Understanding Pulse Code Modulation

Welcome to Digital Communication Playlist. I, Professor Hitesh Dholakia is going to explain you Pulse Code Modulation in this session. So to understand Pulse Code Modulation in Digital Communication, these are my session outlines, where first I'll be going to explain you basics of Pulse Code Modulation, then after we will see Block Diagram of Pulse Code Modulation, then I'll explain you the basic process which is there with pulse code modulation in that first we do sampling quantization and then at the end we do encoding in that process. So in detail we will see all those steps then I'll explain you a few basic standards which is there with pulse code modulation then after we will see how we can identify bit rate and bandwidth in pulse code modulation then we will see advantages of pulse code modulation disadvantages of pulse code modulation and applications of pulse code modulation. So let us begin this session with first agenda that is basics of pulse code modulation. Now see pulse code modulation that is what we are using it to convert analog signal into digital data. So we will be having analog signal like voice signal and that voice signal that we are delivered to convert it into digital data. in terms of 1s and 0s and for that this is very popular method which is what we are using it to convert Y signal into digital data and in that first we will be doing sampling to convert that given analog data into discrete analog signal. Like see if you have one signal which is continuous analog signal then by having a sampling we can convert that into discrete signal and even I have made all the sessions regarding sampling in this playlist of digital communication you can go through it. After sampling we do quantization and this quantization is being done to convert given discrete signal into digital signal. where you will be finding there are predefined quantized level will be there and we will be translating that discrete samples into predefined digital samples. And after that we do encoding. So that encoding will translate quantized samples into digital data. So this is the basic thing which we do it in pulse code modulation. Now let us try to understand all those things by block diagram. and by waveforms. So see we have analog input over here. So you can see here continuous time analog input that we have. Now this input that will be given to sampling. So we will be having sampled output. In my previous sessions you can see I have explained sampling as well as there are different types of methods of sampling where we can have ideal sampling, natural sampling. and flat top sampling. so out of those we can use any method to convert given analog signal into discrete sampled output. so here in sampling we choose sampling frequency. so at fixed time interval we will be having a samples you can see. now this sampled output that will be given to quantizer. so Quantizer will quantized this sampled data and it will be converting this data into predefined sets of values. So what based on approximation and few sets of rules we convert this sampled output into quantized output. So in quantized output there will be fixed levels you can see. C is in sampled output. Amplitude will be continuous. There will be no fixed data which is there with samples. But once you do quantization, there will be fixed intervals and those fixed interval that will be assigned to particular value which is there with sampled out. So, for example, you can see this sample that is assigned with this amount of amplitude in quantizer. Second is given with this amplitude. So likewise we give sampled output to quantizer and it will be converting into fixed values. So here for example if I say I have three bits to represent given data. So three bits can have total number of samples that is equals to 2 to the power 3. So total 8 number of samples are there which can be used to convert this sampled signal into quantized signal. So here you can see there are total 8 levels. So here there are only 8 possible amplitudes to which we will be giving amplitude. Now once we have this quantized data then by having encoding we can convert that into digital. Like see if I say this first level that is having 000 data then second level is having 001, third is having 010. Likewise we can assign direct digital data to quantized signal and this digital data that is what we can carry for further communication. So ultimately this is what complete process which we do it in pulse code modulation to translate analog input into digital output. Now here few more things that we need to take care of one is if at input side frequent high frequency components are there in that case we need to place low pass filter over here before we give it to sampling. So if you provide low pass filter over here then that will eliminate high frequency components which is there in analog input and in that case we will not be having any sampling error as we have removed high frequency components and even sampling is been done based on what is maximum frequency which is there at input side. So it should not have very high frequency to avoid sampling noise. So there is a possibility of alizing as if we don't provide sufficient sampling frequency and as if you increase sampling frequency in that case there will be issue regarding bandwidth. So all those things that we will see step by step. So now see process of pulse code modulation. So where first we do filtering operation and that is what we do it at input side where if we have some high frequency components in input signal in that case by using low pass filter we can eliminate high frequency components and after that we give it to sampling where sampling frequency is so essential in sampling. that justifies bandwidth of false code modulation. So even how to calculate bandwidth and how to calculate data rate that even we have delivered to see later. Then after we do quantization where discrete signal will be converted into quantized signal and that quantized data is now ready to encode. So after encoding we will be having digital data. So this is the basic process which we do it in. pulse code modulation. Now let us see all those things step by step. So see we will see first sampling in pulse code modulation and this is what we are doing it to convert continuous analog signal into discrete signal and discretization is been done with respect to time interval TS that is even referred as sampling time interval and based on sampling time interval we can calculate sampling frequency or sampling rate that is fs is equals to 1 by ts and as per Nyquist rate we should choose this sampling frequency greater than 2 times of fm where fm is a maximum frequency present in analog input. So here sampling frequency should be greater than or equal to 2 times of fm where fm is maximum frequency present in analog input and if you don't do this in that case there will be no reconstruction of analog data which is there at input side. So to reconstruct signal after having a sampling fs should be greater than or equal to 2fm. There are three basic sampling methods and I already I have made separate session for all three methods. First is ideal sampling and in ideal sampling you will be finding impulses are there at each instant. So you can see if my analog signal is this. You can observe then if I provide sampling over here at instant sampling in that case you will be finding C at particular instant only there will be samples right and time period in between two sample is sampling time period Ts and 1 by Ts that is sampling frequency and this sampling frequency should be greater than or equal to 2Fm. Now if we see second method so that is natural sampling method where in natural sampling method pulse of short width with varying amplitude will be there. So if you see that by this diagram this is what my analog signal and this is my pulse but if you observe pulse width amplitude is varying you can see with this pulse even you can see amplitude is varying. right so in natural sampling pulse width will be fixed but its amplitude will vary as you can see it in this waveform now third sampling method that is flat top sampling method where pulse of short width will be there but its amplitude will be fixed and this is what Flat top sampling and this is what happening based on sample and hold output and to understand all these three methods I have made separate session for it. You can go through my playlist where in sampling I have explained all three methods in detail and this is how waveforms are appearing in different sampling methods. Now once you do sampling At output side there will be sampled output where you will be finding there will be pulses with finite amount of amplitude and those pulses with finite amount of amplitude will be given to quantization. Now see in quantization we are dealing with to approximate values which is predefined. So see the process of measuring numerical values of samples and giving them a table value in suitable scale. So for example, if I say my amplitude is varying from zero to one voltage, and if I represent that by three bits, then total number of levels to assign given values is one. Sampled output to quantized output will be 2 to the power 3 so that will be 8. So with 0 voltage I will assign 000 digital data with voltage 0 and as 1 voltage is divided into 8 different levels second level will appear at 0.125 voltage. And with the gap of 0.125 voltage next level will appear. So likewise we will be finding there are total 8 levels which will be defining values of voltage varying from 0 to 1 voltage. So the finite number of amplitude of interval is referred as quantizing interval. There can be two categories. linear quantization and non-linear quantization. And even in my future videos, you can study both in detail. Here, I will just give you basic idea. Like see in linear quantization, level interval will be at finite and fixed value. While in non-linear quantization, level of interval will vary with respect to variation. So, it is not constant. So, in linear quantization, we can say like see if voltage is varying from 0 to 1 with 3 bit resultant, in that case total number of levels will be L is equals to 2 to the power 3 means 8 levels are there. While in nonlinear quantization levels will not appear at finite interval it may have different values of amplitude with different intervals. So difference in between sampled output and quantized output is quantization distortion. Like see for example if my sampled output is 0.129 voltage. But after approximation my quantized output is 0.125 voltage which is predefined. Then the difference in between these two that is quantization distortion. So you will be finding in this particular case it will be 1. 0.129 minus 0.125 that is equals to 0.004 voltage. So that is how quantization distortion is appearing. If you want to reduce quantization distortion then you will have to increase number of levels and if you increase number of levels then To represent given sample we need more number of bits and if you increase more number of bits then you will have to need then you will have to have more bandwidth to transmit data. So to decrease this distortion we can increase number of levels by increasing number of bits. So this is what the basics which is there with quantization. Now Let us see the standards which is there with pulse code modulation. See there are main two standards which is there with PCM and this is what we are using it in digital audio data. And those two standards are there with European standards and with American standards. So these are the two major standards which is there with pulse code modulation to digitize audio signal. Their sides slightly differ. in details but working will remain same. So working is same and that is what we have already seen. In terms of details it will be a bit different. European PCM is having 30 channels and North American PCM is having 24 channels. Japanese PCM is having 24 channels and in India we follow European standards with 30 channels. So There are major two standards European standards and American standards and in India we follow European standards with 30 channel. Here I am not going to show you all the details regarding all those PCM standards for that in future I will make separate session for that. Now how to calculate bitrate and bandwidth of PCM. When we talk about PCM bandwidth and bitrate then bitrate will be based on sampling frequency and number of bits required to represent sample. So bitrate will be n into fs where n is number of bits required to represent one sample and fs is sampling frequency. Now bandwidth of PCM that depends on which type of encoding is being done. So once you identify bitrate then that data rate will be encoded in particular scheme. So that scheme might be having some finite bandwidth and as for that we will be having resultant bandwidth of PCM. So bandwidth is depending on type of encoding that is been used and from bandwidth will even come from bitrate only. So bitrate can be calculated directly like N into FS. But bandwidth that is also depending on encoding scheme. So digital signal requires more bandwidth but we pay price for robustness and digital communication. There are many advantages which is there with digital communication. So we convert analog signal into digital and then we do digital communication because of so many advantages and already I have explained many advantages of digital communication in initial videos of this playlist. Now let us see the advantages which is there with pulse code modulation. Uniform transmission quality can be maintained by having a pulse code modulation and compactability of different class of traffic in network can be assigned. Like see if you use text data that will be digital if you use video data that will be digital and in parallel with that by converting audio signal into digital we can have compatibility of different class of traffic in a same network. So this is what possible due to PCM. Integrated digital network is utilizing that facility of voice translation into digital by using pulse code modulation. Increased utilization of existing circuit can be done and you will be finding good performance over a poor transmission path. So if transmission path is very noisy, still we can have proper communication due to pulse code modulation where our output will be digital signal and digital signal is having many advantages like we can correct error, we can detect error while that is not possible in case of analog signal. So we can have very good performance over poor transmission channel. There are few disadvantages even like you will be finding you need to have a large bandwidth for transmission and if you want accurate white signal in that case even we need to have more number of levels and if you increase number of levels in that case there will be large bandwidth which is required for transmission. Noise and crosstalk leaves low but attenuation will increase. So here we reduce noise and crosstalk but because of pulse code modulation technique we increases attenuation of signal. There are many applications which is there with pulse code modulation regarding audio. One can use that in compact disk to store that in compact disk. We have digital telephony due to PCM. In digital audio applications, there are many applications which is there with pulse code modulation. I hope that you have understood this session. Thank you so much for watching this video. Please do give your valuable suggestions. The reason is your suggestions are giving motivation to me to work out for my students. And definitely that will be helpful to my students. Thank you so much for watching this video.

So in detail we will see all those steps then I'll explain you a few basic standards which is there with pulse code modulation then after we will see how we can identify bit rate and bandwidth in pulse code modulation then we will see advantages of pulse code modulation disadvantages of pulse code modulation and applications of pulse code modulation. So let us begin this session with first agenda that is basics of pulse code modulation. Now see pulse code modulation that is what we are using it to convert analog signal into digital data. So we will be having analog signal like voice signal and that voice signal that we are delivered to convert it into digital data. in terms of 1s and 0s and for that this is very popular method which is what we are using it to convert Y signal into digital data and in that first we will be doing sampling to convert that given analog data into discrete analog signal.

Like see if you have one signal which is continuous analog signal then by having a sampling we can convert that into discrete signal and even I have made all the sessions regarding sampling in this playlist of digital communication you can go through it. After sampling we do quantization and this quantization is being done to convert given discrete signal into digital signal. where you will be finding there are predefined quantized level will be there and we will be translating that discrete samples into predefined digital samples. And after that we do encoding.

So that encoding will translate quantized samples into digital data. So this is the basic thing which we do it in pulse code modulation. Now let us try to understand all those things by block diagram.

and by waveforms. So see we have analog input over here. So you can see here continuous time analog input that we have. Now this input that will be given to sampling. So we will be having sampled output.

In my previous sessions you can see I have explained sampling as well as there are different types of methods of sampling where we can have ideal sampling, natural sampling. and flat top sampling. so out of those we can use any method to convert given analog signal into discrete sampled output. so here in sampling we choose sampling frequency. so at fixed time interval we will be having a samples you can see.

now this sampled output that will be given to quantizer. so Quantizer will quantized this sampled data and it will be converting this data into predefined sets of values. So what based on approximation and few sets of rules we convert this sampled output into quantized output. So in quantized output there will be fixed levels you can see.

C is in sampled output. Amplitude will be continuous. There will be no fixed data which is there with samples.

But once you do quantization, there will be fixed intervals and those fixed interval that will be assigned to particular value which is there with sampled out. So, for example, you can see this sample that is assigned with this amount of amplitude in quantizer. Second is given with this amplitude.

So likewise we give sampled output to quantizer and it will be converting into fixed values. So here for example if I say I have three bits to represent given data. So three bits can have total number of samples that is equals to 2 to the power 3. So total 8 number of samples are there which can be used to convert this sampled signal into quantized signal. So here you can see there are total 8 levels. So here there are only 8 possible amplitudes to which we will be giving amplitude.

Now once we have this quantized data then by having encoding we can convert that into digital. Like see if I say this first level that is having 000 data then second level is having 001, third is having 010. Likewise we can assign direct digital data to quantized signal and this digital data that is what we can carry for further communication. So ultimately this is what complete process which we do it in pulse code modulation to translate analog input into digital output.

Now here few more things that we need to take care of one is if at input side frequent high frequency components are there in that case we need to place low pass filter over here before we give it to sampling. So if you provide low pass filter over here then that will eliminate high frequency components which is there in analog input and in that case we will not be having any sampling error as we have removed high frequency components and even sampling is been done based on what is maximum frequency which is there at input side. So it should not have very high frequency to avoid sampling noise.

So there is a possibility of alizing as if we don't provide sufficient sampling frequency and as if you increase sampling frequency in that case there will be issue regarding bandwidth. So all those things that we will see step by step. So now see process of pulse code modulation. So where first we do filtering operation and that is what we do it at input side where if we have some high frequency components in input signal in that case by using low pass filter we can eliminate high frequency components and after that we give it to sampling where sampling frequency is so essential in sampling.

that justifies bandwidth of false code modulation. So even how to calculate bandwidth and how to calculate data rate that even we have delivered to see later. Then after we do quantization where discrete signal will be converted into quantized signal and that quantized data is now ready to encode. So after encoding we will be having digital data.

So this is the basic process which we do it in. pulse code modulation. Now let us see all those things step by step. So see we will see first sampling in pulse code modulation and this is what we are doing it to convert continuous analog signal into discrete signal and discretization is been done with respect to time interval TS that is even referred as sampling time interval and based on sampling time interval we can calculate sampling frequency or sampling rate that is fs is equals to 1 by ts and as per Nyquist rate we should choose this sampling frequency greater than 2 times of fm where fm is a maximum frequency present in analog input.

So here sampling frequency should be greater than or equal to 2 times of fm where fm is maximum frequency present in analog input and if you don't do this in that case there will be no reconstruction of analog data which is there at input side. So to reconstruct signal after having a sampling fs should be greater than or equal to 2fm. There are three basic sampling methods and I already I have made separate session for all three methods.

First is ideal sampling and in ideal sampling you will be finding impulses are there at each instant. So you can see if my analog signal is this. You can observe then if I provide sampling over here at instant sampling in that case you will be finding C at particular instant only there will be samples right and time period in between two sample is sampling time period Ts and 1 by Ts that is sampling frequency and this sampling frequency should be greater than or equal to 2Fm.

Now if we see second method so that is natural sampling method where in natural sampling method pulse of short width with varying amplitude will be there. So if you see that by this diagram this is what my analog signal and this is my pulse but if you observe pulse width amplitude is varying you can see with this pulse even you can see amplitude is varying. right so in natural sampling pulse width will be fixed but its amplitude will vary as you can see it in this waveform now third sampling method that is flat top sampling method where pulse of short width will be there but its amplitude will be fixed and this is what Flat top sampling and this is what happening based on sample and hold output and to understand all these three methods I have made separate session for it. You can go through my playlist where in sampling I have explained all three methods in detail and this is how waveforms are appearing in different sampling methods. Now once you do sampling At output side there will be sampled output where you will be finding there will be pulses with finite amount of amplitude and those pulses with finite amount of amplitude will be given to quantization.

Now see in quantization we are dealing with to approximate values which is predefined. So see the process of measuring numerical values of samples and giving them a table value in suitable scale. So for example, if I say my amplitude is varying from zero to one voltage, and if I represent that by three bits, then total number of levels to assign given values is one.

Sampled output to quantized output will be 2 to the power 3 so that will be 8. So with 0 voltage I will assign 000 digital data with voltage 0 and as 1 voltage is divided into 8 different levels second level will appear at 0.125 voltage. And with the gap of 0.125 voltage next level will appear. So likewise we will be finding there are total 8 levels which will be defining values of voltage varying from 0 to 1 voltage. So the finite number of amplitude of interval is referred as quantizing interval.

There can be two categories. linear quantization and non-linear quantization. And even in my future videos, you can study both in detail.

Here, I will just give you basic idea. Like see in linear quantization, level interval will be at finite and fixed value. While in non-linear quantization, level of interval will vary with respect to variation.

So, it is not constant. So, in linear quantization, we can say like see if voltage is varying from 0 to 1 with 3 bit resultant, in that case total number of levels will be L is equals to 2 to the power 3 means 8 levels are there. While in nonlinear quantization levels will not appear at finite interval it may have different values of amplitude with different intervals.

So difference in between sampled output and quantized output is quantization distortion. Like see for example if my sampled output is 0.129 voltage. But after approximation my quantized output is 0.125 voltage which is predefined.

Then the difference in between these two that is quantization distortion. So you will be finding in this particular case it will be 1. 0.129 minus 0.125 that is equals to 0.004 voltage. So that is how quantization distortion is appearing. If you want to reduce quantization distortion then you will have to increase number of levels and if you increase number of levels then To represent given sample we need more number of bits and if you increase more number of bits then you will have to need then you will have to have more bandwidth to transmit data.

So to decrease this distortion we can increase number of levels by increasing number of bits. So this is what the basics which is there with quantization. Now Let us see the standards which is there with pulse code modulation.

See there are main two standards which is there with PCM and this is what we are using it in digital audio data. And those two standards are there with European standards and with American standards. So these are the two major standards which is there with pulse code modulation to digitize audio signal. Their sides slightly differ.

in details but working will remain same. So working is same and that is what we have already seen. In terms of details it will be a bit different.

European PCM is having 30 channels and North American PCM is having 24 channels. Japanese PCM is having 24 channels and in India we follow European standards with 30 channels. So There are major two standards European standards and American standards and in India we follow European standards with 30 channel. Here I am not going to show you all the details regarding all those PCM standards for that in future I will make separate session for that. Now how to calculate bitrate and bandwidth of PCM.

When we talk about PCM bandwidth and bitrate then bitrate will be based on sampling frequency and number of bits required to represent sample. So bitrate will be n into fs where n is number of bits required to represent one sample and fs is sampling frequency. Now bandwidth of PCM that depends on which type of encoding is being done. So once you identify bitrate then that data rate will be encoded in particular scheme.

So that scheme might be having some finite bandwidth and as for that we will be having resultant bandwidth of PCM. So bandwidth is depending on type of encoding that is been used and from bandwidth will even come from bitrate only. So bitrate can be calculated directly like N into FS.

But bandwidth that is also depending on encoding scheme. So digital signal requires more bandwidth but we pay price for robustness and digital communication. There are many advantages which is there with digital communication.

So we convert analog signal into digital and then we do digital communication because of so many advantages and already I have explained many advantages of digital communication in initial videos of this playlist. Now let us see the advantages which is there with pulse code modulation. Uniform transmission quality can be maintained by having a pulse code modulation and compactability of different class of traffic in network can be assigned. Like see if you use text data that will be digital if you use video data that will be digital and in parallel with that by converting audio signal into digital we can have compatibility of different class of traffic in a same network. So this is what possible due to PCM.

Integrated digital network is utilizing that facility of voice translation into digital by using pulse code modulation. Increased utilization of existing circuit can be done and you will be finding good performance over a poor transmission path. So if transmission path is very noisy, still we can have proper communication due to pulse code modulation where our output will be digital signal and digital signal is having many advantages like we can correct error, we can detect error while that is not possible in case of analog signal.

So we can have very good performance over poor transmission channel. There are few disadvantages even like you will be finding you need to have a large bandwidth for transmission and if you want accurate white signal in that case even we need to have more number of levels and if you increase number of levels in that case there will be large bandwidth which is required for transmission. Noise and crosstalk leaves low but attenuation will increase. So here we reduce noise and crosstalk but because of pulse code modulation technique we increases attenuation of signal. There are many applications which is there with pulse code modulation regarding audio.

One can use that in compact disk to store that in compact disk. We have digital telephony due to PCM. In digital audio applications, there are many applications which is there with pulse code modulation.

I hope that you have understood this session. Thank you so much for watching this video. Please do give your valuable suggestions.

The reason is your suggestions are giving motivation to me to work out for my students. And definitely that will be helpful to my students. Thank you so much for watching this video.

Transcript for:Understanding Pulse Code Modulation

Transcript for:
Understanding Pulse Code Modulation