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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?

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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.

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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.

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  • \$\begingroup\$ The logger is used for sleep recordings, so a bright blinking led could be annoying for the the patient. I thought of a similar scheme: let the recorder beep, and match this with the camera's audio-signal. The beep should be high-frequency, so that's unhearable for human beings. The question is if the microphone still will be able to pick it up. \$\endgroup\$ – Muis Jul 26 '12 at 10:04
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    \$\begingroup\$ Use an IR-led. It is invisible to humans, but the camera still can pick it up. \$\endgroup\$ – suha Jul 26 '12 at 10:10
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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

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  • \$\begingroup\$ I assume this only works with analogue camera's? \$\endgroup\$ – Muis Jul 26 '12 at 10:00
  • \$\begingroup\$ Yes, I interpreted "standard camera" to mean analogue CVBS/RGB \$\endgroup\$ – Toby Jaffey Jul 26 '12 at 10:18
  • \$\begingroup\$ No, it meant it more like 'regular', as: available in every household. Sorry, I will edit the question. \$\endgroup\$ – Muis Jul 26 '12 at 10:34
  • \$\begingroup\$ Many lines in a frame are not used for the actual image. In Europe we have a system called Teletext that uses these lines. Check this Wikipedia article: en.wikipedia.org/wiki/Teletext#Technology \$\endgroup\$ – jippie Jul 26 '12 at 18:58

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