Pulldown

CClub wrote on 7/4/2008, 4:59 PM

This question arose during a recent discussion involving the new DVDA upgrade, and it’s a bit of a tangent, so I figured I’d start a new post: 1) You capture 24p footage via Cineform and you’ve already removed the pulldown. 2) You want to render a DVD via Vegas into a DVDA template so that it doesn’t re-render. The Vegas DVDA templates include pulldown. I asked this question on the Cineform forum, and David Newman http://www.dvinfo.net/conf/showthread.php?t=125295responded[/link] by saying, “DVD always has pulldown as there is no 24p profile. When 24p is added to a NTSC DVD, the pulldown is added back with the correct repeat flags so progressive DVD players work well. I don't know whether that control is to add the repeat flags and therefore should be on, or for detect 24p from 60i sources and therefore should be off.” Does anyone know the answer to this?

Also, I wasn’t aware that if I’ve removed the pulldown from 24p footage, that when I prepare the computer file to go onto a DVD it RE-INSERTS the pulldown. If you're prepping footage for a DVD, what's the point then in removing it initially if you're just going to have to reinsert it? Would this be the case for Blu-Ray also?

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Comments

Former user wrote on 7/6/2008, 6:37 AM

John, I am running test now, but I can't see how 300 frames of video could be rendered at the same fixed bitrate as 900 frames and end up being the same file size regardless of frames per second.

If I am wrong, which I probably am, I will post back.
See my next post. I was wrong.
Dave T2

John_Cline wrote on 7/6/2008, 6:42 AM

Dave, Bitrate is set in bits per second, not bits per frame. Of course, you could calculate bits per frame, but the encoder is always set in bits per second. (Or kilobits per second or megabits per second, but it's always per second.)

Former user wrote on 7/6/2008, 7:04 AM

Okay, I stand corrected and apologize for being wrong. As John Cline pointed out, I was thinking number of frames and not length of program.

Dave T2

johnmeyer wrote on 7/6/2008, 10:00 AM

I will absolutely guarantee that average bits per second is the ONLY thing that affects file size. I've done the test many, many times because I was as amazed as some who have already posted and couldn't believe it. Also, it bit me hard in a project I was doing 5-6 years ago when I first discovered this.

However, it is a common misconception and you will often find -- especially over at Doom9.org -- people discussing how they can test the quality of video noise reduction by measuring how much smaller it makes the encoded file. What makes them come to this incorrect conclusion is that they are using encoders which have a "constant quality" setting (see TMPGEnc, for example). In that case, reducing the random differences between frames -- which noise reduction will do -- enables a constant quality encoder to to get the same "quality" with fewer bits, i.e., it actually reduces the average bits per second and still gives the same visual quality.

GlennChan wrote on 7/6/2008, 5:12 PM

If you're talking average bitrate, then the file would be the same size regardless of the number of frames encoded. Bitrate is expressed in number of bits over a period of time, usually one second. One hour of video at a fixed bitrate would be the same file size whether it's 29.97fps or 24fps.
Oh wait a second... you'd be correct there.

But I think the point is that framerate does affect compression efficiency. That's what I saw as I left the encoder on VBR.

johnmeyer wrote on 7/6/2008, 6:13 PM

But I think the point is that framerate does affect compression efficiency. That's what I saw as I left the encoder on VBR.

Try it again using 2-pass VBR. As I mentioned in one of my earlier posts, Vegas' single-pass VBR is VERY inaccurate in how it allocates bits for encoding and doesn't actually come very close to achieving the average bitrate setting you specify. By contrast, the CBR or 2-pass VBR will give you dead accurate results which you can verify both by measuring the file size, as well as using a utility (like the latest G-Spot) that accurately measures the average bitrate by scanning the entire file (which takes time) rather than simply reading the file header (which almost always reports a number that has nothing to do with the actual average bitrate -- it's usually the peak bitrate).

TeetimeNC wrote on 10/28/2008, 7:16 AM

This thread from a few months back starts with a question about the pulldown in the default Vegas DVDA 24p template. It wandered off into some other areas and I can't discern a conclusion regarding the original question. My question is: if you are rendering to 24p for burning a DVD, when would you ever insert pulldown since the progressive scan DVD player inserts pulldown when playing to a 60i device. Was this intended for the old pre-progressive scan players? If so, why would it still be the default template for 24p DVD?

Jerry

johnmeyer wrote on 10/28/2008, 8:48 AM

You would insert pulldown if you were going to print to DV tape, for example.

TeetimeNC wrote on 10/28/2008, 10:07 AM

Makes sense John, but does it seem odd to you that this is the default 24p template for DVDA?

>You would insert pulldown if you were going to print to DV tape, for example.

Jerry

kairosmatt wrote on 10/29/2008, 7:08 AM

I'm away from DVDA now, but is it just adding pulldown flags and not actually adding the extra frames?
I guess this heart of the question.....

TeetimeNC wrote on 10/29/2008, 2:44 PM

I'm getting confused here. When I create a new project in DVDA 5.0 it asks me for the project video format. In the dropdown all the choices are 50i or 60i - no progressive. I seem to remember that i used to make progressive DVDs in an earlier version of DVDA. Is (or was) it possible to create progressive scan DVDs in DVDA?

Jerry

In an earlier post John Meyer said:
>I'm not sure I understand. Maybe it's just a matter of semantics. Clearly I can create a 24 fps progressive DVD that plays on NTSC DVD players. The DVD spec specifically provides for this, and as stated directly in the spec, this is how almost all movies are encoded

johnmeyer wrote on 10/29/2008, 5:38 PM

My question is: if you are rendering to 24p for burning a DVD, when would you ever insert pulldown since the progressive scan DVD player inserts pulldown when playing to a 60i device.

Another, probably better example, is if you have mixed media. You see this all the time, where a DVD has both film and video sources, all in the same program. For instance, a documentary on a performer from the 1960s who is still performing today will include film of his old performances, and video of his current performances. These are intercut, back and forth. The only way you can encode this is as 29.97. If you were preparing the 24p portions of this program and you wanted to render them out and then recombine them on the timeline, it would certainly make sense to include the actual pulldown frames.

When I create a new project in DVDA 5.0 it asks me for the project video format. In the dropdown all the choices are 50i or 60i - no progressive. That must be something new. In my older version of DVDA, it just asks for resolution. You can drop 29.97 or 24p files into the project, and it will encode each one correctly (the 24p with pulldown flag).

I don't think it is possible to create a DVD that contains 24p (23.976) video without the pulldown flag. Thus, you either add the extra pulldown fields by specifying 29.97 in the render as dialog in Vegas (making sure to NOT turn off "smart resample" in each event), OR you encode to 23.976 with the pulldown flag. If you encode WITHOUT the pulldown flag, DVDA will recompress and tell you that the reason for the recompression is that the "media is not a DVD-compliant file." Indeed, the DVD spec does not support 24p without pulldown. This is because in 1997 when the DVD players first hit the market, all we had for playback were NTSC (or PAL) TV sets, which could not display anything except 29.97 (or 25) fps interlaced video. However, in this day and age, most TV sets can display all sorts of frame rates and interlaced or progressive video. So, the newer DVD players can be told to ignore the pulldown flag, and you can watch your 24p (i.e., 23.976) progressive video.