Analog and Digital Pulse Modulation Techniques-A Conceptual Video Lecture -Digital Comm. (Part I)

now we are starting the conceptual videos on digital communication before we start the topic we introduce the topic and relate the topic with the previous study which we have done and put up the videos on our website we have seen that analog communication has been covered in three conceptual videos where the part one was amplitude modulation part two was on the angle modulation and part three was on noise okay and the we start with the digital communication this will covered in two conceptual videos part 1 will be on pulse modulation and part 2 will be on digital modulation so present video is the part 1 of the digital communication and this video is on the analog and digital pulse modulation as is obvious from the slide the topic we have seen in analog communication or analog modulation that the carriers were sinusoidal while in the pulse modulation the carrier is pulse train and modulating signal is analog or digital or it could be unlock signal converted to digital form if it was an Locker okay so in pulse modulation carrier this pulse train and some parameter of it is varied in a according to the modulating signal schemes which are prevalent our pulse amplitude modulation pulse time modulation and pulse time modulation is divided into pulse width modulation and pulse position modulation ever moderation schemes come under the analog pulse modulation schemes and we also sometimes call them discrete modulation white is discrete because here the time axis is digital while the amplitude accesses and lock okay that is to get a digital signal from this amplitude axis should also be made discrete okay it is done using the quantization and coding and then we get the pulse code modulation the advantage is that the these digital signals have the advantages that we can detect these signals in the presence of noise and this PCM scheme which will be covered in this virtual video is a fully digital scheme so we’ll start with the presentation now this is the topic that log on digital pulse modulation techniques and we what do you think needs others because the need is that most of the signals which was available in modern communication systems are digital in nature because we have so many digital techniques available with the development of microprocessors

computers and micro electronics the integrated circuits which are available in digital form are more easily available and cheaper also so we are rather moving from analog communication power to digital communication even if the signal unlock be transmitted digitally that means convert into digital form and then transmitted this advantage we already said this reduces distortion and the improvement in the signal noise ratio is obtained with these techniques we just like a summary like there are three schemes pulse amplitude modulation pulse width modulation and position modulation they are said to be like analog modulation techniques and pulse code modulation Delta modulation other schemes which come under the I would say the fully digital streams we had seen that in sub C W modulation that some parameter modulated wave of wearing like amplitude frequency or face in love modulation here in unlocked pulse modulation here some parameter of each pulses modulated are varied by a particular sample value of the message okay so now we will see this pulse modulation of qu type so these analog pulse modulation this are three types of modulation schemes which we have seen like pulse amplitude pulse width and pulse position and these are the digital modulation schemes pulse digital martial skills like pulse code modulation Delta modulation this is p.m. and this high frequency carriers that is the pulse varied in chorus with the sample well with the message signal this pulse width modulation here the width of the pulse is varied and pulse position modulation is the position of the pulse is varied it is subject that PBS the PPM’s means more effective of pulse and lock moderation schemes in terms of self noise ratios okay now this is anyway a presentation just comparing the these modulation schemes like this is the modulating signal business Inspiral nature in the pulse modulation here you notice here that depending upon the amplitude of the this signal pulse amplitude is smaller if this amplitude of the modulating signal is larger pulse amplitude is larger similarly here this is smaller and so on so this is false immune modulation you can see similarly pulse width modulation that means width of the pulse depends upon the amplitude of the modulating signal and this is pulse position modulation position of the pulse depends on the amplitude of the modulating signal and then pulse code modulation this this is about the pulse code modulation here that’s code modulation we have all the digital data 1 0 one zero one one one and so on okay now now we go over to pulse code modulation or PCM schemes and regarding these pulse and lock modulation schemes now we just very commonly used with the line coding oh we will see towards the end of this lecture that how this schemes are used this pond pulse code modulation scheme is very very common in use and you would observe that good number of problems do come in this topic so it should be very sorry about this so we can see first was what we do is in PCM is in PCM analog signal is first sampled then it is quantized and converted into levels that is quantized means converted into levels and then it is encoded so these are the three processes which are involved in the pulse code modulation so before the

sampling of the signal it is first filtered to limit the bandwidth there is one part you should remember now they start with the we start with the first process that is sampling this process of converting unlock single to discrete signal what is the screwed signal that means when there is analog signal and we sample the unlost signal will get discrete signal because in time access it will be digital but amplitude access it will be unlocked that means no quantization so sampling is common in all pulse modulation techniques we signal will sample the signal to regular intervals and our sample is proportional to amplitude of the signal at that instant of time and lock Singh on the sample says the seconds call the sampling interval and FS sampling frequency is 1 over T s is called sampling rate this is very important relation that sampling rate or something frequency should be twice FM FM is the minimum sampling rate this the minimum sampling rate which should be there to recover the signal and this rate is called the Nyquist rate this will be be using very frequently and the sample spectrum G Omega is repeating periodically without overlapping in this case if the sampling frequency is less than this then the overlapping will occur ordinal signal is centered at Omega zero and having T if you see the spectrum Center Omega zero and having the bandwidth my gram this spectrum can be recovered by passing through low-pass filter okay where I first less than twice FM sample spectrum will overlap and cannot be recovered back and this is said to be the aliasing okay now sampling we can see that what are the different methods for sampling there is the ideal sampling where an impulse at each sampling instant this we can see the ideal M this and audial impulse and we are sampling it with ideal impulse and taking the amplitude of the air lock signal add theses and sub time okay the natural sampling a pulse off short with that you can notice here the pulse of this this width here this is the width of the pulse and this is this sampling the analog signal so this is the natural sampling and the other type of sampling is flat types flattop sampling is your sample and hold so that the sample level top level is held to a particular value like natural insulator natural sampling but we’ll love with a single amplitude value because the amplitude does not change in the range why here because not to see an amplitude is different at different points of this pulse so this is a flap top sampling method okay now so far we have seen that the signal is low-pass type then what should be the sampling frequency here the sampling of a bandwidth bandpass signal if the sampling is to be done for bandpass signal these are we can find the what should be the minimum bandwidth bandpass signal of bandwidth – FM can be completely recovered from examples then the minimum sampling rate should be 2 into bandwidth 2 into bandwidth is 2 FM the for FM so range of the sampling frequencies is in the range of 2 into bandwidth – 4 into bandwidth that means if you sample within this range you will be able to record this signal back this train is sampling our impulse sampling sampling function strain um pulses respect to remains constant throughout the grain it is not practical we cannot have such types of natural sampling the spectrum is weighted by a sinc function if you see the spectrum the spectrum is weighted by same function amplitude of high frequency component reduces and the third one is a flattop sampling that’s a very common here top of the sample remains constant that we have seen in the spectrum high frequency components attenuated due to sin pulse roll-off here also high frequency components are reduced and this effect is called an aperture effect and snouted the pulse width increases opposite works once more that is more redemption of high frequency components that means if the

pulse width as width of the pulse which is a flat time flat up type if it is more than it will have attenuation of more frequency component that is more of a personal effect will be coming now the second process the quantization quantization sampling results a series of pulses of varying amplitude between two limits we got now the voltages within that two limits and we want to divide that range of voltage into different smaller ranges steps so amplitude values and finite between two limits and we map these to a finite set of values okay under search it by dividing the distance between minimum and maximum into L zones and each height of Delta so maximum voltage minus minimum over the L so we divide this that gives me the Delta here we these are the quantization levels of their selected because mid point of his honest assigned a value of 0 to L minus 1 and here you can find the difference this is said to be a metrid hypeeeee where 0 is also available 0 volts here 0 volts is not available because that particular level this is not 0 even this is not 0 so that is certainly Trice so mid pred for odd number of levels you use this kind of quantization and for mid rise you use for even number of for Elevens okay now we just taken just assemble example here suppose we have a voltage V minimum of minus 20 volts and we max of plus 20 volts we want to use l equals to 8 quantization levels the zone is you can say 20 minus minus 20 divided by 8 5 so 8 zones are available so divide these eight zones and to minus 20 to minus 15 minus 15 minus 10 minus 10 and so on the rate zones and the midpoint of zone is decided here so this is the V when do quantization now assigning course to zone there and an important part now each zone is then assigned a binary code the number of bits required to encode the Zone on the number of bits per sample is as follows if L is the number of levels then and B is log of 2 and say and B 3 and B is 3 okay then there are eight zones as number of bits are three that’s one sir eight and these are the cash money zone identifications being shown here assigning quotes was on we can assign the quote this little rope refer to mines 9:15 and so on this thing we can assign go to that zone now and one important factor here when a signal is quantized serve introduce an error and the coded signal is an approximation of the actual amplitude value and the difference between the actual coded value and the referable you said quantization error more zones more bits required to encode so that is the problem so we have to have reasonable bits now quantization error and signal to quantization noise ratio signals with the lower amplitude values will suffer more fun translation error up Delta 2 is fixed normally we find that if the signal at signals miles they show we try to see for smaller amplitude of signal if I try to find me we try to find set of knives ratio that noise will be Delta by 2 condition-wise but if the signal MTB slow you will get personalized ratio so the quantization need not be done linearly you can have a nonlinear quantization well here this problem and goal is to keep ask you on our fixed for all values sample values and two approaches are common quadrants levels for logarithmic curve smaller deltas at lower amplitudes and lord eldon’s at higher amplitudes and this process we call a compounding the compression of the signal at the transmitter and then expanding the signal at the receiver the zones are fixed and height okay so this is about the quantization error bitrate and bandwidth requirement fission the bit rate for PCM signal can be calculated

from the number of bits per sample into the sampling rate the bit rated and green to FS sampling rate the bandwidth required to transmit this single difference among the type of line coding use that will see towards the end the digital signal will always need more bandwidth than the old analog signal what is we are trying to say is that digital signal nearly really need some more bandwidth and actually the surprised we pay for the robustness of the digital transmission it’s not that you get all the advantages and you’ll a tremendous advantage so this is the price we had to pay for it important relations this quantization I thank you delta square del square over 12 signal noise ratio test even a single quantization noise a shuttle by to to try to and as to an R&D base by this the standard relations available in the books betrayed number of bits per sample into sampling rate energy FS and bandwidth for PCM for PCM signal s and into FM where in number of bits in PCM code FM a signal bandwidth and F is the sampling rate okay these are relations now we come to the last scheme that’s a delta modulation the here the sample is compared with the previous sample and the value 1 as zeros transmitted if it is greater or less than the previous sample value bandwidth you care for DM is less than compared to GCM Delta modulation it’s simple security compared to Sherman Kardashian is more gråvik you can various ladies are available like slope over load but the magnitude of the slope is greater than the slope of the staircase this is the signal empty and the slope is greater than the slope of this selected steps so when the mental slope greater than the slope or staircase then this slope over load comes in and granule noise signal very sentiment subsides because within the steps of the signals wearing their surrender lysis comes an idiom just of size made adoptive to take care of the other problem in idiom we take care of that and differential PCM the difference between two successive samples quantize encoded and transmitted they seems foreign voice transmission so anyway these are the important features you can go in details about the Delta modulation because some problems come in determination also now this is the line coding line coding with skalise important relation to no pulse I would say unlock pulse modulation techniques we use the term like source coding techniques PCM being a medium they are set to the source coding technique here analog signal is converted to its digital equivalent digital baseband transmission is called a line coding what is digital baseband transmission that means you are transmitting the digital signal by into by you at baseband frequencies okay basically that means the channel is a low-pass channel so you need a shaping for the binary data and the line coding converts – sequence and digital signals it more convenient to be transmitted over a cable okay to maximize bit rate reduces power transmission DC component now line coding could be unipolar polar or bipolar we’ll see that’s what is like would swarm it’s our return to zero return to zero am i alternate my conversion Manchester etc these are the common techniques now here this the figure shows unique people are and are said non-return-to-zero this is correspond to 1 this also 1 and 0 is 0 this is 1 0 0 0 this is the unipolar why it is so important it is going on only one first production you need a single supply ok now second is the polar Polar means it is having both partial negative voltages so there’s a polar but one high voltage gives me 1 and low voltage gives me 0 ok this is polar in our non-return-to-zero now we have unipolar return to zero here unipolar that means it is only in positive

direction and this you have power half half of the time they pulses the amplitude is a and then it is zero so this corresponds to one Summa this is a 1 and similarly that and 0 is what the 0 level now bipolar return to 0 this is bipolar return to this also return to zero because the pulse comes to a path peak value and then returns to 0 that’s why the name in return to 0 here also it’s the return to zero ok bipolar set return to Jerusalem and this is the Manchester type of cooling here also non-return-to-zero so this is the here comes for one and another one starting like one and so on and zero is zero ok so these have been different schemes line coding schemes so basically digital baseband transmission is also called like line coding and it has basically low-pass channel okay so this is all about this video and we would say that the pulse PCM form pulse code modulation Delta modulation seem to be quite important topics with reference to the gate examination so should be very thorough and these topics are easily available in the books try to solve more of the problems so they so that you are able to practice better although the coverage of the topic is very wide we have tried to concise the matter in a limited time it’s only to give you an idea that what all topics you should cover and details you could see from the books I try to give basic ideas and concepts which can help even understanding because they are very important because many concepts are involved and why how we progressed from one type of model – modulation scheme to other modulation scheme what has been the need for it and why did we do it and what has been the purpose of it I hope I have done my duty and I wish very happy we him – are my years thank you