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Question

Let,

x(n)={1,0n80,elsewhereand h(n)={1,0nN0,elsewhere
where, N8 is an integer.
Given that y(n)=x(n)h(n)and y(4)=4,y(12)=0.
Then the value of N = ____.
  1. 3

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Solution

The correct option is A 3
The signal y(n)=x(n)h(n)

=k=x(k).h(nk)

In this case, this summation reduces to
y(n)=8k=0x(k).h(nk)=8k=0h(nk)

h(n)=0 for,n>N[N+1,samples are there]
x(n)=0 for,n>8[9 samples are there]
y(n)=0 for,n>(9+N1)[9+N+11=9+Nsamplesthere]
y(n)=0 for,n>8+N

Given that,
y(n)=0for,n=12
12>8+N
4 > N
N<4

From here, we may conclude that N can atmost be 3

Given that, y(4) = 4
y(4)=8k=0h(4k)
4=h(4)+h(3)+h(2)+h(1)+h(0)+h(1)+h(2)+h(3)+h(4)
4=h(4)+h(3)+h(2)+h(1)+h(0)
and this equation satisfies only when,
h(0)=h(1) = h(2) = h(3) = 1
and h(4) = 0
h(n)=1,for 0n3
N = 3

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