This is a quickie demonstration of an audio sync issue with a Blackmagic Intensity Extreme and AVFoundation. It's the base case as far as I can tell. The audio lags quite a bit behind the video, and the video has an artifact at the beginning of the recording session.
Although it's not demonstrated here, I see identical results with a custom setup that uses sample buffers via data output delegates.
Run the ./bm_problem_demo:
./bm_problem_demo 'Blackmagic' 'Blackmagic Audio' '720p - Uncompressed 8-bit 4:2:2' '59.94 FPS'
The arguments are the video device, audio device, video format, and video frame
rate. FPS is written as 59.94 FPS or 60 FPS. The others are the names that
the AVFoundation driver reports.
The sample output will be in ~/Movies/output.mov.