Saturday, November 05, 2022

Processing slow motion GoPro video in DaVinci Resolve 18

Got out to the shooting range today with the GoPro Hero 9 Black and captured some 240FPS 1080P video.  Now, all the SlowMo folks on Youtube note that the audio is 'fake' - well, you can actually use a couple of tools to get the audio and video sorted out with slow motion playback of your high framerate video! 

First, in DaVinci Resolve, set your project and timeline to 24FPS:

Next import your clips from your GoPro, and then change the clip attributes to 24 FPS! Yes, this is going to make some very long clips - but they are playing at 1000% slower than realtime.  Set your in and out points, or just drop the clip on the timeline.  Unlink your audio and video by right clicking on the clip, and unchecking the 'Link Clips' bit.
Now you have video, but no audio.  What good is that?  Fine if you just want to lay in a music background track, but if you were doing like I was - shooting with distinct reports you want matching the muzzle flash - you need to stretch out your audio.

In comes Audacity to solve the problem.  Open your MP4 file from the GoPro in Audacity.  Yes, it's an audio editor.  Yes, it opens video files just fine.  You'll see the audio track waveform across the screen.  Now, just a simple retime and export and you have audio to match your video!

Since you are going from 240FPS audio to 24 FPS audio, you'll need to slow it down by 90% - or 0.10 of original playback speed as such:

Export the audio (File - Export) as a WAV, saving it in your source video working directory.  Drop it in the media pool and then drag it to the timeline.  Now you have video and audio at 24FPS!  It will take a bit of sliding the audio file around to match it exactly to the video file; hopefully you have a marker (a clap, a gunshot, something loud and visual) to sync the audio waveform up to the video file. Once synced, you can re-link the new audio clip to the video clip and go about your editing!

Mind you, it's not 100% perfect - you may find the audio drifts out of sync with the video over time, so you will have to make a cut, unlink and readjust the audio, and relink it once re-synced.  I'm sure there are more reliable and repeatable processes out there, but I wanted to share what worked for me tonight. 

With no further ado - a couple of brief videos made from high framerate 240 FPS GoPro footage slowed to 24FPS capturing muzzle flash from a handgun.


Enjoy the process!  (I've spent almost 5 times longer editing this than actually shooting the video!)

- Will