[MPlayer-users] [Question] Why 2 channel ac3

[Robert R. Wal]

On 02.12.24 Anders Rune Jensen pressed the following keys:

> I've been wondering for a why some rip dvd's with only 2 channel ac3
> and not 5.1? Sure they save bandwitdth but I mean if they just want to
> save bandwidth they could have used ogg or mp3. I've heard something
> about some amplifiers been able to "convert" that two channel to 5.1,
> I think it's Dolby Pro Logic II. So does this produce as good quality
> as 5.1? I guess if there is indeed such amplifiers that must be the
> reason, since converting it back to ac3 from ogg would just be to
> lossy I think.

The reason for riping DVD-s with reencoding AC3->AC3 is that the moronic
``release groups'' think it makes them more cool, since almost all Windows
users believes that AC3 means ``DVD quality''.

Which is far from the truth.

AC3 is ``old codec'' slightly better than mp3, but much worse than
Vorbis, AAC or WMA (at the same bitrates). When you recompress 5.1 AC3
streams you get artifacts from the lossy compression, mixed with
additional artifacts from downmixing+normalising six tracks to two.

If you compress it again you get another round of artifacts from lossy
compression.

On the other hand lots of dvds contain AC3 at 192 streams (think tv series
massively published on DVDs these days) so if you leave it intact you
really get the ``dvd quality''.

And now the conclusion:

They are right that they don't use mp3 for the reencoding since it would
be much worse quality than AC3. Had they used e.g. Vorbis at q6
(~192kbps) the quality would be much better than it is with AC3 at 192.

But they will stay with AC3 since for average Joe ``the morron'' Windows
User AC3 _is_ dvd quality.

> My amplifiers when fed with ac3 2 channels just play it as 2 channels
> even if I use dolby Pro Logic (I believe it's not Dolby Pro Logic II).

That is totally different story... The most often used windows tool for
AC3 downmixing/recompressing is besweet and it incorrectly marks AC3
streams. If you use it to create dolby streams (i.e. with discretely
mixed surround channels) it marks them as stereo (i.e. plain two
channels), and vice versa. At least it did this last time I checked.

This is the source of the confusion for the most hardware AC3 decoders.
If they don't see DOLBY flag, you won't force them to decode dolby, no
matter what.

Robert

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