# Queries on FFT when N > 256 using complex_frac16

Latest reply on Jul 19, 2010 by AndreasL

We have tried the 16-bit complex FFT -- both the one in the library and the one described on this forum

Consider two different input (1)  DC = 0x7FFF max 16-bit  (2) DC = 0x7FFDC (you can use  0x7FFF * cos (f) but that just shifts the position in the spectrum of the problem

After performing DFT via FFT -- then thes will grow to (1)  DC = 0x7FFF * 2^ 8   (2) DC = 0x7FFDC * 2^8 and over flow

or else -- since fract 16 -- will get scaled as the algorithm progresses so that you get  (1)  DC = 0x7F * 2^ 8   (2) DC = 0x7FF * 2^8 -- i.e. values get severely truncated

Actually  we need 1024 = 2^ 10 DFT via FTT to boost resolution in frequency comain -- so that get very severe truncation

i.e. we must do

signal >> 12 before using complex_fract16 -- if want to use 1024 FFT -- and that is after doing  signal = audio input >> 8 as the A/D is 24

so 1) Am I missing something -- and the scaling, and associated loss of accuracy,  is not necessary because I should be doing XXXXX

and 2)  if I am missing something to avoid the scaling -- what isthe XXXX ?

and (3) If I am not missing something -- then can something be put into the documentation header files hinting about the loss of accuracy, especially with the 1024 example -- The max value 0x7FED becomes 0x003F after scaling -- loss of 12 bits of precision

I think what I need is an efficient 32-bit FFT for the blackfin -- so I am going to follow up with -- http://ez.analog.com/thread/1547

Another question -- why do the 16-bit  library examples also use software bit-reversing when hardware bit-reversing is available ?

I know from trying to work out hardware bit-reversing in an earlier life -- that the hardware bit reversing does not work unless the data is aligned by the size of the data, and I was not aware of any mention when using those libraries that that sort of data alignment is necessary

The example from the forum web page -- definitely uses software -- does the library do the hardware bit reversing on the index?

Thanks

Mike