The Recording System (Digital Systems DVR)

The use of digital video and images has now become widespread in both the general community and within the Police Service, however, digital recordings are not always better quality than recordings made on analogue formats such as VHS.

Resolution

Image resolution is a major contributing factor to image quality. Higher resolution means that more information is stored about an image, therefore more information can be read about an image. The number of pixels used to represent the image determines the resolution of an image. Digital video for television broadcast is represented by 768 x 576 pixels. The minimum recommended resolution for surveillance systems is 320 x 240 pixels.

Compression

Digital video creates large files, so compression is crucial for the storage of this data intensive information. A high amount of compression will result in less detail in the image. In extreme cases, the picture displays compression artefacts such as pixelation (large blocks of pixels). This should be avoided as the image is degraded beyond its usefulness.

Extracting Evidence

Most DVR over-record their data after a set period of time. It is strongly recommended that the DVR you choose has the capacity to archive footage to a removable hard disk and/or export any relevant sections to CD or DVD. Preferably the files should be exported in a standard format that will replay on other computers (e.g. mov, .avi, .mpg, .wmv, .asf). The exported footage should be accompanied by the corresponding decoding software or viewer application if necessary.

Replay and Viewers

Often DVR designers incorporate proprietary software into their systems that inhibits files form being viewed on other computers, and with common media players such as Windows Media Player and QuickTime. Unless a DVR-specific viewer is made available, the video files cannot be opened or viewed. This adds a layer of difficulty for police who need to replay recordings on their systems for interviews, investigations and in court. Likewise, systems that require specific hardware to enable replay or viewing of files are not recommended, as there is difficulty in converting these files into a format that is viewable on police systems and in the courts.