AVI2DVD CBR vs VBR

[WhipHubley]

I'm interested on others' opinions upon the issue of CBR versus multi-pass VBR encoding of MPEG4 AVI files into MPEG2 files.

would I be right in saying that as the hard work of compressing VOB's into MPEG4 has already been done (multi-pass VBR, efficient use of bitrate etc.) that there is not much you can do with the source when converting to MPEG2?

that is, when dealing with an already compressed source you need only run a CBR encode, as this will provide the same visual quality as a multi-pass VBR encode?

or can a multi-pass VBR encode still provide a quality improvement over CBR when dealing with MPEG4 to MPEG2?

surely multi-pass VBR only makes a difference when compressing (converting a DVD into an XviD movie) rather than "expanding" (converting an XviD movie into a DVD)?

what are your thoughts on this?

thanks.

[Nick]

I would argue most strongly in favour of VBR.

There is no mathematical way of converting MPEG4 to MPEG2 - ie no formula that you could apply to the compressed data to convert it from MPEG4 data to MPEG2 data. Instead, each frame is decompressed and rendered from one compressed format, then served to your chosen encoder for compression into MPEG2. Hence any calculations of bitrate allocation which went on during the original MPEG4 encoding are never passed to the MPEG2 encoder - just a stream of pictures.

Both MPEG2 and MPEG4 are lossy compression formats. Thus encoding to either produces visual artefacts. Accepting the fact that the source file will therefore contain artefacts does not IMO constitute a compelling argument for re-encoding with CBR. This will only mean that that the output MPEG2 file will have no "intelligent" use of bitrate and therefore more artefacts. Indeed, artefacts in the source file are actually often hard to encode and so affected scenes in the source file tend to need very high bitrate in MPEG2 to re-encode!

Furthermore, if you use CCE, using RoBa OPV encoding is barely slower than CBR encoding anyway. So CBR has little to recommend it in my view, regardless of source. Unless the source file is very short such that your CBR bitrate would approach your max average bitrate for the encode.

That's my £0.02 worth.

Cheers
Nick

[WhipHubley]

Thanks for your thoughts on this. I'm in no hurry when I do my encodes, so I think I'll take a look at using a 3-pass VBR with CCE during my next AVI2DVD project. So far I've always used CBR.

Cheers.

[Venom_IL]

we have OPV (http://forum.doom9.org/showthread.php?s=&threadid=73243) :sly: