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Author Topic:   FFT and inverse FFT delay time
tuscland
Member 		posted 18 May 2005 17:42         Edit/Delete Message   Reply w/Quote
Hello,

I have tried a test to see if a direct FFT -> iFFT (without processing) would cause any distortion in the signal. To do this, I took a sample and substracted the FFT->iFFT signal with the original signal delayed by the time in sample of the twice FFT size.
I found out that no distortion was induced as the two signal cancelled together, and successfully proved the theorem x = iFFT (FFT (x)). But only at one condition: the original signal must be delay by 7 times the FFT size.

I was expecting that the delay should be of twice the FFT size, could someone explain me this behaviour?

Thank you very much!


Camille

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SSC
Administrator 		posted 18 May 2005 23:04         Edit/Delete Message   Reply w/Quote
The Kyma FFT and inverse FFT modules are always working on four different times:

* storing the incoming samples into an input buffer
* computing the FFT of the previous input buffer
* shuffling the results of the FFT of the "previous previous" input buffer
* outputting the shuffled results of the "previous previous previous" input buffer

Cascading the FFT and its inverse will require essentially 7 buffers (the output buffer of the forward FFT is essentially the same as the input buffer of the inverse FFT).

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All times are CT (US)

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