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'[OT] Impossible--- Was RE: Encode 4 keypad digits '
1999\02\20@145122 by Dave Evans

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I have changed the Subject to [OT] so that folks
who pay to download messages like this are not
forced to listen to our exchanges (but some may
want to continue for a little longer).

I respect your opinions and I especially appreciate
your attitude and civilized tone. Two words that
instantly raise red flags for me are "always" and
"never". If anyone tells me that something is
impossible, I expect them to back it up.

If truly creative people want to spend some time
exploring ways to beat the odds, or to do the
"impossible", I say, "Go for It." On the other hand,
if a person wants to solve a real-world problem that,
I claim, really can't be done, it might be some service
to him to let him know that maybe he can put his
talents to better use. (He is free to ignore my
freely-given advice; no hard feelings.)

For example, a fellow once asked my advice on
how to implement a power supply on a piece of
remote equipment. The remote unit was connected
to the base unit by a pair of 24 gauge copper
wires with a total resistance of about 300 Ohms.
Total power consumption of the remote unit was
20 watts. He was allowed to apply a maximum
of 130 volts DC at the base unit. What kind of
supply should he design (or buy): Switching?
Linear? He was getting ready to tell me what
values of voltage and current were required
at the outputs of the supply. I told him,
"Never mind, it's impossible." Why? Was I wrong?
What were my assumptions? Should I have let him
continue to dream the impossible dream?

Regarding the several interesting cases you mention:

QAM and other, even more exotic, coding methods can
increase the rate of information transfer on
a noisy channel. Upper limits, for a given
error rate, may be calculable by well known
theorems of Information Theory, but you have
to make some assumptions. Bandwidth, type of
noise (white Gaussian noise is almost like
worst-case; real world signals may fare better),
attainable signal-to-noise ratio, etc., etc.
Equipment is now coming on the market to allow
use of twisted-pair "phone" lines to carry information
at several megabits per second. Hint: they use
a higher bandwidth than the traditional 3300 Hz
voice telephony application.

Error correcting codes do not compress the width
of the memory data bus, they add extra bits in
such a way that they induce some redundancy that
can be used to detect and/or correct certain types
of error patterns.

Doublespace and other lossless compression techniques
identify redundancy in a given piece of data, and
re-encode the information to a more efficient form
in such a way that the original information can
always be recovered exactly. The increase in storage
efficiency is a factor of the redundancy that a
particular method can identify in the data. (Binary
files typically are compressed by a factor of 1:2;
text files usually gain by 1:10 or more.) In a similar
vein, we can compress the 16 bits of four decimal
digits to 14 bits by a simple BCD-binary conversion
routine. We simply can't compress them to less than
14 bits if we have to get all of the information
back out.

We ***can*** put 10 pounds of sugar into a
five-pound bag, OK (easy --- just spill half of it),
but we can't get all 10 pounds back out. At least
I don't know of any compression method for sugar.
See, I should always mention my assumptions when
dealing with physical phenomena.

Yes, indeed, I thought it was "impossible" to reduce
the price of anything with moving parts (like a
hard disk drive), just like the guy who once
predicted that horseless carriages were a horrible
threat to humanity, since "the human body will
disintegrate if it ever exceeds the speed of
a galloping horse." (I lost the reference to that
one when the hard drive carrying my bibliography
crashed.)

"I was born not knowing, and have only had
a little time to change that, here and there."
 --- Richard Feynman

Dave Evans, KW7X
spam_OUTdave.evansTakeThisOuTspamdlcc.com

> {Original Message removed}

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