Originally Posted by gorthocar
I'm not entirely convinced it is true yet because I'm getting much closer to 23.976 with the disable-UAC trick. Unfortunately I get lip sync issues when I do this. With UAC enabled (or if I just disabled UAC and haven't rebooted yet), I see the duplicated frame every 42 seconds or so, but the lip sync is locked on.
I have some theory on why disabling the UAC would cause sound and video to be off-sync. I don't have full grasp on how they work completely so I might be wrong, but please read on.
I'm assuming the refresh rate generation is synchronized to a base clock in the PCH. It would require algorithm changes to allow 23.976 rather than simple 24Hz.
When I do something that makes the UAC message pop up, there is a bit of delay(Vista is way worse than 7 because sometimes the response took forever and it felt like the computer isn't responding).
(now I'm not saying the included delay should be noticeable by the user, but something that gave me the idea)
The UAC would constantly check whether the program was one that needed the message or not. The decision of whether it needed the message would have been made by the developers and Microsoft themselves, but I'm pretty sure it would be constantly checking. When Intel considered the video output to be 24Hz, they factored in the delay executing the UAC code required, on both hardware and drivers. Therefore when the UAC is turned off, the video output frequency changes, but the audio stays same, cause audio might not be part of UAC. Each frame rendered adds a delay, so over time the video and audio off-sync problem gets worse.