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The digital CCTV enigma - Part 1

Not a month passes these days where no articles appear in trade magazines detailing digital CCTV. Very few of these actually discuss the merits of digital CCTV technology over analog technologies in layman's terms.

Most articles seem to focus on the technology and rely on jargon, misinformation and hype, which is weighted in the favour of a manufacturers product, rather than focusing on the more important aspects, such as ease of use, cost of ownership, operational considerations, etc.

Similarly, manufacturers representatives are relying on user ignorance to hoodwink customers, rather than rising to the challenge and educating end-users and thus allowing the customer to make an informed decision.

As an example, consider this scenario, a potential user of digital video recorders (DVRs) had requested two quotations from two manufacturers, the requirements specification called for 32 cameras recording simultaneously for 30 days. The resultant quotations varied significantly, one was 30% more expensive than the other.

The end-user was about to place an order with the first manufacturer, hereafter called (ABC Ltd), as they felt they were getting a bargain, fortunately however the second manufacturer, hereafter called (XYZ plc), requested an opportunity to explain their offering as they felt sure their solution could not be that much more expensive.

Evaluation showed that XYZ plc had quoted 3 x 120 Gb hard disk drives to achieve this requirement; they had also quoted a refresh rate of 3,125 fps. ABC Ltd had stated they could store 32 cameras for 30 days on a 20 Gb hard disk drive. This was impossible, as current technology would not allow this at the same refresh rates as quoted by the first company.

Careful scrutinisation of the quotation revealed small print at the bottom of their specification stating that their storage estimate was based upon a recording at a rate of 1 frame every 2 minutes. In a security environment, a lot can happen in a 2-minute interval and so effective historical analysis would not have been possible utilising this solution.

ABC Ltd was then asked to revise their quotation based upon a refresh rate of 3,125 fps, they did this and it was found that their digital video recorder was not capable of supporting hard disks with sufficient capacity to meet the specification. Consequently ABC Ltd was forced to utilise an external archive device, this drove up the price of their offer to twice that of XYZ plc's original offer.

This is typical of what we call 'spec sheet selling' which seems to be prevalent in the industry at the moment, and seems 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 an expensive option.

This article is intended to dispel some of the myths and cut through the hype surrounding digital video recording, and provide guidance to those considering adopting this exciting technological advance to enhance their operations.

Advantages of digital video

The advantages of digital video over analog are becoming established and now known. To reiterate, video images can be saved on a hard disk by using digital video recorders (DVRs), instead of utilising videotapes that degrade with use and have many moving parts. Digital video techniques enable enhanced and more efficient search facilities. It is easy to search the video image stored on the hard disk drives via time and date criteria. Additionally, multiple copies of the images can be produced without degradation. Transmission of images over LAN/WAN technology enhances the ease of distribution, and the adoption of video player software into operating systems allows the viewing of video clips without proprietary viewing software.

Sending video images over the network

There are several factors to consider when planning the deployment 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 that affect the file size are:

* Compression algorithm.

* Refresh rate.

* Resolution.

Compression algorithm

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, MPEG, etc. For this article it is not intended to justify the merits of each different compression technology as there are many manufacturers shooting it out in the trade press as it is. However, it is worth explaining that there are basically two methods of compression to enable 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. Depending on the application, either method is suitable.

The real issue is what does this mean to the user of a system? Different compression techniques and rates will affect the storage requirements and the bandwidth utilisation. This, in turn, will affect the capital cost and cost of ownership of the system. As in all walks of life, there is always a trade off to be made as we cut the cloth to suit our budget.

Refresh rates

How often the image is updated is known as the refresh rate and is generally quoted in frames per second (fps). It would appear that when selling digital CCTV systems, salespeople are quoting frames per second as being the benchmark of a superior system. To confuse matters even further, manufacturers are quoting in various formats. Some quote frames per second (25 fps provides realtime live video), others will quote fields per second. There are 2 fields per frame and so many will quote 50 fps leaving the unsuspecting buyer with the impression that this unit has twice the refresh rate of a 25 fps. Other manufacturers will quote pictures or images per second. The refresh rate has nothing to do with the quality of the image; it only affects the smooth movement of objects within the field of view. All of this obviously disguises the true picture, it is time that the industry got together and utilised a common format to assist buyers.

Although 25 fps will provide realtime live video there is a high cost to pay in utilising such refresh rates. The more frames that are recorded the larger the file size. Large file sizes will require more storage devices and utilise increased bandwidth on a network. Studies have shown that even though many units available on the market are capable of recording at 25 fps almost 75% of the installed base of such units are throttled back to a level that provide a good compromise between usability and cost overhead. 3,125 fps seems to be the standard that most companies are utilising, this provides adequate surveillance whilst reducing storage requirements, and minimising the impact on a network. However, most potential users are still requesting 25 fps within specifications, without realising the impact, as this is the information they have been force fed by unscrupulous suppliers.

Just in these paragraphs alone we have been bandying about figures that to the uninitiated, are likely to confuse and so lets put some of this into context. For a start, most users of analog CCTV systems tend to use a 16-way multiplexer and a time lapse VCR set at 24 hours, this combination will provide a maximum of 0,54 fps. Therefore, by increasing the refresh rate to 3,125 fps, customers are getting a 500% increase in the refresh rates they are used to at present.

Additionally, we tend to think linearly rather than logarithmically, thus we assume that a refresh rate of a system operating at 25 fps is twice as good as a system operating at 12,5 fps. Although in theory it is, 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 fps. This allowed the animators to draw 15 individual cells rather than 25, thus reducing the time required to generate, and the cost of producing an animated film. When viewed we saw these as realtime moving pictures as the receptors in our eyes could not process the information quick enough for us to see the difference.

When evaluating a system, it is important to analyse the differences between the refresh rates. This should be undertaken by visiting the manufacturer and comparing several cameras all set up at different refresh rates and all looking at the same source. When doing so ensure that there is movement in the field of view, which is likely to represent your actual conditions. Having done this simple test, you will be able to settle on a refresh rate that meets your operational requirements. You should also request a quotation based on two scenarios, one at the optimum refresh rate and another at a rate which you would be pre-pared to accept. Set some parameters for the quote, ie, 32 cameras, all recording at x fps with storage for 30 days.

To be continued in next issue.

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