Transmission of Video Signals by Remote Methods
The previous chapter dealt with the transmission of video signals by various types of cable. There are many instances where it is not possible or desirable to use cable and other methods need to be employed. These can be:
- Infrared beams.
- Public telephone networks.
- High Speed Data Links
- Local Area Networks (LAN)
- Wide Area Networks (WAN)
- Optical fibre cables.
The choice will depend on the final system requirements. This may frequently be coupled with the different cost of several options. In addition, the level of security and continuity of use will have a bearing on the final selection.
With all these systems, it is imperative to study the supplier’s information extremely carefully. For instance, there was a slow scan system that described the picture update time as 20 seconds full picture, 5 seconds quad display. What this really meant was that in quad display one picture was updated every 5 seconds. It still took 20 seconds until the first picture was refreshed! Wherever possible see a demonstration of a system on a customer’s premises. Look carefully at the resolution versus the refresh time.
Free space transmission
There are frequent situations where there is no possibility of making a direct cable connection between the camera(s) and the control position. This particularly applies when real time continuous monitoring is required. A situation needing this approach would be where, for instance, there is a main road between the cameras and the control.
Another situation would be when the two ends of the system are separated by a wide river such as in London. It could be a large industrial site where the cost of cabling would be prohibitive.
Free space transmission consists of a transmitter at the camera end and a receiver at the control end. All free space transmission systems require that there be a direct line of sight between the transmitter and the receiver. Normally there are one transmitter and one receiver for each camera. A typical application is shown in Diagram 16.1.
All types of free space transmission equipment must be very rigidly mounted. This is especially important if the transmitters or receivers are to be mounted on masts or poles.
The distance between the two locations is critical to the choice of equipment. The manufacturer’s specification must always be respected. Performance can deteriorate exponentially if their recommendations are exceeded. A 10% increase in distance could result in a 30% fall off in performance.
There will be situations where there are several units requiring surveillance all controlled from a central source. Great care should be exercised in positioning receivers so that there is suitable separation between the beams from transmitters.
If the example site in Diagram 16.1 required a second camera to be incorporated, this would need another transmitter and receiver. If they were simply added as shown in Diagram 16.2 there is a strong probability that the beams would overlap at the receivers. This would cause problems with the reception of the separate video signals. There are ways in which different systems can overcome this. However, a little thought can prevent the need for special considerations. An alternative method of siting the receivers is shown in Diagram 16.3.
If the receivers are located as shown there will be no chance of cross interference between the two signals.
There is one very important point to consider when setting up any type of free space transmission system. The manufacturers recommended test equipment must be used to align the pairs of units. If the width of the beam is only 1 degree, this is a width of over 17 metres at a distance of one kilometre. Many installers have mistakenly thought that since the receiver is within this band then the reception will be satisfactory.
Most systems will be aligned on a clear day when it is not raining and during daylight. Therefore, the reception will seem fine. A slight deterioration in the weather could reduce the performance considerably after the engineers have left site. Irrespective of the beam width, it should be emphasised that the main signal strength is in the centre part. Only the correct test equipment will ensure that the system will be set up to its optimum for all conditions.
With this type of system, the video is superimposed onto an infrared beam by a transmitter. The beam is aligned to strike a receiver where the signal is transmitted as a conventional composite video signal. The infrared beam is at a wavelength of 860 nanometres which, from Chapter 14, can be seen to be beyond the visible part of the spectrum. The system may be configured as a full duplex set up. Then it is possible to transmit telemetry control signals in the reverse direction to control pan, tilt units. The system can also carry speech in both directions. The actual configuration must be specified at the time of obtaining quotations or ordering.
The performance of infrared beams can be affected by weather and environmental conditions. It is important to check the capability of the link with the manufacturer if an absolute guarantee of reception in all conditions is essential.
The infrared beam is completely harmless and requires no licence or operating restrictions. Selecting the correct beam power for a given range requires some consideration. There is always a trade off between range and quality. One manufacturer, for example, gives the following guidelines. (Table 16.1)
Under each model the range is given in metres for each requirement.
|Requirement||Model A||Model B||Model C||Model D||Model E|
|(1) Economy quality||190||710||1220||2350||3100|
|(2) Full quality||120||320||620||1200||2100|
|(3) High penetration||30||160||300||750||1200|
|(4) High resolution||80||250||390||950||1820|
|(3) & (4) together||-||120||250||600||900|
Table 16. 1 Range Of Infrared Links
This table illustrates the problem of selecting the most appropriate model for a particular application. For instance, the model specified as having a range of 3,100 metres only provides ‘economy quality’ at this range. If high resolution and high penetration are required then the range drops dramatically to only 900 metres. Without this information, it is very difficult for a customer to compare competing quotations all specifying ‘infrared links’. There is a significant price jump from one model to the next.
It can also be seen from the table that infrared links are susceptible to poor weather conditions. It is important therefore that both the installer and the customer are aware of the limitations of this type of link. One argument is that if the cameras are installed outdoors then by the time the link has failed due to bad weather the camera picture has also failed. This is a doubtful basis on which to specify a system. There are two factors that have caused problems in the past with this type of link. Both were intermittent and difficult to figure out the cause of lost pictures in apparently good weather conditions. One was a steam vent outlet that caused the steam to carry through the beam in certain wind conditions. The other was smoke from a chimneystack that obscured the beams also only in certain wind conditions. Neither of these effects was in the sight of the cameras.
Another important point is that the beam width of infra-red links is very small, in order to ensure that enough of the infrared beam falls on receiver to give a good signal to noise ratio. Typically, at a distance of 1500m the spot of infrared light shone on to the receiver may only be a couple of metres in diameter. Consequently, over longer distances above 500m, minute changes in the position of the transmitter may cause the beam to be thrown completely off the receiver and transmission will be lost. This is particularly important if the transmitter is mounted on a steel fabricated building such as a warehouse or hanger. Steel buildings will expand and contract with temperature change and these tiny changes may be enough to adversely affect the position of a transmitter mounted on a steel building.
Infrared links, however, do offer a cost-effective solution to free space transmission. The full knowledge of their possible limitations should be considered. There is no requirement for any form of licence for an infrared link.
Microwave links carry the video and telemetry along a link from a transmitter to a receiver. They are capable of much farther transmission distances from 1 kilometre to 80 kilometres. The frequencies that can be used in the UK are allocated by The Radiocommunications Agency, they also determine the maximum power that may be transmitted, which limits the operational distance. They are largely unaffected by weather conditions. On the other hand, they are more expensive than infrared links.
Similar comments apply that mountings must be rigid and the correct test equipment must be used for installation. Beam width is wider than infrared systems and so building movement is not normally a problem.
Duplex systems can be provided where it is required to operate telemetry controls in the reverse direction. This must be specified at the time of quotation or order.
The requirement for licences should be checked with the manufacturer to find the total cost of a system and any recurring costs. Investigation will need to be carried out if microwave links are to be used close to other microwave equipment such as radar at airports as it will be vital that no interference affects the performance of either system.
This article is an extract from chapter 16 of 'The Principles & Practice of CCTV' which is recognised as the benchmark for CCTV installation in the UK.