Adding frames at a variable frame rate

[Miticoval]

Hi!

I'm creating a video frame by frame, so far so good.
But I'm trying to add frames not all at once, but time by time as soon as I receive them from the camera.
However these frames arrive at a variable rate, so I'm trying to add the new frames using:

_mediaBuilder.Video.AddFrame(bitmapData, _totalTimeElapsed);

That kinda works, but when I playback the video it plays really really slow.

First question: is there a way to declare that the video is going to use a variable framerate?
Second question: is it correct to set the parameter "duration" for every frame as the total time elapsed from the beginning of the recording?

Thank you very much!