Difference between Line Code and Keying

Keying or digital modulation applies to transmitting a digital signal over an analogue passband channel.

When a digital signal is to be transmitted over an analogue baseband channel, the modulation technique is termed line coding.

To me, both of the lines seem same. The only difference I see are the terms passband channel and baseband channel.

I see both of these terms used interchangeably on the internet because we only care for the message that we get from passband channel which is obtained after filtering baseband channel.

So, what's the difference between Line Code and Keying?

• you say "both of the lines seem same", but I nowhere see two lines. Can you pinpoint what seems to be the same? Commented Mar 16, 2018 at 13:03
• @MarcusMüller I have edited the question to separate out the 2 lines that are confusing me Commented Mar 16, 2018 at 13:07
• Thank you! I was kind of blind, though, I should've inferred that :) Thank you anyway! Commented Mar 16, 2018 at 13:16

I think the Wikipedia definition is limited, and anybody using the terms keying and line coding interchangably is wrong.

A line code is an encoding (or mapping) from bits to pulse amplitudes (voltages) whose main purposes are spectral shaping and receiver synchronization.

These are some of the things you can acomplish with line coding:

• Remove DC. Since long transmission lines do not transfer DC very well, and since DC uses power but transmits no information, many systems use line codes that remove it. The simplest example is AMI (alternate mark inversion). In AMI, a bit 0 is mapped to 0 V, and a bit 1 is mapped to +1 V or -1 V in an alternating fashion.

• Spread the signal energy across the entire bandwidth evenly. This is achieved with a scrambler.

• Avoid long runs of consecutive '1's or '0's. This helps the receiver synchronize to the received signal; sync algorithmis typically rely on signal transitions. One example is the 8b10b code. This line code substitutes runs of more than 5 symbols by sequences with transitions.

Note that the last two are different ways of achieving similar results.

It is true DC removal is more commonly used in baseband wireline systems. However, line codes, in one shape or another, are used in all communication systems.

Contrast line encoding with other types of codes: source coding compresses the information before transmission, and error control codes add extra information in order to correct and/or detect channel errors. A typical communications system will include all three types of codes.

Regarding keying: keying is the process of taking the output of the line coder and mapping it to a carrier. In this sense, keying does imply a passband channel.

Analogue passband means high frequency radio waves. Analogue baseband means the signal does not modulate a high frequency radio wave. So basically, keying is for radio wave transmissions while line coding is for transmission over bounded media (transmission lines) like coaxial cables, Ethernet cables etc.