If you've a sharp eye, you may have noticed that when you play back the recording from iShowU that it doesn't always have the same rate that you specified. This is intended behavior and not a bug. Let me explain...
There are many factors that influence frame rate, including:
- Machine speed (CPU)
- Disk speed
- Existing machine usage (what the programs you're recording are doing, using CPU? Using Disk? Both?)
All of these conspire to make getting *exactly* 25fps (or 23.98 or 29.97 - etc) impossible. Modern programs run in such a way that they can be stopped or pre-empted at any time. Thus under higher-load situations, iShowU may not get the time (CPU time) it needs to get frames at exactly the rate you specify.
What iShowU does is a "best effort" at getting the frame rate of your choice. If it misses getting a frame, it extends the duration of the frame in the movie to cover the missed part (it's always done this, right from about v1.13, 3 years ago). Normally this is imperceptible but if you're using Final Cut, then you may run into problems.
The upshot of all this is that if you're NOT using FCP you never really run into any problems (at least, in my experience). If you ARE using FCP then you need to re-compress your footage using either QuickTime 7 or Stomp (or a similar video recompressor). This is so that you generate frames at exactly the rate you want (which can be done easily when recompressing).
For more information about variable frame rates please see:"Variable Frame Rates Explained"