CCTV Transmission Using Fibre Optics

 Classic Security Solutions offers a full range of fibre optic services including multimode, singlemode, single channel, multi-channel, drop & insert, ring and point-to-point, dual redundancy and network management for video, audio & data networks.

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System Design

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The use of fibre optic transmission is now commonplace in telecommunications, data communications and broadcast quality television signal applications. In contrast the use of optical fibre as the transmission media in CCTV security and surveillance applications is a relatively recent development fuelled in the 1990’s by the need for the installation of extensive CCTV systems to combat crime, vandalism and terrorism.

Conventionally cameras are linked to monitors over copper cable links using the lowest cost components available. As system size has increased the distance between cameras and monitors has also increased resulting in an unacceptable degradation of received video signal quality. i.e. received picture quality, for link distances over 100 - 150 m. This has meant that the use of optical fibre transmission has had to be considered even in this most cost conscious of applications.

In these sections we will attempt to de-mystify fibre optic transmission as applied to CCTV system use. We will start by outlining why fibre optics should be used, go on to consider the basic requirements of a fibre optic system and installation practice and finally outline the technology to extend CCTV systems from essentially local installations to extensive, distributed multi-channel signal transmission systems.

The principle reasons for using optical fibre as the transmission media in CCTV applications are:

• The maintenance of picture quality and control data integrity over extended distances:
This is the major reason for using fibre optics which have superior signal amplitude loss characteristics than copper cable. Typically co-axial cable attenuation at a signal frequency of 5 MHz can be 20 dB/km. In comparison fibre attenuation is between 0.3 and 3 dBlkm meaning that fibre optic link distances of 60 km+ can be achieved, depending on the precise details of the application. In addition this low fibre signal attenuation is achieved over a very wide signal frequency range so that optical fibre can be used for the transmission of multiple video signals over long distances.

• Immunity to electromagnetic interference:
Optical fibre transmits signals as light pulses rather than electrical pulses. This light transmission is unaffected by the presence of electro-magnetic fields. As a consequence optical fibre transmission can be used in applications where links are routed near electrical conductors and electrical machines. This includes applications such as railways, tramways, power generation and vehicle manufacture with welding machinery. In addition the fibre cable usually has a metal free construction so that there are no ground loop problems between terminal equipment and the cable will not transmit lightning pulses. This elimination of ground loops makes fibre cable the media of choice for inter building links of whatever distance.

• Security of Information and Operational Safety
Unlike copper cables fibre cables do not radiate any signals as a consequence fibre cables are virtually immune from “tapping” and so the signal content is difficult to access for unauthorised parties. As there are no emissions from optical fibre cable there is no risk that a fibre installation will act as a ignition source. This means that fibre can be used in explosive atmospheres such as chemical and petro-chemical sites providing a truly “Intrinsically Safe” transmission path. Note however, that this Intrinsic Safety, would not extend to the electro-optic termination modems which would need to be safety certified and protected the same as any other electrical equipment.

• Efficient use of duct space.
Optical fibre itself is very small, each glass fibre being only 0.125mm diameter. Protective sheathing is then applied in stages, depending on the application area, to make up the fibre into a usable cable. Typically resulting cable would have a diameter of 3mm for a single fibre core patchlead or 8mm for a 8 fibre cable suitable for internal or external use. In contrast 75 Ohm CT100 coaxial copper cable has a diameter of 6.5 mm. It can therefore be seen that the small size of fibre cable gives significant savings over copper where installation space is in short supply or where duct space is limited. Along with the small fibre cable size comes a weight saving both of which give savings in storage and transportation costs prior to installation.

• Multi-channel capability and “Future Proofing”.
While most CCTV fibres today will be used to transmit one video signal and perhaps a control data signal, the user may wish to upgrade the system to support more camera and control channels. Any glass optical fibre used today is able to transmit multiple optical channels either by using different optical carrier “colours” i.e. wavelength division multiplexing or by increasing the signal frequency using electrical multiplexing techniques. The transmission media is hence “future proofed” and the link will need only additional terminal equipment to expand the link capacity. 

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