24.975fps vs. 24fps (Telecide with guide=3)

[helge79]

I am ripping an NTSC DVD that has apparently been mastered from a PAL source.

After I apply Telecide with guide=3 and Decimate(6), I end up with a clip playing at 24.975fps. I have a strong compulsion to speed it down to 24fps (together with appropriately resynching the audio, of course). My hope is that it would precisely restore the appearance of the original film material. The only thing that bothers me is that I have not seen many rips at 24fps (they are all at 23.976, 25 or 29.97). Is there any specific reason why people do not return all those videos to 24, the framerate at which they were originally filmed (undoing the PAL speed-up would be one example)? I couldn't come up with anything myself, but I thought I'd better ask.

Thanks for your comments!

[Mug Funky]

well, 23.976 is close enough, even if you have perfect pitch (which jack all people have, otherwise more people would notice the chipmunking in PAL movies). also, transcoding the audio lossy-lossy is not a nice thing to do...

if you're storing your audio as lossless and your playback device (computer?) supports pure 24fps, then by all means slow it down to that. :) also, if you intended to transcode the audio anyway (rather than retain the ac3 or mp2 audio), then changing the speed will not hurt it.

[dbzgundam]

I don't know the technical reasons as to why, but 24fps can't turned into 29.97fps (It would give you 30fps)... I'm much too afraid to mention 3:2 Pulldown, or duplicating frames without getting severely corrected by some of the experts. ;)

[Guest]

I don't know the technical reasons as to why, but 24fps can't turned into 29.97fps (It would give you 30fps)... I'm much too afraid to mention 3:2 Pulldown, or duplicating frames without getting severely corrected by some of the experts. ;) You mentioned it, so be afraid!

Of course 24fps can be turned into 29.97fps. You can do it with DGPulldown. The trick is to use irregular (generalized) pulldown, instead of 3:2 pulldown. It's legal and valid MPEG2.

[dbzgundam]

Aha! Thanks for that little tip, but of even more concern to me is...

Why 23.976fps to begin with anyway? ;)

Film IS straight 24p, right? What exact process creates a 23.976fps output?... I read somewhere that a transfer is made by slowing down to 23.976 and then doing the pulldown to 29.97... I'm not sure how correct that is. >>

[Guest]

I read somewhere that a transfer is made by slowing down to 23.976 and then doing the pulldown to 29.97... I'm not sure how correct that is. That is correct. It's driven by fact that NTSC specifies 29.97 fps.

[mg262]

If I remember correctly, the strange frame rate is an artefact of the fact that colour transmission had to happen in such a way as to be backwards compatible with the existing black and white sets... the colour was stuffed into a weird and unholy frequency band where nothing was originally intended to go, and the 24/30 had to be changed to 23.976/29.97 to prevent interference. I could be making that up though...

[Mug Funky]

actually, i'd like to know why the speed change, because no such change happened for PAL land (PAL IS SUPERIOR, YO:)).

are there any video historians in the house?

[mg262]

Read the middle of this, guys:

http://paul-lehrman.com/insider/2001/08insider.html

Edit: One line explanation of beats for that link: if you add together two signals which are very close in frequency, you get something whose amplitude varies at a much lower frequency than either of the original frequencies:

http://www.sfu.ca/sonic-studio/handbook/Graphics/Beats.gif

And that lower frequency could well be in the human-perceptible range. Incidentally, this is how tuning forks work.

[helge79]

if you're storing your audio as lossless and your playback device (computer?) supports pure 24fps, then by all means slow it down to that. :) also, if you intended to transcode the audio anyway (rather than retain the ac3 or mp2 audio), then changing the speed will not hurt it.Thanks for the comment. Yes, I intend to recompress it to XviD+mp3 and watch it on my PC. Then, according to what you're saying, slowing it down to 24fps does not have disadvantages of any kind.

But then, I still do not understand why most people do not do it. Like, why would anyone ever leave a PAL movie at 25fps when recompressing it to MPEG4? Are they just lazy, or do they not care, or is there some subtle yet noteworthy reason that I am missing?..

[stephanV]

Im just lazy, resampling the audio just requires too much work. :)

And the "chip-munking" (as it is refered to previously by someone) is not THAT bad. The difference is 12*log2 (25/24) = 0.7 of half a tone. Wowie!

On the other hand, playing it at PAL speed saves you 4% of the time. Imagine what you could do with all of that spare time. ;)