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.
Microwave
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
Radio
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?
GSM
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.
PSTN
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.
ISDN2e
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.
LASER
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.
ATM
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.
HDSL
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.
Internet
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 164.54.96.215, 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.
