[vivekiny2k]

Originally Posted by NetfreakBombay

We are nit picking here, but:

DSL IS a digital "service" that is provided over analog "physical layer".

can you give us an example of a digital signal provided over a purely digital medium?

reminds me of condor's signature.

[Samurai]

Originally Posted by NetfreakBombay

DSL IS a digital "service" that is provided over analog "physical layer".

What is an analog physical layer? analog or digital refers to the signal, not the medium.

[NetfreakBombay]

Originally Posted by vivekiny2k

can you give us an example of a digital signal provided over a purely digital medium?

Ehternet.. where physical layer itself takes care of framing etc so no modem required there.

In case of ADSL ATM layer works on top of these things.

Originally Posted by Thad E Ginathom

So it is sort-of on of those depends-how-you-look-at-it questions?

IMHO thats the case.

Originally Posted by Thad E Ginathom

being outside our hearing frequency, it was, in principle an entirely different means of transmission to the singing tones of an analogue modem.

Thats right, its completely independent of frequencies used by typical dial up modems.

We can compare this situation with analog and digital cordless phones.

In case of analog, signals are directly modulated over radio waves and it becomes difficult to know what is signal and what is noise.

In case of digital cordless phones, signals are converted to digital, and multiple "channels" can be created over the same frequency. So it can be detected that error occurred and signal can be re-sent.

[Sam Kapasi]

I've never really seen a post-mortem on any YetiBlog® - I should write about all this technical stuff more often.

[tsk1979]

Write a YetiBlog about audio . That will lead to some really interesting Post Mortem

[Thad E Ginathom]

Isn't fibre-optic a purely digital medium carrying a purely digital sigal?

Light on = 1
Light off = 0

I've always made that assumption. It wouldn't surprise me to know it was completely wrong!

Same for the tx line in a serial connection? on=1, off=0; the lines in a parallel connection?

I may be oversimplifying the electrics, about which I haven't a clue, by saying 'on or off' --- it may be a difference in voltage, or whatever, but if a line has two states, one of which indicates 1, the other 0, that's digital communication, yes?

Sam.... sure, there's a certain interest in this stuff, but it will never get or hold the attention that your human adventures receive

[tsk1979]

Even in the Fiber optic communication system, light is "modulated".
So you have information + Base carrier signal.
In real world there is nothing called a purely digital signal.
It is not possible to have a purely digital signal in the real world except in textbooks.
A purely digital signal is one in which data changes from 0-1 in zero time.
That cannot happen.

[NetfreakBombay]

Originally Posted by tsk1979

A purely digital signal is one in which data changes from 0-1 in zero time.
That cannot happen.

Don't think thats a requirement. There is "Baud Rate" to measure home many times a line can flip its state.

And any medium becomes "digital" depending on the way we use it.

Example :
Plain copper wire is analogue if voltage 0.5 is valid as well as 0.0 or 1.0.

Same wire becomes digital if only 0.0 and 1.0 are treated as valid.

So medium is same, its the way we use it makes it analogue or digital.

OT : This makes digital "quality" poorer but consistent. An MP3 quality can be worse then vinyl, but with MP3 you know if file gets corrupt (checksum/parity/hash etc). With vinyl, if there is some scratch then player will play it like any other signal.

[tsk1979]

I was talking for a purely physics point of view.
Mostly in Communications technology, "Digital signal" is rarely transmitted. Usually you have a analog carrier signal, over which you modulate your "information".
The information is digital.
Over short distances, its possible to have a "digital signal". but over long distances, it would make the SnR unusable, and you will have very high attenuation.

[Hurrycane12]

This is a sure way of learning for non techies like me

[Samurai]

Originally Posted by tsk1979

Over short distances, its possible to have a "digital signal". but over long distances, it would make the SnR unusable, and you will have very high attenuation.

That is why we have repeater stations every two KMs, to fix the digital signal back to original. Digital signals can be fixed as new, but there is no such hope for analog. Once corrupted, it is spoiled, spilled milk.

[Vid6639]

Originally Posted by Samurai

That is why we have repeater stations every two KMs, to fix the digital signal back to original. Digital signals can be fixed as new, but there is no such hope for analog. Once corrupted, it is spoiled, spilled milk.

Repeater stations for digital signals? AFAIK it is virtually impossible to transmit digital signals over kms.

Repeater stations are needed for the analog signal as they also degrade when transmitted over long distances.

[tsk1979]

viddy is correct. Mostly Analog carrier is used for transmissions. In optical fibers however you have WDM with repeaters every few kms.

[Samurai]

Originally Posted by vid6639

Repeater stations for digital signals? AFAIK it is virtually impossible to transmit digital signals over kms.

What are you talking about man? How do you think telephony signals are transmitted all over the world? I work with digital telephony using T1 and E1 on a daily basis.

Hard to Find Parts, Semiconductors, IC's, Alpha Wire, BlackBox Products | Electrospec

Originally Posted by vid6639

Repeater stations are needed for the analog signal as they also degrade when transmitted over long distances.

Repeater stations for analog? This is new for me, how are they making sure they are not repeating the noise? I know noise can be fully removed in digital while repeating, but how do you do it in analog?

Edit: Oh, you mean the carrier part is repeated.

[tsk1979]

Samu, isn't the carrier analog?
Just like ADSL etc.,
T1 uses PCM right(8Khz)