power & energy signals

Started by shikha April 16, 2005
pls can any body make me understand difference between power signals &
Energy signals.
why  r random signals called power signals.

		
This message was sent using the Comp.DSP web interface on
www.DSPRelated.com
geeez i wish the chat-room lingo would stay in the chat room.

in article VeidnVJa8dkOkfzfRVn-uw@giganews.com, shikha at
shikhadrdo@yahoo.co.in wrote on 04/16/2005 09:01:

> > pls can any body make me understand difference between power signals & > Energy signals.
a finite energy signal: +inf total energy = integral{ x(t)^2 dt} < infinity -inf a finite power signal: +T/2 average power = lim 1/T integral{ x(t)^2 dt} < infinity T->inf -T/2
> why r random signals called power signals.
because, like a sine wave, if they are left turned on forever, they will deliver an infinite amount of energy. but their average power is finite. -- r b-j rbj@audioimagination.com "Imagination is more important than knowledge."
shikha wrote:
> pls can any body make me understand difference between power signals
&
> Energy signals. > why r random signals called power signals.
Trying to make it easy: An energy signal has a finite energy. Signals of a limited length also carry a finite energy, and so they are energy signals. A signal that decays exponentially, for example, also has a finite energy. A power signal is not limited in time (it is *always* on, from the Big-Bang to Judgement Day and beyond), and has an *infinite* energy. Since an infinite energy has no meaning for us, then we use the energy per unit of time, i.e., power. Examples: A square pulse is an energy signal. A square wave of infinite length is a power signal.
"shikha" <shikhadrdo@yahoo.co.in> wrote in message 
news:VeidnVJa8dkOkfzfRVn-uw@giganews.com...
> > pls can any body make me understand difference between power signals & > Energy signals. > why r random signals called power signals. >
Well, first of all, this is the first time I can recall ever hearing of signals labeled like this. So, I would suspect the labeling "energy signals" and "power signals". Signals are signals as functions are functions as sequences are sequences. But maybe I've missed something along the way. Robert suggested something in adding "finite" to his definitions. Now that changes things a lot! Mind well. Oh darn! Now I find that folks have been using these terms. Here is one from Stanford at: http://www.stanford.edu/class/ee179/lecture7.pdf VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV 1. Energy and Power Signals An energy signal x(t) has 0 < E < 1 for average energy inf E = int[|x(t)|^2]dt -inf A power signal x(t) has 0 < P < 1 for average power inf P = lim [1/2T]*int[|x(t)|^2]dt T>inf -inf - Can think of average power as average energy/time. - An energy signal has zero average power. A power signal has infinite average energy. Power signals are generally not integrable so don&#2013266066;t necessarily have a Fourier transform. - We use power spectral density to characterize power signals that don&#2013266066;t have a Fourier transform. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Oh my ... and where did those 1's come from? I guess I'd have to do a bit of studying on this because it doesn't make any sense to me right off. I see an equation for E that doesn't involve time - OK. Then I read a description that says "average". I always thought that power was energy per unit time. Now we have "average power as average energy/time" Well the two aren't exactly inconsistent but power=energy per unit time is already an expression of an average isn't it? So now we have?: average power is average average energy??? For this to be the case, there have to be two time frames otherwise average average energy (aae) is what? aae=[1/T]*sum[(1/T)*sum|x(t)|^2] or [1/T^2]*sumsum[|x(t)|^2] T T T T I'm supposed to know better than to waste time on silly things. Here's another description from: http://www.signal.uu.se/Courses/CourseDirs/ModDemKod/2005/Lectures/lecture1/slides.ppt#14 A signal is an "energy signal" if, and only if, it has nonzero but finite energy for all time 0<Ex<inf: T/2 inf Ex =lim int[|x(t)|^2]dt = int[|x(t)|^2]dt T>inf -T/2 -inf A signal is a "power signal" if, and only if, it has finite but nonzero power for all time 0 < Px < inf: T/2 Px =lim (1/T)int[|x(t)|^2]dt T>inf -T/2 General rule: Periodic and random signals are power signals. Signals that are both deterministic and non-periodic are energy signals. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ It seems this is supposed to help when looking at the autocorrelation and spectral densities. So, OK. I learned something, I hope Shikha did too! Fred
Fred Marshall wrote:

   ...

> - An energy signal has zero average power. A power signal has infinite > average energy.
Huh? Power goes with the square of magnitude (into a resistive load). It is a positive number for all non-zero magnitudes. To have zero average power, a signal must be brief and averaged over all time, or everywhere zero. ... Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
Fred Marshall wrote:


> A signal is an "energy signal" if, and only if, it has nonzero but finite > energy for all time 0<Ex<inf:
...
> A signal is a "power signal" if, and only if, it has finite but nonzero > power for all time 0 < Px < inf:
Hunh? again. A sinusoid fits neither of those verbal descriptions. ... Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
"Jerry Avins" <jya@ieee.org> wrote in message 
news:ieKdndZsCawKcf7fRVn-ug@rcn.net...
> Fred Marshall wrote: > > >> A signal is an "energy signal" if, and only if, it has nonzero but finite >> energy for all time 0<Ex<inf: > > ... > >> A signal is a "power signal" if, and only if, it has finite but nonzero >> power for all time 0 < Px < inf: > > Hunh? again. A sinusoid fits neither of those verbal descriptions. >
Jerry, First off, these were all "quotes" ...... and I was questioning the whole thing from the get go. Second, why doesn't a sinusoid have finite but nonzero power for all time? er.... if I know what that even means! Again, if power is energy per unit time then finite for all time (in chunks of time) seems OK to me. Fred Fred
Jerry Avins <jya@ieee.org> writes:

> Fred Marshall wrote: > > ... > > > - An energy signal has zero average power. A power signal has > > infinite average energy. > > > Huh? Power goes with the square of magnitude (into a resistive > load). It is a positive number for all non-zero magnitudes. To have > zero average power, a signal must be brief and averaged over all time, > or everywhere zero.
This is correct, Jerry, as I understand it. Any finite-temporal-extent signal is considered a finite-energy, zero-power signal because of the first reason you stated. -- Randy Yates Sony Ericsson Mobile Communications Research Triangle Park, NC, USA randy.yates@sonyericsson.com, 919-472-1124
Jerry Avins <jya@ieee.org> writes:

> Fred Marshall wrote: > > > > A signal is an "energy signal" if, and only if, it has nonzero but > > finite energy for all time 0<Ex<inf: > > > ... > > > A signal is a "power signal" if, and only if, it has finite but > > nonzero power for all time 0 < Px < inf: > > > Hunh? again. A sinusoid fits neither of those verbal descriptions.
Why don't you think a sinusoid has finite but non-zero power? When Fred stated "... for all time..." I'm assuming he means "when averaged over all time." -- Randy Yates Sony Ericsson Mobile Communications Research Triangle Park, NC, USA randy.yates@sonyericsson.com, 919-472-1124
Fred Marshall wrote:
> "Jerry Avins" <jya@ieee.org> wrote in message > news:ieKdndZsCawKcf7fRVn-ug@rcn.net... > >>Fred Marshall wrote: >> >> >> >>>A signal is an "energy signal" if, and only if, it has nonzero but finite >>>energy for all time 0<Ex<inf: >> >> ... >> >> >>>A signal is a "power signal" if, and only if, it has finite but nonzero >>>power for all time 0 < Px < inf: >> >>Hunh? again. A sinusoid fits neither of those verbal descriptions. >> > > > Jerry, > > First off, these were all "quotes" ...... and I was questioning the whole > thing from the get go. > > Second, why doesn't a sinusoid have finite but nonzero power for all time? > er.... if I know what that even means! Again, if power is energy per unit > time then finite for all time (in chunks of time) seems OK to me. > > Fred
Fred, I was questioning the quotes, not you. A sinusoid is sometimes positive, sometimes negative. At the instant of transition, it is zero, violating the condition "finite but nonzero power for all time". Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;