Thursday, March 21, 2019

sampling - Absolute convergence of periodic sinc interpolation


An N-periodic complex discrete-time sequence [x0,,xN1] can be resampled to an M-periodic sequence [y0,,yM1] with M>N, using sinc interpolation:



ym=n=sinc(NmMn)xnmodN=k=N1n=0sinc(NmMnNk)xn


where mod denotes the modulo operation and:


sinc(x)={1if x=0,sin(πx)πxotherwise.


Eq. 1 can be seen as resampling an N-periodic continuous-time signal from samples xn at times n+Nk to samples ym at times NmM.


For example, a 2-periodic complex discrete-time sequence [x0,x1] can be resampled to a 4-periodic sequence [y0,y1,y2,y3]:


Eq. 1, N=2,M=4

{y0=x0y1=k=(sinc(2k+32)x0+sinc(2k+12)x1)y2=x1y3=k=(sinc(2k+12)x0+sinc(2k+32)x1)


The two series in Eq. 3 converge conditionally, with for example these possible rearrangements of the first series that give conflicting results if x0x1:


k=0(f(k)+f(k+1))=x0+x12,k=0(f(k)+f(2k+1)+f(2k+2))=(x1x0)ln(2)2π+x0+x12,k=0(f(k)+f(3k+1)+f(3k+2)+f(3k+3))=(x1x0)ln(3)2π+x0+x12,


with shorthand f(k)=sinc(2k+32)x0+sinc(2k+12)x1.


Under which condition does the series given by Eq. 1 converge absolutely?





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