[Hans Blom, 960711]
(Bill Powers (960710.1430 MDT))
Uh? Another misunderstanding? What you call "disturbance", I call
"noise". For me, this translation is automatic; the literature that
I have read does not talk about "disturbance" but about "noise".As Rick Marken pointed out, we're talking about different things.
Disturbances do not have to be randomly distributed or have a mean value
of zero as you usually seem to assume; in fact most of them aren't of
that nature, because they're caused by perfectly regular and systematic
physical processes. Some of them, like gravity, are constant.
Words, words, words. They create major confusion. May I remind you of
the standard PCT schematic (I ought to have asked Rick for one; now I
am making his point ;-), of which the "world" part always looks like
this:
perception action
> --------- |
---<---| world |--<----
···
---------
>
---<--- disturbance
Thus the perception is made up of two (virtual, inseparable) parts:
the result of what the world, with its laws, makes of an action, and
the result of what (part of) the world makes of a "disturbance". This
is, as I understand it, your definition of what a disturbance is.
Right? It is that part of my perception that is not due to my action.
Formula:
p = f (a) + d, where f can be any function, static or dynamic.
Now, if you mean that a disturbance can be constant, like gravity,
where is the "world"? Doesn't gravity belong to it? Since gravity is
constant, my actions can perfectly well compensate for it, isn't it?
To me, that is a confusion between f and d. Rick's confusion was
between p and d.
Examples: I move a chair, but a gusty wind moves it as well. I am at
the steering wheel, but bumps in the road steer as well. Etc.
The disturbance in the above diagram makes sense to me only if it is
variable. I truly do not understand a lapse like this from both Rick
and you! And if I don't understand you now, I have never understood
you. Imagine that: talking past each other for, what is it, four, five
years?
At any rate, what I think of as "noise" in perception would be simply
uncertainty about the state of the perception.
Remember that I've always talked about TWO types of noises? Now
you talk about the other type. It does not act on the "world", but
only on my perception. The world is not (directly) influenced by how
I see it. My dizziness does not shake the world. The effect of both
noises (technically: system noise and observation noise) may be the
same or not; usually not.
Let me show you the difference if there are no actions at all.
x stands for the (dynamics and statics of) the world, p on my
perception, ns stands for system noise, np for observation noise,
f for the laws of nature, g for my perceptual input function.
system noise: x (k+1) := f (x (k)) + ns; p := g (x)
observation noise: x (k+1) := f (x (k)) ; p := g (x) + np
See the difference? In the first case, an unpredictable "noise" is
acting on the world itself, which influences how the world evolves in
the future; my perception of the world is fully accurate. In the
second case, the world evolves fully predictably, but my observation
of it is inaccurate. Your "disturbance" is represented by the first
case, I have always assumed; it represents the unpredictability of
the world, NOT the unpredictability of the perceptual apparatus.
Other things another time. This is basic for me.
Greetings,
Hans