On 11 Oct 96, Dave Hatch wrote to Paul Wankadia --



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

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

 DH> Yup.  That's the idea.



So this is the way I see it (you'll inevitably have to correct me) --



(1) Sine wave is generated at carrier frequency.

(2) Data stream is modulated onto wave using AM.

(3) Resulting "funny-looking" wave is mutilated further by various other

    modulation and error-resistant means.

(4) Signal travels to remote end and is decoded.



PW> So, while the specs say xxxx Hz is the carrier, after modulation, there

PW> no real carrier as such?

 DH> There is - mathematically.  And the "errors" from that
pure sinewave



That's kinda like saying that (theoretically) entropy could DEcrease, but

(statistically) it never happens? :)



 DH> carrier are the carriers of the actual information.   Some of the

 DH> earlier modems even actually worked that way.



The ERRORS?!?  NOW I'm confused... <%-(



PW> What is the "available bandwidth" on your average
average-quality line?

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

 DH> A cruddy one is usually useable from 300hz to 3000.  A good one will

 DH> stretch that to 250 to 3600.



Righto.  That reminds me -- when a modem picks up the phone, it emits a few

different tones.  What are those tones in Hz and what are they for?  Does the

calling modem wait to hear the answering modem's "qualifications"???



PW> So that polynomial is just one of many equations necessary for modulatio

 DH> Yup.



Got any samples?



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

PW> gradient of the wave at amplitude 0, etc.???

 DH> No - try it like this.   Two identical sine waves, phase shifted.  One



So partial cancellation results (out of phase?) ???



 DH> the carrier, the other the quadrature component.  Now AM both of them.



What does "AM-ing" a sine wave do to it?  And what exactly is the
quadrature

component for (you prolly explained for QAM, but that was another message or

wasn't quoted again) ...???



 DH>  Then matrix multiply the pair to get a single value, which is the

 DH> real instantaneous voltage sent to line.   Believe it or not, the

 DH> process is reversible at the other end - and is even moderately

 DH> resistant to noise in the middle.



How is the process reversed?



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

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

PW> break down the signal into simple sine waves or something?

 DH> No - what it actually does is to encode the data into a differential

 DH> on a circle, which represents the original carrier plus its quadrature

 DH> component. Modulation yanks a point away from the quiescent circle.



What is a "differential on a circle"???  But I guess you mean that the

original signal can be found by pulling the circle into a straight line and

looking at how the "differential" moves?



 DH> The live output looks like a bunch of bright star points, hence the

 DH> name of "constellation".



Now where did astronomy come into this?!? <%-\



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

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

 DH> Exactly.  A CD could be, and actually sometimes is, sent over a studio

 DH> quality phone line, using this same technology.



What sort of quality are we talking about for "studio" quality? 
Do you mean

high bandwidth or just "virtually noise-less"?



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



[errors deleted]



Oh.  So the modems have to work around all THAT?



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

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

 DH> That's the idea.  It's more like the chirp transform of a pulse for



And the chirp transform is...?



 DH> the more interesting bits - but what it's measuring is amplitude loss

 DH> variation with frequency, and delay variation with frequency, and

 DH> seeking to linearize both.



Ahem... <%-\  Englaise si vou plait :)



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

 DH> Yep.  So is the back-and-forth blat-blat that happens during a retrain.



I have M1 as the setup (not M2), so I don't usually hear retrains...



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

 DH> Trellis Code Modulation.  That's a description of one of the options

 DH> in the QUAM we were discussing above.  The idea is that you use the

 DH> possible modulation points sparingly, so a single noise pulse must

 DH> push the signal through _two_ wrong states instead of one to

 DH> successfully register an error.



That means you just resend the packet, right?



 DH> Direct application of s/n vis data rate signal theory to increase the

 DH> noise resistance of a link at the expense of the data rate.



Doh! :)



>> CONTINUED IN NEXT MESSAGE <<



Chow.



Junyer Hakker.



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