trying to figure out fps and audio

[Rosey]

ok, im trying to wrap my head around this. please, feel free to correct me at any time......

ok, a typical ntsc avi is encoded @ 23.976 fps, becuase you can drop the additional 6 fps and your eyes wont notice and that saves room for higher bitrate frames. a pal movie is 25 fps. finally, audio is always (majority) 75 fps and its the timecoding that changes during conversion.

now, if i take that ntsc 23.976 fps avi and make a dvd compatible mpg, it must be 29.97 fps. the audio will play fine with this since all that happened is some video frames were duplicated, but 1 second will always equal 1 second. the video is still the SAME length, it just has more frames squeezed into the same amount of time.

so, i think im pretty clear up to here(maybe not). but,if i take a 25fps pal movie, and change the fps to 29.97 this actually speeds the movie up causing it to be shorter than the origional also throwing the audio out of synch. (can you tell ive been fighting to correctly convert a pal movie?)ok, pretty much im lost after this...

why wouldnt the change in video for pal to ntsc just cause duplicate frames ie the ntsc 23-29 fps conversion? ok, ill quit here....

[KpeX]

Point #1: Audio does not have a framerate. This is one of the most annoying things I read every day. When people talk about changing the framerate, they are not changing the framerate, audio doesn't have a framerate, what they are doing is stretching or shrinking the audio to match a video framerate conversion.

Point #2: NTSC Avi's are 23.976 only in the case of film (movie) material, movies are shot at 24 fps commonly, but since NTSC dvd standards require 29.97 fps, flags are added to make the video appear to be 29.97 fps, this process is called Telecine. If one encodes an NTSC dvd with these flags in place, duplicate frames and wasted bits will occur. Thus the user implements some kind of IVTC (InVerse TeleCine, undoes the telecine) to eliminate frames and restore a 23.976 framerate, which is very close to 24. No original frames are removed in the process. Pal movies also originate at 24 fps, I'm not sure what most pal users do, I'm assuming the difference between 25 and 24 fps is not a significant quality reduction. more info here: http://www.doom9.org/ivtc-tut.htm

Point #3
I don't have much expertise with Pal to NTSC conversions, but if the video is sped up or slowed down, you will have to do the same with your audio, which most people would call a frame rate conversion. I don't know of any way of adding duplicate frames to a 25 fps stream to make it 29.97. I would think the best way to convert would be to slow down to 24 or 23.976 fps and then telecine to 29.97 using pulldown or something similar. hth,

[manono]

Hi-

why wouldnt the change in video for pal to ntsc just cause duplicate frames ie the ntsc 23-29 fps conversion?

It does, if you do it right. You stick AssumeFPS(23.976) into the .avs script, and then run Pulldown on that after the encoding. That sets the flags to putput 29.97fps. That is, only 23.976fps is stored on the DVD, but the software tells the player to output the extra duplicate fields to make the thing play as 29.97fps. This is a much better solution than changing directly to 29.97fps. The audio does have to be adjusted, as the video now plays about 4% slower than it did when PAL. But BeSweet can do it easily enough.

KpeX had it right, but I thought I'd elaborate a bit more.

[Rosey]

when i used besweet (ac3machine, if that makes a difference) to convert a six min pal audio to 29.976 fps it would slow down...then speed up....and almost sound like a skipping cd....i thought i might have dont something wrong but after reading all this i went back and tried it again and its the same thing.
thanks for all the great answers btw.

[manono]

Hi-

You don't convert the audio to 29.97fps. You convert it to 23.976fps. BeSweet has a Preset for that. "PAL->NTSC 25.000->23.976". Or you can use AC3Machine's "Change Framerate from 25000 to 23976".

[Rosey]

hmmm....i believe you that the 23.97 fps works better, but besweet gui wizards also has a pal to ntsc conversion of 25 to 29.976, and since my video was going to be 29 i thought my audio should be too...just seemed to make sense as i was choosing options. ill have to try the other way now.

btw, it sure would be nice to have some sort of dvd emulator. as in, an incorrectly produced dvd or created movie is much more likely to play fine on your computer than it will -in the standalone. anybody know if something like this exists?

[Hiro2k]

Originally posted by Rosey
btw, it sure would be nice to have some sort of dvd emulator. as in, an incorrectly produced dvd or created movie is much more likely to play fine on your computer than it will -in the standalone. anybody know if something like this exists?

Well you could always create an ISO file and then mount that with Daemon Tools to see if your DVD Player can read it.

[Rosey]

well, i dont think that would prove anything. i can screw up royally with say....the videos dimensions, enough so that NO standalone dvd player would touch it; that same dvd is going to play just fine in a computer.