I'm using CS4 media encoder, and I am really impressed with the F4V codec (is it called codec for flash video?). I'm curious about the data rate adjustment in relation to frame rate.
My question is, I put a video clip thru the encoder to get an f4v file. When the variable bit rate is adjusted down, the file size goes down correspondingly. However, I can go from an NTSC frame rate of 29.97 down to 15 fps, and there is no file size reaction. That's lowering the number of frames per second by half, yet it has no bearing on the end size of the f4v file. How does that work, why is cutting the number of frames in a project in half not affecting the size of the end result?
Same can be said of frame size, it can go down from ntsc native 720x480 to say 400x366 and there is no file size reaction.
If you assign data rate as bit per second, changing the frames doesn't change the number of seconds. You simply have the same bits spread out amongst fewer frames. It does mean more bits per frame so the "quality" of each frame might improve.
Of course the temporal resolution gets worse (motion is not as smooth at lower frame rates).
Actually since compression is both within and between frames, while there may be more bits per pixel per frame when you lower the frame rate, there is greater distance between (the pixel has moved farther) when there's fewer frames so the codec is seeing more motion (therefore greater pixel difference) from frame to frame having less redundancy to toss out.