Laptops beat screwdrivers

A friend showed me one of the first slow scan receivers in operation – that was 28 years ago! The image was traced out line by line on an old radar tube and took eight seconds to build up. The bandwidth of the signal was around 2K and the picture quality was poor. But it was so impressive. Most of the rules which limited that early system still apply today, such as needing more bandwidth for a faster transmission rate, and sacrificing picture quality for speed.

Nowadays it is possible to transmit IMAX (high resolution film) quality images, providing you have a bandwidth of 20MHz. Most of us have to get by with substantially less than that, so what are the options? If you are serious about getting images from one point to another, then an ISDN connection will do the trick. Expect image update rates at about one per second.

For anyone still wishing to stick to PSTN (old fashioned telephone lines) a transmission system using conditionally updated images is preferable.

On the horizon are some telephone lines that stay permanently connected to an internet service provider. ADSL at 512K and a cable TV link at 1Mbit are already available in some areas.

These high-speed lines direct to the internet would offer us near real time image transmission but the ISPs (Internet Service Providers) may be a limiting factor. ISPs make their money from hosting web sites. The more web space you require for your site, the more money you pay.

Technically you can short circuit this and use your PC as the web server itself, and hey presto, unlimited web space. It did not take long for the ISPs to get wise to this and drop the connection when an unauthorised web server was spotted.

You may be thinking: "What has this got to do with transmission of CCTV images?" Well unfortunately web servers look similar to video servers sending CCTV images. The good news is that some ISPs are already looking at providing special services for CCTV transmission. Whatever happens, with the current rate of communications expansion, the bandwidth limitations applying to older telephone systems look set to disappear.

Even with the maximum number of image frames (pictures) from a European CCTV camera running at 25 per second, we already have transmission media able to handle that data rate (see table).

Transmission speeds
Perhaps the first thing that anyone asks of a video transmission system is "How fast does it send images?" The perception persists that it should be 25 images a second to be considered real time. In reality most people perceive real time images at speeds as low as 10 to 12 pictures a second.

The culprit for the 'anything less than 25 images per second is not real time' is almost certainly the movie world. Early movies appeared to flicker as the light source in the projector had to be shuttered as the film was advanced to the next frame.In modern CCTV systems we suffer no such flicker as the image is constantly displayed on the screen by means of a digital frame store. When the image is refreshed there is no blank period and hence we see no flicker at lower update rates.

While the movie world seems to have settled for 24 frames per second, higher update speeds are only required for the fast action virtual reality simulators found in theme parks and the IMAX movie theatre (see table).

Types of compression
To speed up picture update rates, all modern video transmission equipment uses picture compression. In the CCTV world there are two main compression systems in use at the moment; Wavelet and JPEG. High compression rates (small image sizes and therefore faster transmission rates) affect both differently.

Wavelet images start to smear as compression increases and can look a little like a badly-focused camera. JPEG images start to look a little blocky. It really is just a matter of choice which type you prefer at the moment, and some outrageous claims are being made for some compression systems.

If you do compress and reduce a picture to a minimal representation of the scene that it covers, you will be able to transmit a lot of pictures quickly, but they will be of pretty poor quality.

Fortunately as communications bandwidths are increasing rapidly, the need for high compression rates is only required when sending images over conventional telephone lines.

Networks
At this point it may be worth mentioning that to take advantage of many of the new modes of communication, it is worth making sure your new bit of transmission technology will connect to and send pictures across a computer network. With network capability it is much more likely to be called a video server than a video transmission system. It really is doing a similar job, transmitting video.

As mentioned earlier you can connect your system to ISDN or PSTN lines, but if your customer has a network, why not save him some call charges? Larger companies with many branches are often network-connected by WAN (wide area networks). Their system administrator will advise if it is possible to transport the video on their network.

Then you will need to obtain addresses for the equipment (much like programming in destination telephone numbers) and the network will do the hard job of routeing the images from source to destination.

Some of the smarter equipment will actually ask the network to allocate it an address and you just give it a name. This is incredibly useful if your system is mobile (maybe on a bus) and connected by wireless network.

Instead of constantly reallocating addresses as the vehicle connects to the nearest local network, the system could be named after the vehicle's registration plate.

Whether the vehicle is in London or Glasgow you will get the pictures back thanks to the network dynamically allocating the vehicle a local network address.

Video servers
These are best described as fourth generation CCTV product designed to work specifically on a computer network. The video server is a device that takes in several video sources, typically CCTV cameras, and then distributes the images on demand across a network.

The features vary dramatically from manufacturer to manufacturer, so check all the goodies you need are available and most important, don't get stung on price. A good video server should cost no more than a good multiplexer and time-lapse VCR combined.

So what are the features to look for on a Video Server? A primary requirement would be to send images to another unit on incoming alarm. (This could be another PC on the network or a dial out via modem to a central station.) If it is to form part of a remote monitoring system there should be the facility to arm or set the system either by internal timer or external input. If the unit has a sufficiently large hard disk it should be able to out-perform a multiplexer/VCR combination.

Most units can be ordered with different size disk drives, presently up to 15 times the storage on a conventional VHS tape. If you need more storage, check that the device can interface with something like a RAID array. RAID arrays will typically give you 60 times the storage of a VHS tape.

If archiving images is important to your application, check that this can be done via the network to another device. Once across the network you can choose any storage device from tape archive to large hard disk arrays.

Watermarking
One question that is always asked about any digital video product is: "Can the images be used as evidence in court?" Now the answer is yes, at least if the unit is capable of watermarking images.

Put briefly, the watermarking process produces a unique number specific to the images recorded to disk. The images are then transferred to an evidence bag along with the unique watermark.

If at any time the validity of the images is in question the watermarking read process can be run again, generating a number unique to those images. If the number is identical the evidence is intact, if different the evidence is invalid.

Some units can read incoming text on a serial port, or via the network, and tag that text onto images. Applications that come to mind are the cash register on screen details, reminiscent of Tillscan. Alternatively any access control system can send card details to the video server via the network and then those details can be tagged with images.

Other finer points will depend on the specific installation such as; telemetry control of cameras, control of output relays, STU interface, EOL (end-of-line tamper proof) alarm inputs, audio interface, and battery back-up in case of power failure.

Future trends
Advances in technology move quickly and the integration of CCTV systems into network based product is shifting the market. Already companies that have traditionally provided large computer networks are integrating their own network based cameras into systems without any involvement from CCTV installers.

However, it is far from all bad news for installers! This huge potential market, still largely untapped, offers a massive growth area for our industry. You will be able to spot the successful installers - the engineer's most important tool won't be a screwdriver, it will be a laptop.