Everything you wanted to know ...

In my previous article (June 2002) I considered some of the broader areas of using digital video recording, including some of the operational issues such as evidence handling etc. For Part 2 my intention is to delve a little deeper into some of the more technical aspects of installation, including networking, the impact of the "new generation" of IP CCTV equipment and other factors such as on-site conditions ...

Q: My client wants to use his digital recorders over a network. What are the implications?
GJ: There is currently a great deal of talk in the industry about networkable CCTV products, but in truth because many digital video recorders have been network ready for some time, this has provided an advanced warning to the CCTV industry of the many pitfalls involved with networking.

The first thing to consider is what exactly a client wants to achieve by putting his recorders on the network. The common perception is that they want to receive live images from the remote sites but close questioning often reveals the simple truth – that access to recordings is all that is generally required, as no one has the time or the inclination to sit watching the pictures all day. What is becoming increasingly common is the requirement for occasional live surveillance with the bulk of network access being primarily to recover recorded images.

This is extremely good news from a network point of view. Many people (including IT managers), fail to appreciate the difference between the streaming of a live, almost real time video signal (which uses lots of space on the network) and recovering a video sequence which was recorded at a much lower frame rate, which generally uses far less bandwidth.

A good digital recording system will certainly offer both facilities, but the amount of network bandwidth available will dramatically affect what you can actually do. Generally, moving compressed images recorded at a time lapse update rate may well have little or no impact on the organisation's network operation (depending on the actual DVR you use) whereas streaming live footage will.

I'm obviously generalising somewhat here, as the actual bandwidth required to transmit any images, recorded or live, is a product of the image quality, image update rate and compression applied to the pictures. But the difference between the requirement to view recorded footage as opposed to "live" footage should be considered from the outset. It may be that live viewing at much lower frame rates than "real time" is a perfectly acceptable trade off in some circumstances.

Q: Is security compromised by connecting to networks?
GJ: It shouldn't be. Most digital recorders now employ some degree of password protection, although bear in mind that it's ultimately your clients' responsibility to restrict access and keep his own network secure. Professional digital record-ing systems usually have a viewer application available, enabling users to connect to the recor-ders using their own PC, which should contain exactly the same passwording, audit trailing and other security features as the recorder itself.

Q: Can video recovered over a network still be used as evidence?
GJ: Theoretically yes, but as with all evidence, the way in which it is handled post event will be just as important as the evidence itself. If footage is presented as evidence, whoever is presenting it must be able to "attest to its originality". In practice that means that your user must be able to swear in court that the pictures he has provided are either the originals or direct copies of them.

This is actually quite hard with many DVR's, even on a local basis, and obviously becomes far harder when remote network access, often over hundreds of miles becomes a factor in the way evidence is gathered.

People fail to appreciate the difference between streaming live, almost real time video which uses lots of space on the network and recovering a video sequence which was recorded at a much lower frame rate

Gary James

This is why audit trailing is essential for the provision of evidence. A record of which users accessed which images and from where, is a key factor in any systems ability to produce credible evidence. Even though the pictures presented will be copies of the original footage, as long as the audit trail associated with them is unbroken there should be no problem. The provision of such an audit trail is certainly not common to all DVRs, particularly over networks, so this aspect must be considered carefully.

Remember – there is no problem going to court with a copy of anything as long as you can (a) state where the original is and (b) prove beyond any reasonable doubt that the copy you are standing behind is an accurate one. In such circumstances the audit trail associated with the evidence goes to its weight, not its admissibility in the first place – meaning that one day soon a vital piece of evidence without these safeguards may contribute to the collapse of a trial.

Q: Are DVR systems compatible with all existing CCTV systems?
GJ: On the face of it, yes. All DVRs use an industry standard 1v peak to peak video signal. However at many sites this healthy video signal is often the exception rather that the rule. It's true to say that, in general, digital video recorders are actually far less forgiving than their analogue multiplexer cousins when it comes to dealing with problems associated with the inbound video signal.

Analogue video monitors are actually far better at adjusting themselves to overcome problems with video signals, such as low video or high levels of signal. Within the DVR environment this level of tolerance is not so common. So, when retrofitting a DVR into an existing system don't assume that just because the pictures look OK when connected directly to a monitor that everything is fine.

Recently, this aspect of digital recording has improved considerably and, as with most other crucial aspects of product design, the serious players in the market understand the sort of conditions that exist in the real world and design their systems to cope accordingly. However, if you're faced with reversing a shiny new DVR into a really old existing system, pay particular attention to the state of the video signals.

Q: With all these Hard Disk Drives involved, should I consider an un-interruptible power supply (UPS)?
GJ: Emphatically yes. Regardless of whether your DVR is PC based or not, it's using a standard PC type hard drive, or in some cases, several. The problem here is that there are many power conditions which can cause malfunction or even failure which are not often directly attributed to power issues. Power delivery is a separate subject in its own right but can be broadly categorised as Sags, Blackouts, Spikes, Surges and Noise.

A study by Bell Labs indicates that Sags are the most common problem. Usually caused by the start up demands of some electrical devices, a Sag starves equipment of the power it needs to function. On an ordinary desktop PC for instance, this can sometimes be experienced as a frozen keyboard or system crash.

No one has the time or the inclination to sit watching pictures all day.

One of the direct consequences for DVRs is read errors associated with data written to disks. If a transient is experienced whilst a drive is being written to, even a small surge in power can result in the magnetic field on the write head suddenly increasing to ten or twenty times its normal value. This can result in a disk error, which the system may or may not be able to recover from.

As a result of this, many manufacturers recommend the use of a UPS. All too often however, even the relatively modest cost of these devices is ruled out in the race to stay as "competitive" as possible.

Q: What about 'RAID' arrays and other forms of data back up?
GJ: Ironically, given the comments above, we see 'RAID' array written into specifications for hard disk recording schemes far more often than we see "uninterruptible power supply". RAID, which stands for Redundant Array Of Inexpensive Disks is a method of storing data in such a way that a duplicate backup is continually produced. Interes-tingly, this approach only provides a degree of protection against disk, not general system failure.

There are a few different categories of RAID, with 5 being the highest specification. Primarily these high redundancy systems were designed as a data storage solution for applications that move vast amounts of very small groups of data around, such as an airline ticketing service. It's highly debatable as to whether this level of redundancy is required within a CCTV system and often RAID arrays are specified where high capacity is required but where the actual DVR does not have enough on board capacity. However as hard drive sizes increase there is less requirement to resort to these expensive external arrays.

Backing up data for a long period of time is another problem entirely. Occasionally users feel the need to store all of their images for vast periods of time, regardless of their obligations under the Data Protection Act. There is no in-expensive way around this problem, as the costs involved in either equipping the system with sufficient capacity to store up many months of images is often prohibitive. Providing some form of removable media for what could amount to hundreds of hours of data is again very impractical and costly, as well as going against many of the central advantages of digital recording, such as instant access and no physical storage problems.

Probing of the clients actual requirements can pay dividends here, as in the real world there is little point storing up CCTV footage on a medium which no one else, such as the police, can read.

Q: What about the future of digital recording … and how will the new networkable CCTV products affect things?
GJ: My own view here is definitely one of evolution not revolution. There are now many IP addressable CCTV cameras and camera servers around, which won't integrate very well into either existing CCTV systems or a digital video recorder of almost any vintage. The immediate impact of these new products on today's DVR market, which is the subject of this article, is going to be largely unaffected by the introduction of network cameras and video servers, as these new devices more often than not come with their own software for recording and control. There is a danger here of these new devices in effect creating separate CCTV systems alongside the existing ones in certain circumstances and, as many manufacturers would admit, take-up on them so far has been relatively slow. This need for separate system architecture is possibly the reason for the low growth rate in the video over IP sector and as such has largely no impact on the current DVR market.