Setting the Rules on Digital Evidence

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Analogue and Digital - What's the Difference?

The issue of digital images as evidence is in focus as this new technology takes off in the security world. Demand for digital is rising rapidly as the cost of commercial applications falls (particularly for storage and maintenance). The quality of digital technology is clear to see quite literally with superior images that are more flexible to store and transfer.

So what is the difference between traditional analogue video images and images obtained from digital surveillance technology - and why all the fuss?


Traditional analogue Images are recorded in some physical form, such as frequency, amplitude or in the case of a photograph, the activation of photo-chemical emulsion.


A digital Image is recorded as a series of binary digits (called bits) - either ones or zeroes. The image is then focused onto an electronic sensor comprising individual light-sensitive elements known as pixels (picture elements). These act as switches to modify an electrical current on or off and the information is processed by a computer. It can then be displayed on a screen, stored in a variety of media or printed out.


The Select Committee Report, 'Digital Images and Evidence', seeks to clarify the difference (see panel above) and makes recommendations to the Government on the way forward with digital CCTV images. For a court, the key word is 'traceability' - having a cast-iron audit trail that takes you right back to the original recording. This means that whatever happens to an image if it is enlarged, printed out, even tampered with - the original remains for a court to examine. Because digital technology is so new, people are having to get to grips with the fact that a digital image consists of a series of ones and noughts that are converted by a computer into an electronic image. But that doesn't mean they should be any less valid than a traditional analogue image.

Far from saying digital images cannot be used as evidence the Report lays out guidelines about ensuring their authenticity. Like analogue images, suitable procedures should be followed in collecting and monitoring what is captured on camera. Indeed, the Select Committee established that digital images have already been used as evidence in court. For example, images from a system installed in the car parks at Heathrow Airport have been successfully used as evidence.

It seems certain that the increasing popularity of digital technology coupled with the fact that images can be replayed countless times with no diminution in quality means its widespread use and acceptance as evidence is inevitable. Analogue or digital images are unlikely to be the only evidence presented in a court case. In fact, they are far more likely to be used before a trial to make a person admit their involvement in a situation.

From our understanding of the Report, the Government is saying that methods of storage and authentication of surveillance images should continue as before. Many of the issues created by new digital technology will be governed by the new Data Protection Act.

The Data Protection Act is significant because unlike analogue images, digital images are covered by the Act. This seeks to protect individuals from the use of personal information without their consent, such as their names and addresses. It is very detailed about the way data must be handled and stored. By falling within the remit of the Act, digital recordings are therefore governed by very stringent guidelines and controls.

What It All Means for Installers

What is important is that end users of digital surveillance equipment know what is expected of them in terms of the way they record, store and use digital images. It's not so much installers but the impact on their customers that need to be considered. Installers should make sure their customers know what is expected of them.

We're helped here by a number of specific recommendations made by the Select Committee and endorsed in the Government's official response. Digital technology has the capacity for encryption and security coding so some kind of electronic audit trail involving file coding of digital images is suggested. A permanent physical record of the data that cannot be amended is one idea - this could be some form of write-once read many times' (WORMS) disk. Creating an audit trail would reduce the chances of undetected tampering of images.

These are some of the main Report recommendations which the Government has said it hopes will help to form 'best practice' in the security industry and elsewhere:

  • Responsibility for proving the reliability and authenticity of data is with the body that captures, processes and modifies it. A suitable audit trail is essential
  • Where digital images are considered as evidence, courts should place greater weight on evidence that can be shown to be derived from an authenticated original. Juries should be informed of anything to doubt the authenticity of digital images
  • As with analogue images, proper records must be maintained showing who was in control of the equipment at the time of an incident and subsequently in charge of any images created, and who is responsible for the storage and retrieval of those images
  • The Data Protection Act 1998 should provide the regulatory framework to cover CCTV-derived images, including digital data. The Government supports the idea of devising some kind of incentive such as endorsing codes of good practice that are based on the quality, integrity and authenticity of data. Factors here might include:
  • The way in which systems are tested, including on-site, by installers or users
  • The way systems are set up, calibrated and maintained
  • Environmental conditions
  • Operating procedures
  • Training of users
  • Automatic quality warnings

What installers need to make their customers aware of is not just the fact that digital and analogue images differ but to ensure that the same careful approach is taken to the way any image is captured, stored and maintained. They need to make sure their customers understand the importance of ensuring traceability of surveillance images.

This article was supplied by Norbain and edited with permission. It originally appeared in 'Security Matters' magazine. Norbain are manufactures the Columbus Digital Storage Unit which was voted 'Product of The Year' at Ifsec (UK's national security exhibition)

To view a full guide from the 'Police Scientific Development Branch' for practitioners handling digital images as evidence go to [1]

Please also see our article "Digital Images as Evidence".