Audio/video going out of sync over time in iMovie

[BanjoDudeAhoy]

Hello!
I'm not sure if this is more of an audio or video forum question but since the outcome is a video I'll ask here.

I've got a video of myself playing the guitar, recorded on my iPhone. While I was recording the video, I was also recording my guitar along to an mp3 track in GarageBand via an audio interface.

I exported my recording from GarageBand as mp3 and combined it with the video from my iPhone in iMovie. Then, I lined up the start of my playing and the start of the song correctly - but near the end audio and video are noticeably out of sync.

Can anyone think of a reason?
Is there anything I've done wrong somewhere along the line?

 

[bogdanw]

Try to re-encode the video from the iPhone, then combine the resulting file with the mp3.

 

[Dave Braine]

Try adjusting the speed of the video in iMovie. An increase/decrease of maybe 1 or 2% may solve the problem. I have done this many times to get a video and music track to end together.

 

[BanjoDudeAhoy]

Thanks, both of you.
I’ll give your suggestions a go.

Interestingly, this worked out of the box in Resolve - but it did say that the frame rate of my video wasn’t the same as the Resolve project. I wonder if it was that…

 

[bogdanw]

A video recorded with high frame rate (>60-fps) on an iPhone can show in iMovie with different speeds.

 

[cthompson94]

Could it be because to my knowledge iPhones do not record at a constant frame rate so maybe Resolve could recognize that it was an iPhone video and adjusted accordingly, but if that is the case...it would be weird if iMovie cannot do that.