[FFmpeg-devel] [PATCH] Move Kaiser-Bessel Derived window to mdct.c

Justin Ruggles justinruggles
Fri Jan 11 00:12:40 CET 2008

Robert Swain wrote:
> Hello,
> On 10/01/2008, Michael Niedermayer <michaelni at gmx.at> wrote:
>> On Thu, Jan 10, 2008 at 04:38:16PM +0000, Robert Swain wrote:
>>> The SoC AAC code uses a slightly more flexible version (variable
>>> alpha, window size, iterations) of the Kaiser-Bessel Derived window
>>> used in libavcodec/ac3dec.c. It is desirable to have the code shared
>>> between ac3dec.c and aac.c rather than duplicated. I extended the
>>> capability of the code in ac3dec.c and moved it to mdct.c. Please find
>>> a patch for this attached.
>> moving the code and functional changes must be seperate
> OK, I will post more patches for this later this evening.

That sounds good. I know Benjamin has been discussing doing this for a
while, but I'm guessing there was not a need to share it until native
aac was ready.

>> And IIRC the num of iterations are just a matter of how good the
>> approximation should be, what does AAC say about the ierations?
> I think you're right but, from looking at the AAC spec (page 162 of
> sp04), all I can see is a sum from 0 to inf in the zeroth order first
> kind bessel function I0. I've looked around a bit but unless I'm
> missing something, it doesn't say anything about the suggested number
> of iterations to use. Maxim used 50 in aac.c, Justin used 100 in
> ac3dec.c.

100 is a bit excessive. It could likely be changed to 50 and still match
 the specification table. Honestly, I don't even care if it doesn't
match exactly...floating-point tables in specs annoy me...:)  If
precision matters they should have specified it in fixed-point like all
the other AC3 tables.


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