Forums

Phase in FIR

Started by Edison May 21, 2007
glen herrmannsfeldt <gah@ugcs.caltech.edu> writes:

> Randy Yates wrote: > (snip) > >> Assuming that the independent variable is time and we know the sample >> rate [1], time delay is absolutely precise and well-defined when >> discussing a linear phase digital filter, and all that is required to >> answer the OP's question. > > Since you already put physics into the discussion, remember that > time delay is reference frame dependent.
Are you saying that time delay must be measured relative to some reference? Agreed. However, I assumed that most people in this group would know that, for a filter, it's relative to the input since this is usually taught around the 2nd semester of undergraduate engineering school. -- % Randy Yates % "She tells me that she likes me very much, %% Fuquay-Varina, NC % but when I try to touch, she makes it %%% 919-577-9882 % all too clear." %%%% <yates@ieee.org> % 'Yours Truly, 2095', *Time*, ELO http://home.earthlink.net/~yatescr
Randy Yates wrote:
> glen herrmannsfeldt <gah@ugcs.caltech.edu> writes:
>>Randy Yates wrote:
>>>Assuming that the independent variable is time and we know the sample >>>rate [1], time delay is absolutely precise and well-defined when >>>discussing a linear phase digital filter, and all that is required to >>>answer the OP's question.
>>Since you already put physics into the discussion, remember that >>time delay is reference frame dependent.
> Are you saying that time delay must be measured relative to some > reference?
No, time dilation from special relativity. Not a problem for most DSP systems, but note that GPS requires consideration of general relativity to work. That is, time measurements depend on the gravitational field. -- glen
Jerry Avins wrote:

> glen herrmannsfeldt wrote:
(snip regarding absolute time delay)
>> Since you already put physics into the discussion, remember that >> time delay is reference frame dependent.
> :-) I suppose it is safe to assume that input and output > ports have zero relative velocity.
GPS requires consideration of general relativity, but to consider just special relativity... If the system is one box, then yes the input and output should have zero relative velocity(*). If the system is a GPS receiver and a set of GPS satellites, with inputs of some satellites connected to outputs of others, then no. (*) Not counting the effects of rotating reference frames. -- glen
glen herrmannsfeldt <gah@ugcs.caltech.edu> writes:
> [...] > No, time dilation from special relativity. Not a problem for most > DSP systems, but note that GPS requires consideration of > general relativity to work. > > That is, time measurements depend on the gravitational field.
That's physics, not DSP. That is, a real system necessarily has to exist in the physical universe and is therefore under the physical laws of that universe. Not so with DSP, i.e., we can conceive and analyze algorithms without any real, physical system ever being created or even hypothesized. That's not to say, as your GPS example illustrates, that physics is irrelevent to engineering - just a different set of considerations. -- % Randy Yates % "She tells me that she likes me very much, %% Fuquay-Varina, NC % but when I try to touch, she makes it %%% 919-577-9882 % all too clear." %%%% <yates@ieee.org> % 'Yours Truly, 2095', *Time*, ELO http://home.earthlink.net/~yatescr
glen herrmannsfeldt <gah@ugcs.caltech.edu> writes:

> Jerry Avins wrote: > >> glen herrmannsfeldt wrote: > > (snip regarding absolute time delay) > >>> Since you already put physics into the discussion, remember that >>> time delay is reference frame dependent. > >> :-) I suppose it is safe to assume that input and output >> ports have zero relative velocity. > > GPS requires consideration of general relativity, > but to consider just special relativity... > > If the system is one box, then yes the input and output > should have zero relative velocity(*). If the system is a GPS > receiver and a set of GPS satellites, with inputs of some > satellites connected to outputs of others, then no. > > (*) Not counting the effects of rotating reference frames.
Another example of an engineering problem that [surprisingly] requires accounting of relativistic effects, or so I'm told, is CRT displays/monitors. Apparently there's something tied up between it and electron speed/positional accuracy or some-such. Maybe that's passe now, with LCD panels/DLP/plasma the norm... -- % Randy Yates % "Maybe one day I'll feel her cold embrace, %% Fuquay-Varina, NC % and kiss her interface, %%% 919-577-9882 % til then, I'll leave her alone." %%%% <yates@ieee.org> % 'Yours Truly, 2095', *Time*, ELO http://home.earthlink.net/~yatescr
Randy Yates wrote:
> glen herrmannsfeldt <gah@ugcs.caltech.edu> writes: > >> Randy Yates wrote: >> (snip) >> >>> Assuming that the independent variable is time and we know the sample >>> rate [1], time delay is absolutely precise and well-defined when >>> discussing a linear phase digital filter, and all that is required to >>> answer the OP's question. >> Since you already put physics into the discussion, remember that >> time delay is reference frame dependent. > > Are you saying that time delay must be measured relative to some > reference? > > Agreed. However, I assumed that most people in this group would know > that, for a filter, it's relative to the input since this is usually > taught around the 2nd semester of undergraduate engineering school.
I think Glenn was pointing out that the delay could vary if input and output separated at relativistic speeds. He didn't discuss the likelihood of that happening because it would have spoiled the joke. Jerry -- Engineering is the art of making what you want from things you can get. &macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;
glen herrmannsfeldt wrote:
> Jerry Avins wrote: > >> glen herrmannsfeldt wrote: > > (snip regarding absolute time delay) > >>> Since you already put physics into the discussion, remember that >>> time delay is reference frame dependent. > >> :-) I suppose it is safe to assume that input and output >> ports have zero relative velocity. > > GPS requires consideration of general relativity, > but to consider just special relativity... > > If the system is one box, then yes the input and output > should have zero relative velocity(*). If the system is a GPS > receiver and a set of GPS satellites, with inputs of some > satellites connected to outputs of others, then no. > > (*) Not counting the effects of rotating reference frames.
"Delay" implies that the signals remain in step. Time dilation actually changes the frequency, as does Doppler. Relativistic effects can't, in general, be modeled as simple time delays. Jerry -- Engineering is the art of making what you want from things you can get. &macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;