Friday, February 8, 2013

Frames per second

Many investigators assume that all their video evidence comes in one flavor - 29.97 frames per second. But, is this true?

DVRs are usually scalable and upgradeable utilizing specific or proprietary software. They typically have video capture circuits or cards that can process 60, 120, 240, and 480 frames per second. These numbers represent the total number of frames per second that can be accommodated for all of the cameras or channels per system. For example, the 120 frames per second DVR with 16 cameras has an approximate frame rate of 7.5 frames per second. This means that each camera can be converted at 120/16 or about 7.5 frames per second.

Then there's the question of the reliability of the DVR. Did it actually record 7.5 frames per camera per second? In my tests, the answer is usually NO. System overhead and other internal issues usually result in a number that's between 5 and 8 frames per second - a number that randomly varies over time.

Tools like Amped FIVE can give you precise frame counts, and even accurately tell you the frames per second of the clip. Production tools, like Premiere Pro or Final Cut, force your clip to conform to certain specified standards. Also, there are LEVA courses that teach a method of conforming files for use in Avid's Media Composer.

Still and all, it's better to know than to assume.

No comments: