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

 --- Michel Lespinasse <[email protected]> escreveu: =20
> 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.
>=20
> 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=20
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=20
I'm not very hopeful of it working there either.


Also, AFAIK AC3 is as unoptimal as MP3 on VBR. The bitrates are fixed,=20
and you can't increment bitrate in small steps like you can do with=20
MPC, Vorbis and AAC. That usually leads to waste of bytes.

Just my 2 cents...

Best regards;

Roberto.