[MPlayer-users] Re: Documentation update.

D Richard Felker III dalias at aerifal.cx
Sat Sep 13 19:49:54 CEST 2003

On Sat, Sep 13, 2003 at 12:09:03PM -0500, Jonathan Rogers wrote:
> [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> D Richard Felker III wrote:
> >The only purpose is for encoding mp3 with lame. In the first pass
> >("pass 0") you encode the audio with vbr, and you get the exact audio
> >bitrate. Then you can tune the video bitrate for pass 1 and 2 so that
> >it exactly fills the CD, maximizing quality. With OGM, you do the same
> >thing by encoding your vorbis audio in a .ogg file before you even
> >begin video encoding.
> Why can't one do the same with MP3? Use LAME to encode the audio, then 
> encode video in two passes. Then, put them back together with mencoder.

You can! It's just a little more work.

> >The problem with the 3pass code, though, is that it doesn't just
> >encode the audio. It also makes a table of input/output frame
> >correspondence for A/V sync, but without decoding any video, just
> >using info from the demuxer. This will work under ideal conditions,
> >but if any frames get dropped by the decoder (e.g. they're damaged
> >beyond recovery) or by the vf chain during pass 1/2, the sync
> >info from pass0 will be bogus and will result in a bad output file.
> What is the purpose of the input/output frame table? What does it do for 
> you that an ordinary two pass encode doesn't? Does this have something 
> with kludging VBR MP3 into AVI?

No, I really have absolutely no idea why it was done that way. Ask
whoever wrote it (Arpi?).


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