Analog Video Transmission
Analog, or composite, video is still the most common source today. A J.P. Freeman report claims that new designs are still at least 70% analog cameras. This type of video is called composite video because it combines separate signals, such as vertical-horizontal synchronization and color (red, green and blue), into one using industry-standard encoding such as NTSC. Most existing installations have analog cameras.
Here are the most common methods for transmission of analog video.
In the early days of CCTV, coaxial cable was the most common method of transporting video. There are three common standards of coax: RG-59/U, RG6/U and RG11/U. These cables have a copper core molded into a dielectric material and shielded with a copper braid used as a ground. The common connector used with these coax cable models is the BNC type.
When properly installed, coax cable can transport composite video up to its rated distance. The rated distances for a 10MHz signal are:
• RG-59/U = 441'
• RG-6/U = 789'
• RG-11/U = 1200'
The rating is the length at which the signal drops 3 db in level. Proper installation methods need to be followed to reach these distances. Multiple splices or tight bends in the cable can affect these ratings.
Unfortunately, designers and installers tend to take coax cable farther than its rated length thinking there is little effect. In addition, many installers are not properly trained on the installation of BNC connectors. If improperly installed, the BNC junctions can cause additional loss in signal strength as well as poor shielding. Many coax installations today carry noisy video because of poor installation.
Coax cable loses high frequency as its length is increased. Some manufacturers offer video amplifiers that can amplify the video signal and pre-emphasize its high frequencies, allowing you to extend the length of the cable. This technique, when done properly, can improve the cable length. In these applications, the VA must be installed at the camera end and the equalization must be adjusted using an oscilloscope to optimize the high frequency pre-emphasis.
• Overall, coax is the least expensive solution for transporting video in distances under 2,000 feet.
• Since most cameras (domes and conventional) have BNC outputs, the simplest method is coax.
• Coax doesn’t require a transmitter and receiver as do other techniques.
• Coax is susceptible to EMI and ground loop problems even though it is a shielded cable.
• As pointed out above, it can be installed incorrectly, resulting in numerous problems.
Twisted pair transmission is another popular method for transporting video. It uses a twisted pair of wires instead of coax. The wire is less expensive than coax, but the system requires a transmitter on the source side and a receiver on the receiving side to modify the baseband video to run on the twisted pair. The rated distances are similar to those of coax; however, with heavy enough wire and a bigger power supply, distances up to 4,000 feet are quoted.
• Twisted pair wire is less expensive than coax cable, and with proper equipment and wire size, it can support a video signal at a greater distance.
• Twisted pair is more forgiving in the installation than coax.
• Typical installation uses CAT5 connectors, which are easy to install compared to BNCs.
• The twisted pair system requires a transmitter on the source side and a receiver on the receiving side. This adds cost, often making the complete system more expensive than coax.
Fiber-optic transmission uses glass fibers as the medium instead of copper. The technology converts an electronic signal to light modulation, which travels down the optical fiber. The main two methods used in fiber-optic video transmission are single mode and multi mode, and the distances between them vary. Fiber optics used to require skilled personnel to install the fiber and fiber connectors. Since the cable is glass, splicing or cutting the cable must be done accurately to minimize light loss (signal loss). Most manufacturers today have solved this problem with simple-to-use tools.
• Fiber optics can transmit high-quality video over miles.
• It is immune to normal EMI.
• Fiber optics is more secure than copper. To intercept the signal one must break the glass fiber, which is not an easy task.