Topic: video timing vs manual chrono (timex stopwatch)

Hi! I am a swimming coach currently conducting my graduate studies (experimental research). I tried to verify video timing (kinovea software) compared to manual chronostopwatch then difference is at 2-3 seconds which is a very big variance when it comes to swimming.

Here's how i did: I had a 5meter marked (above and underwater) located at the center of the pool then when the swimmer head (wearing colored cap) crosses the 1st mark the timer (timex chronostopwatch) starts then stops when it hits the 2nd mark. I am using a sony action camera filming at 120fps. Based on my manual record it is at 7seconds then based on kinovea timing (kinovea stopwatch) same procedure it is at 4 seconds. Is there anyone who can help me out with this issue? since it would be much easier for me to gather data thru video than manual timing as to avoid the reaction time of the tester.



2 (edited by joan 2017-Oct-17 17:46:08)

Re: video timing vs manual chrono (timex stopwatch)

One immediate thing to check is if the video file produced by the filming camera is marked as 120fps or not. Usually for such speeds the camera will create a file marked as 30fps or 60fps, even if the content is 120fps, when reading the video normally it appears in slow motion.

Assuming Kinovea 0.8.25, go to menu Video > Configure video timing. In the lower panel, check the "Framerate read in the file: XXXfps". If this is not 120, then this is the source of the difference.

In this case, in the same dialog box but in the top panel ("High speed camera"), change the Capture framerate to 120. This should make Kinovea understand the mapping between the time in the file and the real time of the action captured.

Edit: thinking of it, the difference would go the other way compared to what you describe, so it's probably not the culprit.

Another thing is if the SD card on the action camera is not capable of writing fast enough. In this case it will drop frames and the resulting video will be faster than normal.

To check this, record a running stopwatch (tip: you can type "stopwatch" in Google to get one), and review the video in a regular player. If you record 10 seconds of the visible stopwatch but the video doesn't run for that long then the problem is at the recording step in the camera.

Just using class 10 SD card may not be enough depending on the resolution you are using. You can check this page: fastest microsd cards for info about write speed, ratings, etc.