More Signals Help

Discussion in 'Math' started by crazyengineer, Jul 4, 2011.

  1. crazyengineer

    Thread Starter Member

    Dec 29, 2010
    156
    2
    Hi! I need help understanding this signal

    [​IMG]

    Originally I thought the signal was this
    [​IMG]

    But it's actually this
    [​IMG]

    Can anyone help me understand how they got this graph? I thought since x[n] is one when n>=0, I pay attention to u[1-t] from 0 to 1.
     
  2. guitarguy12387

    Active Member

    Apr 10, 2008
    359
    12
    This is completely arbitrary.

    You need to tell us what x[n] is for us to explain the graph. We assume u[n] is the step function and x[n] is some other arbitrary signal. Without its definition, these plots are completely meaningless

    I think you may be misunderstanding the concept of the unit step function
     
  3. kevinarms

    New Member

    Jun 29, 2011
    6
    2
    this is just the original signal - x[n] - multiplied by a modified step function. u[1-n] is a step function that is time reversed around n=1. So u[1-n] = 1 for n<=1 and u[1-n] = 0 for n > 1 (instead of one for n>=0). So for g[n] = x[n]u[1-n], g[n] = x[n] for n <= 1 (where u[1-n] = 1), and g[n] = 0 for n>1 (where u[1-n] = 0).
     
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  4. crazyengineer

    Thread Starter Member

    Dec 29, 2010
    156
    2
    Opps! I looked in my book and found x[n] *slaps his head* Sorry for the mistake!

    This is X[n]
    [​IMG]
     
  5. kevinarms

    New Member

    Jun 29, 2011
    6
    2
    So x[n]u[1-n] is simply a pointwise multiplication of this given x[n] with u[1-n]. u[1-n] has a value of 1 for all n<=1, and a value of 0 for n >1. So with this given x[n], the result will be almost the same as x[n] (since you are multiplying most sample values by 1), except for the resulting function changes to 0 at n = 2,3, since u[1-n] = 0 at these two points. This gives you the result that you originally posted.
     
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  6. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    I think the gentleman is talking about sidebands, as in RF and AM theory. Hard to be sure though.
     
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