Archive for Expert Witness

Digital Video Forensics: “Is this video clip reliable?”

When we receive a request from an attorney or a forensic engineer to review digital video material, we are most often asked, “Is this video clip reliable?” Over the years, we’ve learned that this can mean many different things. The material in question is often a short piece of video in the form of a digital file that can be played using common media players, such as Windows Media Player. In some cases, the material is accompanied by proprietary viewer software that is required to view the video. On occasion, the video is actually in a DVD format, complete with title and menu.

But what our clients really want to know is one or more of the following:

“Is this video clip a true and complete copy of the original?”

“Has this video clip been altered or edited?”

“Can I rely on the time and date that appear in the video? To what degree of precision?”

“Are the proportions of the picture correct? Can I use it to measure distances?”

We generally deal with civil cases and not criminal investigations. In a criminal investigation, it is usually enough for investigators to obtain identification from the video. This may be facial identification of a perpetrator, the approximate height and weight of the individual, or simply a general description of the clothing the perpetrator was wearing. In some cases, investigators seek to identify a vehicle, by model, make, or color. The investigating agency may extract important images for enhancement or to distribute in aid to the investigation. But it is very rare that a criminal investigator concerns himself or herself with the precision of the date and time stamp, or whether a single frame may be missing from the video sequence.

In civil lawsuits, it is a different matter. Unsurprisingly (and according to the “CSI” shows), video recording systems are everywhere and frequently record video of incidents unintentionally. We have, for example, worked with numerous video files from security systems that recorded vehicle accidents in the background, a purpose for which they were not originally designed or installed. A civil lawsuit in connection with the accident might require the involvement of forensic engineers, who will normally perform a survey of the accident site to obtain accurate measurements of the positions of the vehicles before, during, and after the accident. Since vehicle speed may be a contributing factor in an accident, the engineers want to estimate the vehicle speed(s) at different locations. Speed is often estimated on the basis of skid marks or the amount of damage sustained by the vehicle(s), but the availability of recorded video gives the forensic engineer the opportunity to estimate vehicle speed by time interval. Knowing that speed=distance/elapsed time, and having accurately measured vehicle position during the investigation, all we need is a precise measure of the time interval between the frames of the video clip that show the vehicle at those measured positions. What could be simpler?

As it turns out, a lot. Most digital video recording systems were never intended to be used to measure time intervals to the precision required to differentiate between a vehicle going 45 MPH in a 45 MPH zone and a vehicle going 56 MPH in a 45 MPH zone. In fact, most date and time stamps inserted in video recording systems do not display with any more accuracy than one-second intervals, though some may display to a greater precision. If we do have a system with sufficient precision, there may then be the question of accuracy. Being precise to the millisecond is one thing. Being accurate is another. The task is further complicated if the video clip has been exported by the video recording system in a format different from that which was used in the original recording. It is very common for a video recording system that uses variable frame rate (i.e., the intervals between successive frames are not uniform) to export video clips to a video file format that uses a constant frame rate. The exported files are quite useful for identification purposes, but may useless for performing the calculations required to accurately estimate vehicle speed.

We frequently receive video clips in which the date and time stamp advances 15 minutes, or some other fixed time period, but the video clip actually plays in much less time using a software media player. We have several clips in which the audio portion of the file is shorter than the video itself, sometimes by more than 10%. The questions are then, “Which time intervals are correct? Can we state with confidence that the actual time interval between the vehicle at this location and that location is X.X seconds? What is our confidence interval for our estimate?”

Strangely, old-fashioned videocassette recorders are often more reliable and useful for the purpose of estimating time intervals than are modern digital video recording systems. A standard VHS recorder was designed to record video at 29.97 frames per second, and we have the further advantage of knowing that the camera was providing video to the recorder at an identical rate. Newer IP video cameras and digital recording systems normally work with variable frame rates and may even add the time stamp to the images only after they have been received at the recording unit, adding the problem of network latency to the mix.

A gentleman who was very experienced with digital video and had worked for years in the industry once told me that he would, “…never try to estimate time intervals in digital video with BOTH precision and accuracy.” While this might be an extreme view, it certainly reflects the challenges that face us.

We have attempted in this article to identify some of the important considerations in establishing the “reliability” of digital video used in forensic accident investigation. In subsequent articles, we will discuss some of these topics in more detail and introduce new topics of interest.

Posted in: Expert Witness, Security Technology, Video Forensics

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