I’ve ran across an interesting issue and I’m curious if anyone knows why and how it happened… and maybe ways to prevent it in the future. I filmed a short interview on Sunday, about 18 minutes long. I just did one continuous take, stationary camera—just about as uncomplicated as you can get. For audio, I used a Tascam DR-60DmkII with a shotgun mic, and an iPod using Apogee MetaRecorder with a lavalier mic attached. The audio from each device came out perfect. The video came out perfect too. It’s kind of hard to mess-up an easy set-up like that. The problem comes in post. I’ve imported the 18 minute video and audio into FCPX. I dragged the clips into the project library, got them in sync via the clapboard and started watching. Then, around 4 minutes and 20 seconds the audio starts falling out of sync with the video—ever so slightly. So, I scrolled close to the end of the footage and played from there and found and literally everything was out of sync. The two audio tracks were out of sync with each other and the both were out of sync with the video. How does that happen? All audio and video was recorded at the same time with no problems, and it's all there (no gaps or anything missing). I can fix it easily since the audio will be used over different footage in some places and you won’t always see the interviewee talking. I’m really just curious how it happens and how to prevent it in the future.
Post-Production : Audio and Video Strangely Out of Sync? by Christopher Dyer
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It sounds like your sample rate of the audio and what the timeline is set too may be different. Sample rate (SR) is the audio equivalent of videos frame rate, and if it's recorded one way and played back another you will get that phasing issue. Check your raw audio files to find its SR (usually they are 44.1Kz or 48Kz) and compare it to your timeline settings. Adjust the timeline if needed.
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Most likely Kevin is right on the nose - but if that's not the case, make sure your framerate is set properly on the wav file coming in, it is stored on the wave header (meta data) information (This is highly unlikely as the covert usually happens properly). I am not sure with FCPX (as I am an audio guy in PT), but sometimes there is frame rate that gets applied could be screwing you up. Specially if it takes that long for the video to go out of sync.