Thursday, November 8, 2012

variable frame rates

An interesting quirk in digital multimedia evidence (DME) when it meets the average first responder's workflow can cause unnecessary angst when using the Omnivore.

Here's what I mean.

Let's say that a crime happened and was captured by a CCTV system - and that the Omnivore is your tool of choice for retrieving the DME for use by your agency. The crime lasts only a minute or two, but the investigators want 15 minutes before and after the crime - thus your capture duration would be 32 minutes, give or take a few seconds. With me so far?

Enter a variable frame rate.

Some systems employ a variable frame rate for recording / playback. Thus, if nothing's happening, the frames per second (fps) might be set at 1 or 2 fps. Then action occurs in front of the camera, and the fps jumps up to 15-30 for the duration of the action.

So, in our scenario, there's little going on for about 15 minutes, then 2 minutes of action, then little going on for 15 minutes. The DVR manufacturer wants to maximize storage space, so it's 15 minutes x 2 fps + 2 minutes x 30 fps + 15 minutes x 2 fps (as opposed to 32 minutes x 30 fps. With me? Here's where this is going.

If you run the Optimization routine in Omnivore based on the initial seconds of the DME, it will give you an average playback of 2 and a capture rate of 4 fps. But what happens when 15 minutes later, the fps jumps to 30? Omnivore, if you have it selected to stop when it drops frames, will stop and ask you if you want to save your work. It thinks it's dropped a frame as it's being flooded with frames and isn't set to receive them at the new rate. It's doing what it's supposed to do.

The simple solution to this problem, if you can call it that, is to base your Optimization on the time within the video where the crime actually occurs - or there's action happening. This will be the highest rate, generally. You should also note the internal settings of the DVR. What does it say the frame rate should be for the camera that you are capturing? Is there a different fps setting for motion, alarms, panic buttons, etc.? Write this stuff down. Then compare what Omnivore says about the frame rate with your notes.


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