Re: Can you trim a FFT?
- From: John Bailey <john_bailey@xxxxxxxxxxxxxxxx>
- Date: Mon, 19 May 2008 16:50:10 -0400
On Mon, 19 May 2008 03:52:33 -0700 (PDT), shoo <JGBurgess@xxxxxxxxx>
wrote:
discrete signal -> window -> FFT on window -> display spectrum .move
on and repeat. (a STFT is not what im after)
When I am displaying the specturm I only want the 5Hz-30Hz range
displyed.
I would nievily like to know if there is a method of applying the fft
with the intention of only requiring a predetermind range of
frequency. hense no waste in the fft.
This is not a FFT answer but have you considered digital difference
filter technique? It is incredibly simple to program a digital
difference filter which will respond to a single frequency. Do a
ladder of these and viola!
Programmed digital difference filters are of the form x'=x+ky; y' = y
- ky (interpret x' as the next value of x in time sequence)
The following is the code in basic which accomplishes the math part of
the above to find a Fourier Series.
C = 3.141592653589#
FOR N = 0 TO 9
T = C * N / 128
P(N) = COS(T)
Q(N) = SIN(T)
L(N) = 2 * (1 - P(N))
NEXT N
FOR Z = 0 TO 256
F = (Z < 64)<br>FOR N = 0 TO 9
U(N) = U(N) - L(N) * X(N)
X(N) = X(N) + U(N) + F
NEXT N
NEXT Z
M = X(0) / 256
PRINT USING "###.########"; M
FOR N = 1 TO 9
V = (U(N) - (1 - P(N)) * X(N)) / Q(N)
X = X(N)
M = SQR(((X * X) + (Y * Y)) / 256)
M = M / 16
PRINT USING "####.#########"; M
NEXT N
I hope this works for you. I will be happy to provide additional
theory.
.
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- Can you trim a FFT?
- From: shoo
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