thats because - it doesnt 'slip' if the audio and video are in the same movie.
quicktime keeps them together-
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if the audio and video are on separate layers - then thats different...and depends completely on whether your hardware can playback those layers without dropping frames-
typically with sata discs arent good enough- to keep long term stability-
if you get a machine with an SSD then you reduce the chance of dropping frames by many times-
If you are doing audio and video - i would very very strongly recommend the use of SSD's - to minimise any dropped frames caused by rotational latencies.
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in the most recent versions of catalyst- there is a smpte reader for the audio input - so midi timecode isnt required.