> 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.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Reply by Jerry Avins●May 22, 20072007-05-22

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.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

> 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

> [...]
> 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

Reply by glen herrmannsfeldt●May 22, 20072007-05-22

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

Reply by glen herrmannsfeldt●May 22, 20072007-05-22

>>>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

> 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

Reply by glen herrmannsfeldt●May 21, 20072007-05-21

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.
-- glen

Reply by Jerry Avins●May 21, 20072007-05-21

glen herrmannsfeldt wrote:

> 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.

:-) I suppose it is safe to assume that input and output ports have zero
relative velocity.
Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Reply by kl31n●May 21, 20072007-05-21

"Randy Yates" <yates@ieee.org> wrote in message
news:m3fy5piz3z.fsf@ieee.org...

> "kl31n" <kl31n(get rid of this to write me back)@hotmail.com> writes:
>> [...]
>> I'm saying this without any sort of arrogance, but unfortunately
>> people tend to think that DSP is some sort of magic that happens
>> beyond the rule of physics and that's why weird questions are usually
>> originated.

> That's like saying "differential equations is some sort of magic that
> happens beyond the rule of physics." Well, it does happen beyond the
> rule of physics, in that domain called "mathematics." It just so
> happens that many physical things are modeled by differential
> equations, but that doesn't make them physical. Same with DSP.

I get your point, but that's not quite what I meant. My intention is simply
to underline that when the model tries to describe something physical - and
DSP does - it's often useful to discern the correctness of the results that
one can get out of the model crosschecking them against their physical
meaning.

>> The term "time delay" is ambiguous
>
> 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.

What's the definition of something ambiguous? Something that needs
assumptions to be interpreted. :)
Given the OP's message to which I answered, it was my understanding that he
was confused just about how the constant group delay influenced the phase
delay and that's where I came in. It wasn't my intention to criticize the
expression "time delay", which, as you say, is legitimate, but to move the
OP's attention on the phase delay with respect to the group delay.
Best regards,
kl31n