On 04 Oct 96, Dave Hatch wrote to Paul Wankadia --



PW> So the scrambler turns "0000000011111111110000000" into

PW> "0101010101010101010" (or something like that) ???

 DH> You got it.  That's the desired outcome.  And the practical outcome,

 DH> unless your gambling luck is GASTLY bad.



Hmmm...  And that 0101010101010101010 is then turned into a mish-mash of

different frequencies (as detailed below by you) ???



PW> Oh?  I thought the "spectrum" was just one frequency --
the carrier?

 DH> The act of modulating splatters the carrier into a smear of

 DH> frequencies.  To make matters more interesting, in the presence of the

 DH> more modern modulation techniques, it's almost a case of "will the

 DH> real carrier please stand up".



So, while the specs say xxxx Hz is the carrier, after modulation, there is no

real carrier as such?



 DH> In theory, ideal modulation will place signal energy evenly from one

 DH> end of the available bandwidth to the other.  ("Evenly"
is a rather



What is the "available bandwidth" on your average average-quality
line?  I'm

not talking about the sort of string-quality lines of the
"outback" here...



 DH> esoteric bit of math in and of itself.)  This maximizes the use of the

 DH> channel capacity.



So that polynomial is just one of many equations necessary for modulation?



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!



 DH> QAM "Quadrature Amplitude Modulation".  Sneaky pete in operation -



Who? <%-\



 DH> mathematically consider several different signals, encode each as AM

 DH> at a particular phase angle.  (Uh.. yeah.  What he said..:-)   Turns



Wait a second...  Several different signals?  So you're splitting up the

"carrier" into parts and encoding those in Amplitude Modulation
with various

"phaser angles"???  What is a phase angle -- frequency of the
wave, gradient

of the wave at amplitude 0, etc.???



 DH> out the math is not only doable, but it can be physically implemented

 DH> as well.  Conceptually - well, it talks a good line of math...:-)   It

 DH> also makes pretty pictures on the right kind of test set...:-)



So you just plop your probes down on the line somewhere and watch them on

the test-set?  I guess you then use Fourier decomposition or something to

break down the signal into simple sine waves or something?



 DH> PCM - "Pulse Code Modulation".  Modern digital data
transmission for

 DH> phone lines.  Sample the signal, digitally encode, send the digital

 DH> info.



Is that like for CD-DA -- sample at so many Hz at this many bits, etc.???



 DH> Line probing - the attempt to quantify the particular line impairments



I.e. delay, noise, etc.?  What sorts of "impairments" are we talking about

here?



 DH> of this exact connection, prior to making efforts to at least roughly

 DH> compensate for some of 'em.  Send known signals each way, then listen

 DH> to the other end and compare what you actually get vis what you know



So noise in a simple sine wave would result in a queer wave, the modem would

then work out what the "waveform" of the noise is and then compensate???



 DH> was sent.  Basic drill for setting up a high speed modem link - the

 DH> horrendous and weird noises during modem initial handshake nowdays are

 DH> doing just that.



So the "boing boing" is a "set" signal for line probing???



BTW sorry to keep bothering you, but what does TCM mean?



Chow.



Junyer Hakker.



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