Understanding MPEG2 re-encoding with less bitrate

[bcanneyus]

Hi everyone,

As you can see from the number of my posts, I am quite new to this topic (although I have read up on a lot of material about the various codecs and encoding procedures), so please forgive me if I seem to be asking stupid questions about obvious things.

It would be nice if someone could explain to me one thing about re-encoding an MPEG2 stream to e.g. SVCD with less bitrate.

As far as I know one is well advised to use VBR with multiple passes, as long as there's no concern about encoding time and one wants to achieve good quality.

Now what surprises me is this: Let's say I am backing up one of my DVDs. If I am not mistaken, that would be encoding an existing MPEG2 stream which is already VBR, correct?

So why would I frameserve this as an *.avi or *.avs to some encoder that then has to perform multiple passes to determine the best bitrate distribution for the new (re-)encode?

Why would we not just "read" the existing bitrate distribution from the existing (DVD) MPEG2 stream, simply scale it down and re-encode with only one pass using the pre-read and downscaled bitrate to get a good VBR encode?

Thanks to all for any answers,
Bob

[smiller667]

There is such a tool ... look for rempeg. However, for SVCD-compliant bitrates, the resulting video quality is inferior to the frameserving methods using CCE or another decent encoder.

[bcanneyus]

Thanks for your reply smiller667,

You're right, I know of ReMPEG and its issues (not producing optimal quality), but, and that's why I'm asking, - isn't there any way to feed the bitrate data to a better encoder, like the one you mentioned? Let's say one could create a 'scaled' *.vaf file from the mpeg2 source file for the better encoder?

Greets,
Bob

[ulfschack]

Sounds like a good thought to me. But are we sure that DVDs are encoded with VBR? I mean 9.8 Mbit/s is the highest allowable bitrate for this standard. On a regular pressed dvd you can fit about 9 GB which yeilds a little more than two hours at this rate. So should the duration of the movie be anything less than two hours it would be completely pointless to use anything but CBR. I dunno ... writing makes me think better

cheers

[smiller667]

Most, if not all DVDs are VBR ... you can check it with a bitrate viewer like this one: http://www.visualdomain.net/

[ulfschack]

Ok, good to know. However I do think that my point remains quite valid seeing as for most movies the bitrate would "bump its head against the ceiling" so to speak, leaving very little trace of the sought after bitrate distribution, no?

I mean what would be the point for the encoder to slump to lower bitrates during non-complex scenes when there's space galore allready.

cheers

[Kedirekin]

I suspect the authoring studios just use a standard suite of HW/SW tools, and by default the tools do VBR. So there is no real reason; they do it that way because they always do it that way.

[EyeLykeB0ngs]

Actually, when studios author DVD's, there is actually a job for an encoder. He is the one who takes the Hi-Def (in most cases)transfers, down converts them to our TV standard, and watches the movie, and uses a program that allows him to control the bitrate in real time, as they watch the film. You can read up about it at www.thedigitalbits.com www.dvdfile.com and the other big sites