[FFmpeg-devel] (i)(m)dct help for g.722.1

Ramiro Ribeiro Polla ramiro
Tue May 15 22:02:12 CEST 2007


Justin Ruggles wrote:
> Ramiro Ribeiro Polla wrote:
>> Hello,
>> When I got to the dct part of g.722.1 (which is currently in 
>> cook.c:687), I realized it doesn't use the same transform as cook (at 
>> least I think so). After the window, overlap, and add, the output is 
>> faster than it should be, making the last half of the 320 samples look 
>> like a fade-out.
>> I wanted to know if FFmpeg already has the same transform used in 
>> g.722.1, or if I'm just using it the wrong way.
>> The standard says it's a "type IV DCT":
>> u(n) = \sum_{m=0}^{319} \sqrt{\frac{2}{320}} cos( \frac{\pi}{320} 
>> (m+0.5)(n+0.5) ) mlt(m)
>> for 0 \leq n \leq 319
>> Or else, how would I go about implementing it?
> I suspect Benjamin's advice is correct, but in case not, here is an
> example of how a type iv dct is related to a standard mdct.  See the
> mdct_512() function (and ignore mdct_256() which is an AC-3 oddity).
> Also, this example is for the forward transform, but the same concept
> applies to the inverse.
> http://aften.svn.sourceforge.net/viewvc/*checkout*/aften/libaften/mdct.c?revision=98

Thanks for the tips. I tried what Benjamin suggested, and it seems the 
problem is because g.722.1 uses 320 samples, and the mdcts in FFmpeg 
work only with powers of 2. Cook uses powers of 2. Vorbis also supports 
only powers of 2.

Any thoughts on a solution besides implementing my own dct? Can the mdct 
and fft functions be modified to accept any number of samples?

Ramiro Polla

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