[FFmpeg-devel] [PATCH] NellyMoser audio decoder v2

Benjamin Larsson banan
Thu Sep 27 10:21:36 CEST 2007

Lo?c Minier wrote:
> On Sun, Sep 23, 2007, Benjamin Larsson wrote:
>> Thanks for the hint. I can now see that this is clearly a mdct based codec.
>> unpack_coeffs(buf, aptr);     ->   pre rotation
>> center(aptr);                 ->   fft permutation
>> inverse_dft(aptr);            ->   inverse fft
>> complex2signal(aptr);         ->   post rotation
>> apply_state(s->state, aptr);  ->   window and reorder
>> ff_nelly_state_table          -> should be regular sine window
>> ff_nelly_pos_unpack_table/
>> ff_nelly_neg_unpack_table     -> pre/post rotation factors
>> ff_nelly_inv_dft_table        -> fft twiddle factors
>> ff_nelly_center_table         -> fft bit reverse table
>  Interesting!
>> All in all a standard mdct by fft. Should be replaceable by the ffmpeg
>> mdct routines.
>  I already replaced inverse_dft() by the fft functions from dsputils in
>  my local repo, but I guess this is now pointless as the almost all the
>  code should be replaced by a mdct instead.

Hi, I had some time yesterday and looked closer at the code. While there 
is no doubt that the code is a TDAC implementation it doesn't seem like 
the code can be replaced by the mdct code in ffmpeg. So I guess that the 
fft replacement is what can be clean up for now. I'll sort out the table 
generation for the rotations and the window.

Benjamin Larsson

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