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Unformatted text preview: ECE600 Introduction to Digital Signal Processing Autumn 2011 Homework #1 Sep. 28, 2011 HOMEWORK SOLUTIONS #1 1. Here, the input/output relationship of H is y [ n ] = 2 m =0 (3 m ) x [ n m ] = 3 x [ n ]+2 x [ n 1]+ x [ n 2]. (a) For H to be linear, we need that H{ x [ n ] + w [ n ] } = H{ x [ n ] } + H{ w [ n ] } for all and and x [ n ] and w [ n ] . From the I/O relationship, we know that H{ x [ n ] + w [ n ] } = 2 summationdisplay m =0 (3 m )( x [ n m ] + w [ n m ]) = 2 summationdisplay m =0 (3 m ) x [ n m ] + 2 summationdisplay m =0 (3 m ) w [ n m ] = H{ x [ n ] } + H{ w [ n ] } . Thus, the system is linear. (b) Time invariance means that, if H{ x [ n ] } = y [ n ], then H{ x [ n d ] } = y [ n d ] for any integer shift d . Investigating whether this is the case here, we first use H{ x [ n ] } = 2 m =0 (3 m ) x [ n m ] to evaluate H{ x [ n d ] } = 3 x [ n d ] + 2 x [ n d 1] + x [ n d 2] and then we use y [ n ] = 3 x [ n ] + 2 x [ n 1] + x [ n 2] evaluate y [ n d ] = 3 x [ n d ] + 2 x [ n d 1] + x [ n d 2] . Since these two quantities are equal, the system is indeed timeinvariant. (c) Causality means that, for any time d , the output y [ d ] does not depend on the future inputs { x [ n ] } n>d . From the I/O relationship, we see that y [ d ] depends only on the current input x [ d ] and the past inputs x [ d 1] and x [ d 2], and thus the system is causal. (d) Stability means that a bounded input guarantees a bounded output. To examine whether the system is stable, we can assume a generic bounded input and check to see whether the output is bounded. Say that the input is bounded. This means that there exists finite M x such that  x [ n ]  < M x for all n . Then, from the I/O relationship,  y [ n ]  =  3 x [ n ] + 2 x [ n 1] + x [ n 2]  3  x [ n ]  + 2  x [ n 1]  +  x [ n...
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 Fall '08
 Clymer,B
 Digital Signal Processing, Signal Processing

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