Shift param in FFT block in GRC

GNU Radio
1

When I add a FFT block in the GRC (gr_fft_vxx if I understand things
correctly) I am offered a boolean parameter ‘shift’. When I attempt to
discern the code, there is a comment:

// apply a fft shift on the data  - for the fwd FFT and
// apply an ifft shift on the data  - for reverse.

What does this parameter do?

             Kind Regards,

                     John
2

On Tue, Jun 12, 2012 at 08:19:21PM +1200, John Shields wrote:

When I add a FFT block in the GRC (gr_fft_vxx if I understand things
correctly) I am offered a boolean parameter ‘shift’. When I attempt
to discern the code, there is a comment:

// apply a fft shift on the data - for the fwd FFT and
// apply an ifft shift on the data - for reverse.

What does this parameter do?

It applies an FFT shift on the data for the forward FFT and an IFFT
shift on the data for reverse FFT.

Did that help?

If not, a quick Google search for ‘fft shift’ explains that this moves
the DC carrier to the centre of the spectrum (instead of the far left).

MB

–
Karlsruhe Institute of Technology (KIT)
Communications Engineering Lab (CEL)

Dipl.-Ing. Martin B.
Research Associate

Kaiserstraße 12
Building 05.01
76131 Karlsruhe

Phone: +49 721 608-43790
Fax: +49 721 608-46071
www.cel.kit.edu

KIT – University of the State of Baden-Württemberg and
National Laboratory of the Helmholtz Association

3

On 12/06/12 21:00, Martin B. wrote:

shift on the data for reverse FFT.

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Thanks Martin,
Yes it does help. I had seen some ‘funky stuff’
in Marcus’ Simple_RA and wondered why. Now I think I know.

                              Kind Regards,

                                        John