Encoding vs. Modulation: Key Differences in Data Communication

This article compares encoding and modulation, explaining the differences between them in data communication. It provides examples and highlights their respective advantages.

Introduction

We encounter two primary signal types: analog and digital. These signals serve diverse purposes, and it’s often necessary to convert between them for specific applications. Encoding and modulation facilitate these conversions. This leads to four possible data/signal combinations:

• Encoding analog data as analog signals (e.g., telephone systems).
• Encoding digital data as analog signals (e.g., modems).
• Encoding analog data as digital signals (e.g., codecs).
• Encoding digital data as digital signals (e.g., digital transmitters).

A typical data communication system includes a transmitter, a channel, and a receiver.

Encoding Process

Encoding transforms analog data into a digital signal format. This process uses sampling and quantization, as seen in Pulse Code Modulation (PCM). An encoder is used at the transmitter end, and a decoder at the receiver end. The encoder prepares the digital data for transmission over digital lines. These are used in technologies like DSL (Digital Subscriber Line) or Ethernet cables connecting devices on a LAN (Local Area Network).

Figure-1 : Encoding decoding process

Digital signals consist of voltage/current pulses with discrete levels (e.g., two levels for binary). Each pulse represents a signal element. An encoder translates binary data into these signal elements.

Examples of Encoding

Various schemes exist for encoding digital data into digital signals:

• Unipolar: NRZ
• Polar: NRZ, RZ, Bi-phase (Manchester, Differential Manchester)
• Bipolar: AMI (Alternate Mark Invert), Pseudoternary
• Multilevel: 2B1Q, 8B6T, 4D-PAM5
• Multitransition: MLT-3

Figure-2 : Polar encoding for NRZ and RZ signal

• Unipolar: A binary ‘1’ is represented by the presence of a pulse, and a binary ‘0’ by the absence of a pulse (On-Off Keying). This category includes Unipolar NRZ and Unipolar RZ.

• Polar: A binary ‘1’ is represented by a positive pulse, and a binary ‘0’ by a negative pulse. This includes Polar NRZ and Polar RZ.

• Bipolar: Uses three voltage levels: positive, negative, and zero. Binary ‘0’ is encoded as a neutral zero voltage. Binary ‘1’ alternates between positive and negative pulses. This is also called a Duobinary signal or Alternate Mark Inversion (AMI). Includes Bipolar NRZ and Bipolar RZ.

Advantages of Encoding

• Self-Synchronization: Prevents long strings of zeros or ones by incorporating more transitions, making it easier for the receiver to synchronize with the transmitter.
• Baseline Wandering Prevention: Line encoding techniques prevent baseline wandering.
• Error Detection: Detects some or all errors in the transmission.
• DC Component Elimination: Avoids creating DC components (low frequencies).
• Noise Immunity: Some encoders are resistant to noise and interference.
• Implementation Complexity: Encoding techniques can range from simple to complex based on the number of levels.
• Refer to advantages and disadvantages of Unipolar, Polar and Bipolar coding and NRZ and RZ coding techniques.

Modulation Process

Modulation is the process of shifting baseband signals to a higher frequency range. Modems perform modulation at the transmitter and demodulation at the receiver. There are two types of modulation: analog and digital. The modulated analog signal can be transmitted through optical fiber cables or unguided media. Analog modulation takes an analog baseband signal and produces a modulated analog signal. Digital modulation takes a digital baseband signal and gives a modulated analog signal.

Figure-3 : Modulation demodulation process

Examples of Modulation

Modulation schemes vary depending on the input data type (analog or digital):

• Analog Modulation: AM, FM, PM, etc.
• Digital Modulation: ASK, FSK, BPSK/PSK, QPSK, 16-QAM, etc.

Figure-4 : Analog Modulation (AM-Amplitude Modulation)

The figure depicts amplitude modulation in which input is analog baseband frequency and output is analog carrier frequency.

Figure-5 : Digital Modulation (ASK-Amplitude Shift Keying)

The figure depicts digital modulation in which input is digital data and output is analog modulates signal.

Advantages of Modulation

• Efficient Transmission: Converts low-frequency baseband signals to high-frequency carrier signals suitable for wireless transmission without noise or interference.
• Smaller Antennas: Eliminates the need for large antennas required for low-frequency transmission.
• Bandwidth Efficiency: Higher-level digital modulation schemes (QPSK, QAM) reduce bandwidth requirements and increase spectrum efficiency.
• Increased Data Rate: Higher-level modulation schemes increase data rates by mapping more data bits onto single signal elements (e.g., QPSK maps 2 bits per symbol, 16-QAM maps 4 bits per symbol).
• Reduced Error Probability.
• Refer to advantages and disadvantages of AM and FM , ASK modulation , FSK modulation and PSK modulation .