Encoding Methods and Modulation Schemes

Encoding methods and Modulation schemes

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Computers use digital signalling, digitals signals are represented by the base 2 numbering system “Binary”. Binary bits are shown as 1 or 0, where 1 means ON and 0 means OFF. In a network where digital devices exist, digital signalling will be used. Dependant upon the encoding method used, a high voltage on a communications medium can represent a logical 1 and a low voltage of can represent a logical 0. Different methods of encoding exist these are used for different types of communication between electrical devices. Distortion, noise, and cross talk on a cabling medium are factors that prevent the accuracy of transmitted data to be intact. For these reasons different encoding methods exist. An example is when 2 wires are used to transmit music data to a speaker

Digital signals don’t always have to be carried over to the receiving end by electricity, light can also be used for digital communication. Fibre Optics use light to transmit data through optical fibre within the cable. The strength of the light ray can also be a determining factor to what logic the ray represents, either 0 or 1.

Digital signals are complex waveforms that can be described as a discontinuous waveform having a finite range of levels. You can recognise a digital signal by the wave created, if the wave keeps going up and down in straight lines and forms squares and rectangles these are digital signals. Digital signals assume two states (logic 0 and logic 1). Where the signal changes from one state to another, the signal can be said to be discontinuous. As the signal state changes from 0 to 1, a finite time is required for the voltage to increase to the maximum value, and a corresponding time is required for the voltage to fall when the signal state changes from 1 to 0. Each voltage pulse in a digital signal is a signal element. Encoding each data bit into signal elements transmits binary data.

There are many encoding schemes that go with digital signalling. The encoding scheme is simply the way in which the signalling elements are used to represent data bits.

Below I will be listing different encoding methods that exist.

Below is a diagram showing the digital waveform of an NRZ-L (Non-Return-to Zero- Level) encoding method. NRZ-L is a variation of the NRZ encoding method. The NRZ (Non-Return-to Zero) encoding method is a form of digital transmission. Binary 1’s and 0’s are represented by low and high level states on the line. These states are formed on the line by specific and constant DC (Direct-current) voltages that are transmitted from the NIC (Network interface card). The difference between NRZ and NRZ-L is that, NRZ does not consider the first data bit to be transmitted as a polarity change whereas NRZ-L does.

NRZ logic can be represented by more negative or less positive voltages or vice versa. Example diagrams are shown below.

Logic 0 = +5.0 volts

Logic 1 = +0.5 volts

Logic 0 = 0.0 volts

Logic 1 = -3.0 volts

Below is a diagram of the NRZ-I (Non-Return-to Zero- Inverted) encoding method. NRZ-I is another variation of the NRZ encoding method. The NRZI (Non-Return-to Zero-Inverted) encoding method is also a form of digital transmission. Binary 1’s and 0’s are represented by changes in level and constant states on the line. For example a binary 0 is encoded as no change in the level of voltage, whereas a binary 1 is encoded when the level changes from the previous constant state. A binary 1 can be encoded either when the voltage drops or increases on the line. This encoding method is used with FDDI and USB.

Logic 0 = Constant voltage over time

Logic 1 = Drop or rise in voltage

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There are many other types of encoding methods, a few of them are Manchester encoding, Pseudoternary encoding, Bipolar вЂ" AMI etc. Manchester encoding encodes a binary 1 by the change of a voltage from a low level state to a high state, a binary 0 is encoded by the change of a voltage from a high level state to a low level state.

Carrier Modulation Schemes

Amplitude Modulation

Amplitude modulation is a form of carrier modulation scheme. It is a method of impressing data on an alternating current waveform. In Amplitude Modulation, the carrier does not fluctuate in amplitude. The modulating data appears as signal components at slightly higher or lower frequencies than the carrier. There are two signal components, these are the lower and upper sidebands. The lower sideband is the frequencies that are below the frequency of the carrier, the upper sideband is the frequencies that are above the frequency of the carrier. The lower and upper sidebands are mirror images of each other.

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Amplitude Modulation

0 1 0 0 1 1 1 0 1 0

By demodulating the amplitude of a wave, a binary sequence can be found.

Frequency Modulation

Frequency Modulation is another form of a carrier modulation scheme. Frequency modulation uses one frequency for binary 1 and another for binary 0. Frequency Shift Keying is another word for Frequency Modulation. Transmission rates of Modems are measured in bits per second (bps). Frequency modulation is still used for low speed data transmissions of up to 300 bps. The Trellis Coded Modulation is used nowadays for today’s high speed Modems.

Analogue signals are continuously varying electromagnetic waves that can be propagated over various types of communications media. These waveforms have a positive peak and a negative peak with an infinite range of levels in between. The infinite