signals from one or many cameras, while also carrying power to cameras and PTZ controls. Such a cabling specification would follow the stringent EIA-TIA 568B structured cabling standard, which governs the cable and the distance over which it can carry video, as well as the faceplates, patch panels, labeling, routing and testing.

Structured cabling also makes for an easy transition from analog video signal transmission to network-based digital signal transmission. If you run video on structured cable, making the jump to a network video transmission system requires nothing but the addition of analog-to-digital conversion devices and some re-wiring at patch panels to connect the structured cabling to the Ethernet cables carrying network data.

Any copper cable ! or fiber-optic cable, for that matter ! can carry video signals in analog or digital formats. But a computer network only carries digital signals. To move from a "home run" video transmission cabling system to a network transmission system, video signals must be converted from analog to digital formats.

Both analog and digital video signals use electricity to represent a video image. Analog signals represent video by varying the strength of electrical impulses. By contrast, digital signals represent a video image with a code composed of on and off electrical signals. The code follows the format of binary computer code, or 1's and 0's.

Video cameras produce analog signals. Sometimes, it makes sense to transmit that signal just as it is. In other cases, however, a digital conversion can cut costs. Suppose one wanted to install 50 cameras in the parking garage next door to a building. For one reason or another, the electrical connections between the office and the garage consist only of five twisted pair cables. That's enough to grab analog video from five analog cameras. It's also enough to bring in digital video from 50 digital cameras ! a single twisted pair cable can handle digital signals from 10 cameras.

While this system transmits digital signals, it has not been routed through the network.

Sometimes, there are advantages to using a computer network to transmit video. Chief among these is the ability to view networked video over the Internet, from anywhere in the world, with no distance limitations.

IP-enabled cameras can be connected directly to a network and programmed with an IP address, allowing an Internet browser to access the video over the Internet.

Moving video signals across a network solves distance problems by enabling anyone with a browser, password, and Internet address to view video from an IP digital camera. There is an alternative that does not involve putting video on a network ! the use of fiber-optic cabling.

Beyond the distance limitations of twisted pair (6,000 to 8,000 feet) or structured cable (around 300 feet), fiber-optic cabling comes into play. Fiber-optic systems convert electrical video signals into light, transmit the light through strands of glass fiber, and reconvert the light to electrical impulses at the receiving end.

Among several kinds of fiber systems, there are two basic choices: multi-mode and single-mode. Multi-mode, the low-cost alternative, transmits light using an LED. Multi-mode cable provides good performance up to 2.5 miles using transmitter/receiver systems costing about $600. For distances beyond 2.5 miles, single-mode fiber using a laser light source becomes necessary. Single-mode fiber systems can triple the cost of multi-mode systems.

Wireless video systems support security in three ways. They can move video from a camera site that lacks cabling, they can transmit video to security officers patrolling in vehicles and they can transmit video from mobile surveillance sites ! such as a school bus.

Basic wireless systems can connect cable-less sites to monitors through point-to-point transmission systems. These simple systems send analog signals from a camera to a monitoring or recording point.

More complex systems create wireless networks useful to patrolling security officers. For example, an IP-enabled camera with a transceiver can transmit video to a wireless distribution device known as an access point or "hot spot." A computer equipped with a network interface card, an access point serves as a transmission medium for a wireless network. It supports a number of transmitting cameras and receiving computers.

In order to move security officers out of a command center to patrol a campus on foot, one could create access points or hot spots around the campus, each connected to a group of cameras. When a patrolling officer comes within range of a hot spot, a wireless interface card in his or her handheld computer picks up video from the system.

Security officers patrolling in vehicles could only receive video if they stopped their vehicles within a hot spot. To get around that problem, a different wireless structure might involve giving officers laptops equipped with transceivers like those used by the cameras in the system. As patrolling officers pass different sets of cameras, their transceivers would receive and transmit video from different cameras. Set up properly, all the officers would be able to view video from all of the cameras by way of overlapping signals created in this roving mobile network.

Relatively low-cost wireless video systems do require strict security precautions.

Thanks to the following people and companies that contributed to the research for this article:

Guy Apple, vice president marketing and sales, Network Video Technologies Inc., Menlo Park, Calif. NVT specializes in twisted pair based video transmission technology.

Charlie Clawson, account manager for engineering, Actelis Networks Inc., Fremont, Calif. Actelis specializes in video transmission using Ethernet over copper wire.

David O'Keefe, national sales manager, low voltage, Communications Supply Corp., Carol Stream, Ill. CSC specializes in datacom product distribution and support.

Victor Milani, vice president product and market development, GE Security Commercial Solutions, Fiber Options Product Line. This GE division specializes in fiber-optic transmission systems.

Paul Smith, chief operating officer, DVTel Inc., Paramus, N.J. DVTel specializes in network- based security solutions.

Steve Surfaro, group manager enterprise projects, Panasonic Ideas For Life, Secaucus, N.J. This Panasonic division specializes in wireless video technology.

David Winikoff, consultant, CIS Security Systems Corp., Springfield, Va., CIS is a security systems designer and integrator.

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