[Matroska-devel] Re: Re: MPEG2 in MKV!

spyder spyder at matroska.org
Thu Oct 30 18:56:25 CET 2003

> I copied and pasted what you put in IRC.
> [00:11] <spyder482> here's the way it actually works according to mpeg
> docs: [00:12] <spyder482> [A][B][Cr][Dr] yields [Aa][Bb][Cc][CD][Dd] on
> decode

Then I was wrong :P

> > Bad idea to remove the pulldown.  MPEG decoders expect constant framerate
> > and you can't change the framerate to 24fps becasue you would need to
> > drop some frames which would break the encoding scheme of mpeg2.  (ie,
> > removing data referenced by other frames)
> We just need to change to framerate as specified in the Sequence headers.

No...then you introduce sync errors.  Not all files will be played in crappy 
DirectShow filters.  MPEG video has a framerate for a reason, it's not just 
informational.  If you change the framerate but leave more frames then you 
intorduce sync problems.  If you drop the extra frames then you break the 
stream.  Besides, we shouldn't have to destroy any mpeg compliance just to 
put the stream in MKV.  As I said before, the ONLY thing allowed to change 
from sequence header to sequence header is the QUANTIZERS!  You can't change 
framerate, resolution, colorspace, progressive vs. interlaced etc..  Only 
those 128 bytes for the quantizer are allowed to change.  Otherwise the 
stream is b0rked and no longer spec compliant.


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