Any method to FFT/DFT a long sequence

m26k9 wrote:
>> The first F in FFT, the part that makes it fast, is building
>> up larger transforms from smaller ones.  Once you understand
>> that, the answer to your question is obvious.
>>
>> What I am still interested in seeing is 150000000 sample
>> FFT's, such as a whole CD.  At that size, you have to be
>> a little careful with memory usage.
>>
>> -- glen
>>
>>
> 
> Thanks a lot Glen.
> 
> I honestly have no clear understanding as to how FFT is doing the DFT. 
> I will look in to that. 
> 
> I am actually working on OFDM systems. So the transmitter is doing a the
> DFT on its N-point data by feeding these N-samples to the FFT module. My
> concern is, if the transmitter is equipped with a FFT module that takes in
> N samples (designed to take N-samples) at a time, if there is an efficient
> method to perform a 2N FFT operation with that N-point FFT module.
> 
> Maybe I am understanding this all wrong. My idea of an N-point FFT module
> is that of a DSP, with N inputs an I feed my N-points these inputs in
> parallel and I get a N outputs. I know in scalable-OFDM I can have
> different values of N(512,1024, etc.), but is there a 'maximum' number of
> inputs to an FFT module?
> 
> Thank you again. I think I will look into the internal of a FFT.

The inputs to a Fourier transform are individual samples taken at 
regular intervals in time. They are acquired as a serial stream. If fed 
to a digital-to-analog converter, they would reproduce the original 
signal. (Provided that the proper filters are used before the 
analog-to-digital converter and after the digital-to-analog converter.) 
You probably also want to read earlier chapters about sampling.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
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Glen Herrmannsfeldt wrote:
>What I am still interested in seeing is 150000000 sample
>FFT's, such as a whole CD. At that size, you have to be
>a little careful with memory usage.

Glen - three of the following might do it:

http://www.dilloneng.com/fft_ip/ulfft/ultralong-fft-ip-core-for-xilinx-fpgas

With a radix3 front end, you'd split the data into three streams and
then you could do a 64M FFT on each using three of the above. (PS: I
have no commercial interest or connection with the above company, and
have never used any of their products).

As for m26k9, of course you can combine smaller transforms into larger
ones, but I agree with Jerry that you need to do a lot more reading.
One of my favorites was an old textbook:

L. R. Rabiner, B. Gold, Theory and Application of Digital Signal
Processing.  Englewood Cliffs, NJ, Prentice-Hall, 1975.

Sure, it's dated with regard to some things, but it's got several good
sections on FFT (often available at a local university library, or
used copies at amazon.com).

Kevin

Thanks a lot guys.

Seems the number of FFT points are not a limitation since they can be fed
serially. I will have to read about FFT first before going any further.

Thanks for links Kevin. Didnt know such a 'huge' number of FFTs can be
supported.

Appreciate the help guys.

On Fri, 28 Nov 2008 02:29:31 -0600, "m26k9"
<maduranga.liyanage@gmail.com> wrote:

>
>
>Thanks a lot guys.
>
>Seems the number of FFT points are not a limitation since they can be fed
>serially. I will have to read about FFT first before going any further.
>
>Thanks for links Kevin. Didnt know such a 'huge' number of FFTs can be
>supported.
>
>Appreciate the help guys.

Hi,
  You might have a look at:

http://www.dsprelated.com/showarticle/63.php

Good Luck,
[-Rick-]