Transmission of Video Signals part 2
The previous section 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.
- Optical fibre cables.
The choice will depend on the final system requirements. This may frequently be coupled with the different cost of several options. Also, 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 11.
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 11 required a second camera to be incorporated, this would need another transmitter and receiver. If they were simply added as shown in diagram 12 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 13
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 output as a conventional composite video signal. The infrared beam is at a wavelength of 860 nanometres which, is above 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 14)
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|
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 chimney stack that obscured the beams also only in certain wind conditions. Neither of these effects was in the sight of the cameras.
Infrared links therefore do offer a cost-effective solution to free space transmission. However, they should be used with full knowledge of their possible limitations. 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 50 kilometres. 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.
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.
Transmission by telephone systems
This is one of the most rapidly developing areas in CCTV. By the time of publication there will have been even more advances. Therefore, the object of this section is to provide an introduction to the concepts and terminology of the subject.
The main telephone system in the UK is provided by British Telecom. However, private companies such as Mercury are allowed by law to use the main network to provide a competitive service.
There are two main methods of transmitting speech or data through the system. The original is the Public Switched Telephone Network, abbreviated to PSTN. The latest system is known as the Integrated Services Digital Network, abbreviated to ISDN. The fundamental difference between the two systems is that the PSTN uses analogue signals whereas the ISDN uses digital transmission. The most significant benefit of ISDN is in the speed of transmission, which is many times that of the PSTN.
There are many reasons why it may be necessary to send pictures down the telephone system. Some of these are summarised as follows.
- There is no line of sight to allow the use of infrared or microwave links.
- The distance is too great for either of these methods.
- It is not necessary to maintain a continuous monitoring of the site.
- To remotely verify the status of a site in the event of an alarm.
- To periodically 'patrol' remote sites from a central station.
The most common use of the telephone system to transmit pictures is in association with alarm systems connected to a central station. The huge increase in the installation of intruder systems in recent years has been under pressure from Police and insurance companies. As the number of installations grew so did the number of false or accidental alarms. Nearly all commercial premises that have an alarm connected to a central station are covered by private operators.
Transmission By PSTN
The common term used for transmitting video signals through the telephone system is 'Slow Scan'. This was supposed to describe the slow rate of sending video pictures by this medium. Slow scan systems are invariably used on sites that are unmanned and for alarm verification. The system requires a transmitter at the site and a receiver at the central station. A video signal cannot be transmitted down a telephone line as a composite video. It must be converted to a series of digits that can be sent as a line of numbers. To do this, the slow scan transmitter converts the video signal to an RS232 signal, which is a stream of binary digits. This is known as Analogue to Digital Conversion abbreviated to ADC. This is connected via a device called a 'modem' to the PSTN. The modem converts the binary digits to a series of tones, which is the only way to pass a signal down the PSTN.
The transmitter is receiving pictures from all the cameras on a site continuously, but not carrying out any processing. On receipt of an alarm signal the transmitter locks onto a predetermined camera. It then automatically dials a pre-programmed telephone number of the central station. Here a receiver converts the digital signal back to an analogue signal for display on a monitor. Other information is also transmitted such as the status of alarms, camera number, time of alarm, etc. When the receiver number is dialled and connected there are several protocols that must be processed. Each site has a unique number that is stored in the receiver. The receiver must check that a valid site is calling. There are also other codes that prevent unauthorised receivers making contact with the transmitter. This checking, connecting and checking can take a significant time, sometimes up to one minute. The time could be further increased by congestion in the system. Due to the time needed to transmit the signal it is not feasible to simply connect the camera output to the transmitter. The method used is for the transmitter to store one frame of video on receipt of an alarm or other input. This frame is held in a digital frame store until the telephone contact is made and then transmitted line by line. Some transmitters have the capacity to store several frames and transmit them in sequence. The speed of transmission is restricted by the bandwidth of the PSTN and not much can be done about that.
Transmission By ISDN
Now, the whole of the UK trunk network is digital. To connect to the service the local exchange must be digital. By the time of publication it is anticipated that most business premises will be served by a digital exchange. To use the features of ISDN the exchange at both ends must be digital. The main benefit of ISDN links for security is speed of connection and communication. With each exchange on the system, the dialled up connection is virtually instantaneous when the last number is dialled. The speed of transmission is about three times that through PSTN lines. The combination of these increases can dramatically reduce the time from an alarm on site to the receipt of the first picture at the central station.
It is not possible to provide details in too much depth in this article about ISDN. However, the following information will illustrate the fundamental principles.
The basic system is known as ISDN 2. This provides two channels, each of which can transmit 64,000 bits of data per second, (64 Kb/s). Up to eight separate pieces of equipment can be connected to each line. Any two of these can communicate simultaneously with each item using one of the lines. Therefore, it would be possible to have one line transmitting video data and the other line in telephone contact. The equipment at the premises is connected to the telephone network by a small unit called Network Terminating Equipment.
To connect video signals to the system, an interface called a 'Codec' or terminal adapter is necessary. This will normally be offered as part of the slow scan package. Even so, the average time to complete the handshake procedure will about sixteen seconds.
The benefits of using ISDN links for CCTV transmissions are higher resolution, higher transmission rates and combinations of the two.
The other option offered to larger users is called ISDN 30. This is the same in principle except 30 channels are available via a single connector. This offers the facility for live video conferencing with real time colour pictures and simultaneous sound. It should be remembered of course that the telephone bill is clocking away all the time.
It is possible to have a permanent line connected between premises. This eliminates the handshake procedure and transmission of a picture commences immediately on receipt of an alarm. Then the installation and rental charges are much higher but there are no call charges. This installation is feasible when continuous CCTV monitoring is necessary and can be less expensive than fibre optic cabling.
Enhancements To Transmitted Pictures
For both networks, manufacturers have developed ways to overcome the inherent problems of transmission time versus resolution. Each must be a trade off against the other. One method is to use a technique called digital image compression. With this, as each frame is digitised, each pixel is compared with the same pixel of the previous frame. Only those pixels that have changed are transmitted. This dramatically increases the rate of transmission and can approach real time pictures where there is a small amount of change. With normal slow scan transmission a person may be in one frame and gone by the time it has been updated. Using digital image compression, the person may be captured on many successive frames. This is also known as conditional refresh.
One problem with the technique mentioned is when there is a great deal of movement in the scene. This could be from trees or moving traffic. Another development has been to specify certain areas of the screen and only update those. Movement outside the designated area will be ignored and not transmitted as refreshed data.
Control Of Remote Equipment
Most systems can include options for the control of positioning devices at the remote site. With some earlier systems this was not a great advantage because of the slow transmission rate. The current systems using digital compression techniques offer a much more practical method of control. When it is required to control the remote devices, the main screen is frozen and a small 'window' is opened on the screen. This small area of the screen shows the camera view in low resolution. The fact that it is in low resolution and a small area means that it can be updated at very nearly real time. Therefore the camera movement can be easily controlled to the required position. After this the screen is returned to whatever resolution and update time is wanted.
Fibre Optic Transmission
For the purposes of this article the following conventions have been used. Fibre Optics is the technology of transmitting data along cables that consist of Optical Fibres.
Advantages Of Fibre Optics
In the field of communications in general, fibre optic technology has made tremendous advances. Not many years ago every connection had to be made in a sterile, strictly controlled environment. The cost per joint was enormous in relation to a commercial project. This has now changed and fibre optic links are now commercially viable. In fact, there are now many developments that would not have been possible without fibre optic technology. Optical fibre cables are replacing copper in many applications. Optical fibres are much smaller and lighter than copper, therefore easier and cheaper to install in long runs.
A major advantage of optical fibres is that they can carry far more information than copper. This is especially important in the transmission of vast amounts of data. This may seem irrelevant to CCTV but the conversion of video analogue signals to digital signals is becoming more commonplace. This is why the improvements in data communications will have a significant bearing on the developments in CCTV. Optical fibres are completely immune to interference from electromagnetic sources. Running copper cables requires careful consideration in many situations where there are power lines, electrical machinery, etc. The problems just do not exist for optical fibres.
Attenuation is a term that is frequently used in connection with losses in fibre optics. It is simply a method of describing transmission losses along a cable. Attenuate means to weaken or dilute, therefore the lower the attenuation the less the reduction in signal strength and quality. In relation to CCTV this is one of the main reasons for using fibre optic transmission. Video signals can now be transmitted without being boosted over previously unthinkable distances. Twenty to thirty kilometres over one continuous fibre is quite feasible.
Wavelength, Frequency And Bandwidth
The electromagnetic spectrum is reproduced below because fibre optic transmission uses light from a particular part of the spectrum.
The different parts of the spectrum have previously been described in terms of the wavelength. An alternative measurement is the frequency of the part being considered. Frequency is the number of crests of a wave that move past a given point in a given unit of time. The most common unit of frequency is the hertz (Hz), corresponding to one cycle per second. The frequency of a wave can be calculated by dividing the speed of the wave by the wavelength. Thus, in the electromagnetic spectrum, the wavelengths decrease as the frequencies increase, and vice versa.
For example the wavelength of infrared light is 850 Nm, the equivalent frequency is 3.5 x 1014 Hz.
Different frequencies have different bandwidths and the higher the frequency the wider is the bandwidth. The wider the bandwidth then the more information can be carried. Frequencies in the visible part of the spectrum offer a wider bandwidth, therefore they provide more space for the multiplicity of TV signals and reams of data that need to be transmitted.
The frequencies used in fibre optic transmissions are between 850 Nm and 1550 Nm.
Transmission By Light
An optical fibre is a rod of the finest purity glass that technology can produce. The part of the light spectrum that functions best with optical fibres is the infrared frequency. Coincidentally, this is the same range that is used for infrared illumination. In fibre optics, messages whether data or video are first converted from electrical impulses into pulses of light. This function is performed by a minute device that incorporates a laser chip or an LED (light emitting diode). The infrared light is switched on and off at incredibly high speeds, thereby creating the stream of light pulses. These are then focused onto the end of the optical fibre. The lightwaves travel along the fibre to the receiving end. Here the pulses are converted back into electrical pulses by a piece of electronic equipment, strangely enough called a converter. Converters also function more efficiently when dealing with infrared light.
There is one manufacturer that produces a fibre optic transmitter that will fit directly onto the camera. The unit only measures 50mmx30mmx30mm yet can transmit up to three kilometres without the need for set up or repeaters.
The Optical Fibre
An optical fibre is a solid rod of glass, finer than a strand of human hair. Even so, the fibre is extremely flexible. Ordinary glass would lose a signal within a few metres due to impurities scattering the light. The glass used in fibre optics is so pure that a solid block 35 Km thick would appear as pristine and clear as a window pane. The fibres are produced in extremely exacting manufacturing conditions from pure silica, which is a type of common sand.
The optical core in the fibre is only 0.005mm diameter. A bundle of them would pass through the eye of a needle. Cable construction can consist of multiple fibres. Each fibre would be similar to that shown in diagram 18. Cables can also be made up with varying types of protective covering including steel wire armour for direct burial. The dimensions in diagram 18 are approximate and illustrate the difference to the familiar coaxial cable.
Transmission Losses In Fibre
Signal loss or attenuation in coaxial and twisted pair is usually given as so many dB per metre or per 100 metres. With optical fibre it is given as dB per kilometre. A typical value for single-mode fibre would be 0.5dB per kilometre, which is 0.05 dB per 100 metres. The figure given for a typical coaxial cable, URM 79, is 3.31 dB per 100 metres. For instance, for a 6 dB loss (50%) the coaxial cable run would be 181 metres. For the same loss the optical fibre run would be 12 kilometres. This example is over simplified but it explains the significant advantage of fibre optic technology. The typical weight of a single fibre cable is about 10 Kg per kilometre. Again to reflect the different technology, coaxial cable is provided on drums of so many hundred metres. Optical fibre cable is provided on drums of so many kilometres.
Diagram 17 illustrated a simple one to one connection through an optical fibre link. There will be many occasions when it is required to transmit signals from more than one camera. One method would be to use a multiplexer at each end of a single link. The disadvantage of this method is that the pictures are not truly multiplexed. They are sent as a stream of consecutive frames from all the cameras. They can then only be decoded by the same type of multiplexer as the one encoding at the transmitting end. The ability to use the individual pictures is very restricted.
Transmission By Multicore Optical Fibres
There are two main methods of transmitting multiple live pictures by fibre optic technology. The first and most obvious is to use multicore cable. Here again the difference in technology is apparent. The outside diameter of a sixteen-fibre cable suitable for running in a duct is only 10 mm. It only weighs 70 Kg per kilometre. For these reasons running multiple video signals is very much easier using fibre optics than conventional coaxial. This is apart from the greater transmission distances possible. There are applications where the cable runs are within the capacity of coaxial cables. However, running sixteen coaxial cables is time consuming and takes up a lot of space in trunking and in ducts. This is especially so if the route is tortuous and difficult to access. A single, light, optical fibre cable may offer dividends in overall cost.
Transmission By Multiplexed Video
Mention was made earlier about the wide bandwidth available with frequencies in the infrared part of the spectrum. It is not important to understand the meaning of bandwidth to appreciate the application. The following example illustrates how advantage can be taken of this feature. A technique known as 'frequency division multiplexing' is employed to transmit multiple channels of video along a single optical fibre. This is achieved by allocating an individual carrier frequency to each separate video channel. The signal, having been modulated, occupies approximately 35 MHz and carriers are spaced 40 MHz apart. Diagram 20 shows the concept of frequency division multiplexing.
At the receiving end of the system demodulators are tuned to each of the individual carrier frequencies. These detect and recreate the original waveform of the video signal.
The big advantage of this system is that each individual video signal is recreated at the receiving end. These then could be shown on separate monitors or be connected into a matrix switching system and combined with other systems. It is possible to add audio and data channels into the combiner and transmit these over the same single fibre. Transmission distances of up to 35 kilometres are achievable along the one fibre without the need for repeaters. Distances of 50 - 60 kilometres are possible using special equipment.
If telemetry signals are required for the remote control of equipment then one additional fibre will be required per group of receivers. The telemetry signal will be transmitted through a separate optical transmitter and receiver. The type of telemetry considered should be discussed with the optical equipment supplier to ensure compatibility of components.
It was said that the optical transmitters use infrared wavelengths between 850 Nm and 1550 Nm. The frequency division multiplexer can be further extended by making use of different wavelengths of light. This is called 'wavelength division multiplexing' and follows similar principles.
This chapter is supplied by Mike Constant and was originally published in CCTV Today. Mike is the author of 'The Principles & Practice of CCTV' which is generally accepted as the benchmark for CCTV installation in the UK.