On May 27, 9:14=A0am, HardySpicer <gyansor...@gmail.com> wrote:> On May 27, 12:18=A0pm, Tim Wescott <t...@seemywebsite.com> wrote: > > > > > > > On 05/26/2011 03:44 PM, Jerry Avins wrote: > > > > On May 26, 4:02 pm, Tim Wescott<t...@seemywebsite.com> =A0wrote: > > >> On 05/26/2011 12:18 PM, Vladimir Vassilevsky wrote: > > > >>> Tim Wescott wrote: > > > >>>> In the real world, you could say "I have a filter, whose transfer > > >>>> function is H(s) =3D 100 / (s + 10)". And such a filter can be rea=lized> > >>>> (or at least approximated). But if you then go on to say "and it's > > >>>> passive" then the half the engineers in the room would start to ei=ther> > >>>> snicker or show you why you're wrong. > > > >>> I don't see any problem with such passive filter. The only hurdle c=ould> > >>> be a gain at S =3D 0, however even that could be realized by a piez=o> > >>> transoformer, for example. > > > >>>> Passivity isn't automatic in either the Laplace domain or the z do=main> > >>>> -- it's only automatic when you're using all passive components. > > > >>> The mistake of this thread is the omission of the fact that the pow=er is> > >>> a product of voltage and current, not just a voltage or just a curr=ent.> > > >>> Derive the transfer functions for both voltages and currents, and t=he> > >>> activity or passivity of the circuit will be obvious. > > > >> I'm sorry, you're correct of course. =A0I should have made some qual=ifying> > >> comment about everything going into equal impedances, or being > > >> normalized, etc. > > > >> None the less -- the concept of "power" in the digital domain loses =a> > >> great deal of meaning in the transition from the real world, and eve=n in> > >> the real world -- as I have demonstrated and you have pointed out -- > > >> there are pitfalls to effectively keeping track of it. > > > > ... power comes out of the barrel of a gun ... Mao Tse Tung > > > > It is important to use well defined terms. > > > And he defined it well! > > > -- > > > Tim Wescott > > Wescott Design Serviceshttp://www.wescottdesign.com > > > Do you need to implement control loops in software? > > "Applied Control Theory for Embedded Systems" was written for you. > > See details athttp://www.wescottdesign.com/actfes/actfes.html > > Actually I am being Devil's Advocate here a bit because I find it > interesting. > Now if you take a string of numbers we call a digital signal and > divide them by two of course no power is lost > as we are dealing with numbers only. However, if I read a signal into > a computer and attenuate by two and send it out, > then the power of my output signal has indeed changed. I think this > can be explained by the fact that it is not the same signal that comes > out!Wrong. That can be explained by the gain settings at the post-DAC amplifier. Like when you adjust volume while playing digitized music: You don't modify the numbers stored on the CD record (or in the .wav file) even if this would obtain the objective. You adjust the gain at the amplifier stage after the DAC, that drives the speakers. Rune
This is daft FIR power
Started by ●May 23, 2011
Reply by ●May 27, 20112011-05-27
Reply by ●May 27, 20112011-05-27
On May 27, 12:14=A0am, HardySpicer <gyansor...@gmail.com> wrote:> ...> Nevertheless we do use the terms > power in DSP to denote SNR ie 10log10(Po/Pin).Never correctly.> ALso variance is taken > to be average power for a random signal. >There are classes of noise for which the numerical value of the variance is equal to the numerical value of the average power. That does not make it correct to take the variance to be the average power for any random signal. The parameters have different definitions. These may, of course, be your common practices. Dale B. Dalrymple
Reply by ●May 27, 20112011-05-27
On 05/27/2011 12:14 AM, HardySpicer wrote:> On May 27, 12:18 pm, Tim Wescott<t...@seemywebsite.com> wrote: >> On 05/26/2011 03:44 PM, Jerry Avins wrote: >> >> >> >>> On May 26, 4:02 pm, Tim Wescott<t...@seemywebsite.com> wrote: >>>> On 05/26/2011 12:18 PM, Vladimir Vassilevsky wrote: >> >>>>> Tim Wescott wrote: >> >>>>>> In the real world, you could say "I have a filter, whose transfer >>>>>> function is H(s) = 100 / (s + 10)". And such a filter can be realized >>>>>> (or at least approximated). But if you then go on to say "and it's >>>>>> passive" then the half the engineers in the room would start to either >>>>>> snicker or show you why you're wrong. >> >>>>> I don't see any problem with such passive filter. The only hurdle could >>>>> be a gain at S = 0, however even that could be realized by a piezo >>>>> transoformer, for example. >> >>>>>> Passivity isn't automatic in either the Laplace domain or the z domain >>>>>> -- it's only automatic when you're using all passive components. >> >>>>> The mistake of this thread is the omission of the fact that the power is >>>>> a product of voltage and current, not just a voltage or just a current. >> >>>>> Derive the transfer functions for both voltages and currents, and the >>>>> activity or passivity of the circuit will be obvious. >> >>>> I'm sorry, you're correct of course. I should have made some qualifying >>>> comment about everything going into equal impedances, or being >>>> normalized, etc. >> >>>> None the less -- the concept of "power" in the digital domain loses a >>>> great deal of meaning in the transition from the real world, and even in >>>> the real world -- as I have demonstrated and you have pointed out -- >>>> there are pitfalls to effectively keeping track of it. >> >>> ... power comes out of the barrel of a gun ... Mao Tse Tung >> >>> It is important to use well defined terms. >> >> And he defined it well! > > Actually I am being Devil's Advocate here a bit because I find it > interesting. > Now if you take a string of numbers we call a digital signal and > divide them by two of course no power is lost > as we are dealing with numbers only. However, if I read a signal into > a computer and attenuate by two and send it out, > then the power of my output signal has indeed changed. I think this > can be explained by the fact that it is not the same signal that comes > out! > In other words it is a reconstruction or clone of the orginal scaled > down. No power is dissipated at all since in the ideal case let us say > the ADC has infinite input impedance. Nevertheless we do use the terms > power in DSP to denote SNR ie 10log10(Po/Pin). ALso variance is taken > to be average power for a random signal.What you're doing is rejecting the discussion that I've already presented, that when we talk about "power" in signal processing terms we're using a metaphor for "power" in physics terms. In physics terms it's perfectly reasonable to ask "where did the power (or energy) go?". In DSP it's meaningless, because you've lost the connection to physical energy. Examples of infinite impedance ADCs (which, in physical terms, aren't absorbing any power from the signal at all) followed by computing systems with numerical gain, followed by DACs that coincidentally happen to have the inverse of the gain of the ADC are interesting in their own right, but -- beyond demonstrating how the two definitions are significantly different -- are meaningless and trite when you are conflating the DSP definition of "power" with the physics definition. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
