[mythtv-users] Re:MPEG-2 to MPEG-4 Transcoding comparisons

[James L. Paul]

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On Thursday 30 October 2003 04:54, Ben Curtis wrote:
> > I wonder... Is the quality of a 3.7G MPEG2 file transcoded to 2.2GB
> > MPEG4 better or worse than just encoding the original MPEG2 at 2.2G in
> > the first place? I have trouble believing it's better quality and worth
> > the effort, unless for some reason you really want the results as MPEG4.
>
> Does mythtranscode use variable bitrates and/or multipass for encoding the
> MPEG4's?  If so, then a high quality file transcoded to low quality will
> look better than a low quality capture.

True, that's why I was asking the question regarding those specific sizes. A 
3.7GB MPEG2 is plenty for _great_ video quality. It's common for commercial 
movie DVDs to use that for a 2 hour movie at 720x480. For 352x480 it's 
stunning quality.

The reason I asked is because I use 1.8GB/hour for MPEG2, and I find the image 
quality to be comparable to SVHS. Transcoding certainly isn't going to 
improve it, so I don't see why to go to all the effort and CPU just to make 
it smaller. You would only want to do that if your MPEG2 2.2GB capture was 
lacking in image quality, and frankly I have a hard time believing a MPEG4 
2.2GB transcoded from a 3.7GB MPEG2 will look noticeably better than an 
original MPEG2 2.2GB. The only reason I see to transcode from MPEG2 to MPEG4 
is if your hardware only does MPEG2 or does it poorly, and you have a 
specific reason to need an MPEG4 result.

> This is because the transcode can lower the bitrate during slow scenes and
> speed it up during fast ones, then keep an average rate of 2200.  I'm not
> sure if mythtranscode is this advanced though, as I have not yet gotten it
> to not segfault.

I believe the defaults for both MPEG2 and MPEG4 include VBR. My MPEG2 streams 
coming out of my PVR-250 are VBR (notice that you specify the bitrates with a 
'peak' value? ;) I didn't even realize MPEG4 could do CBR, but I very rarely 
use it since the only hardware I have to play MPEG4 are my computers, and 
that's not where I watch my video. :) I don't know if mythtranscode uses 
multipass, but 2-pass can give great results if it's available.

Regardless, it's great that we have such good options. When I find a way to 
remove commercials from MPEG2 natively on Linux, I'll have it all! :)

> Ben
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