Virtual Lab: Line Coding Schemes

Binary Input

Enter a sequence of 0s and 1s (max 16 bits).

Line Coding Scheme

High voltage = 1, Low voltage = 0.

Parameters

Bit Rate 1 kbps

Voltage Levels +V, 0, -V

Time Domain Signal

Power Spectral Density (Conceptual)

Frequency (f)

Signal Characteristics

DC Component --
Clock Sync --
Baseline Wandering --
Bandwidth --

Fundamentals of Line Coding

Definition

Line coding is the process of converting digital data (binary 0s and 1s) into a digital signal (voltage levels) suitable for transmission over a physical medium (like a wire or fiber). It is the interface between digital data and the analog channel.

Signal Elements vs Data Elements

• Data Element: The smallest piece of information (a single bit).
• Signal Element: The shortest unit of the digital signal (a pulse). One or more signal elements are used to encode one data element.

Key Evaluation Criteria

✓ Signal Spectrum: Should not have DC component or strong low frequencies.
✓ Clocking: Must provide sufficient transitions for synchronization.
✓ Error Detection: Capability to detect errors in the data stream.
✓ Noise Immunity: Resistance to interference.

Common Line Coding Schemes

Unipolar NRZ (Non-Return-to-Zero)

Legacy / Simple

The voltage level determines the bit value. High = 1, Low = 0. The signal does not return to zero in the middle of the bit.

Pros: Simple.
Cons: DC component, no sync for long 0s/1s, baseline wandering.

Polar NRZ (NRZ-L & NRZ-I)

Standard

Uses both positive and negative voltages.

NRZ-L: Level determines bit (High=1, Low=0).
NRZ-I: Inversion determines bit (No change=0, Change=1).

NRZ-I is better for noise immunity during transmission because it relies on change rather than absolute value.

RZ (Return-to-Zero)

Good Sync

The signal returns to zero halfway through the bit interval. Uses three values: +, -, 0.

Pros: Good synchronization.
Cons: Requires 3 signal levels (more complex hardware), 2 signal elements per bit (higher bandwidth).

Manchester & Differential Manchester

IEEE 802.3

Transition happens in every bit interval.

Manchester: Transition is in the middle. Low-to-High = 1, High-to-Low = 0.
Diff-Manchester: Inversion at start of interval = 0, No inversion = 1.

Pros: Self-clocking, no DC component.
Cons: Minimum bandwidth is twice that of NRZ (inefficient).

Bipolar (AMI & Pseudoternary)

Long Distance

Uses three voltage levels. Zero voltage represents binary 0.

AMI: Binary 1s are represented by alternating + and -.
Pseudoternary: Binary 1 is zero voltage, Binary 0s are alternating + and -.

Pros: No DC component, less baseline wandering, easy error detection (violation of alternate rule).
Cons: Long string of 0s (AMI) or 1s (Pseudoternary) causes sync loss.

Laboratory Procedure

Step-by-step guide to conducting the Line Coding experiment.

Objective Identification

Read the theory section carefully. Identify the specific characteristics of the coding scheme assigned to you (e.g., NRZ-L, Manchester).

Input Data Generation

Navigate to the Simulation tab. Generate a binary sequence of at least 8 bits. Record this sequence in your notebook. Try sequences with specific patterns:

All 1s (e.g., 11111111)
All 0s (e.g., 00000000)
Alternating (e.g., 10101010)
Random (e.g., 10110010)

Signal Visualization

Select the coding scheme from the dropdown. Observe the generated waveform on the oscilloscope canvas.

"Sketch the waveform in your lab journal. Label the voltage levels (+V, 0, -V) and the time intervals (Tb)."

Analysis & Measurement

Analyze the signal based on the criteria in the sidebar:

Does it have a DC component?
Is there a transition for every bit (Clocking)?
Does it use 2 or 3 voltage levels?

Comparison

Repeat steps 2-4 for at least 3 different coding schemes. Compare their bandwidth efficiency and error detection capabilities.

EEEN 464 - Virtual Lab | Line Coding