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Leo Lam © 2010-2012 Signals and Systems EE235

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Oh beer… An infinite amount of mathematicians walk into a bar. The first one orders a beer. The second one orders half a beer. The third one orders one quarter of a beer. The fourth one starts to order, but the bartender interrupts "Here's two beers; you lot can figure the rest out yourself." Leo Lam © 2010-2012

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Today’s menu To do: –If you still haven’t: join the Facebook Group! –From Wednesday: Describing Common Signals –Introduced the three “building blocks” –General description for sinusoidal signals –Started Periodicity Today: –Finish Periodicity –Manipulating signals!

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Periodic signals Definition: x(t) is periodic if there exists a T (time period) such that: The minimum T is the period Fundamental frequency f 0 =1/T Leo Lam © 2010-2012 For all integers n

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Your turn! Find the period of: Leo Lam © 2010-2012 No LCM exists! Why? Because LCM exists only if T 1 /T 2 is a rational number

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A few more Leo Lam © 2010-2012 Not rational, so not periodic Decaying term means pattern does not repeat exactly, so not periodic

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Summary Periodicity Leo Lam © 2010-2012

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Playing with signals Operations with signals –add, subtract, multiply, divide signals pointwise –time delay, scaling, reversal Properties of signals (cont.) –even and odd Leo Lam © 2010-2011

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Adding signals Leo Lam © 2010-2011 1 1 123 t t + = ?? x(t) y(t) 1 t 123 x(t)+y(t)

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Delay signals Leo Lam © 2010-2011 unit pulse signal (memorize) t 0 1 1 What does y(t)=p(t-3) look like? P(t) 0 34

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Multiply signals Leo Lam © 2010-2011

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Scaling time Leo Lam © 2010-2011 Speed-up: y(t)=x(2t) is x(t) sped up by a factor of 2 t 0 1 1 t 0 1.5 y(t)=x(2t) How could you slow x(t) down by a factor of 2? y(t)=x(t)

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Scaling time Leo Lam © 2010-2011 y(t)=x(t/2) is x(t) slowed down by a factor of 2 t 01 t 01 y(t)=x(t/2) 2 -2 y(t)=x(t)

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Playing with signals Leo Lam © 2010-2011 What is y(t) in terms of the unit pulse p(t)? t 8 3 5 t 0 1 1 Need: 1.Wider (x-axis) factor of 2 2.Taller (y-axis) factor of 8 3.Delayed (x-axis) 3 seconds

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Playing with signals Leo Lam © 2010-2011 t 8 3 5 in terms of unit pulse p(t) t 8 2 first step: 3 5 t 8 second step:

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Playing with signals Leo Lam © 2010-2011 t 8 3 5 in terms of unit pulse p(t) t 8 2 first step: 3 5 t 8 second step: replace t by t-3: Is it correct?

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Playing with signals Leo Lam © 2010-2011 3 5 t 8 Double-check: pulse starts: pulse ends:

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Do it in reverse Leo Lam © 2010-2011 t Sketch 1

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Do it in reverse Leo Lam © 2010-2011 t Let then that is, y(t) is a delayed pulse p(t-3) sped up by 3. 1 1 4/3 1 3 4 Double-check pulse starts: 3t-3 = 0 pulse ends: 3t-3=1

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Order matters Leo Lam © 2010-2011 With time operations, order matters y(t)=x(at+b) can be found by: Shift by b then scale by a (delay signal by b, then speed it up by a) w(t)=x(t+b) y(t)=w(at)=x(at+b) Scale by a then shift by b/a w(t)=x(at) y(t)=w(t+b/a)=x(a(t+b/a))=x(at+b)

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Playing with time Leo Lam © 2010-2011 t 1 What does look like? 2 1 -2 Time reverse of speech: Also a form of time scaling, only with a negative number

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Playing with time Leo Lam © 2010-2011 t 1 2 Describe z(t) in terms of w(t) 1 -213 t

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Playing with time Leo Lam © 2010-2011 time reverse it: x(t) = w(-t) delay it by 3: z(t) = x(t-3) so z(t) = w(-(t-3)) = w(-t + 3) t 1 2 1 -21 3 x(t) you replaced the t in x(t) by t-3. so replace the t in w(t) by t-3: x(t-3) = w(-(t-3))

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Playing with time Leo Lam © 2010-2011 z(t) = w(-t + 3) t 1 2 1 -21 3 x(t) Doublecheck: w(t) starts at 0 so -t+3 = 0 gives t= 3, this is the start (tip) of the triangle z(t). w(t) ends at 2 So -t+3=2 gives t=1, z(t) ends there

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Summary: Arithmetic: Add, subtract, multiple Time: delay, scaling, shift, mirror/reverse And combination of those Leo Lam © 2010-2011

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How to find LCM Factorize and group Your turn: 225 and 270’s LCM Answer: 1350 Leo Lam © 2010-2012

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Even and odd signals Leo Lam © 2010-2012 An even signal is such that: t Symmetrical across the t=0 axis t Asymmetrical across the t=0 axis An odd signal is such that:

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Even and odd signals Leo Lam © 2010-2012 Every signal sum of an odd and even signal. Even signal is such that: The even and odd parts of a signal Odd signal is such that:

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