Buyer beware

What should you look for in a digital recorder? And what standards should you expect from a good quality digital system? There are numerous types of systems available, the trick is choosing the correct one for the application in hand. Some digital boxes stick closely to the style of the VCR recorder, and are merely drop-in replacements for analogue with limited digital capabilities, often ignoring key features such as pre alarm.

Going up a step are the small PC based boxes, which can generally support up to 16 channel inputs. These are ideal for smaller security installations, such as areas within the retail industry. For extensive applications such as large-scale shopping centres or prison security, larger scalable systems are available, with some boxes capable of accessing up to 4000 cameras.

A good digital CCTV system can be configured according to the customer's requirements; therefore the buyer should have a clear understanding of how they would like the site to be monitored prior to a product demonstration. It is vital to define the requirements of any digital system camera by camera in order to gain maximum potential from the system as a whole.

Once the number of cameras has been assessed, time needs to be taken in order to decide where the cameras will be positioned and what recording mode is best suited. For example, is continuous recording required at all times throughout the site or can storage space be saved by having some cameras record only on alarm? If the configuration of the system has been thought through prior to a product demonstration the buyer can ask to see particular recording modes and discuss their exact requirements.

Image quality and capture rates
The buyer also needs to consider the image quality necessary for the application. Good quality images are the result of a number of important factors. The positioning of the camera itself affects the quality along with the correct combination of camera, lens and video transmission. Good images are also dependent on appropriate capture speeds, system resolution, and the amount of image compression used. The resolution you should expect from a digital recorder today is 768 pixels per line (400 TV line resolution similar to SVHS performance). Capture rates also need to be considered at this early stage, as this can affect the overall cost of the system. For example, if the camera is being used to capture slight of hand in a casino the camera would need to be recording between 12.5fps and 25fps in order to pick out detail. High security sites such as prisons, generally capture at 3fps with more general surveillance in banks, shopping centres etc at 2fps.

It is important to establish the type of capture rate required prior to a product demonstration as, generally, if you double the frame rate you also double the cost. In some applications it is best to have a few cameras capturing at a high frame rate with the others at a lower rate, in order to have a few quality images rather than lots of poor ones.

Storage Requirements
It is also important to consider storage requirements, as the bigger the storage capacity, the higher the price. All digital video recording systems store images initially onto hard disk, some systems are capable of supporting massive disk arrays, with RAID (Redundant Array of Inexpensive Disks) technology incorporated if required. This gives high reliability and availability. However, disks are expensive and most companies offer a compromise using disks for 'on line' instant access storage and a form of digital tape for slower 'on-line' or 'off-line' storage.

One of the key reasons people buy digital storage systems is because it reduces the volume of archive material. Another benefit is the ability to store just the images which contain activity, helping to save on space. Many requirements cannot be achieved using disk storage alone unless they're used solely for alarm verification or storing in response to activity detection. Therefore many recording system designs are likely to use a combination of a computer disk for intermediate storage and an archive device for long storage.

Disc capacity
Methods of storing video images can be flexible, with options to suit individual user requirements. Hard disks are constantly improving and getting faster and more reliable. Capacity is forever growing with 600Gbyte discs likely to be available by 2004. It is difficult to establish how much video information can be stored on a disk of any size, as there are various factors to consider.

The longevity of a disk will depend on how the system is being used (event-driven, snap-shot recording, time lapse recording etc) along with compression techniques. This all affects overall file size. Although storage functions vary, digital technology greatly exceeds storage capabilities currently available with analogue systems.

Networking
The ability to network systems over a local (LAN) or wide area network (WAN) is increasingly becoming standard within the security industry, therefore it is important to look at the possibilities networking introduces when choosing a digital system (Fig 1). Transmission methods are comparatively slow and can represent real bottlenecks in the system unless image data is converted to a format optimised for transmission.

There are many different types of digital compression techniques, the most commonly used in the security industry are JPEG and Wavelet for storage as these store a series of individual pictures which can be very high quality and therefore used for evidential purposes. Alternatively, MPEG or H263, which are not suitable for quality recording, are well suited to high compression for streaming data over networks. Video compression makes use of the innate redundancy within real-life images. Compression groups the pixels that relate to these redundancies. As a result of this procedure, a condensed packet of digital video data, which requires less bandwidth for transmission, can be utilised. ( Primary Image can give a choice of quality or speed. High quality JPEG images can be transmitted at a slow rate or conditional refresh with the harder compression technique (Wavelet) can be used to transmit images at a faster rate. The user can make a valued decision as to whether quick transmission of the data is paramount or the sustaining of the image quality.)

Legal Considerations
There is still some apprehension regarding digital images as evidence in a court of law. It is easy to understand initial concerns, as it can be simple to doctor or distort a digital image. If, however, good authentication techniques are used and a proper audit trail is put in place, then images stored digitally are perfectly safe, if not safer than analogue. This view is supported by many of our Government and commercial customers. Digitally compressed and stored images from our systems have been used successfully in court. The 2nd EYES PLUS family of digital recorders use strong cryptography to generate a unique digital signature for every video image frame that is stored. At some later date anyone in possession of the authentication key can ascertain the source of an image and obtain the appropriate audit trail information. This strengthens the value of the image for evidential purposes as any tampering with image or audit trail data can be immediately diagnosed. Images will remain in standard JPEG format and may be copied or transmitted to other computer systems but they will still retain the digital signature for authentication at any time.

Audit trail
A good digital system should incorporate an audit trail into all images, including information such as an image fingerprint, the time and date of when the image was created and the name or number of the camera that took the original image.

It is also important to secure images whilst still on the actual system itself, in order to prevent staff from copying or deleting images at will. Built in security features, such as multi-level user access rights should be available so that only trusted individuals can perform certain operations.

Audit logs further secure the images by logging information, such as when staff log on or off, image printing and configuration changes. The security manager knows who has used the system and which images have been looked at in detail and exported to printer, disk or cd.