inverse fourier

[Guest]

I have a complex numbers vector of size 128.
when using the inverse fourier (and I used it before okay) it returns a
complex numbers vector instead of a real number vector as expected.
any idea?

 

[Dana DeLouis]

I have a complex numbers, vector of size 128.

If you do a Fourier Transform, and then do an Inverse Fourier Transform, you
should end up with the original data.
How did you get the 128 Complex numbers? Where they calculated via fft?

The only thing I can think of is that, do to rounding, you have a very small
Imaginary part.

 

[Guest]

Dana
Thanks,

yes, it is true, you fft and you get a complex, you ifft and get back the
original.
however it should work when you already have a complex vector.

and that is the situation, I had a complex vector that I made with the
function complex(a,b).
the inverse is returning another complex? help!

is there a way to attach the file here to show you?

 

[Guest]

Attachments are strongly discouraged in these newsgroups, and are filtered
out on many portals. 128 lines of text in the body of your reply, together
with a brief discreption does not seem excessive.

When Excel produces a complex value, it creates a string where both the real
and immaginary components are limited to 15 significant figures. However, 17
significant figures are required to uniquely characterize a double precision
value. Hence there is ample room for discrepancies in representation that
would prevent complete cancellation of the immaginary part.

Even if you include 17 significant figures in your constructed complex
values, Excel's string to number conversion routines ignore anything beyond
the 15th figure. VBA's string to number conversion routines do use the
trailing figures to get a more accurate representation, so you could try
doing the calculations from VBA to see if that helps.

Jerry

 

[Dana DeLouis]

...I had a complex vector that I made with the

function complex(a,b).
the inverse is returning another complex? help!

Hi. In general, that would be a true statement.
I'm not exactly sure I understand what you are starting from, and the
desired results.

is there a way to attach the file here to show you?

It's best not to post attachments in this group.
Feel free to send me the file. I'd be glad to take a look at it with any
notes you have.
If we come up with anything, we can post back for the bennefit of others
searching for the same question.

 

[Guest]

here is the data:

DATA IN: 256 by 1:

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DATA OUT: INVERSE: 256 by 1

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17933.0494516059-70.5333625292354i

 

[Dana DeLouis]

Hi. I don't really follow. A random array of complex data will most likely
have an Inverse FFt of Complex data as well. I'm not sure what output you
are expecting. Were you working with Integer values at one point?
The first, and mid-point are real values only, so I'm thinking you took a
fft on some data as well??

3622432.69085871
-5165.02426613522

Were you trying to do Convolution on two sets of data? Do you have a small
sample of data (say 8), and then show what you want?
--
Dana DeLouis

here is the data:

DATA IN: 256 by 1:

3622432.69085871
-186510.890077619+552318.675643871i
136351.90840889+128685.356316703i

<snip>

 

[Guest]

Disagree:

I have done this several times before, this is the first time that I get a
complex return on an inverse. The inverse should return an array of "real"
numbers.

The FT is performed on a time domain array (real), once you perform an FT
you go to the frequency domain (complex array), when you come back (inverse)
it should come back to the time domain (real)

 

[gimme_this_gimme_that]

If you like complex numbers and fourier transforms and if you have
some free time Google JExcel.