phase and magnitude of FFT in TI's F2812

Hi,
Im trying to use ti's eZdsp F2812 to perform FFT on a simple sinewave. The
FFT libraries provided allow me to extract the magnitude square of the fft
but have no such allowance for the phase.
The fft calculations are performed in the "in place computational buffer"
and its supposed to hold the values of the fft result afterwards (real in
the even positions and imag in the odd).

I attempted to perform the magnitude calculations manually
(mag=sqrt(ipcb[0]^2+ipcb[1]^2).. and so on) but my results dont match the
results obtained by the fft libraries mag function. From looking at the
ipcb buffer my results dont seem correct as every second number is very
very large, when it should be zero. However somehow the mag function is
able to read the exact same values and give me the correct result. 

Does anyone know if there is some special way to read the ipcb buffer to
get the actual real and imaginary values out to calculate the mag and
phase myself? The main reason i want to do this is to get the phase as the
fft library has no provision for this.

Any help would be greatly appreciated!
Cheers
Seb

What is the data type in your ipcb[] array? If it is complex, then this may
be where your manual calculation is failing.

>Im trying to use ti's eZdsp F2812 to perform FFT on a simple sinewave.
The
>FFT libraries provided allow me to extract the magnitude square of the
fft
>but have no such allowance for the phase.
>The fft calculations are performed in the "in place computational
buffer"
>and its supposed to hold the values of the fft result afterwards (real
in
>the even positions and imag in the odd).
>
>I attempted to perform the magnitude calculations manually
>(mag=sqrt(ipcb[0]^2+ipcb[1]^2).. and so on) but my results dont match
the
>results obtained by the fft libraries mag function. From looking at the
>ipcb buffer my results dont seem correct as every second number is very
>very large, when it should be zero. However somehow the mag function is
>able to read the exact same values and give me the correct result. 
>
>Does anyone know if there is some special way to read the ipcb buffer to
>get the actual real and imaginary values out to calculate the mag and
>phase myself? The main reason i want to do this is to get the phase as
the
>fft library has no provision for this.
>
>Any help would be greatly appreciated!
>Cheers
>Seb
>
>
>

THanks for your reply. 
The ipcb aray is of type long, as that is how it is in all the examples
ive seen. I dont think the FFT library allows for any other type. 
However after the FFT calculation, every second result should be complex,
which is crazy because Ive never seen a "complex" data type in C or C++. I
suspect they change the data type so that the fft library recognises its
complex, but how is your average programmer meant to know how to convert
it back? There is no mention of this in the FFT manual...
Any ideas about getting the phase from this thing?

Cheers


>What is the data type in your ipcb[] array? If it is complex, then this
may
>be where your manual calculation is failing.
>
>>Im trying to use ti's eZdsp F2812 to perform FFT on a simple sinewave.
>The
>>FFT libraries provided allow me to extract the magnitude square of the
>fft
>>but have no such allowance for the phase.
>>The fft calculations are performed in the "in place computational
>buffer"
>>and its supposed to hold the values of the fft result afterwards (real
>in
>>the even positions and imag in the odd).
>>
>>I attempted to perform the magnitude calculations manually
>>(mag=sqrt(ipcb[0]^2+ipcb[1]^2).. and so on) but my results dont match
>the
>>results obtained by the fft libraries mag function. From looking at the
>>ipcb buffer my results dont seem correct as every second number is very
>>very large, when it should be zero. However somehow the mag function is
>>able to read the exact same values and give me the correct result. 
>>
>>Does anyone know if there is some special way to read the ipcb buffer
to
>>get the actual real and imaginary values out to calculate the mag and
>>phase myself? The main reason i want to do this is to get the phase as
>the
>>fft library has no provision for this.
>>
>>Any help would be greatly appreciated!
>>Cheers
>>Seb
>>
>>
>>
>
>
>
>
>

It is quite common to find Complex data types in DSP vector libraries. I do
not know the F2812 processor at all, but I guess you are using SPRC081
library? 

Take a look in the header file to see the prototype of the function you
are using, and see if a special type is defined for it.

You see, it could be that indexing through the data members of your buffer
is actually creating a 'stride' of 2, which means that you are not
accessing real and imaginary parts individually.

Jeff