Digital-to-Digital (Line Coding Scheme)

What is Line Coding?

The line coding is defined as the process of converting Digital data to Digital signal.

Characteristics of line coding:

· There should be self-synchronization

· There should have error detection capability

· There should be less complexity to implement

· There should not have DC Component

· There should not have Baseline Wandering

· It should need less Bandwidth to operate.

CA’s Scheme

These scheme is based on concept of 2B1Q and Bipolar Schemes.

· Bipolar Scheme concept is for error detection.

· 2B1Q scheme concept is for designing the levels.

Description:

There are 2 binary data for 1 level. Total level for represent the signal is four i.e. +3, +1, -3 and -1.

Every next level depends on the current level. Next level will be changed according to the chart of 2B1Q but there is come addition with these. The addition of bipolar concept. The concept is if any where we see the next double bits voltage level is going to be same as current level, it will alter the voltage level of the next double bits level to the opposite level of the 2B1Q chart.

Altering the voltage level we can achieve the error detection.

DC Component of this Scheme:

In these scheme,

we have 4 levels -> 1 , -1 , 3 , -3

Probability -> ¼(1) , ¼(-1) , ¼(3) , ¼(-3)

DC Component -> ¼(1) -¼(1) + ¼(3)-¼(3) = 0

Signal & Data Rate:

We have 2 bits per signal level

So, N = 2, S = 1

=> r = N/S = 2

Savg = c * N * (1/r) [c = ½ on average]

= ½ * N * ½

= N/4

On an average we can send data 4 times of the signal.

Baseline Wandering:

In this scheme signal is changing its level in every double bits so we can say it has no baseline wandering.

Error Detection:

Data: 00 00 01 01 11 11

Tx: +1 -1 -3 +3 -3 +3

Rx(with error): +1 +1 -3 -3 -3 +3

We can detect the error that consecutive 2 level’s voltage cannot be same. If that error occur we can alter the voltage level to opposite of it.

Self-Synchronization:

In every double bits the level of voltage is changing. We can surely detect the starting of a bits and the ending by transition of the voltage level. So it has self-synchronization property.

On the other hand 2B1Q has no self-synchronization for long same double bits. We overcome it by applying bipolar concept. That is altering the levels if next double bits voltage level is going to be same as current voltage level.

Bandwidth:

The actual bandwidth of a digital signal is infinite, the effective bandwidth is finite.

So, the minimum bandwidth we need

Bmin = c * N * (1/r)

= ½ * N * ½

= N/4

This is much less.

Disadvantages:

· It requires memory to store previous voltage level.

· Amplitude is -3 to +3, it requires high power.