[LAU] Decoding/ripping dolby surround CDs

[Fons Adriaensen]

On Sat, Nov 19, 2011 at 04:11:42PM +0100, Julien Claassen wrote:

> I tried a few calculations to filter out the last channel 
> or isolate on of th3e other two channels, but no luck. I end up with 
> always having the third channel somewhere in the equation.

No surprise. You can't mix 3 (or more) signals into 2 and
recover them independently. 

To decoded your CD to surround you need a Dolby Prologic II 
decoder which consists of two parts:

* A matrix recovering the missing channels (with lots of
crosstalk from the others, but things are done in a way
as to make this acceptable),

* 'Logic' which detects the dominant sound directions and
modifies the matrix to optimise for that. So in a movie
you could e.g. have a gunshot or explosion coming only
from the left surround speaker (most other sound would be
pulled to that direction as well). This part is pretty
useless for music. 

Jconvolver could easily do the matrix part. How the logic
part is supposed to work exactly isn't described in any
open documents AFAIK.

For a music CD the surround channels would probably contain
just room ambience and reverb. These are present in the
stereo signals as well, at a lower level (as required for
stereo) and mostly out of phase. The center channel is 
panned halfway between L and R by the encoding, there is
no need to decode it except for film dialogue.

So you can just listen to this in stereo and not miss
anything.

-- 
FA

Vor uns liegt ein weites Tal, die Sonne scheint - ein Glitzerstrahl.