A digital enigma

By way of example, consider this scenario. A potential user of digital video (ie a member of Security Management Today's readership!) had requested two quotations from two separate manufacturers. The specification called for 32 cameras recording simultaneously for 30 days. The resultant quotations differed markedly, with one coming in 30% more expensive than the other.

The end user in question was about to place an order with the first manufacturer, whom (for the sake of convenience) we shall refer to as ABC Ltd, because it was felt that a bargain was there to be had. Fortunately, the second manufacturer – to be known as XYZ plc – asked for an opportunity to explain its tender offering, sure in the knowledge that its own solution couldn't be much more expensive.

Evaluation showed that XYZ plc had quoted 3 x 120 Gb hard disk drives to achieve the stated recording targets, also citing a refresh rate of 3.125 frames per second. ABC Ltd had stated that it could store recordings from 32 cameras for 30 days on a 20 Gb hard disk drive. Impossible, as current technology would not allow this at the same refresh rates as had been quoted by the first supplier.

Careful scrutiny of the quotation revealed small print at the bottom of its specification stating that the storage estimate was based upon recording at a rate of one frame every two minutes. In a security environment a lot can happen in a two-minute interval, thus effective historical analysis wouldn't have been possible using this solution.

ABC Ltd was then asked to revise its quotation based upon a refresh rate of 3.125 frames per second. They did so, and it was found that the company's digital video recorder wasn't capable of supporting hard disks with sufficient capacity to meet the specification. Consequently, ABC Ltd was forced to use an external archive device. This drove up the price of its offer to twice that of XYZ plc's original project cost.

That, Ladies and Gentlemen, is a typical example of what we call "spec sheet selling" (which seems to be prevalent in the industry at the moment, and appears to follow the introduction of new technology in any market where a buyer can be hoodwinked by misinformation). This format dictates that, in order for an industry to mature, customers have to learn from their mistakes (which is frequently an expensive option).

With this in mind, let's dispel some of the myths and cut through the hype surrounding digital video recording, and offer some desperately-needed guidance to end users who are looking to specify such technology by way of enhancing their security operation.

The advantages of digital video over analogue are fast becoming established and, for the most part, are now well-known. To reiterate briefly, video images may be saved on to a hard disk by using digital video recorders instead of tapes (which degrade with constant use and have numerous 'moving parts'). Digital video techniques also enable enhanced and more efficient search facilities (it's easy to search the video images stored on the hard disk drives via time and date criteria).

Multiple copies of the images may be produced without degradation. In addition, the transmission of images over LAN/WAN technology enhances the ease of distribution, while the adoption of video-playback software in operating systems readily permits security staff to view video clips without proprietary viewing software.

Key factors to think about
There are several equally important factors to consider when planning the use of digital video over a network.

The available bandwidth of the computer network is a key factor in all network video transmission. If bandwidth is limited, the amount of information being sent over the network must be reduced, either by longer update (refresh) intervals or lower image quality. The factors affecting file size are the compression algorithm, the refresh rate and resolution.

Compression is the amount of redundant video information that can be stripped out of a video image before storage and transmission. The higher the compression, the more information is lost.

There are many compression algorithms available such as H263, Wavelet, MJPEG and MPEG, etc. It's not the intention here to justify the merits of each different compression technology, as there are currently many manufacturers doing so within the pages of the trade press.

However, it's worth explaining that there are basically two methods of compression enabling the transmission of high quality video over a network. Either a complete image is transmitted every time (JPEG, Wavelet) or a reference image is transmitted and updated little by little (MPEG, H263) until a major change requires that a new reference image should be transmitted. It would indeed be true to say that either method is suitable depending on the application in question.

The real issue is: what does all this mean for the end user?

Different compression techniques and rates will affect the storage requirements and bandwidth use which, in turn, will affect the capital cost and cost of ownership. As in all walks of life, there's always a trade-off to be made as we cut the cloth to suit our budget.

A little refresher course
How often the image is updated is known as the refresh rate, which – as we've already seen – is generally quoted in frames per second. It would appear that when involved in selling digital systems, manufacturers' commercial teams are quoting frames per second to be the benchmark of a superior system. To confuse matters still further, manufacturers are also quoting in various formats.

For example, some quote frames per second (25 frames per second provides real-time live video), while others will quote Fields per second (Fps). There are two Fields per frame, and thus many systems providers will quote 50 Fps – leaving the unsuspecting buyer with the impression that this unit has twice the refresh rate of a 25 frames per second system. Other manufacturers will quote pictures or images per second. In truth, refresh rate has nothing to do with the quality of the image. It only affects the smooth movement of objects within the field of vision.

When evaluating a digital CCTV system, it’s important to analyse the differences between the refresh rates. Visit the manufacturer and compare several cameras all set up at different rates, and all looking at the same source. When doing so, ensure that t

Obviously, all of this disguises the true picture. It's high time the industry joined forces and produced a common format to assist the purchaser in making an informed specification.

Although 25 frames per second will provide real-time live video, there's a high cost to pay in using such refresh rates. The more frames that are recorded, the larger the file size.

Large file sizes will require more storage devices and use increased bandwidth on a network. Several studies have shown that, even though many units available on the market today are capable of recording at 25 frames per second, almost 75% of the installed base of such units are throttled back to a level that provides a good compromise between usability and cost overhead.

At the present time, 3.125 frames per second seems to be the standard that most companies are using. This provides adequate surveillance while reducing storage requirements and minimising impact on a network. That said, most potential users still request 25 frames per second in specifications without realising the impact, solely because this is the information they've been force-fed by less-than-scrupulous suppliers.

Already in this article we've been quoting figures that, to the uninitiated, are likely to confuse. Let's put some of what's been said into context. For a start, most users of analogue CCTV systems tend to deploy a 16-way multiplexer and a time-lapse VCR set at

24 hours. Such a combination will provide a maximum of 0.54 frames per second. Thus, by increasing the refresh rate to 3.125 frames per second, customers are enjoying a 500% increase in the refresh rates to which they've become accustomed.

In addition, we tend to think in a linear fashion rather than logarithmically. Thus we assume that the refresh rate of a system operating at 25 frames per second is twice as good as one operating at 12.5 frames per second. Although in theory that may be the case, our eyes and brains cannot actually see twice the difference. Try counting to 25 or even 12 in a single second to understand how difficult it is for us to process this amount of information.

Indeed, many of the early cartoon animations were manufactured at 15 frames per second. This allowed the animators to draw 15 individual cells rather than 25, thus reducing the generation time and, therefore, the cost of producing an animated film. When viewing, we saw these as real-time moving pictures since the receptors in our eyes couldn't process the information quickly enough for us to be able to see the difference.

When evaluating a system, it's important to analyse the differences between the refresh rates. Visit the manufacturer and compare several cameras all set up at different rates, and all looking at the same source. When doing so, ensure that there's movement in the field of view which is likely to represent your own on-site conditions. Having completed this simple test, you'll be able to settle on a refresh rate that meets your operational needs.

Don't forget that you should also request a quotation based on two scenarios – one at the optimum refresh rate, and another at a rate which you'd be prepared to accept. Set some parameters for the quote (ie 32 cameras, all recording at x frames per second and with storage for 30 days).

Talking about a resolution
Resolution is the measure of image quality, and is 'calculated' in pixels. Remember that the higher the image quality, the more data there is (to transmit). Images with a lot of data will take longer to transmit, will use more network bandwidth and require extra space on the hard disk for storage. For example, a 320 x 240 pixel image (approximately VHS quality) is one quarter the size of a 640 x 480 pixel image (approximately S-VHS).

Digital CCTV recorders are a PC with one or more hard disks included for storing the video images. As stated, storage requirements will differ widely depending upon the compression algorithm used, the amount of compression selected, the refresh rate used and the resolution adopted. Inevitably, this means that more hard disks will be needed as the specification is increased.

Hard disks are now quoted in terms of the number of transactions written to them before failure, rather than the length of time before failure. Recording video is a very transaction-intensive process, as information is continually being written to the disk(s). The law of averages dictates that the more disks used for storage the more likely it is that one will fail.

As digital CCTV systems mature in the marketplace, we're beginning to recognise that the hard disk is the most common point of failure. Given this scenario, it's important for the end user to select a digital video recorder with the largest capacity disk drives available. It's far better to have fewer large disks than more small capacity disks. Additionally, digital video recorders with easily removable disks that can be replaced without having to strip down the unit should always be a priority.

Some manufacturers will treat three hard disks in a digital recorder as a single volume. Thus, if one disk should fail all of the stored information is lost on all three drives. Other manufacturers treat each disk as a separate volume. If one drive fails then only the information on that drive will be lost, while that stored on the others remains intact. To overcome the problematic issues surrounding disk failure, many manufacturers make use of RAID storage devices that allow for disk failure without loss of video information.

Digital video recorders, then, can only store a limited amount of information. The number of disks that may be mounted within the units limits this. Even though some digital video recorders can accommodate up to 1 Terabyte of hard disk space, this will soon fill up.

Most recorders are actually set up to have two areas of storage, the first for event-driven 'locked' video and the other for continuously recorded video. The system is continuously recording – or 'caching' – the video, and when an event occurs it will 'grab' the video associated with that event from the cache according to pre- and post-alarm video configuration and place it in a locked file.

Locked video space versus free space
Those locked files cannot be overwritten by the system unless the operator manually purges them. The cache, meanwhile, is overwritten on a 'first in, first out' basis. The ratio of locked video space to free space on any given disk is dynamically allocated, and thus after a period of time the disk will fill up with locked video.

As digital CCTV systems mature in the marketplace, we’re beginning to recognise that the hard disk is the most common point of failure. Given this scenario, it’s important for the end user to select a digital video recorder with the largest capacity disk

When this situation occurs it's necessary to purge the files, in which case all of the information is lost or clips can be archived. Should the user wish to access this information at a later time it will be necessary to archive the video. Similarly, where there's a requirement to store large amounts of continuous video, it too can be archived – either automatically or manually.

CCTV information can be archived to two types of media – either tapes (video juke boxes) or arrays of hard disks. Tapes tend to be slow in terms of information throughput and selection, and the actual loading of the tape containing the relevant information. Tapes also have many 'moving parts', and so require regular maintenance. That said, they're a cheaper option than hard disks.

Disks, on the other hand, offer virtually instant access to video clips but at a much greater cost. As hard disk drive costs are continually coming down, the difference in cost is becoming smaller and smaller. Most digital video recorders rely on these being located next to and attached to the field recorders. If, for example, the customer has three digital video recorders they would require three archive storage devices.

Smarter manufacturers are starting to deploy central archiving capabilities, meaning that video images from remote digital video recorders are transmitted across the network and stored centrally on either tape or disk. The likes of IBM have been doing this for 'pure data' for many years now, not only for simplified storage and retrieval but also for disaster recovery. They are now offering their considerable experience and technology solutions to the industry to the point that they can store petabytes of video information, and allow access to many simultaneous video streams – thus enhancing retrieval and analysis of video clips by multiple people.

Strike up the bandwidth
Bandwidth use is that amount of the 'pipe' taken up by the video information being transmitted. The frames per second, algorithm and compression ratios all affect the use. When planning a system, the buyer should derive some estimates of use based upon the scenarios they have selected.

Certain systems are designed to be good network citizens and take bandwidth use into consideration from the outset, whereas others are designed to be replacements for analogue VCR. There's room in the market for both products, but they're two separate products for two separate needs.

A networked digital recorder can be used as a digital video recorder replacement but would tend to be expensive. A digital VCR replacement would struggle to effectively 'sit' on a Wide Area Network.

Another issue that's rarely discussed is the use of a central database. Many manufacturers produce units with a database included. That's all well and good for a site with a single unit. However, if a site has two or more units then it'll need two or more databases (ie one per unit).

The use of a central database means that all recorders may be configured from a single computer. It's also possible to run a report or ascertain current system status, event or configuration information from every unit in the system from a single computer. That must be seen as a great benefit to the end user.

Essentially, this means that the management of events can be accomplished from a single location including the archiving of multiple recorders back to a single archive server.

Thus a single database reduces management complexity, increases usability and therefore reduces the overall cost of ownership.

The great security divide
At present, there still seems to be two distinct camps within the industry, namely CCTV and access control. CCTV manufacturers are merely moving along and producing a plethora of digital VCR replacements. Access control system providers, meanwhile, are still producing their products with an RS232 port to send ASCII commands to the CCTV system that will drive it upon an event.

Very few manufacturers have really embraced the two technologies and produced a single, holistic solution.

For example, the benefits to the user of a single platform that would allow programming, control and monitoring of both the access control and CCTV systems from a single screen would be quite considerable. Consider the cost savings of such a system using a single user interface (ie one lot of training, a single database to administer, a single application to upgrade, duplication of servers and workstations eliminated, etc).

Historical analysis using two discreet systems – albeit interfaced – can be cumbersome. To track an event, information has to be reviewed on an access control system and a report generated, giving the date and time of the event in question. An operator will then have to review that date range on the digital CCTV system to access video clips. As the two separate systems operate on two separate PCs, the clocks on both are likely to drift out of synchronisation (for example, 13:30 on one system may well be 13:50 on the other, further hampering investigation and adding to end user frustration).

True integration will provide the ability to attach video clips to alarms and events on a single workstation, thus an operator should be able to click on to an alarm and launch the video clip of the event that caused that alarm. Using a single database on a single server means that the times of the alarm occurrence and video clip will always be in sync.

To date, several manufacturers have launched digital video servers that are capable of running other applications. For example, it's now possible to have one PC (video server) running digital video, access control, ID management and Guard Tour, etc. Again, lowering the cost of ownership to the user.

Going digital? A wise move
Ultimately, an end user's decision to embrace digital CCTV is a wise one. However, selecting the technology is really down to what works for you in your environment and within your company's budget.