# Phase in FIR

Started by 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
>
> 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
```
```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

>>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
```
```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
```
```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
>> 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;
```