## PSN-L Email List Message

Subject: Re: Digest from 08/06/2007 00:01:22
From: ChrisAtUpw@.......
Date: Sat, 11 Aug 2007 14:51:25 EDT

```In a message dated 2007/08/11, gmvoeth@........... writes:

> I really dont know how you can say these things as the
> absolute truth unless you are a Physics Major with a PHD
> With a degree also in Math to prove your points.
> You might very possibly be both of those but because
> of the way the USA is and its lack of freedoms
> I have no legal way to positively
> identify either you or your background.

Hi Geoff,

You could always look me up in the Oxford University Yearbook?

> It is interesting to note we all live maybe
> 100 milliseconds behind real time and with
> that in mind is it possible at all that we can know the present.
>
> I think because the phase shift is bizarre through the entire spectrum
> of any filtered component that a faithful recreation of actual ground
> motion is out of reach for the common man.
>
> A square wave ground motion will not be square wave recorded
> with a hundred or more of necessary ODD harmonics showing.
> As all those harmonics must be in phase with the fundamental.

If you use a Bessel filter, you get a roughly square wave out for a
square wave in, but with rounded corners, depending on the relationship of the
wave frequency to the corner ferquency. The phase relationship below fc is
~constant.

If you want roughly the same tail off with a Bessel filter, you need
to choose about 0.7 x the corner frequency you would use with a Butterworth
filter.

Regards,

Chris Chapman
In a me=
ssage dated 2007/08/11, gmvoeth@........... writes:

I really dont know how you can=20=
say these things as the
absolute truth unless you are a Physics Major with a PHD
With a degree also in Math to prove your points.
You might very possibly be both of those but because
of the way the USA is and its lack of freedoms
I have no legal way to positively
identify either you or your background.

Hi Geoff,

You could always look me up in the Oxfo=
rd University Yearbook?

It is interesting to note we al=
l live maybe
100 milliseconds behind real time and with
that in mind is it possible at all that we can know the present.

I think because the phase shift is bizarre through the entire spectrum
of any filtered component that a faithful recreation of actual ground
motion is out of reach for the common man.

A square wave ground motion will not be square wave recorded
with a hundred or more of necessary ODD harmonics showing.
As all those harmonics must be in phase with the fundamental.

If you use a Bessel filter, you get a r=
oughly square wave out for a square wave in, but with rounded corners, depen=
ding on the relationship of the wave frequency to the corner ferquency. The=20=
phase relationship below fc is ~constant.

If you want roughly the same tail off w=
ith a Bessel filter, you need to choose about 0.7 x the corner frequency you=
would use with a Butterworth filter.

Regards,

Chris Chapman
```