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**DFT - can't evaluate components in -negative time!!**

hey guys - if you look at the definition/equation of discrete fourier transform, you will see that we can only evaluat values from n=0 to N (see http://en.wikipedia.org/wiki/Discrete_Fourier_transform, to view the equation).....but what have we have a discrete time signal that appears on the negative time axis as well???? according to the equation, we neglect all the negative parts and only start from n =0!!!!

please can someone give me an explanation as this has been bugging me for some time now!!

thanks very much!!!

John