FPS question... how does AutoGK handle it?

[travisbell]

Hey guys, I am just getting into the swing of things with AutoGK and have found it pretty damn awesome so far.

I am wondering something about the way AutoGK handles fps. My source is 29.970 fps, and for my first encode I left the fps details in their default state. It encoded at 24 fps and seem to come out OK. Sooo... what I am wondering is what is happening to those 5 fps, and how do I not notice any kind of stuttering/chop like I usually do when lowering the fps?

I am obviously not familiar with what's going on, so I guess that is my main question.

Thanks!

[manono]

Hi-

If you check the log, most likely AutoGK found a high percentage of progressive frames, and performed IVTC/ForceFILM to remove the duplicate fields/frames. NTSC movies are supposed to be 24fps (23.976fps), but have to be telecined to meet the required 29.97fps output for DVD. If none of that made any sense, then don't worry about it, as AutoGK knows what it's doing.

what I am wondering is what is happening to those 5 fps

They were redundant, duplicate fields/frames, and were removed. It'll actually play more smoothly, with less chop or stutter, with them gone.

[travisbell]

Interesting, it does seem to play smoother! Thanks for the reply, makes more sense now.

While not related to AutoGK, why do HD streams play at 29.970 fps then? Aren't t hey NTSC?

Cheers,

[manono]

Hi-

...why do HD streams play at 29.970 fps then? Aren't they NTSC?

Earlier I said that 24fps films are telecined to 29.97fps because that's the DVD spec for NTSC. It's the DVD spec because the NTSC TV spec is 29.97fps. Actually it's 59.94 fields per second. 2 fields=1 frame. 2 x 29.97fps=59.94 fields per second. HDTV is different, because it only shows full progressive frames, so it's displaying a full 59.94 frames per second. I don't know for sure if that answers your question.

Oh, maybe this is it. If those HD streams were originally shot with film cameras (24fps), then they can also be IVTC'd back to 23.976fps, just as your DVD was. I have an HDTV, but don't receive HD broadcasts. Aren't they broadcast at 59.94 frames per second? What are you doing? Capturing them? Maybe every other frame is being dropped to give you 29.97fps. I've never captured anything in my life, so I'm out of my league here. jggimi, where are you? :)

[jggimi]

I'm right here. In NTSC-land, Standard Definition (SDTV) television is broadcast at 29.97 fps, but High Definition (HDTV) television is not. The HDTV standard, ATSC, allows framerates of 24, 30, or 60 fps. And the standard allows both progressive (such as 720p) and interlaced (such as 1080i) content.

http://www.hdtvprimer.com/ISSUES/what_is_ATSC.html

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Edit: Added this additional comment. 35mm film is projected at 24 fps. To put film into 29.97 fps video, a process called Telecining is used. AutoGK will first examine the flags associated with each frame to see if the there are Telecining marks, and if the original progressive frames can be assembled without examining the image content of each frame (we call this capability forcing FILM mode). If not, it will examine frame content to see if Telecining was performed on the content, and it will use an Inverse Telecine process if so. For a detailed look at how this works, see www.doom9.org/ivtc-tut.htm.

[travisbell]

Well that was certainly more info than I was expecting, but a good read nonetheless!

Thanks guys,

[fewtch]

Always better too much information than not enough. To me, it's a sign of an excellent forum when you ask something and get more than you bargained for :D.