[MPlayer-users] encoding ac3

[anders at gnulinux.dk]

* Kresimir Kukulj <madmax at iskon.hr> [2002-05-31 22:20:34]:
> [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> Quoting anders at gnulinux.dk (anders at gnulinux.dk):
> > Hi
> > 
> > I'm playing around with some dvd's and mencoder and the following question
> > popped up: Is it possible to encode ac3 audio with mencoder? The Docs
> > states that mencoder supports the following codecs: copy, pcm and mp3.
> > What I don't really understand is: "is ac3 a codec?" Or can I encode ac3
> > audio with the mp3 codec? Will the copy "codec" just copy and raw and keep
> > the ac3 "uncompressed"? 
> 
> As far as I know, there are no solutions for linux to encode AC3 (5+1).
> 
> But mencoder can passthrough AC3 soundtrack.
> 
> mencoder ... -oac copy -aid 128 -o bla.avi
> 
> It will copy AC3 stream unmodified from a DVD source. Use -aid to specify
> appropriate audio track.
> 
> Resulting file will be unplayable (jittery) in windows players. Interleaving
> video with audio affects playback greatly (using S/PDIF).
> Since mencoder has no interface to specify how much/often audio will be
> interleaved with video, use attached patch (idea from this mlist).
> Tweak the number if needed. This works for me.
> 
> -----CUT HERE----
> diff -ruN main-cvs20020527/mencoder.c main/mencoder.c
> --- main-cvs20020527/mencoder.c	Sun May 26 00:24:08 2002
> +++ main/mencoder.c	Mon May 27 22:02:32 2002
> @@ -826,7 +826,7 @@
>  	    // CBR - copy 0.5 sec of audio
>  	    switch(mux_a->codec){
>  	    case ACODEC_COPY: // copy
> -		len=mux_a->wf->nAvgBytesPerSec/2;
> +		len=mux_a->wf->nAvgBytesPerSec/10;
>  		len/=mux_a->h.dwSampleSize;if(len<1) len=1;
>  		len*=mux_a->h.dwSampleSize;
>  		len=demux_read_data(sh_audio->ds,mux_a->buffer,len);
> -----CUT HERE----
> 
> I'm not sure, but using ­oac copy for AC3 soundtrack, produces file with
> wrong AV sync.  Transcode tool 'tcprobe' shows for a DVD vob file
> PTS=0.2873 (Presentation Time Stamp?). If I adjust audio (using Nandub) by
> +287msec I get better AV sync.
> Need to test this with more different samples to be sure.
> 
> I also found, that windows players have less problems with files produced
> with Nandub. (Maybe all win players are developed with samples made by that
> tool so ti works better with Nandub's interleaving?).
> 
> > This is what mencoder tells me about the audio:
> > AC3: 5.1 (3f+2r+lfe)  48000 Hz  384.0 kbit/s
> > 
> > Which is a quite high bitrate ;-)
> 
> Not as high as you may think. AC3 contains 5 independent audio streams + 1
> LFE stream.  384 kbit/s are divided between them using some algorithm.
> 
> -- 
> Kresimir Kukulj                      madmax at iskon.hr
> +--------------------------------------------------+
> Old PC's never die. They just become Unix terminals.
> 

Thanx a lot for the info! ac3 is not a top priority for me, was just
wondering :-)

-- 
Anders Rune Jensen
jabber: superduck at myjabber.net | email : anders at gnulinux.dk
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