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Topic: Where is well described FFT divisor 5 ? (Read 4023 times) previous topic - next topic
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Where is well described FFT divisor 5 ?

It is known that FFT not must works only on 2^n values, but also for 2^a*3^b*5^c. For example in FAAD sources are functions: passf2pos,passf3,passf4pos,passf5. In Opus are kf_bfly3,kf_bfly5 etc. FAAD sources of MDCT are more clear than Opus and I want use it, but FAAD uses only 2^n frame length. I check that divisor 3 is also good (libfaad issue 2) but divisor 5 is bad. Where is described algorithm for this, for example in PDF file? Opus's kf_bfly5 is shorter and differs than FAAD's passf5 function. Function passf5 is long and complicated.

Where is well described FFT divisor 5 ?

Reply #2 –
Quote from: Borneq on
It is known that FFT not must works only on 2^n values, but also for 2^a*3^b*5^c.


The discrete fourier transform works for any number of values, not just ones of those forms.  Individual FFTs can work on whatever range of inputs the author wants.  Since almost all audio formats use power of two transforms, those are the ones you'll see in most codecs.

Quote from: Borneq on
For example in FAAD sources are functions: passf2pos,passf3,passf4pos,passf5. In Opus are kf_bfly3,kf_bfly5 etc. FAAD sources of MDCT are more clear than Opus and I want use it, but FAAD uses only 2^n frame length. I check that divisor 3 is also good (libfaad issue 2) but divisor 5 is bad. Where is described algorithm for this, for example in PDF file? Opus's kf_bfly5 is shorter and differs than FAAD's passf5 function. Function passf5 is long and complicated.


AAC (ignoring some obscure variants that are rarely if ever used) has a power of 2 transform length.  Rather then trying to add support for whatever size you're using, I would just grab a version of the MDCT that supports the sizes you need. 

That and the libfaad MDCT is painfully slow.