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I have a Windows program that uses the Apple QuickTime API to play video. On Windows 7, CPU usage is 100% on one core, which I believe is why the playback is choppy. If I turn on XP compatibility mode for this program, the CPU usage is around 20% of one core, and playback is normal.

Using a profiling tool called Very Sleepy (http://www.codersnotes.com/sleepy), I was able to narrow down the high CPU usage to a function in the QuickTime H.264 decoder called JVTCompComponentDispatch.

I can't imagine why there would be a difference in CPU usage when XP compatibility mode is turned off or on. Any ideas?

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Most likely, it's able to use your display processor to do some of the decoding in compatibility mode but forced to do software-only decoding otherwise. To know why that's happening, you'd likely need to know more about your hardware configuration and drivers and how QuickTime was implemented internally, which probably isn't realistic unless you work at Apple and wrote the code.

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Thanks, Nicole. That does make sense. I should also mention that the Apple Quicktime Player plays fine, and it uses the same DLLs as my program. What could be causing this difference? –  user858518 Oct 18 '12 at 0:21
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