1851 SITO/City & Guilds masterclass [part 30] CCTV [paper 6] - Try a fibre diet

Armoured coaxial cable is used mainly for external and subterrain work. It is sometimes referred to as SWA (steel wire armoured cable). Its construction (Fig 1) is the same as standard coaxial cable except for an additional layer of steel wire encasing the cable and a polyethylene sheath encapsulating the whole assembly. Steel wire armoured cable should be used in situations where the cable may be exposed to pressure or the possibility of damage.

For most CCTV applications the impedance of the cable is also important. If you remember when I talked about monitors the signal was terminated in 75 ohms. Termination and cable impedance must be the same, otherwise mismatch will occur. This can lead to signal loss, interference, ghosting or patterning.

Fibre optic cable

While coaxial cable is the most suitable and cost effective cable for CCTV signal transmission over short distances, it is best to consider other methods for distances greater than 1 kilometre.

One of the most suitable cables for these distances is fibre optic. Fibre optic is a fine strand of glass which is highly transparent. There are two main types, referred to as single mode and multi-mode optic fibres.

The single mode fibre optic has a high level of efficiency but can transmit in only one mode. Laser transmitters and receivers are usually required for single mode application. Multi-mode fibre optics are larger and can operate in several modes. They also accommodate cheaper forms of transmission media such as infra-red.

These cables are used mainly over shorter distances while the single mode fibre would be used where distance and performance were critical.

The main types of applications for fibre optics are:

Light guide fibre

Used in instrument panels and lamps, carries visible light only.

Coherent fibre

Normally referred to as coherent bundle because of its construction. This glass fibre will carry an undistorted image of light over a short distance. Ideal for extending the lens with applications in endoscopes and covert surveillance (Fig 2).

passing along the fibre's length. The main light passes straight along the centre of the cable while a little of the light hits the side of the glass tube. This is reflected back into the centre of the cladding (Fig 3). This results in very low transmission losses over long distances.

Fibre optic cable also has the advantage of not being affected by electromagnetic interference (EMI). A receiver which consists of a photosensitive device receives the light and converts it back to a video signal (Fig 4).

There are also a number of other transmission methods available, these are:

• Twisted pair

• Microwave

• Infra-red

• Slow scan systems.

Sometimes it is necessary to convert the video signal into another format to prevent loss of detail. Special units known as modems have to be attached to each end of the line to convert the signal into a lower frequency signal that will not be affected as much. One way in which this can be done effectively is through slow scan systems or SSTV.

These systems can utilise the standard telephone system referred to as PSTN (Public Switched Telephone Network). This means that distance becomes no object and the only requirement for both transmitter and receiver is access to a telephone line.

The signal from the camera is fed into the slow scan transmitter, which converts the signal to a lower frequency so that it can be transmitted on a telephone line.

The time taken to transmit a single frame of information is extremely slow because the bandwidth of the video signal is much greater than that of the telephone line, therefore the video information is transmitted in manageable portions, a little at a time.

The more detail that is transmitted in a picture the slower the transmission time. The result at the monitor is a series of still pictures rather than the real time pictures we are used to viewing.

The speed of transmission is referred to as the update time and this is reflected in the monitor in terms of the time it takes to produce a picture frame.

Slow scan transmission

In order to process and send each frame the transmitter 'grabs' a sample of the input signal from the camera and stores it in a frame store. To achieve this it must convert an analogue video signal to a digital signal. This is done in an analogue-to-digital converter (Fig 5).

Storing the sample frame allows the system time to process the picture information, convert it to a lower frequency, and transmit over the telephone line as a standard telephone signal.

This could be likened to a dam storing the water and releasing it at a controlled rate. The receiver converts the incoming signal back into a composite video signal and displays it on the monitor.

In these systems the update time of the frame is visible on the monitor as each line of information changes.

Improving the speed of slow scan

The speed of the update time is a critical factor in these systems. One way of improving the speed is to transmit only picture information which is different from the last image sent. If a camera is viewing a car park, the only part of the scene that is likely to change will be cars, or people, moving around.

The system will grab a sample of the signal from the camera and compare it with the previous sample. If there is a difference in the signal, then only the differences will be transmitted. If a car appears on the scene, only the information about the car will be transmitted. This reduces the amount of data sent and therefore improves the update time.

In our next session we will consider ancillary equipment and peripherals.