What's the difference?

Digital recording is very much the way forward for the security industry. Just as colour television has replaced black and white, CDs have replaced vinyl records, and e-mails are taking over from traditional letter writing, digital recorders look set to take over and replace analogue VCRs.

Although digital recording is still in its infancy, and analogue recorders are tried and tested, offering a cost-effective solution, people are still turning to digital.

The reasons for this are many; over a large number of cameras, digital recording can be as cost effective, if not more so than analogue. Digital does not have the same restrictions as analogue and offers the end user more choice to record images in the best way for the application ... not in a way that is confined to the limitations of an analogue system.

While there is still much debate about which method of digital recording is best, with each manufacturer rushing to claim their system is the benchmark against which to measure the others, the fact that one day all CCTV recording will be digital seems unquestionable.

Installers can be reluctant to fit a digital recording system, as the consensus seems to be that it will be a costly and complex procedure. To consider installing a digital system entails a change of thinking about the entire process of CCTV recording. It also means that a recording system can meet all, not just some, of the needs of the end user, and make the difference between winning and losing a major contract.

But for those that remain unconvinced, let's look at the ten key differences between analogue and digital recording, and try to understand why digital is being heralded as the way of the future for the security industry.

IMAGE QUALITY AND RESOLUTION

A new analogue S-VHS machine with a brand new tape will provide better image quality than even the highest resolution digital recorders. However, after just one use, recordings made on the analogue system quickly degrade. With a digital recording system, this image degradation never takes place.

Surprisingly, some digital images are of a considerably poor quality, due to being overly compressed. But with a good compression algorithm, such as JPEG, and a reasonable compression ratio, the superior quality of digital images compared to VHS is easy to see, particularly with still images or slow motion sequences.

Different recording systems offer different resolution; some good digital systems can provide full resolution, full screen images, while others provide small sized images, which, if blown up to full-screen size look terrible. Using a low resolution and high compression may reduce the amount of storage required for an image, but can make identification tricky.

DIVISION OF FRAME RATE/RECORDING MODES

As most CCTV installers know, on a PAL system, images are captured 25 times a second, giving us a 'frame rate'. With a 16 input multiplexer and analogue VCR in 24-hour mode, images are captured once every five seconds. With an analogue recorder, this is the best that is available. However, by thinking digitally, there are countless new options.

With digital technology, the 'share' of the 25fps available can be spread between the different cameras at varying rates. Any configuration is possible, as long as the number of frames per second add up to no more than 25. A capture rate of 1.5fps may be perfectly suitable for recording images of people. But for capturing images of moving vehicles, this may not be adequate, depending on the vehicle's speed.

Continuous recording may not be needed from all cameras. A digital system can use an alarm input to trigger recording for just part of the time, in the evenings for example, when activity would be unexpected. Alternatively, the system can be configured in time-lapse mode to provide a basic continuous capture rate from all cameras. Yet another option is to set pre-determined capture rates and, using an event trigger, to speed up the recording rate on the associated camera, to capture the event in its entirety.

PRE/POST EVENT RECORDING

Another useful feature of digital, but not analogue, recorders is the pre or post event recording. Instead of triggering recording just after an incident has happened, a digital system's alarm trigger feature can automatically capture several seconds or even minutes of activity before and after the event. This feature lends itself to being used with access control systems to ensure a good view of anybody or anything causing alarms.

Pre-event recording is vital in major sites where several seconds can pass between the alarm being triggered and matrix switchers routing the video through to the control room. Even if the perpetrator has long since gone, the cause of the alarm can still be ascertained with accuracy. This is ideal for remote site monitoring. When combined with a modem, telephone line and remote station, the system automatically dials the remote station and replays the images leading up to and after the event. The guard can then decide whether to act on this or to just continue monitoring and recording the live video.

SYNCHRONOUS RECORD/REPLAY/ARCHIVE

One innovative feature of some (but not all) digital recorders is the ability to continue recording while replaying an image. With an analogue system, if we wanted to replay footage, it would involve manually stopping the recording, then moving the selected tape into another machine. Then we would have to replace the tape we wanted to view with another tape so as not to miss any events. Finally, we would have to try to guess whereabouts on the tape the incident was, before we could review it.

With a good digital system, events can be instantly accessed and reviewed without stopping recording or archiving. But a word of caution: many digital manufacturers claim they can do this, but to do so, have to slow right down the speed at which they record, or suspend or slow down archiving. Better digital systems can perform record, replay and archive without compromise.

INSTANT ACCESS TO IMAGES

Images recorded on a digital system to hard disk can be accessed instantly. Different manufacturers offer different ways of doing this, but generally, searches can be made by time, date or camera to view images from an event instantly. Compare this to analogue, where recording has to be stopped, and the tape manually rewound to a time where the incident may have happened. Searching through digital video is quick and easy, with most systems automatically indexing recorded events. Searches can be made by event, such as the use of an identity card or a trigger from a PIR or VMD. This ease of use makes it an increasingly popular choice with the end user for speed and convenience.

IMAGE SECURITY

Unlike an analogue recorder, where successive copies deteriorate in quality, digital recording provides copies that are absolutely equivalent to the original and should fulfil the requirements for 'best evidence' when produced in court ... and, just like analogue images, digital images can also be tampered with and altered. Some sort of protection of the image is needed to ensure that, once recorded, the image stays secure and also contains an accurate record of where and when it was captured. This can be added as the image is recorded in the form of a watermark, digital signature, encryption or audit trail.

Last year, the House of Lords Select Committee produced a report entitled 'Digital Images As Evidence'. One of their major conclusions was: 'We were pleased to find that digital images, which we initially thought might create difficulties for the court, do not'.

IMAGE STORAGE

Methods of storing digitally recorded images are much more flexible than analogue methods, with options to suit different budgets and requirements. Disks can currently store up to about 20 hours of recording at 25 frames per second, and almost a month at one field per second time-lapse. With the advent of RAID disks, the potential for even greater on-line storage time is also possible. Disks can then transfer archive information to another medium.

Digital tapes offer the same high image quality of other digital media but are, by their nature, sequential devices. They are good for storing large amounts of data in small compact cassettes; they are easily manageable and can be electronically indexed to improve search times. Auto-changing jukeboxes may also be used to continuously archive large volumes of data. And unlike analogue tapes, digital tapes can be recorded over time and time again, with no loss of quality or time-consuming degaussing.

IMAGE TRANSMISSION

With the advent of ISDN and PSTN, transmission of images from sites on the other side of the world is now possible over wide area networks. Images may also be transmitted over local area networks, and viewed on everyday computers. At the click of a mouse, a site in London can check up on, and view images from, a site in San Diego. Images can be e-mailed directly to other interested parties or saved as single bitmap images and printed out. Soon, images being relayed over the Internet and other exciting transmission technology will become everyday options for those with digitised images.

DEGRADATION OF IMAGES

Scratched or stretched tapes, worn recording heads, faded images and slow response to alarms are just some of the problems faced by police when they start to analyse scenes captured on conventional analogue recorders. With a digital recorder, a series of identical 'copies' are made from camera to memory to disk to tape, without any of the visible degradation or 'noise' found on a copy of an analogue image.

In general, the image recorded on a good digital system is much sharper than its analogue forebears. In particular still images or slow motion sequences don't exhibit the jitter commonly seen on standard VCRs; nor are fast searches streaked with frame-bars. Furthermore, it will be exactly the same quality after hundreds or thousands of viewings.

THE FUTURE

One of the most obvious differences between digital and analogue recorders is their future potential. Analogue has long since reached the point of maximum development, and can really go no further. Digital, on the other hand, opens up many exciting possibilities, not only now, but also for the years ahead. Many large forward-thinking organisations are already buying digital recorders that will integrate with existing equipment, in a move to future-proof their systems. With networking and remote transmission all set to become as commonplace within the industry as a multiplexer, this move to the next millennium of recording technology is already in full swing.

CONCLUSION

As the new millennium progresses, digital systems will replace and change how we do many things, from shopping over the internet, to designing our own personalised television programmes with digital TV, and also how we record security footage for CCTV applications. Once an image has been digitised, countless opportunities to send, save, and view that image become possible. Earlier worries over the admissibility of digital images have, with adequate safeguards, proven to be unfounded. What better way to embrace the technology of the future than to make the change to digital now?