Transmission of video signals

This is not meant to be a text book on transmission but is intended to remove some of the mystery associated with various methods of transmission. Many approximations and simplifications have been used in writing this guide. This is to make the subject more understandable to those people not familiar with the theories. For general application in the design of CCTV systems it should be more than adequate and at least point the way to the main questions that must be addressed. The manufacturers of transmission equipment will usually be only too keen to help in final design.

This first part deals with the transmission of video signals by cables. Part 2 deals with the transmission of video signals by other methods such as microwave, telephone systems, etc.

Diagram 1 illustrates the many methods of getting a picture from a camera to a monitor. The choice will often be dictated by circumstances on the location of cameras and controls. Often there will be more than one option for types of transmission. In these cases there will possibly be trade offs between quality and security of signal against cost.

A field of video is created by the CCD being scanned across and down exactly 312 ¹/₂ times and this reproduced on the monitor. A second scan of 312 ¹/₂ lines is exactly ¹/₂ a line down and interlaced with the first scan to form a picture with 625 lines. This is known as a 2:1 interlaced picture. The combined 625 line is known as a frame of video and made up from two interlaced fields. The total voltage produced is one volt from the bottom of the sync pulse to the top of the white level, hence one volt peak to peak(p/p). The luminance (brightness) element of the signal is from 0.3 volts to one volt, therefore is 0.7 volts maximum. This is known as a composite video signal because the synchronising and video information are combined into a single signal.

In the case of a colour signal, further information has to be provided. The colour information is superimposed onto the video signal by means of a colour sub-carrier. A short reference signal, known as the chroma burst, is added to the back porch after the horizontal sync pulse to detect the difference in position or phase.

The transmission system must be capable of reproducing this signal accurately at the receiving end with no loss of information.

Note that the imaging device is scanned 625 times but the actual resolution is defined by the number of pixels making up the device.

The video signal from a TV camera has to provide a variety of information at the monitor for a correct TV picture to be displayed. This information can be divided into: Synchronising pulses that tell the monitor when to start a line and a frame; video information that tells the monitor how bright a particular point in the picture should be; chrominance that tells the monitor what colours a particular part of the picture should be (colour cameras only).

The composite video output from the average CCTV camera covers a bandwidth ranging from 5Hz to many MHz. The upper frequency is primarily determined by the resolution of the camera and whether it is monochrome or colour. For every 100 lines of resolution, a bandwidth of 1MHz approximately is required. Therefore, a camera with 600 lines resolution gives out a video signal with a bandwidth of approximately 6MHz. This principle applies to both colour and monochrome cameras. However colour cameras also have to produce a colour signal (chrominance), as well as a monochrome output (luminance). The chrominance signal is modulated on a 4.43MHz carrier wave in the PAL system therefore a colour signal, regardless of definition, has a bandwidth of at least 5MHz.

From the above it will be obvious that to produce a good quality picture on a monitor, the video signal must be applied to the monitor with little or no distortion of any of its elements, i.e. the time relationship of the various signals and amplitude of these signals. However in CCTV systems the camera has to be connected to a monitor by a cable or another means, such as Fibre Optic or Micro Wave link. This interconnection requires special equipment to interface the video signal to the transmission medium. In cable transmission, special amplifiers may be required to compensate for the cable losses that are frequency dependant.

All cables, no matter what their length or quality, produce problems when used for the transmission of video signals, . the main problem being related to the wide bandwidth requirements of a video signal. All cables produce a loss of signal that is dependent primarily on the frequency, the higher the frequency, the higher the loss. This means that as a video signal travels along a cable it loses its high frequency components faster than its low frequency components. The result of this is a loss of the fine detail (definition) in the picture.

The human eye is very tolerant of errors of this type; a significant loss of detail is not usually objectionable unless the loss is very large. This is fortunate, as the losses of the high frequency components are very high on the types of cables usually used in CCTV systems. For instance, using the common coaxial cables URM70 or RG59, 50% of the signal at 5MHz is lost in 200 metres of cable. To compensate for these losses, special amplifiers may be used. These provide the ability to amplify selectively the high frequency components of the video signal to overcome the cable losses.

There are two main types of cable used for transmitting video signals, which are: Unbalanced (coaxial) and balanced (twisted pair). The construction of each is shown in diagrams 2 and 3. An unbalanced signal is one in which the signal level is a voltage referenced to ground. For instance a video signal from the camera is between 0.3 and 1.0 volts above zero (ground level). The shield is the ground level.

A balanced signal is a video signal that has been converted for transmission along a medium other than coaxial cable. Here the signal voltage is the difference between the voltage in each conductor.

External interference is picked up by all types of cable. Rejection of this interference is effected in different ways. Coaxial cable relies on the centre conductor being well screened by the outer copper braid. There are many types of coaxial cable and care should be taken to select one with a 95% braid. In the case of a twisted pair cable, interference is picked up by both conductors in the same direction equally. The video signal is travelling in opposite directions in the two conductors. The interference can then be balanced out by using the correct type of amplifier. This only responds to the signal difference in the two conductors and is known as a differential amplifier.

This type of cable is made in many different impedances. In this case impedance is measured between the inner conductor and the outer sheath. 75 Ohm impedance cable is the standard used in CCTV systems. Most video equipment is designed to operate at this impedance. Coaxial cables with an impedance of 75 Ohms are available in many different mechanical formats, including single wire armoured and irradiated PVC sheathed cable for direct burial. The cables available range in performance from relatively poor to excellent. Performance is normally measured in high frequency loss per 100 metres. The lower this loss figure, the less the distortion to the video signal. Therefore, higher quality cables should be used when transmitting the signal over long distances.

Another factor that should be considered carefully when selecting coaxial cables is the quality of the cable screen. This, as its name suggests, provides protection from interference for the centre core, as once interference enters the cable it is almost impossible to remove.

In a twisted pair each pair of cables is twisted with a slow twist of about one to two twists per metre. These cables are made in many different impedances, 100 to 150 Ohms being the most common. Balanced cables have been used for many years in the largest cable networks in the world. Where the circumstances demand, these have advantages over coaxial cables of similar size. Twisted pair cables are frequently used where there would be an unacceptable loss due to a long run of coaxial cable.

The main advantages are:

1) The ability to reject unwanted interference.

2) Lower losses at high frequencies per unit length.

3) Smaller size.

4) Availability of multi-pair cables.

5) Lower cost.

The advantages must be considered in relation to the cost of the equipment required for this type of transmission. A launch amplifier to convert the video signal is needed at the camera end and an equalising amplifier to reconstruct the signal at the control end.

It is extremely important that the impedances of the signal source, cable, and load are all equal. Any mismatch in these will produce unpleasant and unacceptable effects in the displayed picture. These effects can include the production of ghost images and ringing on sharp edges, also the loss or increase in a discrete section of the frequency band within the video signal.

The impedance of a cable is primarily determined by its physical construction, the thickness of the conductors and the spacing between them being the most important factors. The materials used as insulators within the cable also affect this characteristic. Although the signal currents are very low, the sizes of the conductors within the cable are very important. The higher frequency components of the video signal travel only in the surface layer of the conductors.

For maximum power transfer, the load, cable and source impedance must be equal. If there is any mismatch some of the signal will not be absorbed by the load. Instead it will be reflected back along the cable to produce what is commonly known as a ghost image.

It is essential that coaxial cables and balanced cables should only be used with the correct type of equipment. Unpredictable results will occur if the incorrect cable type is used. For instance, if the intention is to use a balanced cable, this cannot be connected directly to a coaxial cable or an amplifier designed to drive a coaxial cable. Some form of device is required to be connected between the two cable types so that both cables are correctly matched. This piece of equipment may be an amplifier or video isolation transformer.

Every joint in a cable produces a small change in the impedance at that point. The mechanical layouts of the conductors change where it is joined. This cannot be avoided. However, the changes in impedance should be minimised by using the correct connectors. When in line joints are being made, ensure the mechanical layout of the joint follows the cable layout as closely as possible. The number of joints in a cable should be minimised, as each joint is a potential source of problems and will produce some reflections in the cable.

Cable and amplifier performance are usually defined as a certain loss or gain of signal expressed in Decibels (dB). The dB is not a unit of measure but is a way of defining a ratio between two signals. The dB was originally developed to simplify the calculation of the performance of telephone networks, where there were many amplifiers and lengths of cable on a network.

The calculations become extremely difficult, and often produce very large figures using ordinary ratios, when many of them have to be multiplied and divided to work out the signal levels of the network. However these calculations become relatively simple if the ratios are converted to the logarithm of the ratio, which can then be just added and subtracted. This therefore, is the reason for using the decibel, which in simple terms is:

10 x log (ratio)

This dB (power dB) is often used to measure power relative to a fixed level. It is not a measure in its own right. If the impedance at which the measurements are made is constant, the dB becomes 20 x log (ratio). This is the dB (voltage dB) which is normally used to define cable loss or amplifier gain in the CCTV industry.

The advantage of using this method becomes obvious when working out the performance of a network containing more than one or two items. Many people who do not use dBs all the time have problems relating them to real ratios. The key figures to remember are:

If the ratio is 2:1, then 20 x log 2= 20 x .310 = 6.021, e.g. 6dB.

If the ratio is 10:1, then 20 x log 10= 20 x 1 =20, e.g. 20 dB.

If the ratio is 20:1, then 20 x log 20= 20 x 1.3=26, e.g. 26 dB.

Similarly a ratio of 100:1 is equal to 40 dB.

Therefore, put in reverse, some common ratios are:

6 dB is a loss or gain of 2:1

20 dB is a loss or gain of 10:1

26 dB is a loss or gain of 20:1

40 dB is a loss or gain of 100:1

Diagram 5 illustrates the relationship between the measure of signal to noise in dB and as a ratio.

The following example illustrates a typical network and how to calculate the losses and gains.

To work out the net loss or gain of signal on a network, add the amplifier gains and subtract the cable losses.

1st cable -- loss 12dB, 1st amplifier -- gain 6dB

2nd cable -- loss 20dB, 2nd amplifier -- gain 26dB

3rd cable -- loss 6dB.

The result would be: -12dB + 6dB - 20dB + 26dB - 6dB = -6dB

i.e.. 1/2 the input signal is present at the end of the 3rd cable. This calculation is much easier than if the original ratios were used:

When a video signal is amplified the noise, as well as the signal, is increased. If the amplifier were perfect then the resulting signal to noise ratio would remain unchanged. Amplifiers are not perfect and can introduce extra noise into the signal. The amount of noise introduced increases as the amplifier approaches its maximum gain setting. A typical amplifier or repeater operating at maximum gain may reduce the signal to noise ratio by about 3dB. Consequently, it is not advisable to run such equipment at the maximum levels. This is similar to the results of turning the volume up too high on a domestic HI FI. A lot of interference is evident and most units are only operated at up to about half their maximum rating.

In the same way as the net gain or loss in a network can be simply calculated by adding the dB values arithmetically, so can the reduction in signal to noise ratio. In the previous example if the original s/n ratio is 50 dB at the camera then after two amplifiers the s/n ratio could be reduced to 44dB. After four amplifiers this could be reduced to 44 - 12 = 32 dB. At this signal to noise ratio the picture would show a lot of 'snow' and be close to the limit of a usable picture. This then is the limit of the distance that a video signal may be transmitted using this type of transmission. Therefore, besides calculating the losses and gains of the network the reduction in s/n ratio must also be calculated. This example assumes that the worst case is considered. Manufacturers' data or assistance should be sought if equipment is to be used at maximum settings.

The term dB is very often misused as a measurement, which it is not. This practice is very common. However, the correct way of stating a measurement is +/- YdB's relative to a base level. It is a common, though technically incorrect, practice not to mention the base level, which can lead to the assumption that the dB is a unit of measure.

Overall cable performance is usually defined for its ability to pass high frequency signals. After selecting the correct type of cable with the desired impedance, the next most important factor is the cable transmission loss at frequencies within the video band. Most cable manufacturers provide figures at 5MHz and 10MHz. The 5MHz figure is the most important for CCTV use. The cable losses will be defined as a loss in dB at 5MHz per 100 metres. Care should be taken when dealing with cables of American origin as these are often defined as loss per 100 feet. Generally, the larger the size and the more expensive the cable, the better will be its performance. This holds true for most cables as larger conductors produce the least loss.

If the loss is given for a frequency but not the one required, the conversion is as follows. Assuming the cable is rated at 3.5 dB loss per 100 metres at 10MHz, then the loss at a frequency of 5MHz would be:

Note that before using this conversion the cable specification should be checked to ensure that it will transmit satisfactorily at 5 MHz. Some cables are designed specifically for high frequency transmission only, and will not be suitable for the lower frequencies used in CCTV.

The important factors when selecting a cable for a particular installation are:

1) Establish the type of cable to use, coaxial or twisted pair.

2) Select a range of cables of the correct impedance.

3) Select the correct mechanical format, i.e. normal cable to be laid in ducts or single wire armoured for direct burial etc.

4) Consider the distance the cable is required to run and calculate the length of cable required.

Do not forget to make allowances in this calculation for unseen problems in installing the cable. A minimum of a 10% allowance should always be made. This provides a safety margin to cover inaccurate site drawings, sections of the cable running vertically and other problems likely to be met during installation.

5) When the length of cable has been established, assess the high frequency loss from the cable data.

6) Once the cable loss has been estimated, then the equipment requirement can be established.

The data for twisted pair cables is not always easy to obtain. However, most telephone type cables are highly suitable for video transmission. Even the internal telephone subscriber cable can be used over quite long distances for video, with the correct equipment. (Typical losses at 5MHz are 4dB per 100 metres.) If in doubt about the suitability of a twisted pair cable, the general rules are that suitable cables will be unscreened and will have a very slow twist to the conductors, 1 to 3 twists per metre.

Many twisted pair cables are advertised as "Wide Band Data Cables." These are usually of American origin and are heavily screened. They are designed for use with computers and are generally unsuitable for video use. If a cable is to be used about which there is some doubt, it is worth testing the cable with the equipment to be used before installation. Although this may be considered as a waste of time, it can avoid a costly mistake in the installation.

Tests can be run with the cable on drums as the performance will improve when the cable is taken off the drums and installed. When faced with using existing cables on a site, the only safe way to establish if they are suitable is to run an actual test with the equipment it is intended to use.

The problems that can be encountered when attempting to use existing cables include:

Cables that have absorbed water or moisture.

The cable route is much longer than it appears.

Other cables have been connected in parallel.

Bad joints.

If in any doubt, run a transmission test.

When considering the preceding details regarding cable performance, it is obvious that special equipment is required to transmit video signals over long cables. The type of equipment required is dependent on the length of cable involved and the required performance.

This equipment falls under two headings:

1) Launch Equipment

Launch equipment is designed to precondition the video signal for transmission over the cables.

2) Cable Equalising Equipment

Cable equalising amplifiers are designed to provide variable compensation to make up for the losses after the video signal has been transmitted over the cables.

When selecting the cable and equipment for a particular installation the following rules apply:

1) Select the cable to be used, noting the high frequency loss associated with the length of the cable selected.

2) Select the line transmission equipment required to compensate for the cable loss.

3) Sometimes it is possible to save on the installation cost by using a cheaper cable with more powerful equipment.

4) Determine the level of performance required.

5) For colour transmission, it is wise to allow a margin of 6dB extra equalisation in the equipment over the projected cable losses.

6) For high quality monochrome transmission no margin is required other than the 10% for variations in cable length mentioned previously.

7) An acceptable monochrome picture can be obtained with a net loss of 6dB over the transmission link.

Example:-

Cable = 1000 Metres of URM70 = Loss of 33dB at 5MHz.

Equipment required for full equalisation = Launch Amplifier with +12dB at 5MHz + Cable equalising amplifier with +32dB of equalising at 5MHz.

This combination of equipment provides a total of +44dB at 5MHz against a cable loss of -33dB giving +11dB at 5MHz in hand.

This configuration will provide a first class colour picture. In fact it would work well up to a cable length of 1200 metres.

The normal transmission levels for video signals in the CCTV industry are:

Coaxial Cable:- 1 Volt of composite video, terminated in 75 Ohms, positive going, i.e. Sync tips at 0V and peak white at 1 Volt.

Twisted Pair Cables:- 2.0 Volts balanced, terminated in the characteristic impedance of the cable, normally between 110 and 140 Ohms.

A selection of commonly used cable specifications is given below.

The object of using special transmission amplifiers is to be able to produce a video frequency response that is a mirror image of the cable loss. The net result is that the video output will be a faithful reproduction of the input and effectively the cable loss disappears completely. The above is a much simplified version of what happens in a correctly installed transmission link.

The example in Diagram 8 shows that the equaliser response is produced by being able to adjust the gain of the amplifier at different frequencies. In this case the amplifier has five sections operating at 1, 2, 3, 4, and 5MHz.

If the higher frequencies of the video signal are sent at an increased level, this will reduce the high frequency noise by reducing the amount of amplification required at the end of the cable. This method of changing the video signal is known as pre-emphasis.

A cable equalising amplifier acts rather like the audio "Graphic Equaliser" with which most people are familiar. It enables the gain of the amplifier to be adjusted independently at different frequencies within the video band. The object of this is to be able to produce a mirror image of the cable response.

Each amplifier requires setting up to match the cable with which it is to be used. Once set, it should never require readjustment unless a drastic change in the installation is made.

Correct cable equalisation cannot be achieved without the use of special test equipment. This enables the various adjustments to be set to optimum. Some people claim to be able to set up this type of equipment "by eye". No matter how experienced a person is, the results obtained by attempting to use this method will be always inferior to those produced with the proper test equipment.

This produces a special wave form that is designed to show problems in a video transmission link. The timing and period of the chroma burst are especially important in the transmission of colour signals, particularly if multiplexing equipment is incorporated in the system.

This is required to observe the wave form from the pulse and bar generator and should have a bandwidth of at least 10MHz.

The object of setting up the video line transmission equipment is to obtain a true replica of the Pulse and Bar wave form after it has been transmitted through the amplifiers and cable. If this is achieved, a satisfactory picture will be produced by the monitor.

The pulse and bar generator should be connected in place of the camera. The resultant wave form is viewed on the oscilloscope at the output of the amplifier before the monitor. If a launch amplifier is being used, the output level of this should be set first to 1 Volt with no pre-emphasis. The gain of the cable equalising amplifier should then be set to give 1 Volt output.

The equalising controls should then be adjusted in ascending order, i.e. low frequency (LF) lift first to obtain the best equalisation. Each control affects a different portion of the video signal, to obtain the best results. The controls may need adjusting more than once as there is a certain amount of interaction between them.

Once the controls are set to optimum in the equalising amplifier, the high frequency (HF) lift control in the launch amplifier should then be adjusted to give the required pre-emphasis. The HF lift controls in the equalising amplifier should then be able to be set to a lower level. Care must be taken to ensure that the launch amplifier output is not overloaded as this may produce peculiar results.

When a video signal has to be transmitted over extremely long or poor quality cables, it is necessary to use a repeater amplifier within the system. The distance along the cable at which it should be installed can be calculated from the cable loss figures. When using repeater amplifiers, an extra allowance of 3dB should be made for the cable loss. It is better to insert a repeater amplifier in a cable run before the video signal deteriorates too much, than to attempt to equalise a very poor quality signal. There is no actual limit to the length of cable and number of repeater amplifiers that can be used. The problem that occurs is that the signal to noise ratio deteriorates with each amplifier.

The practical limit is approximately 4 repeater amplifiers in cascade with a launch and equalising amplifier at the ends of the cable. This configuration can easily operate over cable lengths of 50 Km or more if the correct type of cable is used. This applies equally to coaxial or balanced cables.

The method of setting up a system with repeater amplifiers is identical to adjusting a single equalising amplifier. The pulse and bar signals are inserted in the cable at the position of the last repeater amplifier. This enables the final equalising amplifier to be adjusted. When this is completed, the pulse and bar unit is moved up the next section of cable to enable the last repeater to be set up. The procedure is then repeated working along the cable towards the camera position until the launch amplifier is reached. Great care should be taken when setting up a transmission link using repeater amplifiers. This is because once an error has been introduced into the video signal by an incorrectly adjusted amplifier it cannot be corrected by miss-setting another amplifier. Errors are normally additive and a slight mis-setting of several amplifiers will produce unacceptable results.

When installing TV cameras or other equipment on large sites, the potential of the earth connection provided for the equipment can vary by quite large voltages (up to 50 Volts). This can produce high currents in cables connected between different points on the site and will produce interference on the video signal.

Most video equalising amplifiers have differential inputs that can reject a certain amount of interference due to earth potential variations (up to 10 Volts). However, it is good practice, and a safe precaution, to break the earth connection using a video transformer or opto-coupled equalising amplifier on long cables. It is not safe or legal to remove earth connections from equipment and rely on the earth provided by the video cable.

This latter procedure, which is still common practice in the CCTV industry, is in breach of the electrical safety regulations and is extremely dangerous and should on no account be used

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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:

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.

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.

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.

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.

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.

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.

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.

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.

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.

For the purposes of this article the following conventions have been used. FIBRES OPTICS is the technology of transmitting data along cables that consist of OPTICAL FIBRES.

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.

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.

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.

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.

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.

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.

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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.