[Matroska-devel] Re: [mpc-devel] Protocol of my 2+ hrs telephone conversation with Frank Klemm

Roberto José de Amorim rjamorim at yahoo.com
Mon Nov 17 19:28:34 CET 2003


 --- Michel Lespinasse <walken at zoy.org> escreveu:  
> I think the AC3 format has a lot of potential too, but currently the
> free encoders for it kinda suck. One of the interesting features of
> AC3 is that the decoder performs a bit allocation using a default
> standardized psy model - if the encoder has a better psy model, it can
> transmit deltas to apply to that bit allocation, but the idea is that
> in the end it should be cheaper to transmit these deltas than to
> transmit a whole bit allocation information. However, all free
> encoders are crappy in the respect that they dont even have their own
> psy model - they just use whatever is the result of the default psy
> model in the AC3 decoder.
> 
> Anyway. I believe a mdct based format such as AC3 would have a lot of
> potential if combined with a smart encoder using a good psy
> model. Regarding what frank told you, I'd be curious if he was
> considering a straight AC3 format encoder, or doing an AC3-derived
> format with stream format improvements similar to what he's done in
> mpc (i.e. the various shaped tables etc)

It's worth mentioning that AC3 doesn't cope well with VBR. Even though
it might be accepted by the standard, several decoders (most 
notably the ones from Dolby) choke on VBR streams.

I did a test once at doom9. A friend "hacked" the ffmpeg AC3 encoder
so to provide a silly VBR mode (he just randomized bitrates). Then,
I tried to feed this VBR streams to decoders.

Decoders based on liba52 worked well (valex' in_ac3, ac3filter...)
AC3dec worked well
Azid didn't work well. I don't remember now what was the error.
Encoders based on Dolby libraries (PowerDVD, WinDVD) choked, with
weird noises coming from the speakers and eventual crashes

I couldn't get anyone to test it through S/PDIF on receivers, but 
I'm not very hopeful of it working there either.


Also, AFAIK AC3 is as unoptimal as MP3 on VBR. The bitrates are fixed, 
and you can't increment bitrate in small steps like you can do with 
MPC, Vorbis and AAC. That usually leads to waste of bytes.

Just my 2 cents...

Best regards;

Roberto.



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