ipps fft function

[sever70]

Hi,

I am trying to take FFT of a real signal and also the output should be a real signal. Thus, I useone of these functions :

ippsFFTFwd_RToPerm_32f

ippsFFTFwd_RToPack_32f

ippsFFTFwd_RToCCS_32f

However the output signal doesn't match with the Matlab output of fft function. Any idea?

Thanks in advance

[Thomas_Jensen1]

What is it that doesn't match?

Numbers?

[sever70]

Yes, the numbers doesn't match..

[sever70]

Matlab output for a length of 8 array

4.000000

-1.414214

4.000000

1.414214

-4.000000

1.414214

4.000000

-1.414214

And by using these ippsfunctions,

ippsFFTInitAlloc_R_32f( &pSpec, 3, IPP_FFT_DIV_INV_BY_N, ippAlgHintNone );

ippsFFTFwd_RToCCS_32f(pX, pY, pSpec, pBuf);

I got these outputs:


4

0

-1.41421

-1.41421

4

-4

1.41421

-1.41421

[Thomas_Jensen1]

You should have taken a look at the IPP documentation. For CCS, your output is as expected.
Look at the IPP definition of CCP, you'll see it there:

http://software.intel.com/sites/products/documentation/hpc/compilerpro/en-us/cpp/lin/ipp/ipps/ipps_ch7/ch7_packed_formats.html#CCS

[sever70]

Thank you Thomas. But shouldn't there be a one to one match with matlab fft functions. Or which function I should use for this purpose?

[SergeyKostrov]

Quoting sever70

Hi,

I am trying to take FFT of a real signal and also the output should be a real signal. Thus, I useone of these functions :

ippsFFTFwd_RToPerm_32f

ippsFFTFwd_RToPack_32f

ippsFFTFwd_RToCCS_32f

However the output signal doesn't match with the Matlab output of fft function. Any idea?

As far as I know the Matlab does calculations usingDouble-Precision variables. The set of IPPSfunctions
you're using are Single-Precision ( ..._32f ). Could you submit a complete IPP Test-Case?

Best regards,
Sergey

[Thomas_Jensen1]

There would only be a match if the output of matlap and of ipp was in the same "domain", I mean, of the same type.

FFT and DFT uses different data domains (types) for efficiency purposes.
For instance, if you data is purely real, you can hold its complex version is a so-called packed domain. This is much smaller the the full complex version and thus faster (better cache utilization etc.).
However, if you print packed data, it looks not at all like normal complex data. You would have to convert packed to complex before printing it.

I don't really know useful purpose of the CCS format, but if you simply select normal complex output format, and then print that (possible first converting to real format), I'm sure your comparison would understandable.

Do you understand my text?

[Ying_H_Intel]

Hi Sever70,

Not sure Matlab's out put format. But same suggestion as Thomas,
The format is as below

Arrangement of Forward Fourier Transform Results in Packed Formats - Even Length

Index	0	1	2	3	. . .	N-2	N-1	N	N+1
Pack	R0	R1	I1	R2	. . .	I(N-1)/2	RN/2
Perm	R0	RN/2	R1	I1	. . .	RN/2-1	IN/2-1
CCS	R0	0	R1	I1	. . .	RN/2-1	IN/2-1	RN/2	0

Forward Fourier transform Result Representation in Packed Formats - Odd Length

Index	0	1	2	3	. . .	N-2	N-1	N
Pack	R0	R1	I1	R2	. . .	R(N-1)/2	I(N-1)/2
Perm	R0	R1	I1	R2	. . .	R(N-1)/2	I(N-1)/2
CCS	R0	0	R1	I1	. . .	I(N-1)/2-1	R(N-1)/2	I(N-1)/2

and you may use the Transform Functions to get real complex result.

Intel IPP Transform Functions

Function Base Name	Operation
Support Functions
ConjPack	Converts the data inPack format to complex data format.
ConjPerm	Converts the data in Perm format to complex data format.
ConjCcs	Converts the data in CCS format to complex

Best Regards,
Ying

[sever70]

Thanks Thomas and Ying.Finally I got correctFFT values for real data by usingCCS format.