Video Transmission

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.

Transmitting the unbalanced 75 ohm video signal over the high impedance/balanced impedance of twisted pair cable require the use of a balum at each end of the wire. Also, because twisted pair cable has a lower bandwidth than the higher frequency components of the video signal, those components will be attenuated more than the lower frequency components. The transmitter and receivers used to push video over twisted pairs cabling has circuitry ( balums ) to match the unbalanced signals to the balanced cable impedance and to compensate for the high frequency attenuation. For longer cable runs, active balums with amplifiers are employed. An added advantage these twisted pair products is the transmitter and receivers add a degree of surge immunity.

Generally, twisted pair is a thin, flexible cable that is relatively easy to run between walls. The video over twisted pair products usually employ CAT5-type connectors that are found in most modern buildings. Again, installation practices are important. Susceptibility to electromagnetic interference increases as the cable's maximum pulling tension or bend radius is exceeded.

 

Fiber Optics

Fiber optics replaces the copper in coax and twisted pair cabling with glass or plastic fibers to transmit video. Fiber optics became practical for use in communications during the 1970s. The method works by converting an electronic signal to light modulation. Fiber optic cable may contain up to 1,000 fibers within a single cable to accommodate high-bandwidth applications. The cables are available in a variety of sheathings for application in building plenums, burial in trenches or underwater installation.

Single-mode fiber is a particularly good method for long-distance transmission of video, capable of delivering high-quality video over many miles. Fiber optics are immune to electromagnetic interference and are often employed to harden an installation prone to lightning strikes. But it is more expensive than copper-based cable, and special methods of splicing fibers and of connecting them to other equipment are needed. Transmitters and receivers are also required, adding further to the expense — especially units that allow multiple video signals and control data to be transmitted over one cable.

 

Microwaves and RF

When cabling is impossible for reasons such as distance and difficult-to-access terrain, microwave transmission is a possible solution. Microwave equipment provides good bandwidth and, through the use of relay links, can cover large distances. However, it requires line-of-sight between relays, can be affected by rain, snow, fog, dust and other environmental factors. These downsides limit microwave transmission to only a few security applications.

One other little-used transmission method is radio frequency (RF). It can also be very effective for transmitting large-bandwidth video over long distances in areas where cabling is impossible. An RF system requires transmitters, receivers and antennas. The entire system must be precisely tuned and may even require a license from the Federal Communications Commission. Due to the expense and difficulty setting up a system, RF is used to transmit video only when other options are unavailable.

 

The IP Revolution

Advances in computer technology have made it possible to digitize and compress a video signal so that it can be sent over still other methods in a cost-effective manner. Among the choices available for transmitting digital video are Internet Protocol (IP), telephone lines, wireless and satellite. Here is a look at each of these possibilities.

IP video is currently among the hottest technologies in the industry. It takes advantage of a corporate Local Area Network (LAN) or Wide Area Network (WAN) to transmit video virtually anywhere in the world. IP-addressable cameras connect directly to the network. Cameras can be added in the same plug-and-play manner as a new printer or PC. Analytics can be added to help automate the monitoring of the video.

The corporate IT department can manage the system and maintain security encryption to allow only authorized users to access the transmitted video. With a password, users can view the video over a host of devices such as a PC or laptop and Internet-ready PDAs or mobile phones. Standard network servers can be used to record large quantities of video.

Video requires a large piece of the corporate bandwidth, which can become an issue as most IT managers ferociously guard the network. However, there are algorithms that can further compress the video and ameliorate much of the bandwidth argument.

 

Wireless Transmission

Digital video has also opened up the possibility of wireless transmissions. Wireless is available in three major configurations – point-to-point, point-to-multipoint and mesh networking.

As their names suggest, a point-to-point network allows for the wireless transmission of video or other data between two separate nodes. Point-to-multipoint networking permits communication between a base node and two or more satellite nodes. These networks, which are relatively inexpensive to set up, work well when cabling is not a practical choice due to hostile terrain or other issues.

Mesh networking is a relatively new way to transmit data between nodes, which may number in the hundreds for a large network. The data moves from node to node until it reaches its destination. The manner in which all nodes are connected keeps the network operating even if one or more nodes go bad. This makes for a reliable network. This type of system is expensive but is finding application in city-wide and campus installations.

Satellite transmission is one more available wireless transmission method. It requires sending video data from an earth-bound transmitter to an orbiting satellite, which bounces the video to an antenna/receiver elsewhere on Earth. Today, only the government and very large corporations can afford to use satellite transmission as the costs run into thousands of dollars to transmit just a few minutes of video.

 

Telephone Lines

Telephone lines, including POTS (plain old telephone service), ISDN and T1, can be used to stream video. The limited bandwidth of POTS means the video will have to be reduced in size or resolution and/or stream in less than real time. The cost of transmitting video over POTS lines is low and an option for long-distance transmission.

By reducing resolution, ISDN, with its 128Kbs bandwidth, can transmit real-time video. Full-resolution, real-time video is possible over a T1 line with its 1.5 Mbs bandwidth; however, both ISDN and T1 lines can be very expensive.

It is not unusual for larger, more complex, systems to employ more than one transmission method. But no matter the size of the system, it always a good idea before starting a new job to seek the advice of an experienced systems integrator. He or she will know the value of choosing the correct transmission infrastructure. It easily can mean the difference between a well functioning, quality system and one that quickly becomes a disappointment.

 

Jim Coleman has more than 30 years of experience in the security industry. He is the president of Operational Security Systems, which is headquarted in Atlanta and also operates offices in Florida. Coleman is also a past president of SecurityNet , a 22-member international group dedicated to providing a single source of electronic security for government and corporate installations .

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