Reply by robert bristow-johnson●June 3, 20132013-06-03
On Jun 3, 11:46�am, "rigor" <95223@dsprelated> wrote:
> Hi r b-j,
>
> Thanks !
>
> I combine the outputs per the below and got the correct result. Is the
> program I am having the real-input FFTs you called ?
i dunno, are you running MATLAB?
> A = fft(real(x))
> B = fft(imaginary(x))
> result = (real(A)-imaginary(B)) + i * (real(B)+imaginary(A))
... because in MATLAB, this should also work:
result = fft(x)
r b-j
Reply by rigor●June 3, 20132013-06-03
Hi r b-j,
Thanks !
I combine the outputs per the below and got the correct result. Is the
program I am having the real-input FFTs you called ?
A = fft(real(x))
B = fft(imaginary(x))
result = (real(A)-imaginary(B)) + i * (real(B)+imaginary(A))
Reply by robert bristow-johnson●June 2, 20132013-06-02
On Jun 2, 8:05�pm, "rigor" <95223@dsprelated> wrote:
> Hi DSP people,
>
> Is there a way to do complex FFT (with imaginary components) with a real
> only FFT alogarithm ? Thanks !
uhm, we need to talk a bit about what you mean by a "real only FFT".
there are FFT programs that take N real values, x[n], going in and
tightly packed (no zero-padding the imaginary part) and returns N/2
complex values, X[k], where X[0] is packed with the real part of X[0]
and the real part of X[N/2] (the imaginary parts of those two
components are known to be zero for real x[n]). for X[1] ... X[N/
2-1], these values are understood to have complex conjugates that are
not redundantly computed since x[n] is given as real.
now, you can do a complex FFT with that alg by running it twice, once
with the real part of x[n] and another with the imaginary part of
x[n]. combining the outputs is no sweat. but here's the dumb thing
about it: normally these real-input FFTs (i would not call it "real
only" FFT) are *built* outa a regular complex FFT of half the size.
so you separate the even-n samples of x[n] from the odd-n samples.
the former goes into the real part and the latter goes into the
imaginary part of N/2 complex samples and then you send that to a size
N/2 FFT and fix the output.
so you might already have a complex FFT that your real-only FFT is
using.
r b-j
Reply by rigor●June 2, 20132013-06-02
Hi DSP people,
Is there a way to do complex FFT (with imaginary components) with a real
only FFT alogarithm ? Thanks !