I’m not one to use the phrase “blew my mind” but … no, I’m not going to use it today either. But I watched this video and was .. shocked.
INTRODUCTION
The precis of the video is that syncing music videos to the audio track is not as straightforward as it sounds. In an earlier video, John Hess shows that physchoacoustics tells us that the audio beat should precede the related video event by a few milliseconds. So video editors usually cut audio so it’s one frame AHEAD of the video event. BUT in this video he finds out that YouTube shifts the audio so that it’s one frame BEHIND the video!
So, does this mean we need to cut audio TWO frames before video? and what about Vimeo? And does H.264 vs ProRes make a difference here?
I wanted to find out.
METHOD
I made 3 videos in DaVinci Resolve using a simple drum beat and some still photos. One had the video cuts exactly on the beat, on had the cuts with the audio one frame behind the video, and another with the audio one frame ahead of the video. I then rendered these into both H.264 and ProRes and uploaded all of those to YouTube and Vimeo. I’m not going to link to these videos (sorry) because I screwed up a couple of things and that messes up the results if you don’t know what I did.
RESULTS
Before uploading I had a look at the files that came out of DaVinci Resolve – using Reaper. The synced ProRes file look perfectly synced, but in the H.264 file, the audo was about 40ms behind the video – about one frame at 25fps. I don’t think this is exclusively a Resolve issue as Hess noted similar results with Premiere Pro.
On YouTube that particular difference didn’t seem to manifest itself. But the files where the audio and video where exactly synced, the audio looks clearly delayed to my eyes. On Vimeo there was a very slight delay but only really visible if you’re looking for it.
With the files where the audio was ahead by one frame, the YouTube videos looked much better. There was still a slight delay in the audio but, like the Vimeo above, it was quite reasonable. On Vimeo the files looked precisely synced.
Now this might be down to me using 25fps, I do this because I’m in the UK and that’s the TV convention here but I wonder (and haven’t had time to test this) whether 30fps might be better (or worse) on YouTube, it being American.
CONCLUSION
For best sync on both YouTube and Vimeo, cut your video so that the audio is one frame ahead of the video. In other words, when cutting video to audio, make your video cuts one frame BEHIND the beat. On YouTube (at 25fps) it’ll look fine, and on Vimeo it’ll look great.
Of course cutting video to audio is a subjective process and sometimes you want to cut across the beat – be creative folks!
All tests done on a a 2019 i9 Macbook Pro running DaVinci Resolve Studio 16.2.6 on OS/X 10.15.6, with 350Mb/s internet connection, and watched using Mozilla Firefox 79 full-screen after waiting for all online processing to complete.
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