Why IVTC???

[miahallen]

So, I read the sticky at the top concerning IVTC, and I read the attached guide/tutorial...but one question still remains in my mind. Why do it? Why not just leave the material in whatever form it comes off the DVD in, is it a matter of quality? And for those of you working toward displaying these movies on an NTSC TV, wouldn't it NEED to still be in an NTSC compatible (read 29.97fps) format?

For instance: If the movie producers had to perform the telecine operation on their film for the DVD authoring process (I interpreted this correctly right?)....in order for the movie to properly display on our NTSC TVs. Will it still display correctly if we undo the proccess through IVTC?

Now I also realise that many more modern TVs (read HDTVs) are capable of IVTC internally, and therefore should be well suited to a 23.97fps source. So am I safe to assume that most people into this stuff have one of these newer display devices? Or do you encode these movies with the intention of playback on your PCs?

[scharfis_brain]

[ ] you've read the IVTC-guides thoroughly
[x] you do not know, how tough it is to compress interlaced (combed) video.

[manono]

Hi-

Since you're posting in the GKnot forum, then I'll assume the intended output is AVI. Let's also assume that you're encoding something that was originally a movie, playing in a theater at 24fps (as opposed to video, originally 30fps).

scharfis_brain laid out one of the most important reasons why you IVTC and make it progressive 23.976. Interlacing takes many more bits to compress for the same quality, as compared to compressing progressive material. In addition, by encoding to 23.976fps, as opposed to 29.97fps, you're encoding 20% fewer frames, spreading the available bits among 20% fewer frames, resulting in higher quality for the same file size (or the same quality with a smaller file size). Most (but not all) people are making their AVIs for a fixed file size, like 1 or 2 CDs, so we use any way we can to achieve the same quality with a smaller file size. And as that tutorial says, ForceFILM in DVD2AVI (DGIndex) when you can (progressively encoded on the DVD), and IVTC when you have to (either not entirely progressively encoded, or hard telecined (interlaced encoding) on the DVD).

If the movie producers had to perform the telecine operation on their film for the DVD authoring process...

During the encoding, the flags are set to output 29.97fps, even though the actual images might (and should) be stored at 24fps on the DVD. The telecine (conversion of 4 frames to 10 fields) takes place in the player, prior to output to the display.

And for those of you working toward displaying these movies on an NTSC TV, wouldn't it NEED to still be in an NTSC compatible (read 29.97fps) format?

No. Although a progressively encoded AVI will certainly look better on a progressive display (computer monitor, or HDTV), plenty of people are playing their AVIs using the video out connection on their computer graphics card to a standard interlaced TV set, or through a standalone DVD/MPEG-4 player, also to an interlaced TV set. And they're playing Force FILM'd or IVTC'd 23.976fps progressive AVI.

Now I also realise that many more modern TVs (read HDTVs) are capable of IVTC internally,...

Well, yes and no. Very few Progressive Scan DVD Players or HDTVs can perform a true IVTC. Most are pure flag readers, and do pretty much what DVD2AVI's Force FILM does. But if the DVD wasn't encoded progressively to begin with, or has places where it drops to video (all to common with anime, for instance, and even many Hollywood film DVDs), then about all flag readers can do is to deinterlace, with the resulting degradation that will produce.

[niamh]

I find the use of the "interlacing" word applied to IVTC confusing for newbies, as opposed to "combing"

For you two it's obvious, but I still remember when I first dived into IVTC (and still many praises and thx to your great guide manono, hakko and jggimi), the things it took me a little while to figure out (PAL people can be thick):

-Telecined stuff is progressive, before or after decombing
-IVTC is not deinterlacing

resulting quandary: for example, manono, in the very same post above, you mention:
Interlacing takes a many more bits to compress for the same quality, as compared to compressing progressive material

and then:
then about all flag readers can do is to deinterlace, with the resulting degradation that will produce.

If the difference between both meanings for the same word isn't clear in one's mind, then it's hell to figure out :p

I hope you don't think I'm being smart(I'm not), but you know your stuff so very well, it's easy to forget what happens when you're clueless :)



Of course miahallen has read the guide, but his questions lead me to believe he hasn't got totally the hang of the process yet :)

[neuron2]

That's why Decomb is called "Decomb". :D

[miahallen]

Some of my knowledge remains from a few years back when "progressive scan" DVD players and HDTVs first hit market, I was in retail sales, and had a resonable grasp on the basics. But this stuff is definitly taking it to the next level. However I do follow what you're telling me, so thanks.

With that being said, the compression factor was a good enough explanation. But thatnks for the elaboration.

[jggimi]

When video material is truly interlaced - 59.94 fields per second NTSC, or 50 PAL - each field is recorded at a different point in time. True interlaced material is the type of content you may not want to deinterlace, if your display device is interlaced.

But material that has been Telecined can be returned to its original progressive frames.