Hi Denis, > I don't want to overstate the problem: by and > large, the cameras do hit 15 fps quite accurately, it's just that there > is some jitter. > > To back this up with a little data, you can see the capture times for > a single camera here: http://denism.homeip.net/stuff/capture-times, the > differences between consecutive frames here: > http://denism.homeip.net/stuff/capture-differences, and I've generated a > histogram of those differences here: > http://denism.homeip.net/stuff/interframe-histogram.png against a > logarithmic Y axis. The histogram shows that the vast majority of the > inter-frame differences are in the 0.06-ish area, but there are a number > faster and slower than that.
Thank you for this data! Looks like 1.6% of all frames are lost. I noticed that the jitter always start with a very long delay, and they seem to be quite regular. I have attached a histogram over the time between the jitters, and the jitter seems to occur at multiples of 80 frames. Hard to say where it comes from, but it might be a buffering problem? > Whether or not this jitter is big enough to cause a problem depends on > how synchronized you need frames from the cameras to be: the jitter I > show above is quite tolerable for my purposes. While I haven't calculated the tolerable amount yet, I think I probably need to synchronize the image streams after they have been captured. > I actually had a > breakthrough last night, when I realized my audio/visual synch wasn't > caused by the video frames being non-uniform, but rather that the audio > I recorded from the USB mics isn't correct. When I matched the video up > against the audio from a DV video camera filming the same scene, > everything looked fine (wrt synch). Yay! Good to know. But actually the video frames should be approximately 1.6% faster than the audio, based on the data you posted. Maybe that amount is harder to notice, but after two hours, the difference would be 115 seconds? ciao Pablo
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