Analog and Digital Modulation Techniques

Analog and Digital Modulation Techniques Computer Networks Assignment 1 Amit Kumar Jha 201201087 Introduction Modulation is the process of varying some parameter of a periodic waveform in order to use that signal to convey a message.It is used mostly in the transmission of data from one point to another. Although there are many types of modulation, there are two basic types; analog modulation and digital modulation. The main difference between analog modulation and digital modulation is in the manner that they transmit data. With analog modulation, the input needs to be in the analog format, while digital modulation needs the data in a digital format. Because of the differences in the input signal, the output signal is also quite different. In analog modulation, any value between the maximum and minimum is considered to be valid. It is not so with digital modulation as only two values are considered valid; one value to represent “1” and another to represent “0.” All other values are considered noise and are rejected. Because most signals that we transmit are analog in nature, like one’s voice, it is far simpler to do analog modulation than digital. If you want to transmit a voice using digital modulation, you’d need to pass it through an analog-to-digital converter before transmission and a digital-toanalog converter at the receiver to recover the original signal. The additional stages needed for transmitting digital modulation increases both the cost and complexity of the transmitter and receiver. The major advantage that digital modulation has over analog transmission is how it achieves greater fidelity. With analog modulation, any noise or interference that falls in the given frequency bandwidth gets mixed with the actual signal. Although there are a number of ways to mitigate noise, it will still cause some amount of degradation. Because digital modulation only recognizes 0’s and 1’s, any noise is virtually eliminated once the receiver discerns whether a “0” or a “1” was transmitted. Unless the signal is very badly distorted, the output signal will be literally identical to what was transmitted. Under both analog modulation and digital modulation, there are a number of other modulation techniques each with its own strengths and weaknesses. But each technique has its basic commonalities of transmitting either a digital or analog signal.

Type of Modulation Techniques Sr. No

Modulation Techniques

Type

Notation

01

Analog Modulation

(i) Amplitude Modulation

A.M.

Techniques.

(ii) Frequency Modulation

F.M.

(iii) Phase Modulation

P.M.

Digital Modulation

(i) Amplitude Shift Keying

A.S.K.

Techniques

(ii) Frequency Shift Keying

F.S.K.

(iii)Phase Shift Keying

P.S.K.

02

Analog Modulation Techniques Amplitude Modulation Amplitude Modulation modifies the amplitude of the carrier to represent 1s or 0s.In the above example, a 1 is represented by the presence of the carrier for a predefined period of 3 cycles of carrier. Absence--or no carrier--indicates a 0.

Advantages: 

Simple to design.

Disadvantages:  

Noise spikes on transmission medium interfere with the carrier signal. Loss of connection is read as 0s.

Frequency Modulation Frequency Modulation modifies the frequency of the carrier to represent the 1s or 0s. In the above example, a 0 is represented by the original carrier frequency, and a 1 by a much higher frequency (the cycles are spaced closer together).

Advantages:  

Immunity to noise on transmission medium. Always a signal present. Loss of signal easily detected

Disadvantages:  

Requires 2 frequencies Detection circuit needs to recognize both frequencies when signal is lost.

Phase Modulation Phase Modulation modifies the phase of the carrier to represent a 1 or 0.

The carrier phase is switched at every occurrence of a 1 bit, but remains unaffected for a 0 bit. The phase of the signal is measured relative to the phase of the preceding bit. The bits are timed to coincide with a specific number of carrier cycles (3 in this example = 1 bit).

Advantage:  

Only 1 frequency used Easy to detect loss of carrier

Disadvantages: 

Complex circuitry that is required to generate and detect phase changes.

Digital Modulation Techniques Amplitude Shift Keying(ASK) This is a form of amplitude modulation that represents digital data as variations in the amplitude of a carrier wave. In an ASK system, binary symbol 1 is represented by transmitting carrier wave of fixed amplitude and fixed frequency for the bit duration T seconds. ASK assigns a unique patter of binary digits to a finite number of amplitudes. Each pattern of the bits form a symbol which is represented by a particular amplitude. The equation of the wave generated by the ASK is given by: S (t) = m (t). Ac cos (2πfct).

Frequency Shift Keying (FSK) In this scheme digital information is transmitted through discrete frequency changes of a carrier wave. In binary FSK, the simplest FSK scheme a pair of discrete frequencies are used to transmit binary information. In this scheme 1 is called the mark frequency and 0 is called space frequency.

Phase-shift keying (PSK) It is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). PSK assigns a unique pattern of binary digits to a finite number of phases. Each pattern of bits forms the symbol that is represented by the particular phase.

Comparison of Various Techniques With respect to Analog Techniques it has been found that frequency modulation is much better than amplitude modulation. The main factor for that is that the amplitude modulation has much more chances of being disrupted by noise which effects

Amplitude causing it to change the signal present in the amplitude modulated wave. But AM can travel larger distances then FM due to the fact that FM waves have a tendency to interfere with physical objects due to their lower wavelengths. If we consider Digital Techniques we find that FSK is better than ASK because it is not disrupted easily by interference and is easily generated with modern electronic equipment. But it is not right to define which technique is better just because of the fact that each of the techniques have use cases where other techniques are not ideal to be used. In general digital techniques are valid only when digital information has to be sent and analog techniques are valid when the signal is analog.

References http://en.wikipedia.org/wiki/Modulation http://en.wikibooks.org/wiki/Communication_Systems/What_is_Modulation%3F NCERT 12th Class Textbook