User talk:Dastew

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the T factor in Zero-order hold
hi Dastew,

two things: please ask questions like you have at Zero-order hold in the talk page, not on the article itself. if you want to know why there is this different factor of T, this is discussed quite a bit in Talk:Nyquist–Shannon sampling theorem and also on the USENET comp.dsp. the basic reason is that nearly all textbooks (Pohlmann: Principles of Digital Audio is a notable exception) sample with the Dirac comb without this leading factor of T, that causes each image to be scaled by 1/T, which requires the reconstruction LPF to have a passband gain of T to get the original x(t) out. here we include the leading T factor and the reconstruction LPF has a passband gain of 1. now this does not make any difference regarding the function of the ZOH, but there is confusion that this convention in the textbooks do with that gain factor that we do not have here in WP. this was discussed quite extensively at the sampling theorem talk page. anyway, welcome to Wikipedia. r b-j 18:09, 24 January 2007 (UTC)

btw, you can respond here if you want, no need to go to my talk page. r b-j 18:14, 24 January 2007 (UTC)


 * Well this is wrong!!!  Matlab does not agree with you. Octave does not agree with you.
 * and I don't agree with you. I have built and used differnt programs that are based on the ZOH  without the T in the denominator and they work!!! Therefore this is wrong!
 * Doug Stewart B.E.Sc. P.Eng.


 * it is not wrong. (and try not to underestimate other editors here - there are some pretty smart cookies here.)  and MATLAB and Octave have nothing to do with it (they both work with dimensionless numbers and T is not dimensionless).  you need to think about what it means to have a hypothetical filter (the brickwall LPF used in reconstruction) with a dimensionful passband gain of T.  how may dB is T?  does the number of dB gain that corresponds to a gain of T depend on what units you use to express T?  should it?
 * consider that you have a filter where the species of animal coming out is the same species going in (voltage-in, voltage-out or dimensionless-in, dimensionless-out). now ask yourself what is the dimension of the impulse response of such a filter?  i respect your P.Eng. but you might have something that you could learn here. r b-j 18:11, 25 January 2007 (UTC)

Would you consider puting both versions on the page? And explain that your version has to have this extra T becouse of outher problems related to how you are using it but not realy part of the ZOH?

I just took a look around the web and looked at about 30 places that disagree with you. I found 0 zero that agreed with you. Given that every resource that I can find disagrees with you( I could list them all here but why bother), could you explain to me Why: Why do " MATLAB and Octave have nothing to do with it"? Why are all these people wrong and you are the only one that knows the truth?

Doug


 * anything can be considered. a similar note used to be in the Nyquist–Shannon sampling theorem article and was taken out here and was discussed a little here (there was some objection to including this note about scaling Talk:Nyquist–Shannon sampling theorem    ).
 * so what has happened, we wanted consistancy between the sampling theorem article and the zero and first order hold articles, since the holds simply ask the question "what LTI system can you put in place of the reconstruction LPF, without changing the baseband gain, that will convert the ideally sampled signal into the piecewise constant (or piecewise linear) function that represents the output of a DAC?" if you're going to be gain neutral about this, your reconstruction LPF has to have a passband gain of 1 instead of T which is the convention in most, not all, textbooks (and that T factor is pushed ahead into the ideally sampled signal).  then when you ask the question above, the answer is a filter with dimensionless gain and, in fact, a gain of 1 (0 dB) at DC (and close to it for most of the passband).  it's not messed up by any factor of T.  there was pretty much consensus to just take out this "Note about Scaling" and scale this the logical way even if it was not the convention in most textbooks. r b-j 01:08, 26 January 2007 (UTC)

What you have done is asked the wrong question. What you should be answering is: What is the LTI system that will produce the "stair step* results of an a/d d/a. This is all in the Time domain. But what you are asking for is a particular result in the frequency domain. We have the other methods for that. (bilinear Matched pole zero etc.)

The only system that produces the correct step response (in the time domain)is the ZOH and now you want to change it so it won't even work there. When I build a system that has to work in the frequency domain criteria, I do not use the ZOH.

You could put the two systems in your web page and state what they are good for.


 * this is becoming a content issue. i am moving (copying) the comments to the article talk page that does not yet exist.  then i'll get the attention of the other editors hanging out at Nyquist–Shannon sampling theorem.  we need consistency and we don't want a POV fork.  so we, as a group, need to decide (or maybe "re-decide") which way we're gonna do this.  i still believe there is a technical issue that you are missing (but i cannot say, for sure), but perhaps getting more editors involved can straighten this out. r b-j 20:43, 26 January 2007 (UTC)