Syncing camera to Velodyne Lidar's Pulse Per Second signal
I would like to resolve time differences between camera frames and external Lidar frames. I could do this if there were a way to include in the camera's metadata the state of the SYNC hardware input pin.
By using a one-shot to convert an external PPS (Pulse Per Second) signal (a common output on GPS modules for 1us syncing) to a 1 Hertz square wave, we could create a unified timestamp that allows us to keep track of the timestamp of any frame from an external device. This method would have about 20ms of jitter but frame times could be better resolved over a period of several seconds by dynamically stretching the PPS pulse width to find frame edges.
What would be better would be to have a frame timestamp metadata entry that was labeled "microseconds since last (falling or rising) edge of Sync pin)". This would allow us to create a microsecond level accurate timestamp of all frames from various devices regardless of whether or not they are hardware-frame-synced together.
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Could you re-post your request to the RealSense GitHub forum please as a 'Feature Request'. Doing so allows the RealSense developer team to officially track the request over time. You can do so by visiting the link below and clicking the 'New Issue', button, indicating in your message that it is a Feature Request.
https://github.com/IntelRealSense/librealsense/issues
It is certainly possible to sync between a RealSense depth camera and a non-RealSense camera (e.g a StereoLabs ZED depth camera) using system time, frame number and hardware timestamp.
A user of a ZED camera who was doing such a sync said that "it did seem that comparing the TIME_OF_ARRIVAL timestamp with the ones from ZED frames, I could sync the two outputs with ~70msec difference (~1 frame@15FPS)".
Edit: I should also mention that Intel has a new RealSense lidar camera called the L515 launching in the week of April 27 2020. Like the 400 Series depth cameras, it supports multi-camera sync and will have support for 99% of the RealSense SDK's features at launch.
https://store.intelrealsense.com/buy-intel-realsense-lidar-camera-l515.html
Edit 2: Information on frame metadata can be found in the RealSense documentation page below if you have not seen it already.
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