EEs: Discrete Time Question

[jmcoreymv]

I just started my discrete time class and the book is really horrible at explaining stuff. Im stuck on the first problem which is just like the rest so does anyone have any idea how to do this (it looks like its really easy)

Set up an expression for the signal using a digital freqency F < .5 for x[n]=cos(4n*pi/3)

I figure If I can get some help with how to solve the first one, the rest should make sense too.

 

[BullsOnParade]

YOU ARE FAK'D. that is all i have to say about that.
<- school starts monday

 

[BullsOnParade]

try some Z transforms with n as a repeating unit impulse maybe ?

 

[jmcoreymv]

Originally posted by: BullsOnParade
try some Z transforms with n as a repeating unit impulse maybe ?

Havent learned anything about that yet...

 

[BullsOnParade]

hmm then that $#!t can't be that hard, you're overlooking something simple, check the chapter out again.

 

[Alydis817]

i thought the question is kinda vague...but that just may be because it uses some different terminology from when i took the course...

but, basically..it wants you to set up an expression, or find an equation... that is equivalent to x[n] = cos(4n*pi/3)... using a digital frequency implies (at least to me) that they want you to digitize it -- or sample it.

the frequency of your signal is 4/3 * pi..which if you evaluate it, is 4.189. so, your signal is x[n] = cos(4.189n).

if you sample this signal at a frequency of 0.5, you're going to get a weighted sum of impulses, spaced 0.5 apart. so, basically...you're looking for an equation that describes that weighted sum.

typically, you do this by fourier series... which can be a longer explanation than i'm willing to type... but this is a good reference point:

http://www.sosmath.com/fourier/fourier1/fourier1.html

 

[jmcoreymv]

Originally posted by: Alydis817
i thought the question is kinda vague...but that just may be because it uses some different terminology from when i took the course...

but, basically..it wants you to set up an expression, or find an equation... that is equivalent to x[n] = cos(4n*pi/3)... using a digital frequency implies (at least to me) that they want you to digitize it -- or sample it.

the frequency of your signal is 4/3 * pi..which if you evaluate it, is 4.189. so, your signal is x[n] = cos(4.189n).

if you sample this signal at a frequency of 0.5, you're going to get a weighted sum of impulses, spaced 0.5 apart. so, basically...you're looking for an equation that describes that weighted sum.

typically, you do this by fourier series... which can be a longer explanation than i'm willing to type... but this is a good reference point:

http://www.sosmath.com/fourier/fourier1/fourier1.html

Thanks for the reply! I guess I also dont understand why we are sampling the signal, I thought it is already a discrete time signal since it uses n not t.

 

[jmcoreymv]

^