On Oct 01 11:26 96, Paul Wankadia of 3:640/772.3 wrote:

PW> On 29 Sep 96, Dave Hatch wrote to Paul Wankadia --

PW>> Yes -- could someone (anyone?) explain what that polynomial supposedly
PW>> called "scrambling" is for?
DH>> The scrambler function is to control the statistical probability
DH>> function of the encoded signal.  Without a scrambler, the probability
DH>> of steady 1 and steady 0 are pretty high.  With an adequate
DH>> pesudo-random overlay, NOTHING gets a favoured look-in.

PW> So the scrambler turns "0000000011111111110000000" into 
PW> "0101010101010101010"
PW> (or something like that) ???

You got it.  That's the desired outcome.  And the practical outcome, unless
your gambling luck is GASTLY bad.

DH>> Bingo - the maths pay off, and the spectrum is used evenly.  Without,

PW> Oh?  I thought the "spectrum" was just one frequency --
the carrier?  
PW> Please  explain to this know-nothing at 3:640/772.3 :)

The act of modulating splatters the carrier into a smear of frequencies. 
To make matters more interesting, in the presence of the more modern
modulation techniques, it's almost a case of "will the real carrier
please stand up".

In theory, ideal modulation will place signal energy evenly from one end of
the available bandwidth to the other.  ("Evenly" is a rather
esoteric bit of math in and of itself.)  This maximizes the use of the
channel capacity.

PW> [other bits about STN ratios deleted]

DH>> A "perfect" modem transmission would be sending pure
white noise as
DH>> the encoded signal.  The scrambler pretends this is so, to a moderate
DH>> degree of success.

PW> I've picked up the phone a few times in the middle of modem sessions and it
PW> certainly sounds like pure white noise to me...  Of course, that could be
PW> because I was transferring a nice big file... 

DH>>   And what do half of those TLAs actually MEAN
PW>> what some of them STAND for, but what do they MEAN?) ...?
DH>> Lots of people are too lazy to string together jawbreaker words, AND
DH>> they like to look smart at the same time...:-)

PW> Yeah -- I was actually hopping for some enlightenment there :)  For a 
PW> start,
PW> what does QAM stand for AND what does it mean?  Ditto for PCM and TAM and
PW> line probing and all that sort of thing!

QAM "Quadrature Amplitude Modulation".  Sneaky pete in operation
- mathematically consider several different signals, encode each as AM at a
particular phase angle.  (Uh.. yeah.  What he said..:-)   Turns out the
math is not only doable, but it can be physically implemented as well. 
Conceptually - well, it talks a good line of math...:-)   It also makes
pretty pictures on the right kind of test set...:-)

PCM - "Pulse Code Modulation".  Modern digital data transmission
for phone lines.  Sample the signal, digitally encode, send the digital
info.

Line probing - the attempt to quantify the particular line impairments of
this exact connection, prior to making efforts to at least roughly
compensate for some of 'em.  Send known signals each way, then listen to
the other end and compare what you actually get vis what you know was sent.
 Basic drill for setting up a high speed modem link - the horrendous and
weird noises during modem initial handshake nowdays are doing just that.

Regards,
Dave Hatch.

--- Msgedsq 3.20