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

Ramiro Ribeiro Polla ramiro
Sun May 13 18:36:43 CEST 2007

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

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?

Ramiro Polla