[xine-user] Re: libac3 trouble

[Bill F. <bil...@ca...>]

On Fri, 11 May 2001, Rich 'Forge' Mingin wrote:

> Yes, left/center/right should be three equal inputs for the 2 speaker
> downmix. This is a common 5.1>2 downmix error.

OK.  I took a gander at the code in imdct.c and downmix.c, but I
don't yet grok what the code is actually doing.  If I have the time
later, and if I can figure out what the code is doing (maybe a big
if), I may try some experimentation.

						-Bill



> On Thu, 10 May 2001, Bill Fink wrote:
> 
> > On 05/09/2001, Guenter Bartsch wrote:
> > 
> > > BTW: do you have any ideas about proper ways to do downmixing? simply
> > > adding the channels and dividing isn't always the best solution as it
> > > often results in a very low-volume center playback
> > > (left_channel = l_surr + left + 0.5 center / 2.5 or something like
> > that) -
> > > I guess we'll need some logarithmic scaling here or something of that
> > kind
> > > ... I hope this won't harm if we do downmixing first and then do the
> > > imdct...
> > 
> > I've been wondering about this.  Is it actually correct to treat all 5
> > inputs equally, which is what:
> > 
> > 	left_channel = (l_surr + left + center/2) / 2.5
> > 
> > effectively does.  Or would it be more correct to treat total left,
> > center, and total right equally for 3 basic inputs, with total left
> > and total right each having 2 subcomponents.  This would make the
> > equation something like:
> > 
> > 	left_channel = ((l_surr + left)/2 + center/2) / 1.5
> > 
> > which simplifies to:
> > 
> > 	left_channel = (l_surr + left + center) / 3
> > 
> > I may be all wet since I admit not knowing anything much about the AC3
> > specs, but it would give the center channel more weight, which is the
> > problem I've noticed with voices being too low in action scenes with
> > a loud score, and someone else mentioned that voices are generally in
> > the center channel.