does the bit stream before line coding is in voltage form?

[hassan jamal]

Hi,
As we know that line coding is used to enable the data which is in the form of a bit stream to pass it through electrical cables so it converts the bits into digital waveforms.
But i ve a confusion in my mind, as i heard from one of my friends that the digital data (in the form of bits) before line coding are also in the form of voltage but they dont have specific voltage levels for example a bit '0' may have a voltage of 0.25 or 1.5V similarly a bit '1' may have the voltage level of 5.1v or 5v, but after applying line coding that different levels of voltage are converted to specific voltage waveforms for example 5v for '1' and 0v for '0' so he meant to say that line coding means quantization. is it so? is my friend right??????
In my opinion i ve the concept that the data which is in the computer's memory that has to travel through a cable (ie the data before line coding) is purely a digital stream of bits 0000101010......... and nothing else

Kindly help me out of this ambiguity plz explain me what is the correct concept behind it , thank u in advance

[jagadeshc]

yes whenever we need to transmit any data (0s and 1s) we need to convert them into any form which can be recogonised by the Reciever at the other end .Electrical signal is one form where os and 1s are having certain voltage levels and when needed to transmitt it needs a carrier to carry those 0s and 1s (Bit stream) .There are different forms of data storage one of them is magetic energy levels.any way to transmit these datas it has to be converted into electrical energy that is in the form of volatge levels as mentioned by your friend.