Q:

Sync video with data

When I have a datalogger and I want to sync a video-file to the recording, what would be an easy method? The logger and the camera both have an internal clock, but they may not be set exactly to the same time, so I cannot use those timestamps to sync the start, and during long recordings they may drift apart.
The solution seems to be to let the datalogger record an extra sync-signal, but what should this channel contain (no. of frames?) and how do I get this information from a regular camera? 

A:

A simple solution could be a blinking LED in the video. If the perspective is static the LED could be in the scene, otherwise some (fixed) position in the frame. You trigger the LED and log this event with the datalogger. Then use something like simpleCV (OpenCV vs. Matlab vs. SimpleCV) to detect the blinking event, thus you know the significat frame numbers. From there you can correlate the video data to the logged data.

A:

One solution would be to feed some timestamp output from the data logger into the vertical blanking information in the video. Then, when replaying the video, you can extract the timecode (with something similar to a closed caption decoder) and match the two up.
http://en.wikipedia.org/wiki/Vertical_interval_timecode
Here's an example of a device which can insert these timestamps http://www.adrielec.com/box28lit.htm

A:

Often, cameras can pick up infrared, so if a visible-light LED is out of the question (i think it could be quite dim and unintrusive), IR LED might be another option.

