I can confirm that iPhones do this too. When I did my recent tour of the Modor NF-1m, I used the iPhone’s microphone to record my voice (worked well enough) so my voice was in sync with my actions; and I recorded the synth into a simple audio program on my Mac. I was able to get their starting point sync’d, but the synth’s audio slowly lost some of its synchronization throughout the video.
It wasn’t enough to ruin what I was trying to show in that particular one. I just had to shoot it that way as I wanted to get it done and I still didn’t have a working mic solution at the time.
I’m going to check out that recommended ‘Filmic’ app.
But I just wanted to confirm that the dynamic frame rate exists in default iPhone setup too, making it harder to sync external audio files.
It was your comments, Gir, on another thread that prompted me to watch out for this!