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I am trying to get 2 video streams into sync at low cost. I would like to visually measure how good can I synchronize my cameras by pointing them to a millisecond capable stopwatch.

Does anyone know what is the refresh rate of an "old style" monochromatic LCD stopwatch (example)? I don't have one so I haven't tried to capture these yet. Wikipedia says an LCD could have a refresh rate as high as 200Hz. If LCD is slow, LED probably would be faster depending on circuity used.

So far I used online stopwatch, but these are limited by (usually) a 60Hz refresh rate of a computer LCD monitor. 60Hz lowers the resolution to about 16ms (1s/60Hz).

  • This question appears to be off-topic because it is about electronics unrelated to video. The only question I see you actually asking is "what is the refresh rate of a stopwatch?" This has nothing to do with video even if the application of it does. You may want to try the Electrical Engineering StackExchange with a focus on the question of "which display type is better for use in a device for high speed sync". – AJ Henderson Aug 13 '14 at 17:56
  • Also, if you really want highest possible accuracy, video is not the way to do it, sync your audio tracks. Video samples around 24 to 60 times a second, audio samples around 41,000 to 96,000 times per second. Simply play a sound with a quick start and stop so that you can cleanly align on the edges of the sound. This should work well unless the cameras you are using have bad sync internally. – AJ Henderson Aug 13 '14 at 18:08
  • I agree with you actually. Thanks for a topic recommendation for EE SE. – Kozuch Aug 13 '14 at 20:12
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You can only sync video to an accuracy of a frame, or somewhere in the area of 15-40 mSec depending on frame rate. So as long as whatever you're shooting can update and refresh at least that quickly, it's not important to do much better. Even shooting a mechanical metronome will probably get you as close as anything else. The refresh rate of the display shouldn't be a factor, if you're examining both clips on the same screen.

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  • Please note I have two cameras in stereo setup and with OpenCV I am showing a new frame for measurement only once in 2s so I can read the captured time easily. Say the exposition time is 1/1000s (1ms). In case of 60Hz monitor I may only be sure that my frame drift is less than 16ms if both frames show same time. I want to push this 16ms even to a smaller number - but this can only be done by getting more Hz on display that is captured by cameras, possibly 1000 Hz for ms accuracy. – Kozuch Aug 13 '14 at 12:24
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    A faster display won't change anything. All that matters is the frame rate of the camera. Your captured motion is quantized at that rate, and observing the quanta more quickly or more often doesn't make those quanta any different. – Jim Mack Aug 13 '14 at 12:45
  • Agreed - assuming you're shooting at 60 fps, you'll only be grabbing a frame roughly every 17 milliseconds. Your display can update all it wants, but as long you're updating at least every 17 milliseconds (60 Hz), you'll be fine. There will be no difference between a 60 Hz and 1000 Hz display shot at 60 fps. As long as the refresh rate of your monitor (in Hz) is greater than or equal to your frame rate (in fps), you should be fine. – John Barton Aug 13 '14 at 13:44
  • I still think you both are wrong. You are still talking about a single camera. What I care about is the sync offset of TWO cameras. Example: Time is 0. I push a trigger at time=1.000s. First camera takes its first frame at time=1.000s. Second camera takes its first frame at time=1.005s. The 0.005 or 5ms is the offset I care about. Though on a display that has 60Hz both cameras show 1.000 value in their frame because the dislpay has not updated yet and will update at time=1.016s. That is my point. – Kozuch Aug 13 '14 at 20:07
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    @Kozuch - One camera or two, the frames occur at 16ms intervals (assuming 60fps). It makes no difference if the cameras are taking at different times: if you observe, side-by side, still frames from the two cameras, you will always see the images as captured at their respective times. Any time difference you see on the display can't be affected in any way by the display's speed or refresh rate. In any case, AJ has given you a good suggestion to use audio rather than video to determine the offset. – Jim Mack Aug 13 '14 at 21:39
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As Jim Mack said, you can only sync video to an accuracy of a frame, which can be pretty tricky. If your two video streams are half a frame off, for instance, there's really no way for you to correct this difference any further (this is the worst case scenario).

My suggestion would be to use some physical object or motion instead of a stopwatch. A "slate" or "clapboard" is a very common way of synchronizing audio and video and can easily be used to sync two cameras.

For this case, some other physical movement might be easier to use - maybe put both cameras on the ground, shooting a ball dropping in front of a ruler or something along those lines.

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