A GoPro camera also has its own internal clock where again the sync between GPS is quite important. Recent data from Pierre shows some challenging problem that two cameras (one taken by Pierre and the other by his friend) showed inconsistency in time that makes the time sync between three sensors quite challenging: two GoPro cameras and one GPS. So I first found the time gap between two cameras by analyzing the sound wave correlation (See the below screen shot of Audacity for Mac).
So I could realize that the time difference between two cameras is 00:01:22 considerating the difference in their creation time. However, this does not mean that we can simply shift the creation time of one video file of two by that much becuase sadly the file creation time (which you can identify using ffmpeg -i inputfile.MP4) is not exact.
To verify the problem in setting the exact clock of GoPros, I tried to sync the GoPro clock with the UTC time (Take a video shot of http://www.timeanddate.com/worldclock/city.html?n=215 using GoPro). I set the clocks of two cameras at the same moment but the output later checked using the video shows 23 seconds difference between two clocks (Weird!). Thus the exact start time of Pierre's cases should be retrieved from the GPS coordinate by matching the scene in the video. Once we find a one good matching spot between GPS and GoPro, we can use it for the entire collections.
Unfortunately, GoPro cameras do not have a way to set the time resoultion accurate to seconds. Its interface supports down to Minute setting. Thus to prevent further troubles, here I am setting a number of rules for time synchronization.
- Our two cameras are marked at the backside as "L" and "R" each. Its memory card is also marked like. Thus DO NOT mix up the memory card with the other camera.
- At the start of video shots, take a video of exact time. You may use the GPS clocks. Press "Enter" key of Garmin GPS for 3 seconds, date and timestamp will be shown at the bottom. But it is a little tricky to catch that small fonts in a reflective GPS screen. So you may want to sync with iPhone but never use its internal clock. Rather open the browser and take a video shot of http://www.timeanddate.com/worldclock/fullscreen.html?n=215 (Bookmark this link with your iPhone).
As of today (July 8th, 2011), both cameras are tested a number of times. Check out below information after calibration. Hope the cameras keeps this info without further changes. So it is important never taking out a battery from a camera.
- Left GoPro camera: -1 seconds delay. After taking a movie, you have to change the creation time by adding 1 second from the original.
- Right GoPro camera: -1 seconds delay. After taking a movie, you have to change the creation time by adding 1 seconds from the original.