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From: glen herrmannsfeldt on 6 May 2008 13:31
Fred Marshall wrote:
(snip)

>>Ok, that's good. Am I right in thinking I'll only take half of the
>>output of the IFFT (as it should be mirrored), meaning with a
>>frequency resoultion of 0.2, I can get 2.5 seconds of impulse
>>response?

> ***No. That would be closer to correct for the results of an FFT but not
> for an IFFT. You should be very careful about throwing out what seems
> "redundant" because it really isn't. That's because generally:

> ***The time series is real but not purely even or odd.
> Thus the frequency series is complex with the real part even and the
> imaginary part odd around zero/fs and is zero at fs/2.

You might be interested in the Hartley transform, and
corresponding FHT:

http://en.wikipedia.org/wiki/Discrete_Hartley_transform

where the transform of real data is real, of even data is
even, and of odd data is odd. (And of imaginary data
is imaginary.)

The web page indicates that FHT is about the same number
of operations as FFT optimized for real data, but might
be faster in some cases.

-- glen

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