Video not properly synced to data in composite video export

I recently made a video of a trebuchet, with a PocketLab in the weight basket.  I had the PocketLab set to record the three-axis accelerometer at maximum sampling rate.  I recorded a test firing using the iPhone PocketLab app on an iPhone 6.

My apologies for the video quality; this was an early test and I didn't worry about lighting or anything.  However, you can clearly see that the bump in acceleration is not synced to the trebuchet's motion, and the data stops before the video.  Also, the graph does not extend all the way across the screen.

It looked correct (synced, and with the data across the screen) while I was recording it.  The problem was only visible when I saved it to the iPhone (which I later exported to YouTube).

Let me know if you need me to test this under other conditions.

Original Post

Hi Joel, 

Thanks so much for your question! 

Something odd does seem to be going on with your video sync. It doesn't appear to be synced and the graph doesn't scroll across the entire screen. I haven't seen this error before.

Was the video sync accurate before you export by selecting "Save data combined with video"? Was the data scrolling across the entire screen before exporting?  

Would you mind trying another quick test? Try doing a really short video using the acceleration graph. Just move the PocketLab back and forth a couple times. 

Let me know the results of your second test and I can do some investigating on my end. 

Thanks so much, 


I'm so sorry it took me so long to respond!  I wrote a response last week, but forgot to click "Post Reply".  

It seems that the problem is triggered by turning off the PocketLab during the video render process (which I did to save its battery once the experiment was successfully captured).  The export will fail, but a subsequent export will exhibit the problem shown.

I've made a video showing the problem, and a video of the steps used to make the first video.

Here's the steps to reproduce:

  1. Optionally, set the sample rate to high.  This isn't necessary, but does make more dramatic effects.
  2. Activate camera.
  3. Record experiment.
  4. Tap Share, Share Video Combined with Data, Proceed.
  5. While the render is running (spinning circle), turn off the PocketLab.  (A dialog saying "PocketLab Disconnected" will pop up, but you can't interact with it during the render.)
  6. Wait for the render.  Note that the graph displayed doesn't extend all the way across the screen.
  7. Once the render is done, click Okay to dismiss the "PocketLab Disconnected" message.
  8. You will not have a chance to export the rendered file at this stage; the following directions re-render it for export.  However, at this point, just pressing "Play" will show the issue.
  9. Tap Share, Share Video Combined with Data, Proceed.
  10. Wait for the render.
  11. Tap Save Video.

After I finished making this, I made some observations that suggest that it may not be necessary to render the video; it may be sufficient to turn off the PocketLab and immediately press Play.  I have not verified this, though.

For your consideration, the most boring films you'll see all week:


Hi Joel, 
Thanks so much for your detailed response and for taking the time to create these videos! These are very helpful. The PocketLab team also very much appreciated the music on the second video! We were mesmerized. 

In terms of the bug, this makes a lot of sense. Thanks for pointing this out. The video function works by overlaying the graph image with each frame of the video. When there are disruptions to that process, it can cause issues. For example, if you change the orientation of your phone during the rendering, and the screen flips between portrait to landscape, sometimes the aspect ratio on the screen will be off. 

We hadn't seen this particular bug before though, so thanks for finding it. I followed your steps and was able to reproduce the bug. I also tried disconnecting the PocketLab after the recording but before the export/rendering, but it still produced the bug. For now, I would just say if you want to save your videos, make sure the PocketLab is connected until the video finishes rendering. The affect on the PocketLab's battery would be minimal. We will look into the bug in more detail. 

You can watch the video in the app itself after the PocketLab disconnects, like you described. However if you want to save the video, or watch it in slow motion, you would need to export/render it. 

Let me know if you have any other questions! 

Thanks so much, 


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