CCTV TODAY - TRANSMISSION ARTICLE
With twisted-pair cabling for digital data and telephones on most premises this method has found widespread popularity. To ensure good picture quality make sure that the units are properly adjusted by an engineer who is competent with a pulse/bar signal generator and oscilloscope.
Beyond simple, short runs where good ol' co-ax is king, it has several advantages:
Ø twisted-pair cable is often cheaper than co-ax of an equivalent robustness.
Ø greater distances can be run without extra amplification (e.g. over 1000m for colour signal, as opposed to 250-300m for RG59 co-ax.)
Ø it shows greater immunity to electromagnetic interference in 'harsh' environments.
A single sheath containing many twisted-pairs (e.g. common telephone cable) is much cheaper to install than the equivalent in co-axial cables especially, for instance, if armoured for direct burial. This also makes the running of spare paths for/in the future much more comfortable. The cable might also carry audio, control signals as these are commonly accommodated on twisted-pairs.
For many years optical fibres have been used to carry analogue video signals over distances of several kilometres. Incidentally, microwave or laser transmission has been used where line-of-sight from A to B is available and other circumstances are correct.
There are many providers of optical fibre systems. BT already has the nationwide advantage of a large network of ducts, but local cable TV and telecoms companies may have their own and choose to compete. While the creation of new ducts can be expensive, it is not beyond the wit of man, especially in tandem with other services.
For links that must stretch from town to town larger telecoms service providers may be able to help, but remember how many of them have recently suffered financial difficulties, and you don't want to be left high and dry in the future. In addition, many descendents of nationalized utility companies have added data networks alongside their existing countrywide infrastructure and may offer suitable services too.
The Public Switched Telephone Network provides a direct A to B dial-up path capable of carrying a range of frequencies suitable for the human voice. To hire and use the wire is pretty cheap of late, especially where telco's do special fee packages to better suit your usage pattern. However, the voice-carrying legacy means that data is the poor relation and the best modems can only push ~38kbps out of your premises (at best, with a perfect non-shared line. Have you got one?). Remember that 56kbps is the receiving speed only, and your CCTV is transmitting. If you can live with period circa 30 seconds to establish a new connected between two modems, and a possible engaged tone if the line is used for other purposes and not barred to incoming calls or ex-directory, then such a solution may be very cost-effective. This is especially true is the data is classified as non-urgent, such as scheduled 'patrols' of remote sites, or to gather alarm activated pictures that are stored in a memory buffer until retrieved, or to carry out overnight downloads of bulk data for off-site archiving.
Don't forget that in mixing and matching the technologies described in this article may offer you the opportunity to receive images at the preferable 56kbps where they are coming from your own remote server, or that at your ISP (Internet Service Provider), and that server is being fed with pictures from its local cameras or remote ones via a faster link, e.g. a DSL connection that are so commonplace now.
Where you are trying to squeeze every last ounce (can I only legally say gram now?) out of your PSTN connection the interface box will digitally compress the images. To do this is will throw out most of the original pixels, remove colours and reduce the picture refresh rate. So make sure you see it in action before you buy a pig-in-a-poke, as these techniques will be drastic.
The Integrated Services Digital Network provides digital data paths in a similar way to above. However, the dial-up time is quick at only a few seconds and your data is intelligently routed around the telco's network if any points along the way are refusing to play ball which increases the odds of a successful connection. The signal is digital (as opposed to the analogue, noise-suffering PSTN modem) and runs at 64kbps both transmitting and receiving. The calls are charged like voice calls, i.e. pretty cheap, but if you are expecting to clock up more than 4000 hours a year then a dedicated 'always-on' leased line may be more economical.
Again, 64kbps isn't really considered very generous for many CCTV applications. This is especially true where 'live' work is important, e.g. pan/tilt/zoom control of remote cameras is required, or picture detail and/or fast picture refresh are crucial. However, a standard ISDN2e circuit comes with two 64kbps channels as standard, and these may be 'bonded' by a suitable CCTV interface controller to give one 128kbps path, simply costing twice as much for the phone call.
In a recent real-world project the end-user wanted a cheap way of getting 256kbps video on an occasional basis from any one of numerous sites to any one of a selection of receiving sites. Instead of installing a new private network that would be quiet (idle) much of the time, they were happy to install cheap ISDN2e boxes in pairs (2 x 64kbps x 2 = 256kbps) and then pay small line rental costs and minimal call charges only. Go up to an ISDN30 line and you get 1.5Mbps. It's all there for the asking, and much cheaper than it used to be.
Most of us have a phone in our pocket that uses the Global System for Mobile communications (GSM, known by some as 2nd Generation or 2G). Some of us will have wanted to believe the WAP hype or connected our laptops to the Net and realised that their native 9.6 to 14kbps data rate is very limiting. Some telco's offer High Speed Circuit Switched Data (HSCSD) at 28.8kbps, rising to 43.2kbps soon. Old copies of CCTV Today reported that such equipment is continuing to be developed and used for wireless camera installations. It appears that the limiting factors of data compression must be carefully considered if such systems are going to meet their operational requirements. After all, you may be proud of your hard-earned and very attractive Aston Martin DB7, but don't use it to try and beat Mr. Schumacher's F1 machine round Monaco. You'll probably be disappointed. To mix metaphors, remember that it's 'horses for courses'. As an afterthought, let's not forget that cellular phones are not welcome in hospitals and that 'jammers' are now available that may scupper your surveillance. Also, GSM encryption can be routinely broken, and your data may be subject to eavesdropping. This is true of all 'free-space' or wireless techniques.
For the past couple of years we have had GPRS(General/GSM Packet Radio Service) which is capable of 115kbps, and is the first to offer 'always-on' wireless connections that are billed by volume of data carried rather than connection duration. This could be very attractive, but at the moment prices do not appear to have dropped as far as £1/Mbyte, so 1 day's viewing at 115kbps would tot up to £1,212. Hmm. Methinks a different proposition is required.
Now, the first 3G services are promised to emerge soon and will likely offer 384kbps in EDGE (Enhanced Data for GSM Evolution) form, with the later roll-out of much talked about WCDMA (Wide Band Code Division Multiple Access) that can offer 2Mbps to buildings and less to pedestrians and less to vehicles. It is reasonable expect that the providers are going try and recoup their large investment quickly from the pockets of the early adopters, but if the data costs are anything like those for GPRS above, then 2Mbps will seem rather unattractive! I expect that things will develop somewhat differently, but search as I might, prices are proving elusive at the moment.
Deployable or mobile CCTV, both cameras and monitors, are likely to change enormously when these technologies are used. As with any radio frequency communications coverage will not be complete, but the nature of the consumer demand will mean that the infrastructure develops in a way that will see many new members of the CCTV community. Let's hope that our generally good name is maintained. Crystal ball experts welcome.
The existence of licence-exempt parts of the radio spectrum has given us more options for the transmission of video (not forgetting the control signals that often accompany them, travelling in the opposite direction). In the UK, equipment transmitting at 1.394GHz carries analogue video quite successfully. Line-of-sight is not strictly necessary as the radio waves can reflect off buildings, etc. and find their receiver, but this reduces the available range. Multiple reflections, especially from moving vehicles, can cause 'ghosting' and picture degradation so care must be taken when doing surveys and propagation tests. Unfortunately, these can vulnerable to eavesdropping and jamming, and adjacent units may interfere with each other in competing for the airwaves.
At an alternative frequency of 2.4GHz digital transmission of video is possible at 25fps. Data errors are corrected maintaining good pictures and up to three units may be used in close proximity. The radio emission is 'spread spectrum' and make eavesdropping very difficult. Analogue systems are now available in this 2.4GHz band too.
The placement of antennae is vital to the good performance of these units. Often, the higher the location the better. For deployable systems a central, or a number of distributed receivers, may be located around the vicinity with their signals being returned to a fixed control room by 'conventional' means.
As with these and the other wireless techniques outlined in this article, the importance of site surveys, RF propagation tests and the help of an experienced expert cannot be over-emphasized if disappointment is to be avoided.
This technology has been used reliably over many years and has been the main alternative to copper cabling and optical fibres over long distances or difficult terrain. These can be used over several kilometres to convey both analogue video or digital data at rates high enough to carry many camera streams simultaneously. Indeed, analogue systems are capable of simultaneously carrying several full resolution images at full frame rate. The erection of microwave antennae may require a licence and planning permission be granted before you begin the work. The path from transmitter to receiver must be line of sight with no obstructions, although repeaters at the tops of hills or on tall structures are an option. Remember that obstructions are not always there on the day you do the survey. Flocks of roosting birds, new high-rise buildings, ships, etc. may all hove into view at a later date. In marine applications, common as an alternative to underwater cabling, the ebb and flow of the tide may cause changing microwave reflection paths that significantly affect your signal. This warning leads onto the absorption of microwaves over long distances by water in the form of heavy rain. Consider the effect of a degraded analogue signal (snow on the picture) or digital data (a much slower data transfer and higher error rate). Can you application tolerate these occasional events?
Another popular high-bandwidth line-of-sight transmission method, with the added benefit of being licence free (although planning permission to mount the equipment may still be required), is to modulate a laser light source and 'beam' the data. At the top end of the market (circa £20,000) these units can shift 1,000Mbps which is a lot of video in any format. The same potential problems raise their heads again (into the path of our beam) where clear line-of-sight is absolutely essential. As this is light or infra-red radiation then thick fog can also bring things to a halt. Some locations are predictably prone to fog at certain times of the year.
The laser beam is very narrow (spreading out to a diameter of only 1 or 2 metres over a distance of 1000 metres, a divergence of 1-2 milliradians for the techies) and consequently is difficult to eavesdrop, but this makes alignment stability absolutely vital. If your transmitter's mountings move occasionally because their host building sways in the wind or warps when the temperature changes (similarly for the metal mounting brackets) then your receiver may lose sight of the incoming beam very easily.
It's interesting to know that such units were used in the aftermath of the 11 September destruction of New York's communications infrastructure to re-establish fast data links across town weeks before the subterranean optical fibres were repaired.
Currently, the two most talked about in the computing world are WiFi and Bluetooth.
Bluetooth is for connectivity between units over a distance of about 10 metres. The first generation of devices provide 1Mbps with ~721kbps one-to-one, rising to 2Mbps in the second generation, operating at 2.45GHz in a frequency-hopping mode. This may be useful for close-quarters use of a video enabled PDA, for instance.
Wi-Fi networks operate in the license-exempt 2.4GHz and 5 GHz radio bands, with an 11 Mbps (802.11b) or 54 Mbps (802.11a) data throughput similar to the basic 10baseT Ethernet networks used in many systems. Units are becoming available that provide long range data paths over distances of 100's or 1000's of metres with the appropriate antennae, although data rates are designed to drop as distance increases and the environment changes. The deployment of WiFi base-stations to act as repeaters can easily double the size of your wireless network area.
You might imagine that network security is a major issue, as evidenced by the recent proliferation of 'war chalking' activity in the public areas of metropolitan business centres and the unauthorized use of corporate networks and Internet bandwidth. Not good in anybody's world, especially the CCTV surveillance arena. Nonetheless, exhibitors at the recent inaugural IIPSEC exhibition were proudly demonstrating live video feeds to palmtop units using exactly the same hosts that we find available in the high street for only a few hundred pounds, and that many organisations deploy to their staff en masse for daily office use.
As the cutting edge techniques
in this article become more and more commonplace and 'ho-hum' to the gadget
addicts of this world, the prices will fall while the capabilities and
imaginative applications will go up. (That statement is not intended to
demean their genuine value; the gadgets nor the addicts.) The older technology
at the beginning of this piece will doubtless become cheaper in order
to stay competitive, and so everyone will stand to benefit if they play
their cards right. Don't use the new stuff because it is new. Use what
will do the job the best. As Brucie says: "Tonight's the night,
© Simon D. Lambert 2003