Deinterlace MPEG2 streams without reencoding

[cosmin]

Hi,

According to the DVD FAQ, the 24->29.97 telecine in MPEG2 is done by embedding the repeat_first_field flag in the stream, without doing the actual 3:2 redundant pulldown.

My question is: are there any tools to hack the MPEG2 stream in order to go back from 29.97fps to 24fps, by resetting the repeat_first_field flag and changing the frame rate accordingly?

This looks appealing, because (1) no quality is lost whatsoever, and (2) it should be lightning-fast.

Best regards

[Poplar]

I think it's much more complicated than rtf flags. So many smart guys have worked on IVTC, but none of the solutions is simply stripping off those flags. And MPEG2 is inherently *interlaced*. You can change those flags if you get wrong field order. Restream can do it pretty fast.

[cosmin]

I think it's much more complicated than rtf flags. So many smart guys have worked on IVTC, but none of the solutions is simply stripping off those flags.

If you encode an already-interlaced source, coming from a telecined film, then yes, IVTC can be very difficult. Moreover, if you have an inherently interlaced source, such as home-made digital video, there isn't even an IVTC to talk about.

But if you start with a progressive 24fps source and encode it to 29.97fps, by setting repeat_first_field and not by physically replicating the fields prior to encoding, then it shouldn't be any theoretic difficulty to revert this back. repeat_first_field means exactly this thing: the repeated fields do not physically exist in the stream, but are supposed to be artificially reconstructed by the decoder. This is why VCD has 23.976fps, but SVCD and DVD don't have this frame rate: they don't need it.

I said theoretic difficulty, because I don't know if this was done in practice.

Thanks for the tip about Restream.

[Poplar]

By reading some discussions on deinterlacing, the problem is that many streams are mixture of progressive and video. How to correctly match fields in such streams is not a simple undertaking, certainly not automatic.
That's what I have to say right now. Gotta go to work.....

[cosmin]

If the stream is a mixture of interlaced an progressive images, the repeat_first_field is missing, so this case is irrelevant to my question.

But I found the answer to my own question: there is a program named pulldown that can modify the telecine flags and hence is capable to go back an forth between 23.976 and 29.97fps.

Again, that is, IF the repeat_first_field field is present in the MPEG source.