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.
One way of solving distortion problems is through the use of time-based correctors (TBCs). A TBC is a device that inserts or corrects missing synchronization (timing) signals in video, adds burst (color) signals, and realigns the signal. Some high-end VCRs have TBC devices built in, and higher quality PC capture cards will have limited TBC capabilities. Most professional video production studios use standalone devices. Analog TBC devices are the least expensive, but still cost around $2,000. They use phase lock loops to insert clean horizontal and vertical sync signals. Digital devices tend to run as high as $6,000. These convert analog signals to digital, buffer the frames, and digitally calculate new signals.
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.
Some distortion problems can be improved through forensic video analysis. Intergraph Video Analyst® System offers filters such as Average and Straighten that can reduce noise and help realign the frame. The Separate filter helps separate out images captured on multiple camera systems. While these filters can improve a video image when you have no control over how the image is captured, it is always better to start with TBC-corrected images. For one thing, video analysis can''t correct all problems. For another, analysis processes can take considerable time and effort. In one recent case, Intergraph analysts spent several days recovering only 20 minutes of an hour-long tape - less than one frame every 30 seconds. The tape had nearly been destroyed prior to Intergraph receiving it, but fortunately, it was enough to prove our client''s innocence.
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
Ultimately, the best way to avoid distortion in forensic analysis is to go to the source. Most commercial business establishments and federal organizations use some sort of video surveillance system. These systems typically use a single tape on which surveillance images are recorded over and over each day. Whether to save money or because of constrained resources, users of surveillance systems tend to change videotapes only when a tape breaks or is no longer usable. This can damage the VCRs as well as the tapes. In some of the worst cases Intergraph analysts have seen, the same tape had been used for a year or more, making image recovery particularly difficult.
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.