VOSCOM Fiber Optics Blog

Fiber optics for CCTV applications are predominantly used in extended local installations linking cameras back to monitors with dedicated fibres for each link. A typical fiber optic transmission system layout is shown in Fig. 6.

This example illustrates the main features of any fibre optic system, which are as follows:

1. The fibre optic link and its associated terminal equipment fit between the camera and the associated monitor/controller and provide a transparent signal path i.e. the camera and controller do not know that the signals have been transmitted over fiber.

2. The camera output is a 1V peak to peak composite video signal.

3. Movable cameras have a telemetry receiver is mounted near to the camera movement mechanism. This telemetry receiver connects to the system controller to provide control of the camera pan/tilt and zoom PTZ functions.

4. At the control end of the link camera selection and movement is looked after by the system controller and video signal outputs from the controller are displayed on a local monitor(s).

5. Electrical to optical and optical to electrical converters provides the interfaces to the optical fibre transmission fibre.

6. At the camera end of the link the E/O converter is usually a single channel unit packaged in a small enclosure which can be conveniently mounted near to the camera or telemetry receiver. These E/O converters are not usually environmentally sealed and so need to be protected from the elements often mounting them in the telemetry receiver enclosure. In their most cost effective form a PTZ cameras E/O converter will use two multimode fibres to give a uni-directional video connection plus a bi-directional control data channel.

7. As an alternative these control and video link functions can be carried over a single fibre using optical transmission at two wavelengths, WDM – wavelength division multiplexing. These WDM links are more expensive than single wavelength links but they do save on fibre usage and they also can make the best use of a previously installed fibre infrastructure.

8. The E/O converter data interface must be compatible with that used by the system controller; these are often non-standard.

9. Fixed cameras can use a miniature E/O transmitter, which can connect directly to the camera BNC signal output. This link requires only one fibre.

10. The camera end E/O converter is connected to the transmission fibre through a patch box. This patch box provides a point of termination for the transmission cable and so prevents strain and wear and tear being placed on the transmission cable when installing, servicing or moving the terminal equipment. Optical connections between the E/O converter and the patch box are made with duplex patchleads (which are short fibre cable lengths terminated at each end with an optical connector). The patch box will only be a relatively small enclosure because it will only need to provide connectivity for a few fibre cores.

11. At the control room end of the link fibres from a large number of cameras will be concentrated. Equipment must therefore be packaged accordingly and most often this means the use of 19” rack mount units. E/O converters are manufactured in modular card format, which enables multiple video channels to be accommodated in a 19” cage. Typically one 3 U high rack can accept plug-in E/O converters for up to 30 video only channels or 10 video/data channels (or a mixture of both).

12. The fibre transmission cables are also handled in 19” rack enclosures because now we will be organising many fibre cores. These enclosures are called patch panels and they again provide a physical buffer between the transmission cable and the terminal equipment. Here the cable will be bought into the rear of the patch panel via a compression gland and the fibre cores will be broken out into the secondary coated cores. These cores will then be terminated with connectors, which are then connected into in-line adaptors mounted through the front bulkhead of the patch panel enclosure. This termination may either be carried out by the direct attachment of connectors to the fibre tails or factory terminated connectors tails will be spliced to the transmission fibre cores. If splices are used then the splice enclosures will be mounted in clips on the patch panel base. Patchleads then connect the patch panel bulkhead connections to the E/O converter optical connections. Copper leads then complete the connections to the system
controller and monitors.

As part of the cable installation the installer will have measured the installed cable loss, a function of position using a piece of test equipment called an OTDR (Optical Time Domain Reflectometer). This measurement serves to finger print the system and provides a point of reference for future system maintenance. It also provides the value of the end to end loss of each optical fibre used. The total loss must not exceed the optical margin specified by the equipment manufacturer, otherwise the transmitted picture quality may be impaired. In a correctly installed multimode system link lengths of 4 km for 850nm products and 8 km for 1300nm products are readily achieved.

The sole purpose of the fibre optic link in a CCTV fibre optic transmission systems is to transfer electrical signals between two remotely separated points, A and B, with no degradation in the transmitted signal quality. In this way the fibre optic link becomes transparent to the user. An analogous situation is with a telephone call where you want to be able to talk to another person anywhere as though they were standing next to you.

The basic components of a CCTV fibre optic transmission system are as follows:

·  Electrical to Optical Converter (Transmitter) at the camera end of the link. This unit takes the analogue 1 v peak to peak signal from the surveillance camera and converts it into a light signal that varies in proportion to the camera output signal. The light signal is generated by an LED (light emitting diode) or laser transmitter which is designed to couple a maximum of the generated light into an optical fibre.

·  The optical transmission fiber and fiber optic cable. The optical fibre guides the light from the LED or laser transmitter with a minimum of loss to the monitor or matrix controller end of the link. The optical fibre itself is protected by a variety of sheathing materials to provide a cable construction appropriate to the specific application. The fibre cable is connected to the terminal equipment using de-mountable screw or bayonet fixing connectors.

·  Optical to Electrical Converter (Receiver) at the monitor end of the link. This unit takes the optical signal from the optical fibre and converts it into an analogue electrical signal that is compatible with the monitor input requirements. The light to electrical conversion is carried out by a semiconductor detector which is called a photodiode, or an avalanche photodiode. Subsequent electronic circuitry regenerates the output signal. Products from the better quality manufacturers compensate for optical fibre losses and transmitter output intensity variation with time and temperature by providing automatic gain control to give a standard 1 v peak to peak output format as generated at the camera output.

·  Control data and audio connections. Cameras in CCTV installations are either fixed, viewing a specific scene, or movable, so that different scenes can be viewed under the direction of the operator who would be sited in the remote control room. In the case of fixed cameras then the fibre optic link is required to transmit video only information from the camera to monitor, this requires only a single fibre link for each camera to monitor path. In the case of a movable camera then a return signal must be provided from the control room to the camera usually over a second optical fibre. It is usual for these return control links to provide remote control of the camera PTZ – pan, tilt and zoom functions plus
camera enclosure wash/wipe activation.

If camera control is used then the fibre optic link interface electronics must be compatible with the protocols used by the controller manufacturer. These functions are transmitted over the return fibre link using a standard digital transmission format such as RS232, RS485/422, 20 mA current loop and most recently Echelon Lonworks FTT10A. In addition some controller manufacturers require a return data channel from the camera to confirm camera movement. This return data is usually encoded by the camera optical transmitter electronics and sent over the same fibre as the video signal.

Help point and door entry installations require the transmission of two-way audio signals over the fibre link. Again optical transmitter and receiver units are available to provide this facility in addition to the video and control data links all over the same two fibres. It is also possible to provide all of these video, data and audio transmission functions over one fibre using different wavelength (colour) lights sources to transmit light in each direction. This technique is known as wavelength division multiplexing; it maximises the use of installed fibre cores but at the expense of more costly fiber optic transmitters and fiber optic receivers.

The principle reasons for using optical fiber 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 fiber attenuation is between 0.3 and 3 dB/km meaning that fiber optic transmitter 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 fiber 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 fiber optic 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 fiber cables do not radiate any signals as a consequence fiber optical 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 fibers 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 fiber 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 fiber optic converter equipment to expand the link capacity.

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 elements of a fibre optic converter 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.