[FFmpeg-cvslog] r12397 - trunk/libavcodec/ac3dec.c

[Justin Ruggles]

Rich Felker wrote:
> On Sun, Mar 09, 2008 at 05:18:18PM +0000, M?ns Rullg?rd wrote:
>> Justin Ruggles <justinruggles at bellsouth.net> writes:
>>
>>> jbr wrote:
>>>> Author: jbr
>>>> Date: Sun Mar  9 18:05:19 2008
>>>> New Revision: 12397
>>>>
>>>> Log:
>>>> downmix before imdct unless different size transforms are used. about 20%
>>>> faster 5.1-to-stereo downmixing.
>>> But still about 13% slower than liba52.  I don't think it will ever be
>>> able to match liba52's speed in downmixing, but at least it's much
>>> closer now.
>> Why can't it be as fast as liba52?
> 
> I don't understand what's meant by "delay samples", but upmixing them
> then downmixing them again sounds incredibly misguided. I suspect
> linearity makes it totally useless to do these two steps.

It is necessary because 256 samples from the previous block are combined
with 256 samples from the current block.  The channel information has to
be the same.  So if the previous block was downmixed before IMDCT, but
the current block will be downmixed after IMDCT, you have to upmix the
previous 256 samples in order to combine with all channels for the
current block.  Then you have the same thing in reverse when you go back
to downmixing before IMDCT.  liba52 does the same thing.

-Justin