# vrf Complex FFT - are there any eggheads out there?

Question asked by papineni_vinod on Aug 4, 2005
Bill,

I am no math expert, but I thought you should sequence the  "DO" before
the "Call FFTComplex" object from the "ComplexWave" object to get
proper timings!?

-Vinod

--- Bill Ossmann <bill.ossmann@philips.com> wrote:

> OK, the FFT of a real signal is Hermitian. That is, if X(f) = FFT(x),
> then
> Re(X(-f)) = Re(X(f))   and   Im(X(-f)) = -Im(X(f))
>
> So "all" you need to do is to perform separate FFTs on the real and
> imaginary parts and combine them using the proper complex symmetry.
> The
> attached VEE program shows how. I found that the MATLAB script ran
> several
> times faster as well as being much easier to code (just a single
> function
> call). Perhaps the "painfully slow" part was waiting for the MATLAB
> engine
> to fire up the first time.
> --
> Bill Ossmann
> Philips Ultrasound
> e-mail:  bill.ossmann@philips.com
> (See attached file: ComplexFFT.vee)
>
> "David Watling" <watling@IceraSemi.com> wrote on 08/04/2005 07:15:42
> AM:
>
> > Hi,
> > This works, but only delivers the positive frequencies.  Do you
> know
> > how to add the negative part of the spectrum?
> >
> > Dave
> >
> >
> > From: Iulia Popovici [mailto:iulia.popovici@urbanet.ch]
> > Sent: 04 August 2005 11:28
> > To: David Watling
> > Subject: Re: [vrf] Complex FFT - are there any eggheads out there?
> >
> > Hi David,
> >
> > If you are speaking about signals used in vector modulations, the I
> > and Q signals are orthogonal, so you can calculate separately
> fft(I)
> > and fft(Q), and then add the two complex spectra. In my opinion
> > you'll get the same result as with a bi-dimensional (complex) FFT.
> Try it
> !
> >
> > Best regards,
> >
> > Julia
> > ----- Original Message -----
> > From: David Watling
> > To: VRF
> > Sent: Thursday, August 04, 2005 9:27 AM
> > Subject: [vrf] Complex FFT - are there any eggheads out there?
> >
> > Hi all,
> > I need to perform a complex FFT on 2 (I&Q) waveforms.  I have been
> > using the Matlab complex FFT, but it is painfully slow.  Does
> anyone
> > know how to use the Vee real FFT to work as a complex FFT?  This
> > question is for someone with superb maths knowledge.
> >
> > Regards
> >
> > David Watling
> >

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