Introduction

When it comes to the criteria of using video images as evidence, the requirements can appear to be something of a minefield. Add the use of digital images to the equation, and question marks start to appear with regularity. Quite often, depending upon whom you are talking to, you will hear a different version of the so-called "digital dilemma" when it comes to using CCTV footage for evidential purposes. However the police and courts are aware of the potential offered by digital surveillance solutions, and do not want to curtail the use of a credible and efficient tool in the fight against crime.

The Police and Scientific Development Branch have published guidelines for the use of digital images as evidence. The guidelines do not specify which compression algorithm or archiving media should be used instead taking the view that final quality and a credible audit trail are more important.

An audit trail for digital CCTV is very much the same as one for an analogue system. However, with digital systems the audit trail must also keep records of which copy is designated as a Master Copy and which is a Working Copy.

Step 1: Obtain the relevant authority to capture images

CCTV systems must obtain, where necessary, the relevant authority to capture images. They should be regularly maintained and an audit trail commenced at the earliest opportunity

Step 2: Create a Master Copy

Because digital images can be replicated without degradation, and due to the fact that the first storage medium is usually an integral part of the DVR, a Master Copy needs to be created. It needs to be stored securely and kept in accordance with evidential procedures. It should be taken directly from the original recording medium and stored onto a removable medium. This removable medium must be a WORM type device. WORM (Write Once, Read Many) devices such as CD-R, DVD-R and DVD+R media add a level of credibility to digital images as once the Master Copy is written the data cannot be affected and changes cannot be made.

A Working Copy, can be produced simultaneously with the Master Copy or subsequent to the Master Copy from the original footage, or can be produced directly from the Master Copy.

• The relevant image sequences
• A text file stating any hardware or software requirements for replay
• Metadata such as time and date information bound to the footage
• Licence-free software to view and make copies of the footage, including editable sequences*
• Instructions on how to copy the footage to VHS cassette plus details of any changes that may occur if the cassette is then played alongside the digital footage.

*The DVR manufacturer should supply this.

• The capacity of the media
• The average image file size of the recorded footage
• The frame rate at which it is recorded.

The following calculation can be used to ascertain how much footage can be stored on any media:

Media capacity in MB x 1,000 (or Media capacity in GB x 1,000,000) / file size in KB / image capacity / pictures per second rate (PPS) / 60 (calculates how many minutes) / 60 again (calculates how many hours).

The following example shows how we calculated a 700MB CD-R would contain just over 1 hour and 4 minutes of footage. In this example, with image quality equivalent to S-VHS performance, average image file sizes are 18KB and for the purposes of this explanation it can be assumed that the recording rate is 10 pictures per second (PPS).

JPEG COMPRESSION, 10pps, STANDARD RESOLUTION 700MB x 1000 = 700,000KB + 18KB (file size) = 38,888 (images) / 10 (pps) / 60 (seconds) = 64.8 minutes.

However taking an average of 40 KB as the file size, and recording in real-time (25 pictures per second) this CD would contain only 11.6 minutes.

JPEG COMPRESSION, 25pps, HIGH RESOLUTION 700MB x 1000 = 700,000KB / 40KB (file size) = 175,00 (images) / 25 (pps) / 60 (seconds) = 11.6 minutes.

*The tests above have been carried out using a DVR from a major manufacturer and are intended as a guide only. Image size and quality may vary between different DVRs.