Dealing with Video Distortion
By Gene Grindstaff, Chief Scientist of Video Analyst® System, Intergraph Solutions Group
You''ve probably seen it countless times. You''re investigating a crime, and the best piece of evidence - surveillance video of the crime scene - is so degraded and distorted you cannot get any information from it. If only the video images could be rescued, it could mean an open and shut case. Luckily, there are ways of improving distorted video, but the inherent problem is one that will require cooperation from users of closed caption TV (CCTV) and video surveillance systems.
Sometimes referred to as time-based errors, video distortion is a common problem with a number of common causes. A tape that has been repeatedly used or paused will have a lot of noise or have tears that change the image. Poor synchronization within a video image can displace individual lines, giving the image a bent effect. When multiple camera systems are not synchronized, lines appear on the screen or the image looks scrambled.
The most common causes of distortion are manmade. As tapes are used and reused, VCRs degrade the videotape. Tapes become stretched and worn over time. The slow mechanical response of the VCR, such as during pausing operations, contribute to this problem. Changes in temperature and humidity can cause degradation of the film oxide. Each time someone makes a copy of a tape, it not only adds to the degradation of the original, but the copy itself is of poorer quality than the original. By making copies of copies, over time the image becomes practically useless.
Aside from the cost, the pitfalls of TBCs are that they can blank out or write over multiplex, time, or other header information. Usually, standalone units support only one type of video system (NTSC or PAL), and they can''t correct for multiple unsynchronized cameras. Overcoming this problem will require a synchronizer with the cameras themselves.
Digital video analysis also can help preserve the original piece of evidence. By digitally capturing a videotape once and then using the digital copy for analysis, an analyst can apply filters and remove them, experiment, repeatedly play a clip, and quickly scan video to find images - all without impacting the original source video. This helps preserve it as evidence.
Help from surveillance system users
A best-case scenario would be for surveillance system users to change tapes daily or weekly. Tapes used for a month or longer risk distortion problems. Investigators and officers should make this clear to all users of surveillance systems in their jurisdiction. The more businesses take care to change tapes in these systems, the better protection they will receive from them.
At the same time, forensic analysts cannot count on surveillance system users to ensure tape quality. A video analysis system can help recover some of this information, but if a VCR is used to capture forensic evidence, a TBC will probably still be required. The simplest, least-expensive solution is to use a VCR or capture card with TBC capabilities, but care must be given that the unit does not remove vital header information. You will have to deal with distortion every day. Be sure to take the steps necessary to minimize its impact.
|Back to previous page|