I'm actually using raspberry pi3 with the camera module v.2 to acquire (digitalyze) some old super8 films. The film projector runs at 18fps. I grabbed off the projector's lens and inserted a camera module inside the lens hole.
Everything works quite well except when I'm digitalyzing a film with sound. I'm recording the sound with an external recorder (Zoom h1 mic) and then putting video and audio together inside a NLE video software.
The problem is this: it seems that raspivid captures video with a variable framerate so, even if the audio and video streams durations are quite the same, when stitched together, they are out-of-sync and if I re-syng a part of the video, other parts go out-of-sync.
What am I doing wrong?