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In my experience even cameras with constant framerates aren't really constant, if you look at the timestamps the delay between frames typically varies a few (dozen) ms. So on average you might have 30fps, but if you sample a random second you might end up with 28-32 frames.

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29.97?

That comes from the old NTSC standard and the history of adding colour in a backwards compatible way:

https://www.youtube.com/watch?v=3GJUM6pCpew

In the case that the above posters are mentioning, this probably has less to do with NTSC compatibility and more to do with bad timing hardware or cameras that attempt to do too much in PIO mode instead of using real encoding hardware.

Glad to see that video linked; it's a terrific explanation.

I wouldn't say bad timing means bad hardware though, even expensive scientific cameras suffer from this. You need rather complex electronics to capture at a high and consistent rate. I think normally the encoding is done by software.

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