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CCTV Transmission Media

FROM HERE TO ETERNITY; An interesting title perhaps, but it is often what we expect of our poor coaxial cable.

In modern town centres we have now far exceeded the maximum cable run for realistic cables and as a result we have graduated to the "black art" of optical fibres for our transmission in town for links of 200m to over 15km.  The telecommunications providers charge what appear to be high rates, but justify it by saying, "we are losing telephony revenue by giving fibres to CCTV".  This causes some Authorities to explore alternative means of transmission of the signals to the control room.

Microwave, radio, GSM, PSTN, ISDN, LASER, Ethernet, ATM  Internet and HDSL are all media that can be used for the transmission of CCTV pictures.  Not all of these will give us full frame rate of video (50 fields per second) and therefore need to be chosen for the appropriate application.   There are also pro's and con's to each, which I will try to briefly cover.

Probably the most commonly used alternative medium is microwave.  The equipment is well proven and generally reliable.  In some circumstances, such as Watford, town centre TV systems are all transmitted on microwave to the control room.  In this case on important factor is frequency management.  With many signals coming into the same receiving point there is the potential for cross talk between signals.  It is therefore important for the individual links to be chosen for the frequency and the directions of transmission to minimise the possibility of interference.  This is best achieved by the Tenderers for any particular system, involving the microwave manufacturer at the survey stage and getting a transmission plan drawn up.  This must include the possible future expansion of the cameras "we cant afford this year but will add next year (or whenever)."  If the potential expansion is not included in the original plan, then it is likely that when the expansion is carried out that it may not be possible to integrate the new links without complications of interference with existing signals.

A problem often overlooked when microwave is used over tidal water is reflected signal paths.  Water reflects microwave, therefore a link will often have both a direct signal and a reflected signal.  In a static situation this is not a problem, but where the tide rises and falls the reflected path changes and causes interference with the received signal.

Problems with microwave also include things such as; it is a "line of sight" transmission, it is a narrow angle of beam and therefore needs firm fixings.  It is important to also remember that physical obstructions can interrupt the beam, these include birds.  Where do birds roost? On top of buildings.  Where do we locate microwave dishes?  On top of buildings!!  What will you do when they construct a high rise building in the path of your link?  All you can do is get permission to put a receiver and transmitter on their roof to relay the signal over the new obstruction

Another medium becoming popular is radio, because of the proliferation of mobile systems.  This is at the high UHF or low microwave bands of 1394MHz or 2.4 and 2.8GHz, of which a number of manufacturers are Radio Communications Agency (RCA) licence exempt, within certain criteria of range or power limiting.  Many of the issues highlighted above for microwave also apply to radio transmissions.  This can include items such as reflected signal path, which will not be tidal but vehicle movement, and also line of sight transmission, but we have the added complication that the link may only operate over 20 metres in a town centre because the antenna is at low level.  "High gain antenna available", but if it is installed on the transmitter, does this now exceed the Effective Radiated Power (ERP) and contravene the exemption of the licence?

The fanfare that accompanied the start of use of GSM as a transmission medium has come to little at this stage. The maximum data rates at the moment over GSM are limited to 19.2k bits per second, ie slower than a dial up telephone link on PSTN.   Hold your breath until the next generation of GSM, which will, supposedly, have data transmission rates of up to 2Mbits per second.  This will give picture update rates of 100 times that of the current links (15 times as fast as ISDN 2e).  When this does happen, watch what happens to the possibly more costly fibre link prices, we may have systems that dial up each camera to access the pictures rather than having 32 continuous images all the time in the control room.  There are rumours of further developments in telecommunications that will possibly supercede GSM and give even better results.

Or in American speak POTS (Plain Old Telephone System).  The telephone system we all know and love, that with the right equipment and a couple of modems, we can send signals to the other end of the country, or even the world,  for the cost of a phone call.  The disadvantage is that the data pipe this gives us is not very big, just 38.4kbps effectively (56kbps modems only work at 56kbps in one direction).  This means that with the data compression we can expect picture updates every seven to 20 seconds depending upon the resolution.  In simple terms this means the higher the resolution, the slower the update rate.

The newer digital telephone system that enables much faster data transmission rates of 128kbps  dial-up (192kbps leased line), that is 3 times the speed.  Thus the update rate is faster......not  always... because when signals are sent as PSTN we sometimes only send the changes, but with many of the ISDN systems the whole picture is sent every time, which means more data and more data takes longer.  The ISDN system is not in place all over the world, but it is "e" European harmonised, so it can be used through Europe (where installed) without compatibility problems.  The USA is starting to install ISDN, but most of their digital telephony is on Switch 56, which is not compatible with ISDN.

 Not the Star Wars weapon, but a low power, often Infra-Red, light source onto which the video signal is modulated.  No licence is required and , according to the spec sheets, will work up to 1km.  Sounds good doesn't it.  However beware there are some considerations to bear in mind.  We have a beam diameter of about 2 milli-radians (or 0.1 degrees) which, in practical terms, is a beam diameter of 2m at a range of 1km.  Because we are using light, any physical obstruction may cause the beam to be broken and the signal to be lost, so birds, fog, heavy rain  and vehicles can all cause our signal to disappear.  The bracketry that is used to mount the equipment is critical as well.  Using aluminium brackets is not recommended, the preference is steel.  This is due to the relative coefficients of expansion, aluminium expands faster than steel and therefore the direct sunlight falling on one side of the bracket causes the expansion of one side of the bracket, with respect to the other.  This results in the bracket twisting and the beam is twisted off target and the signal is lost.  Whilst steel brackets will have a similar effect, the effect is much smaller and therefore more likely to stay on target.

Ethernet, LAN & WAN
Digital data networks are becoming the norm in many companies for routine data transfer and computer file sharing.  If we convert the video signal to a digital data stream, it is possible to use the data network to convey the signal around a building.  The problem with this form of transmission is the amount of data required to be transmitted.  The majority of existing data networks are such as the 10baseT or 100baseT (10Mbps or 100Mbps).  They are used for the transmission of day to day data used in a company and thus there is often some resistance to putting "bl.....y security pictures on my network" from IT managers.  The IT manager has to make sure that the software for viewing and control is only put on the relevant PCs in the company, which again can differ from the priorities of the security manager.

Having identified the right people to have access to the images and the sorted out the politics, there is then the practical problem of how many cameras you can have on your network .  Images at or near full frame rate will require data streams of around 1.5Mbps for compressed data using ITU  H.261 video conferencing techniques and with CIF (small) image sizes.  Therefore you could have up to 6 cameras on a 10baseT system before there would be too much data flying around.  Alternatively other manufacturers will give you full screen video at 15fields per sec at a data rate of 6Mbps using Wavelet compression, which is only 1 camera on the 10baseT network.   But don't forget this is just the CCTV on the network.  "What about my precious data" The Network just clogged and nobody could work.

Beware when you look at these systems.  Remember that you have to keep both systems working therefore check the networks from all aspects.

Not the hole in the wall where we get our the banks money, but Asynchronous Transfer Mode.  A data network that really "cooks with gas", a maximum rate of 2.4Gbps and at that rate it usually has to be provided on an optical fibre backbone. There are a number of manufacturers providing ATM equipment for CCTV such as Baxall & Plettac etc who provide cameras that will either connect directly to the digital network or with their own interface equipment.  These are now what can be truly called digital cameras.  The signal coming out of the RJ45 connector (or whichever they use) is a stream of binary 1s and 0s that cannot be connected to a standard monitor.

Because of the cost of installing the structured cabling and backbones required for such high speed data networks, it is not yet, in my opinion an economic solution to install fresh for CCTV.  However, if you have an existing ATM network, then the option of CCTV over this transmission medium is viable.

This is a new alternative from BT to the well known and loved?  RS1000 fibre system they have been providing for a number of years.  For long distance transmission the RS1000 has been very expensive.  BT have developed RS3000 (HDSL) in parallel with the ADSL system, for use in home and business for "always on" connections to the Internet.  The system is provided as a service, at the moment, and you provide a composite video signal at one end and get the composite video at the other end.  Unlike ADSL the HDSL is capable of transmission at the same rate both to and from the exchange.  It is a point to point link that is cabled on copper to the exchange, so you cannot be further than 3km, along the cable route,  from the exchange.  The system uses BTs own CODEC and can take one, two or five cameras that share the transmission "pipe".  The "pipe" for this service is a 2Mbps shared between the number of cameras the link provides.  Because of the nature of the CODECs used it can give a very good update rate for control of pan and tilt cameras.

I am only going to mention that IP addressable cameras are available, because things are moving so quickly that they will be out of date before I finish writing this statement.  However, there is much specmanship regarding the performance of the cameras.  Very high compression rates and  very small images (QCIF) are just two of the ways of overcoming some of the problems with the World Wide Wait.  More ominously, if a camera has an IP address and is connected to the Internet then there is a possibility that the camera and receiving computer could be found and the security compromised by your security pictures being watched by those who are not authorised.  However watch this space.

Glossary of Terms
ADSL - Asymmetric Digital Subscriber Line, asymmetric because it gives downstream (from service provider to you) rates of 512kbps and upstream rates of 256kbps.  An always one connection to the Internet, also used for Video on Demand (Home Choice) and Astreaming video. Provided by telecommunications companies such as NTL and BT.

ATM - Asynchronous Transfer Mode A data network that breaks the data into packets 48 Byte packets and sends them with routing information (5 Bytes).

CIF -  Common Intermediate Format.  A video format often used in video conferencing systems that easily supports both NTSC and PAL signals. CIF is part of the ITU H.261videoconferencing standard.  It specifies a data rate of up to 30 frames per second (fps), with each frame being a size of 352 pixels wide by 288 pixels high.

CODEC - Coder Decoder.  A piece of video transmission equipment that converts the signals from analogue to digital and uses advanced data compression techniques including Conditional Refresh.

Conditional The technique of reducing the amount of data to be transmitted or stored, by only
Refresh - sending the data that relates to the changes.  In other words only the movement is transmitted (after the original whole picture is first sent).

Data Rates - Data rates in serial transmission, are always expressed as bits per second not Bytes per second so 2Mbps is 2 mega bits per second, not 2 mega Bytes per second.

Ethernet - A data network that was originally provided on coaxial cable

GSM - Global System for Mobile Communication, the current mobile telephone network system.

 HDSL - High-bitrate Digital Subscriber Line.  The big brother of ADSL.  Gives a point to point transmission rate of 2Mbps but is provided as video in to video out.

ITU  H.261 - A video conferencing transmission standard that defines the rates and conditional refresh

ISDN - Integrated Services Digital Network.  A "dial up" digital telephone system that enables transmission at rates of up to 128kbps

IP address - Internet Protocol address.  A series of numbers that uniquely identifies your location such as, which is the web page of the Advanced Photon Source, but could equally be a piece of equipment.

LASER - Light Amplification by the Stimulated Emission of Radiation.  A Laser produces light that is one wavelength (monochromatic), in phase (coherent) and an almost parallel beam.

PSTN - Public Switched Telephone Network.  The standard dial up analogue telephone network that most people have at home.

QCIF -  Quarter Common Intermediate Format.  A video conferencing format that specifies data rates of up to 30 frames per second (fps), with each frame being only 176 pixels wide by 144 pixels high.  This is one fourth the resolution of full CIF.  QCIF support is required by the ITU H.261 video conferencing standard.  Because the image size is one quarter the size of CIF it transfers one fourth the amount of data and is suitable for video conferencing systems that use telephone lines.




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