Discrete non periodic : Discrete time fourier transformation (DTFT)
u can consider the part of the sequence u want to make and then it is assumed to be periodic over tht interval
eg: x
=0,1,2,3,4
if u want to find the DFT of this sequence x
then it is assumed to be periodic like x
=0,1,2,3,4,0,1,2,3,4,0,1,2,3,4.....................
then u take the DTFT . DTFT of this sequence is continuous and periodic in frequency domain which is sampled for N-points ( by using twiddle factor - which divides the unit circle into N parts in Z domain) , this the reason it is called N- point DFT . More the samples accurate is your reconstruction using fourier analysis.
cheers,
Pavan :-D