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By: Jo Chew
After an image has been captured by the camera, it is then converted into an electrical signal and needs to be transmitted to a monitor, switcher, or video recorder. In order for the image to travel from the camera to the other device, it needs to go through some kind of transmission media.
The most common forms of transmission media for CCTV are:
• Coaxial cable
• Twisted pair cable
• Fibre optics
In CCTV, coax is the most often used form of transmission media. Fibre optics is becoming increasingly popular with its far superior characteristics.
I'll go through each one separately:
Coaxial cable is the most commonly used form of transmission media in CCTV. It is also known as unbalanced transmission.
As you can see in the image above, the cable is constructed of a central core, and a "shield" is used to common the ground potential of the end devices - the camera and the monitor, for example. It not only commons the ground potential, but also serves to protect the centre core from EMI (electro-magnetic interference).
Cable manufacturer's typically specify numbers between 90-99% for the percentage of the cables screening, which is how well the shielding protects the core from EMI. It is not possible to have 100% protection from external interference.
With coax cable used in CCTV, 75? (Ohms) is taken as the standard impedance for all equipment producing or receiving signals. This is why coax cable should be with 75? impedance in CCTV. Calculation of impedance is complex, it cannot be measured with an ordinary multimeter, as it is defined by the voltage/current ratio at each point of the cable.
The most commonly used coaxial cable in CCTV is RG-59/U, which can transmit signals up to 200 metres. This distances are for a single run of cable with no amplifiers or in-line correctors.
Another and more expensive form of coax cable is RG-11/U. It is more expensive because it is thicker. RG-11/U can transmit up to 270 metres for a single run of cable.
An even thicker coax cable is RG6/U. This is even thicker than RG-6/U and can transmit a video signal up to 400 metres with a single run of cable.
In CCTV, coaxial cables are usually terminated with BNC connectors.
When installing coaxial cable, try to avoid sharp bends in the cable, as this will affect the cables impedance. Whenever possible, cable should be run inside a conduit of adequate size.
Twisted pair cable (CAT5e)
Twisted pair cable is a good alternative to coaxial cable if the length of the run is more than a few hundred metres. With twisted pair, runs of up to 600 metres can be used without any in-line repeaters.
Twisted pair transmission is also called balanced video transmission. The cable consists of 4 pairs of 'twists'. Because each pair of wire are twisted for the length of the cable, any electro-magnetic interference affects both wires equally. Both wires are subjected to the same interference, which is why it is called balanced transmission. Unlike coaxial cable, where the shield is grounded and commons the zero potential between the two points, the twisted pair video transmission concept does not common the zero potential between the two points.
If the two wires have similar characteristics, and enough twists per metre(the more the better!), they will be subjected to the same levels of interference and voltage drop. This allows for most unwanted noise to be eliminated at the receiving end.
In CCTV cameras usually have a BNC connection on the camera. You cannot directly connect twisted pair cable to this. Whats needed is a video balun on both the sending and receiving end. You still connect a coax cable to the camera, the coax cable then runs into a video balun, which transmits the signal over twisted pair. On the receiving end, another video balun is required to convert the signal to run through a coaxial cable.
Because twisted pair cables generally have 4 pairs per cable. This actually allows you to carry 4 video signals in the one cable. So you can connect to 4 cameras at one location with the run of one cable!
Fibre optic transmission provides the best quality and is the most secure transmission media. Fibre optics have been used in long distance and overseas communications for decades. It has mainly been unused in CCTV for fear of it being a "touchy" technology and "too expensive". Fibre optics are becoming cheaper and simpler to install, so it's use will increase in the future.
Fibre optic transmission uses light to transmit data, rather than electric currents. The advantages of fibre are:
• Very wide bandwidth
• Cables can be run great distances with no loss of data
• No ground loops are possible
• Very secure, fibre optic cables cannot be tapped into without physically intercepting the signal, which would be easily detected
• Light used as carrier of the signal travels entirely within the fibre. Therefore causes no interference to the adjacent wires or other optical fibres.
• Fibre is immune to electronic interference. It doesn't matter if it is running next to a power line or is close to a megawatt transmitter.
• Fibre optic cables are very small and light
• Fibre optic cables are becoming continually cheaper
Fibre optics do have some disadvantages however. Terminating fibre optic cables requires special tools and better precision of workmanship than with any other cable. Also switching and routing of fibre optic signals is difficult. Fibre optic cables are also more fragile, stepping on the cables could easily damage them.