[FFmpeg-devel] Information about FFT implementation in ffmpeg

jogging song joggingsong at gmail.com
Tue Jul 26 17:38:20 CEST 2011

Thanks for your timely reply, Vitor

Today I find that FFT is followed by IFFT immediately, but the
results is not same with original input data. The output is 16 times
of original input data. Is this correct?

Best Regards

On Tue, Jul 26, 2011 at 3:33 AM, Vitor Sessak <vitor1001 at gmail.com> wrote:
> On Mon, Jul 25, 2011 at 10:30 AM, jogging song <joggingsong at gmail.com> wrote:
>> Hi, all
>> I hope to learn more about FFT implementation in ffmpeg.
>> FFT in ffmpeg performs on float type data.
>> When twiddle factors are generated, only one fourth of
>> the length is calculated. Others are copied from generated
>> twiddle factors and only cosine is generated.
>> When the precision is considered, more twiddle factors
>> are required.
> This has nothing to do with precision.
>> Such as in the function ff_init_ff_cos_tabs
>> for(i=0; i<=m/4; i++)
>>   tab[i] = cos(i*freq);
>> When i > m /4, the cosine value calculated from the
>> above equation is not equal to the value copied
>> from the generated value. One or two significant
>> digits are different.
> I expect the sign to be different, not just a few significant digits.
> In the C implementation only the values for i < m/4 are used. Higher
> values are used only in ASM implementations to avoid shuffling.
>> Now I try to generate twiddle factors of half of the length
>> for both cosine value and sine value, and use these
>> twiddle factors to calculate FFT. I need to modify the
>> source code.
> Why?
>> Is there any materials about the FFT implementation in ffmpeg?
> It is a pretty standard split-radix FFT implementation. Google for
> "split radix FFT".
> -Vitor
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