I already know that you know this Michael.  The way that you probally know it
the best would be the simple RC (resistor-capacitor) network.  A typical RC
network would be one where the resistor is in series with your signal and the
capacitor is to ground.  The "physically related" process that you seek is an
understanding of the capacitor and the rate that it charges and discharges.

Example #1:  You have a large diameter garden hose that you are turning on and
off at a slow rate.  When the garden hose is on, water goes out. When off, water
is sucked back up into the hose.  Put the garden hose into a small bucket and
have it turn on and off once per minute.  You can EASILY watch the level of the
water rise and fall within the bucket.

Example #2:  Small garden hose inside a large bucket and you are turning the
garden hose on and off at a high rate (lets say 20 times per second).  You would
not notice the water rise and fall very much if at all.  Mostly due to the small
amount of water going in and out of the small bucket.

Apply this to the real world:  The larger the size of your resistor, the smaller
the diameter of the garden hose (less current).  The smaller the size of the
capacitor, the smaller the size of your water bucket.

For any given size of resistor and capacitor (hose and bucket), there is a rate
of flow in which you will see the level of the cap (or bucket) rise and fall.
For a smaller resistor (lots of currents) and a small capacitor, it doesn't take
long to see the rise and fall and you can have a higher rate of turning the
signal on and off.  At some point, the capacitor doesn't have enough time to
fully charge and discharge and you don't see the level change as much (filtering
is now in progress).

Increasing the resistor (less flow / current) means it takes longer to charge
the cap (or fill the bucket) and you cannot have as fast of a rate of change (or
frequency).  Likewise, changing the value of the cap will have simular effects
(less capacitance means you can charge/discharge faster and have a higher
frequency).

michael brown wrote:

> This may sound like a dumb question, but I have not run across a good "plain
> English" description of low-pass.  I completely understand low-pass
> filtering with regards to RF and AF.  But when it comes to this stuff, I am
> unsure of what it means.  I am under the impression that it is logically
> related to the process of low-pass filtering AF and RF but not necessarily
> physically related.  By my thinking, it is a way of obtaining a general (but
> accurate) "reading" of some type of signal, without processing all the
> minutia.  If someone could kindly point me to some "understandable"
> description of the process along with some simple examples I would be much
> appreciative.  I am thinking that I have probably incorporated this process
> without "knowing" it.  To me, this is like PID a fairly simple idea with a
> complex definition.
>
> Michael Brown
> Instant Net Solutions
> www.KillerPCs.net
>
> "In the land of the blind, he who has one eye is king"
>
> --
> http://www.piclist.com hint: The PICList is archived three different
> ways.  See http://www.piclist.com/#archives for details.

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