How does OBS capture the desktop at a particular framerate?

[IcyAlmond]

I'm trying to use DDA on windows (AcquireNextFrame) to get frames of the screen, but I'm struggling to get it to reach a particular set framerate.

Currently if I put it into a loop, it just gets frames as quick as possible. Even if I calculate a "wait" timing like in Nvidia's example here https://github.com/NVIDIA/video-sdk-samples/blob/aa3544dcea2fe63122e4feb83bf805ea40e58dbe/nvEncDXGIOutputDuplicationSample/main.cpp#L351 it just seems to set a limit on how slow DDA will be in replying with a frame, it seems to imply that if things on screen are moving it will do nothing to limit how fast a frame can be returned, hence it captured at 150fps even though the timeout I gave to DDA was always under 17 ms (for 60fps 16.66ms)

I've also tried sleep_for but it doesn't seem consistent enough? i got about ~40fps when i aimed for 60fps.

how does OBS get an accurate capture rate using dda?

and how does the replay buffer mode work? I understand that it encodes to a memory location and then saves it to disk when a button is pressed, but once the buffer is full, how does it overwrite old data in a way which allows the whole buffer to still be a valid video stream?