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I have security camera footage from a bus. The timestamps are resolved only to the second. Upon export from the bus video software, I can select a frame rate. If I select, say, 10 FPS, I get a sequence of unique frames. However, as I play the bus video it is clearly not constant frame rate (CFR). For example, I know that the bus is going a constant speed, but when I switch between a few frames, I can see that it appears to move a foot between two frames but then 3 feet between the next set of adjacent frames.

I guess I have two questions:

  1. Fundamentally, how do video frames get captured by security cameras? I assume they strive to shoot at CFR. How often is that precisely the case? Say it is shooting at 10 FPS, then it captures once every 100 ms. How often does it screw up and wait 90 ms or 120 ms between shots? This is obviously a big factor in using video to compute bus speed. I cannot assume CFR and use two adjacent frames to estimate bus speed, I need to pick an interval that sufficiently subsumes any frame rate errors, but I have no basis for what those errors might be.

  2. Is there anything I can do with FFMPEG to further diagnose this, or extract the true timestamps?

Thank you.

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    Select the highest frame rate and export.
    – Gyan
    Jul 11 at 19:32

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