Komdat Modulation.ppt

Modulation( Analog Data )

Komunikasi Data

Fakultas Teknologi Informasi

Universitas Budi Luhur

Yan Everhard R, Ir. MT

long-distance transmission: digital telecommunications system

analogue-to-digital conversion: conversion process

• Basic steps in analog-to-digital conversion An analog signal is sampled at regular intervals. The amplitude at each interval is quantized, or assigned a value, and the values are mapped into a series of binary digits, or bits. The information is transmitted as a digital signal to the receiver, where it is decoded and the analog signal reconstituted.

amplitude-shift keying: digital signal modulation methods

• Three methods of digital signal modulation A digital signal, representing the binary digits 0 and 1 by a series of on and off amplitudes, is impressed onto an analog carrier wave of constant amplitude and frequency. In amplitude-shift keying (ASK), the modulated wave represents the series of bits by shifting abruptly between high and low amplitude. In frequency-shift keying (FSK), the bit stream is represented by shifts between two frequencies. In phase-shift keying (PSK), amplitude and frequency remain constant; the bit stream is represented by shifts in the phase of the modulated signal.

Modulation• Communication channels like telephone lines are usually analogue

media. Analogue media is a bandwidth limited channel. In the case of telephone lines the usable bandwidth frequencies is in the range of 300 Hz to 3300 Hz.

• Data communication means moving digital information from one place to another through communication channels. These digital information signals have the shape of square waves and the meaning of "0" and "1".

• If such digital signals were transmitted on analogue media the

square waves of the digital signals would be distorted by the analogue media. The receiver which receives these distorted signals will be unable to interpret accurately the incoming signals. The solution is to convert these digital signals into analogue signals so that the communication channels can carry the information from one place to another reliably. The technique which enables this conversion is called "modulation".

Modulation• Modulation is a technique of modifying some basic

analogue signal in a known way in order to encode information in that basic signal. Any measurable property of an analogue signal can be used to transmit information by changing this property in some known manner and then detecting those changes at the receiver end. The signal that is modulated is called the carrier signal, because it carries the digital information from one end of the communication channel to the other end.

• The device that changes the signal at the transmitting end of the communication channel is called the "modulator". The device at the receiving end of the channel, which detects the digital information from the modulated signal, is called the "demodulator".

Modulation

• With Frequency Modulation, the frequency of the carrier signal is changed according to the data. The transmitter sends different frequencies for a "1" than for a "0". This technique is also called FSK - frequency shift keying. Its disadvantages are that the rate of frequency changes is limited by the bandwidth of the line, and that distortion caused by the lines makes the detection even harder than amplitude modulation. Today this technique is used in low rate asynchronous modems up to 1200 baud only.

• The Amplitude Modulation (AM) technique changes the amplitude of

the sine wave. In the earliest modems, digital signals were converted to analogue by transmitting a large amplitude sine wave for a "1" and zero amplitude for a "0", as shown in the figure. The main advantage of this technique is that it is easy to produce such signals and also to detect them.

Modulation• However, the technique has two major disadvantages. The first is that

the speed of the changing amplitude is limited by the bandwidth of the line. The second is that the small amplitude changes suffer from unreliable detection. Telephone lines limit amplitude changes to some 3000 changes per second. The disadvantages of amplitude modulation causes this technique to no longer be used by modems, however, it is used in conjunction with other techniques.

• Phase Modulation (PM) is a process where two sinusoidal waveforms are compared with each other. The case where the two waveforms are going in the same direction at the same time is known as zero phase shift. With a phase shift of 180 degrees (as illustrated), waveform B starts at the mid-point of waveform A, so that when waveform A is positive waveform B is negative, and vice versa. Two phase states allow the representation of a single bit of digital data, which can have the value "0" or "1". Additional 90- and 270 degree phase shifts provide four phase shift states and the capability to represent four digital data representations.

Amplitudo Modulation

Amplitudo Modulation

Amplitudo Modulation

Amplitudo Modulation

Amplitudo Modulation - Proses

Frequency Modulation

Frequency Modulation

Frequency Modulation – Proses

Phase Modulation

Phase Modulation

Phase Modulation

Phase Modulation – Proses

Quadrature phase-shift keying (QPSK)