Category: principles Of Communication

Sideband Modulation In the process of Amplitude Modulation or Phase Modulation, the modulated wave consists of the carrier wave and two sidebands. The modulated signal has the information in the whole band except at the carrier frequency. Sideband A Sideband is a band of frequencies, containing power, which are the lower and higher frequencies of the carrier frequency. Both the sidebands contain the same information. The representation of amplitude modulated wave in the frequency domain is as shown in the following figure. Both the sidebands in the image contain the same information. The transmission of such a signal which contains a carrier along with two sidebands, can be termed as Double Sideband Full Carrier system, or simply DSB-FC. It is plotted as shown in the following figure. However, such a transmission is inefficient. Two-thirds of the power is being wasted in the carrier, which carries no information. If this carrier is suppressed and the power saved is distributed to the two sidebands, such a process is called as Double Sideband Suppressed Carrier system, or simply DSBSC. It is plotted as shown in the following figure. Now, we get an idea that, as the two sidebands carry the same information twice, why can’t we suppress one sideband. Yes, this is possible. The process of suppressing one of the sidebands, along with the carrier and transmitting a single sideband is called as Single Sideband Suppressed Carrier system, or simply SSB-SC or SSB. It is plotted as shown in the following figure. This SSB-SC or SSB system, which transmits a single sideband has high power, as the power allotted for both the carrier and the other sideband is utilized in transmitting this Single Sideband (SSB). Hence, the modulation done using this SSB technique is called as SSB Modulation. Sideband Modulation − Advantages The advantages of SSB modulation are − Bandwidth or spectrum space occupied is lesser than AM and DSB signals. Transmission of more number of signals is allowed. Power is saved. High power signal can be transmitted. Less amount of noise is present. Signal fading is less likely to occur. Sideband Modulation − Disadvantages The disadvantages of SSB modulation are − The generation and detection of SSB signal is a complex process. Quality of the signal gets affected unless the SSB transmitter and receiver have an excellent frequency stability. Sideband Modulation − Applications The applications of SSB modulation are − For power saving requirements and low bandwidth requirements. In land, air, and maritime mobile communications. In point-to-point communications. In radio communications. In television, telemetry, and radar communications. In military communications, such as amateur radio, etc. Learning working make money

Information Theory Information is the source of a communication system, whether it is analog or digital. Information theory is a mathematical approach to the study of coding of information along with the quantification, storage, and communication of information. Conditions of Occurrence of Events If we consider an event, there are three conditions of occurrence. If the event has not occurred, there is a condition of uncertainty. If the event has just occurred, there is a condition of surprise. If the event has occurred, a time back, there is a condition of having some information. Hence, these three occur at different times. The difference in these conditions, help us have a knowledge on the probabilities of occurrence of events. Entropy When we observe the possibilities of occurrence of an event, whether how surprise or uncertain it would be, it means that we are trying to have an idea on the average content of the information from the source of the event. Entropy can be defined as a measure of the average information content per source symbol. Claude Shannon, the “father of the Information Theory”, has given a formula for it as $$H = -sum_{i} p_ilog_{b}p_i$$ Where $p_i$ is the probability of the occurrence of character number i from a given stream of characters and b is the base of the algorithm used. Hence, this is also called as Shannon’s Entropy. The amount of uncertainty remaining about the channel input after observing the channel output, is called as Conditional Entropy. It is denoted by $H(x arrowvert y)$ Discrete Memoryless Source A source from which the data is being emitted at successive intervals, which is independent of previous values, can be termed as discrete memoryless source. This source is discrete as it is not considered for a continuous time interval, but at discrete time intervals. This source is memoryless as it is fresh at each instant of time, without considering the previous values. Source Coding According to the definition, “Given a discrete memoryless source of entropy $H(delta)$, the average code-word length $bar{L}$ for any source encoding is bounded as $bar{L}geq H(delta)$”. In simpler words, the code-word (For example: Morse code for the word QUEUE is -.- ..- . ..- . ) is always greater than or equal to the source code (QUEUE in example). Which means, the symbols in the code word are greater than or equal to the alphabets in the source code. Channel Coding The channel coding in a communication system, introduces redundancy with a control, so as to improve the reliability of the system. Source coding reduces redundancy to improve the efficiency of the system. Channel coding consists of two parts of action. Mapping incoming data sequence into a channel input sequence. Inverse mapping the channel output sequence into an output data sequence. The final target is that the overall effect of the channel noise should be minimized. The mapping is done by the transmitter, with the help of an encoder, whereas the inverse mapping is done at the receiver by a decoder. Learning working make money

Angle Modulation The other type of modulation in continuous-wave modulation is the Angle Modulation. Angle Modulation is the process in which the frequency or the phase of the carrier varies according to the message signal. This is further divided into frequency and phase modulation. Frequency Modulation is the process of varying the frequency of the carrier signal linearly with the message signal. Phase Modulation is the process of varying the phase of the carrier signal linearly with the message signal. Let us now discuss these topics in greater detail. Frequency Modulation In amplitude modulation, the amplitude of the carrier varies. But in Frequency Modulation (FM), the frequency of the carrier signal varies in accordance with the instantaneous amplitude of the modulating signal. The amplitude and the phase of the carrier signal remains constant whereas the frequency of the carrier changes. This can be better understood by observing the following figures. The frequency of the modulated wave remains constant as the carrier wave frequency when the message signal is at zero. The frequency increases when the message signal reaches its maximum amplitude. Which means, with the increase in amplitude of the modulating or message signal, the carrier frequency increases. Likewise, with the decrease in the amplitude of the modulating signal, the frequency also decreases. Mathematical Representation Let the carrier frequency be fc The frequency at maximum amplitude of the message signal = fc + Δf The frequency at minimum amplitude of the message signal = fc − Δf The difference between FM modulated frequency and normal frequency is termed as Frequency Deviation and is denoted by Δf. The deviation of the frequency of the carrier signal from high to low or low to high can be termed as the Carrier Swing. Carrier Swing = 2 × frequency deviation = 2 × Δf Equation for FM WAVE The equation for FM wave is − $$s(t) = A_ccos[W_ct + 2pi k_fm(t)]$$ Where, Ac = the amplitude of the carrier wc = angular frequency of the carrier = 2πfc m(t) = message signal FM can be divided into Narrowband FM and Wideband FM. Narrowband FM The features of Narrowband FM are as follows − This frequency modulation has a small bandwidth. The modulation index is small. Its spectrum consists of carrier, USB, and LSB. This is used in mobile communications such as police wireless, ambulances, taxicabs, etc. Wideband FM The features of Wideband FM are as follows − This frequency modulation has infinite bandwidth. The modulation index is large, i.e., higher than 1. Its spectrum consists of a carrier and infinite number of sidebands, which are located around it. This is used in entertainment broadcasting applications such as FM radio, TV, etc. Phase Modulation In frequency modulation, the frequency of the carrier varies. But in Phase Modulation (PM), the phase of the carrier signal varies in accordance with the instantaneous amplitude of the modulating signal. The amplitude and the frequency of the carrier signal remains constant whereas the phase of the carrier changes. This can be better understood by observing the following figures. The phase of the modulated wave has got infinite points where the phase shift in a wave can take place. The instantaneous amplitude of the modulating signal, changes the phase of the carrier. When the amplitude is positive, the phase changes in one direction and if the amplitude is negative, the phase changes in the opposite direction. Relation between PM and FM The change in phase, changes the frequency of the modulated wave. The frequency of the wave also changes the phase of the wave. Though they are related, their relationship is not linear. Phase modulation is an indirect method of producing FM. The amount of frequency shift, produced by a phase modulator increases with the modulating frequency. An audio equalizer is employed to compensate this. Equation for PM Wave The equation for PM wave is − $$s(t) = A_ccos[W_ct + k_pm(t)]$$ Where, Ac = the amplitude of the carrier wc = angular frequency of the carrier = 2πfc m(t) = message signal Phase modulation is used in mobile communication systems, while frequency modulation is used mainly for FM broadcasting. Learning working make money

Principles of Communication – Multiplexing Multiplexing is the process of combining multiple signals into one signal, over a shared medium. The process is called as analog multiplexing if these signals are analog in nature. If digital signals are multiplexed, it is called as digital multiplexing. Multiplexing was first developed in telephony. A number of signals were combined to send through a single cable. The process of multiplexing divides a communication channel into several number of logical channels, allotting each one for a different message signal or a data stream to be transferred. The device that does multiplexing, can be called as a MUX. The reverse process, i.e., extracting the number of channels from one, which is done at the receiver is called as demultiplexing. The device which does demultiplexing is called as DEMUX. The following figures illustrates the concept of MUX and DEMUX. Their primary use is in the field of communications. Types of Multiplexers There are mainly two types of multiplexers, namely analog and digital. They are further divided into FDM, WDM, and TDM. The following figure gives a detailed idea about this classification. There are many types of multiplexing techniques. Of them all, we have the main types with general classification, mentioned in the above figure. Let us take a look at them individually. Analog Multiplexing The analog multiplexing techniques involve signals which are analog in nature. The analog signals are multiplexed according to their frequency (FDM) or wavelength (WDM). Frequency Division Multiplexing In analog multiplexing, the most used technique is Frequency Division Multiplexing (FDM). This technique uses various frequencies to combine streams of data, for sending them on a communication medium, as a single signal. Example − A traditional television transmitter, which sends a number of channels through a single cable uses FDM. Wavelength Division Multiplexing Wavelength Division multiplexing (WDM) is an analog technique, in which many data streams of different wavelengths are transmitted in the light spectrum. If the wavelength increases, the frequency of the signal decreases. A prism which can turn different wavelengths into a single line, can be used at the output of MUX and input of DEMUX. Example − Optical fiber Communications use the WDM technique, to merge different wavelengths into a single light for the communication. Digital Multiplexing The term digital represents the discrete bits of information. Hence, the available data is in the form of frames or packets, which are discrete. Time Division Multiplexing (TDM) In TDM, the time frame is divided into slots. This technique is used to transmit a signal over a single communication channel, by allotting one slot for each message. Of all the types of TDM, the main ones are Synchronous and Asynchronous TDM. Synchronous TDM In Synchronous TDM, the input is connected to a frame. If there are ‘n’ number of connections, then the frame is divided into ‘n’ time slots. One slot is allocated for each input line. In this technique, the sampling rate is common for all signals and hence the same clock input is given. The MUX allocates the same slot to each device at all times. Asynchronous TDM In Asynchronous TDM, the sampling rate is different for each of the signals and a common clock is not required. If the allotted device, for a time slot transmits nothing and sits idle, then that slot is allotted to another device, unlike synchronous. This type of TDM is used in Asynchronous transfer mode networks. Demultiplexer Demultiplexers are used to connect a single source to multiple destinations. This process is the reverse of multiplexing. As mentioned previously, it is used mostly at the receivers. DEMUX has many applications. It is used in receivers in the communication systems. It is used in arithmetic and logical unit in computers to supply power and to pass on communication, etc. Demultiplexers are used as serial to parallel converters. The serial data is given as input to DEMUX at regular interval and a counter is attached to it to control the output of the demultiplexer. Both the multiplexers and demultiplexers play an important role in communication systems, both at the transmitter and receiver sections. Learning working make money

Modulation Techniques There are few modulation techniques which are followed to construct a PCM signal. These techniques like sampling, quantization, and companding help to create an effective PCM signal, which can exactly reproduce the original signal. Quantization The digitization of analog signals involves the rounding off of the values which are approximately equal to the analog values. The method of sampling chooses few points on the analog signal and then these points are joined to round off the value to a near stabilized value. Such a process is called as Quantization. The quantizing of an analog signal is done by discretizing the signal with a number of quantization levels. Quantization is representing the sampled values of the amplitude by a finite set of levels, which means converting a continuous-amplitude sample into a discrete-time signal. The following figure shows how an analog signal gets quantized. The blue line represents analog signal while the red one represents the quantized signal. Both sampling and quantization results in the loss of information. The quality of a Quantizer output depends upon the number of quantization levels used. The discrete amplitudes of the quantized output are called as representation levels or reconstruction levels. The spacing between two adjacent representation levels is called a quantum or step-size. Companding in PCM The word Companding is a combination of Compressing and Expanding, which means that it does both. This is a non-linear technique used in PCM which compresses the data at the transmitter and expands the same data at the receiver. The effects of noise and crosstalk are reduced by using this technique. There are two types of Companding techniques. A-law Companding Technique Uniform quantization is achieved at A = 1, where the characteristic curve is linear and there is no compression. A-law has mid-rise at the origin. Hence, it contains a non-zero value. A-law companding is used for PCM telephone systems. A-law is used in many parts of the world. µ-law Companding Technique Uniform quantization is achieved at µ = 0, where the characteristic curve is linear and there is no compression. µ-law has mid-tread at the origin. Hence, it contains a zero value. µ-law companding is used for speech and music signals. µ-law is used in North America and Japan. Differential PCM The samples that are highly correlated, when encoded by PCM technique, leave redundant information behind. To process this redundant information and to have a better output, it is a wise decision to take predicted sampled values, assumed from its previous outputs and summarize them with the quantized values. Such a process is named as Differential PCM technique. Learning working make money

Amplitude Modulation Among the types of modulation techniques, the main classification is Continuous-wave Modulation and Pulse Modulation. The continuous wave modulation techniques are further divided into Amplitude Modulation and Angle Modulation. A continuous-wave goes on continuously without any intervals and it is the baseband message signal, which contains the information. This wave has to be modulated. According to the standard definition, “The amplitude of the carrier signal varies in accordance with the instantaneous amplitude of the modulating signal.” Which means, the amplitude of the carrier signal which contains no information varies as per the amplitude of the signal, at each instant, which contains information. This can be well explained by the following figures. The modulating wave which is shown first is the message signal. The next one is the carrier wave, which is just a high frequency signal and contains no information. While the last one is the resultant modulated wave. It can be observed that the positive and negative peaks of the carrier wave, are interconnected with an imaginary line. This line helps recreating the exact shape of the modulating signal. This imaginary line on the carrier wave is called as Envelope. It is the same as the message signal. Mathematical Expression Following are the mathematical expression for these waves. Time-domain Representation of the Waves Let modulating signal be − $$m(t) = A_mcos(2pi f_mt)$$ Let carrier signal be − $$c(t) = A_ccos(2pi f_ct)$$ Where Am = maximum amplitude of the modulating signal Ac = maximum amplitude of the carrier signal The standard form of an Amplitude Modulated wave is defined as − $$S(t) = A_c[1+K_am(t)]cos(2pi f_ct)$$ $$S(t) = A_c[1+mu cos(2pi f_mt)]cos(2pi f_ct)$$ $$Where,mu = K_aA_m$$ Modulation Index A carrier wave, after being modulated, if the modulated level is calculated, then such an attempt is called as Modulation Index or Modulation Depth. It states the level of modulation that a carrier wave undergoes. The maximum and minimum values of the envelope of the modulated wave are represented by Amax and Amin respectively. Let us try to develop an equation for the Modulation Index. $$A_{max} = A_c(1+mu )$$ Since, at Amax the value of cos θ is 1 $$A_{min} = A_c(1-mu )$$ Since, at Amin the value of cos θ is -1 $$frac{A_{max}}{A_{min}} = frac{1+mu }{1-mu }$$ $$A_{max}-mu A_{max} = A_{min}+mu A_{min}$$ $$-mu (A_{max}+A_{min}) = A_{min}-A_{max}$$ $$mu = frac{A_{max}-A_{min}}{A_{max}+A_{min}}$$ Hence, the equation for Modulation Index is obtained. µ denotes the modulation index or modulation depth. This is often denoted in percentage called as Percentage Modulation. It is the extent of modulation denoted in percentage, and is denoted by m. For a perfect modulation, the value of modulation index should be 1, which means the modulation depth should be 100%. For instance, if this value is less than 1, i.e., the modulation index is 0.5, then the modulated output would look like the following figure. It is called as Under-modulation. Such a wave is called as an under-modulated wave. If the value of the modulation index is greater than 1, i.e., 1.5 or so, then the wave will be an over-modulated wave. It would look like the following figure. As the value of modulation index increases, the carrier experiences a 180° phase reversal, which causes additional sidebands and hence, the wave gets distorted. Such overmodulated wave causes interference, which cannot be eliminated. Bandwidth of Amplitude Modulation The bandwidth is the difference between lowest and highest frequencies of the signal. For amplitude modulated wave, the bandwidth is given by $$BW = f_{USB}-f_{LSB}$$ $$(f_c+f_m)-(f_c-f_m)$$ $$ = 2f_m = 2W$$ Where W is the message bandwidth Hence we got to know that the bandwidth required for the amplitude modulated wave is twice the frequency of the modulating signal. Learning working make money

VSB Modulation In case of SSB modulation, when a sideband is passed through the filters, the band pass filter may not work perfectly in practice. As a result of which, some of the information may get lost. Hence to avoid this loss, a technique is chosen, which is a compromise between DSB-SC and SSB, called as Vestigial Sideband (VSB) technique. The word vestige which means “a part” from which the name is derived. Vestigial Sideband Both of the sidebands are not required for the transmission, as it is a waste. But a single band if transmitted, leads to loss of information. Hence, this technique has evolved. Vestigial Sideband Modulation or VSB Modulation is the process where a part of the signal called as vestige is modulated, along with one sideband. A VSB signal can be plotted as shown in the following figure. Along with the upper sideband, a part of the lower sideband is also being transmitted in this technique. A guard band of very small width is laid on either side of VSB in order to avoid the interferences. VSB modulation is mostly used in television transmissions. Transmission Bandwidth The transmission bandwidth of VSB modulated wave is represented as − $$B=( f_{m}+ f_{v}) Hz$$ Where, fm = Message bandwidth fv = Width of the vestigial sideband VSB Modulation − Advantages Following are the advantages of VSB − Highly efficient. Reduction in bandwidth. Filter design is easy as high accuracy is not needed. The transmission of low frequency components is possible, without difficulty. Possesses good phase characteristics. VSB Modulation − Disadvantages Following are the disadvantages of VSB − Bandwidth when compared to SSB is greater. Demodulation is complex. VSB Modulation − Application The most prominent and standard application of VSB is for the transmission of television signals. Also, this is the most convenient and efficient technique when bandwidth usage is considered. Learning working make money

Pulse Modulation So far, we have discussed about continuous-wave modulation. Now it’s time for discrete signals. The Pulse modulation techniques, deals with discrete signals. Let us see how to convert a continuous signal into a discrete one. The process called Sampling helps us with this. Sampling The process of converting continuous time signals into equivalent discrete time signals, can be termed as Sampling. A certain instant of data is continually sampled in the sampling process. The following figure indicates a continuous-time signal x(t) and a sampled signal xs(t). When x(t) is multiplied by a periodic impulse train, the sampled signal xs(t) is obtained. A sampling signal is a periodic train of pulses, having unit amplitude, sampled at equal intervals of time Ts, which is called as the Sampling time. This data is transmitted at the time instants Ts and the carrier signal is transmitted at the remaining time. Sampling Rate To discretize the signals, the gap between the samples should be fixed. That gap can be termed as the sampling period Ts. $$Sampling:Frequency = frac{1}{T_s} = f_s$$ Where, Ts = the sampling time fs = the sampling frequency or sampling rate Sampling Theorem While considering the sampling rate, an important point regarding how much the rate has to be, should be considered. The rate of sampling should be such that the data in the message signal should neither be lost nor it should get over-lapped. The sampling theorem states that, “a signal can be exactly reproduced if it is sampled at the rate fs which is greater than or equal to twice the maximum frequency W.” To put it in simpler words, for the effective reproduction of the original signal, the sampling rate should be twice the highest frequency. Which means, $$f_s geq 2W$$ Where, fs = the sampling frequency W is the highest frequency This rate of sampling is called as Nyquist rate. The sampling theorem, which is also called as Nyquist theorem, delivers the theory of sufficient sample rate in terms of bandwidth for the class of functions that are bandlimited. For the continuous-time signal x(t), the band-limited signal in frequency domain, can be represented as shown in the following figure. If the signal is sampled above the Nyquist rate, the original signal can be recovered. The following figure explains a signal, if sampled at a higher rate than 2w in the frequency domain. If the same signal is sampled at a rate less than 2w, then the sampled signal would look like the following figure. We can observe from the above pattern that the over-lapping of information is done, which leads to mixing up and loss of information. This unwanted phenomenon of over-lapping is called as Aliasing. Aliasing can be referred to as “the phenomenon of a high-frequency component in the spectrum of a signal, taking on the identity of a lower-frequency component in the spectrum of its sampled version.” Hence, the sampling of the signal is chosen to be at the Nyquist rate, as was stated in the sampling theorem. If the sampling rate is equal to twice the highest frequency (2W). That means, $$f_s = 2W$$ Where, fs = the sampling frequency W is the highest frequency The result will be as shown in the above figure. The information is replaced without any loss. Hence, this is a good sampling rate. Learning working make money

Principles of Communication – FM Radio Frequency division multiplexing is used in radio and television receivers. The main use of FM is for radio communications. Let us take a look at the structure of FM transmitter and FM receiver along with their block diagrams and working. FM Transmitter FM transmitter is the whole unit which takes the audio signal as an input and delivers FM modulated waves to the antenna as an output to be transmitted. FM transmitter consists of 6 main stages. They are illustrated in the following figure. The working of FM transmitter can be explained as follows. The audio signal from the output of the microphone is given to the pre-amplifier which boosts the level of the modulating signal. This signal is then passed to the high pass filter, which acts as a pre-emphasis network to filter out the noise and improve the signal to noise ratio. This signal is further passed to the FM modulator circuit. The oscillator circuit generates a high frequency carrier, which is given to the modulator along with the modulating signal. Several stages of frequency multiplier are used to increase the operating frequency. Even then, the power of the signal is not enough to transmit. Hence, a RF power amplifier is used at the end to increase the power of the modulated signal. This FM modulated output is finally passed to the antenna to get transmitted. Requirements of a Receiver A radio receiver is used to receive both AM band and FM band signals. The detection of AM is done by the method called as Envelope Detection and the detection of FM is done by the method called as Frequency Discrimination. Such a radio receiver has the following requirements. It should be cost effective. It should receive both AM and FM signals. The receiver should be able to tune and amplify the desired station. It should have an ability to reject the unwanted stations. Demodulation has to be done to all the station signals, whatever the carrier frequency is. For these requirements to get fulfilled, the tuner circuit and the mixer circuit should be very effective. The procedure of RF mixing is an interesting phenomenon. RF Mixing The RF mixing unit develops an Intermediate Frequency (IF) to which any received signal is converted, so as to process the signal effectively. RF Mixer is an important stage in the receiver. Two signals of different frequencies are taken where one signal level affects the level of the other signal, to produce the resultant mixed output. The input signals and the resultant mixer output is illustrated in the following figures. When two signals enter the RF mixer, The first signal frequency = F1 The second signal frequency = F2 Then, the resultant signal frequencies = (F1 + F2) and (F1 – F2) A mixer of two signals of different frequencies are produced at the output. If this is observed in frequency domain, the pattern looks like the following figure. The symbol of a RF mixer looks like the following figure. The two signals are mixed to produce a resultant signal, where the effect of one signal, affects the other signal and both produce a different pattern as seen previously. FM Receiver The FM receiver is the whole unit which takes the modulated signal as input and produces the original audio signal as an output. Radio amateurs are the initial radio receivers. However, they have drawbacks such as poor sensitivity and selectivity. Selectivity is the selection of a particular signal while rejecting the others. Sensitivity is the capacity of detecting a RF signal and demodulating it, while at the lowest power level. To overcome these drawbacks, super heterodyne receiver was invented. This FM receiver consists of 5 main stages. They are as shown in the following figure. RF Tuner Section The modulated signal received by the antenna is first passed to the tuner circuit through a transformer. The tuner circuit is nothing but a LC circuit, which is also called as resonant or tank circuit. It selects the frequency, desired by the radio receiver. It also tunes the local oscillator and the RF filter at the same time. RF Mixer The signal from the tuner output is given to the RF-IF converter, which acts as a mixer. It has a local oscillator, which produces a constant frequency. The mixing process is done here, having the received signal as one input and the local oscillator frequency as the other input. The resultant output is a mixture of two frequencies [(f1 + f2),(f1 − f2)] produced by the mixer, which is called as the Intermediate Frequency (IF). The production of IF helps in the demodulation of any station signal having any carrier frequency. Hence, all signals are translated to a fixed carrier frequency for adequate selectivity. IF Filter Intermediate frequency filter is a bandpass filter, which passes the desired frequency. It eliminates any unwanted higher frequency components present in it as well as the noise. IF filter helps in improving the Signal to Noise Ratio (SNR). Demodulator The received modulated signal is now demodulated with the same process used at the transmitter side. The frequency discrimination is generally used for FM detection. Audio Amplifier This is the power amplifier stage which is used to amplify the detected audio signal. The processed signal is given strength to be effective. This signal is passed on to the loudspeaker to get the original sound signal. This super heterodyne receiver is well used because of its advantages such as better SNR, sensitivity and selectivity. Noise in FM The presence of noise is a problem in FM as well. Whenever a strong interference signal with closer frequency to the desired signal arrives, the receiver locks that interference signal. Such a phenomenon is called as the Capture effect. To increase the SNR at higher modulation frequencies, a high pass circuit called preemphasis, is used at the transmitter. Another circuit called de-emphasis, the inverse process of pre-emphasis is used at the receiver, which is

Principles of Communication – Modulation A signal can be anything like a sound wave which comes out when you shout. This shout can be heard only up to a certain distance. But for the same wave to travel over a long distance, you’ll need a technique which adds strength to this signal, without disturbing the parameters of the original signal. What is Signal Modulation? A message carrying signal has to get transmitted over a distance and for it to establish a reliable communication, it needs to take the help of a high frequency signal which should not affect the original characteristics of the message signal. The characteristics of the message signal, if changed, the message contained in it also alters. Hence it is a must to take care of the message signal. A high frequency signal can travel up to a longer distance, without getting affected by external disturbances. We take the help of such high frequency signal which is called as a carrier signal to transmit our message signal. Such a process is simply called as Modulation. Modulation is the process of changing the parameters of the carrier signal, in accordance with the instantaneous values of the modulating signal. Need for Modulation The baseband signals are incompatible for direct transmission. For such a signal, to travel longer distances, its strength has to be increased by modulating with a high frequency carrier wave, which doesn’t affect the parameters of the modulating signal. Advantages of Modulation The antenna used for transmission, had to be very large, if modulation was not introduced. The range of communication gets limited as the wave cannot travel to a distance without getting distorted. Following are some of the advantages for implementing modulation in the communication systems. Antenna size gets reduced. No signal mixing occurs. Communication range increases. Multiplexing of signals occur. Adjustments in the bandwidth is allowed. Reception quality improves. Signals in the Modulation Process Following are the three types of signals in the modulation process. Message or Modulating Signal The signal which contains a message to be transmitted, is called as a message signal. It is a baseband signal, which has to undergo the process of modulation, to get transmitted. Hence, it is also called as the modulating signal. Carrier Signal The high frequency signal which has a certain phase, frequency, and amplitude but contains no information, is called a carrier signal. It is an empty signal. It is just used to carry the signal to the receiver after modulation. Modulated Signal The resultant signal after the process of modulation, is called as the modulated signal. This signal is a combination of the modulating signal and the carrier signal. Types of Modulation There are many types of modulations. Depending upon the modulation techniques used, they are classified as shown in the following figure. The types of modulations are broadly classified into continuous-wave modulation and pulse modulation. Continuous-wave Modulation In the continuous-wave modulation, a high frequency sine wave is used as a carrier wave. This is further divided into amplitude and angle modulation. If the amplitude of the high frequency carrier wave is varied in accordance with the instantaneous amplitude of the modulating signal, then such a technique is called as Amplitude Modulation. If the angle of the carrier wave is varied, in accordance with the instantaneous value of the modulating signal, then such a technique is called as Angle Modulation. The angle modulation is further divided into frequency and phase modulation. If the frequency of the carrier wave is varied, in accordance with the instantaneous value of the modulating signal, then such a technique is called as Frequency Modulation. If the phase of the high frequency carrier wave is varied in accordance with the instantaneous value of the modulating signal, then such a technique is called as Phase Modulation. Pulse Modulation In Pulse modulation, a periodic sequence of rectangular pulses, is used as a carrier wave. This is further divided into analog and digital modulation. In analog modulation technique, if the amplitude, duration or position of a pulse is varied in accordance with the instantaneous values of the baseband modulating signal, then such a technique is called as Pulse Amplitude Modulation (PAM) or Pulse Duration/Width Modulation (PDM/PWM), or Pulse Position Modulation (PPM). In digital modulation, the modulation technique used is Pulse Code Modulation (PCM) where the analog signal is converted into digital form of 1s and 0s. As the resultant is a coded pulse train, this is called as PCM. This is further developed as Delta Modulation (DM), which will be discussed in subsequent chapters. Hence, PCM is a technique where the analog signals are converted into a digital form. Learning working make money