Video Transmission

As CCTV has improved, so have the methods of moving video from one place to another


Officials in a southern California city have installed 90 cameras at known illegal dumping areas in an effort to stop people from inappropriately disposing of their unwanted refuse. The city expects the system to significantly reduce the $300,000 it spends annually to clean up the trash.

A Texas apartment complex has installed a video surveillance system to monitor the tenant parking lot. It paid off when police were able to use recorded video to identify a suspect stealing a plasma television and stereo equipment from one apartment. The suspect was arrested and all the stolen goods were returned to the owner.

One wily bank robber in New Hampshire duct taped small tree branches all over his body, thinking it would make him unrecognizable. The bank's video system still managed to get a good shot of his face, which police distributed to the local media. Acting on a tip from the public, the suspect was apprehended at his home the next evening.

These are three video surveillance systems — among thousands spread across the country — aimed at saving lives, protecting property and encouraging lawful behavior. They may each have a slightly different purpose and employ equipment from different manufacturers, but they all share one thing in common: they need to move high-quality video from the cameras to monitors and/or recorders that may be located anywhere from a few feet to many miles away.

There are a number of choices for video transmission, ranging from simple cable to sophisticated wireless infrastructures. Each method has its advantages and disadvantages. Which method is right for a specific job will be determined by factors such as price, distance to be covered, environment, compatibility, resistance to interference, bandwidth and a number of other issues. Selecting the best choice from among the alternatives is critical to the creation of an effective video surveillance system.

Most existing and new systems still transmit analog video. There are several well-proven transmission infrastructures that are commonly used. These include coaxial cable, twisted pair and fiber optics.

 

Coax

Coaxial cable was invented in the late 1880s and has since been used to transmit video and voice around the world. It consists of a round conducting copper wire, surrounded by an insulating spacer, inside a conducting sheath and shielded by a final insulating jacket. In theory, since the electromagnetic field carrying the signal exists within the space between the inner and outer conductors , it should not suffer interference from external electromagnetic fields. However in reality, interference can be common with this method of transmission.

For short runs between cameras and a command center, coax cable is the least expensive transmission method. The rated distances for the various types of coax cable range from about 440 to 1,200 feet. Without the use of video amplifiers, coax cable begins to lose video quality beyond those distances. And the addition of an amplifier begins to take away coax cable's price advantage. Also, proper installation of coax cable is critical. Sharp bends, multiple splices and overreaching on distance can lead to poor results.

 

Twisted Pair

Another common video transmission format is twisted pair cabling. As the name implies, this form of cabling involves a pair of conducting copper wires wound together. Twisted pair cabling was used as early as 1881 to carry telephone conversations. It is also the most common form of cabling for computer networking. The cable is less expensive than coax, but requires a transmitter at the source (camera) and a receiver at the receiving end (recorder/monitor) that adds to the cost. Generally, twisted pair's rated distance is similar to coax, but with a larger power supply and heavy wire, it can support runs up to 4,000 feet.

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