Welcome to Digital Communication Playlist. Here in this session, I'll be going to explain Differential Pulse Code Modulation, DPCM, where in this session, I'll explain what is the importance of Differential Pulse Code Modulation and what are the advantages and disadvantages which is there with DPCM over Pulse Code Modulation. So, I have already explained all the sessions which is there regarding Differential Pulse Code Modulation.
where if you have input analog signal like see this is what one analog signal one can see and if I take sampling rate fs at Nyquist rate then it will be twice of the bandwidth of the input signal. So what will happen is like see you will be doing sampling of this signal at interval which is twice of the bandwidth but as if you increase this sampling rate what will happen is like number of samples that will increase so i'll give you one example so it will be more clear like see what i mean to say by difference in between dpcm and pcm so if i consider sampling rate which is 2w then you'll be finding sampling is happening let us say it is happening like this so you will be finding samples are separated like this I have just approximated drawn this. It is not exactly the same as for this.
But just to convey message I am just writing this. So sampling rate will be somewhat like this. Now if I consider sampling rate which is higher than this one.
Let us say it is 5W. So in that case number of samples that will be dense. So you will be finding now number of samples are nearer. compared to previous case.
So you will be finding like see this is what the case which is happening over here. So here one analysis that one can say which is as you increase sampling frequency number of samples are getting denser. Now see to have more correlation of signal you can increase number of samples per second means sampling rate could be increased.
So as if you increase more number of samples per second by increasing sampling rate, you will be finding relevance of signal with sampled output will be more. So samples are more correlated to each other. So this is one observation. Now what is there which is there with pulse code modulation and what is there with Differential pulse code modulation. So in pulse code modulation, we transmit digital data of all samples.
While in case of DPCM, we don't transmit digital data of all sample, but we transmit difference in between samples data. Like see, for example, if I say I have this sample, which is having magnitude. 3.1 voltage. This sample that is having magnitude let us say 3.5 voltage.
Let us say this sample that is having magnitude 2.5 voltage. Over here we have magnitude which is 2.2 voltage this is what i'm assuming right now so in case of pcm in case of pcm we encode all these samples with respect to number of levels and we transmit all this data so in PCM in PCM all we do is we provide digital data to all the samples and based on that we send signal while in case of DPCM all we do is we take a difference like see over here if I say this is 3.1 voltage over here it is 3.5 voltage over here it is 3.4 voltage if I say this is 2.9 voltage if I say this is 2.5 voltage is 2.1 voltage is 2.2 voltage. This is what I am assuming right now. Now in differential pulse code modulation we take difference in between consecutive samples. So here you will be observing difference is 3.5 minus 3.1.
So that is what the difference. So that difference is 0.4 voltage. If you see the difference in between these two you So that difference is 0.1 voltage.
In between these two difference is 0.5 voltage. In between these two difference is 0.4 voltage. In between these two difference is 0.4 voltage. In between these two difference is 0. 0.1 voltage. So in differential pulse code modulation DPCM we transmit difference of amplitude in between consecutive samples.
So here all we do is we don't need that many levels as we need it in case of PCM. In PCM we need to have encoding of all samples amplitude. While in case of DPCM, we need to send encoding of voltage. But that encoding is difference in between samples. So here we will be decreasing number of levels.
Like see here level magnitude is 3.1 in case of BCM. 3.5 in case of this sample. In case of this sample, it is 2.5. But when we send it by DPCM, we just have 0.4, 0.1 voltage.
So that... amplitude level that is far lower than PCM. So ultimately we reduce number of levels in terms of voltage. So we need less amount of bit rate to satisfy that condition.
So by using DPCM we will be reducing data rate as well as we will be providing more correlation in between consecutive samples. So this is what the basic difference which is there with DPCM and PCM. So let us see all those basic points.
Like see when the input analog signal is sampled at a rate higher rate than the Nyquist rate, the successive samples become more correlated. Like see if you increase sampling rate, successive samples are getting more correlated to each other. That one can see. Next point is there exists a vary.
little difference between the amplitude of successive samples. In case of DPCM, you will be finding there is very little difference in between successive samples. So amplitude difference in successive samples is very small.
So that is what one can observe. When all samples are quantized and encoded, there exists more redundant information in the transmitted signal. Like see here If you do PCM by this in that case you will be finding more amount of data that we need to transmit after encoding. So even you have more relevance in case of PCM you will be finding like more redundant memory that we need to encode. While in case of DPCM number of levels of voltage will decrease that will provide advantage of decreasing of redundancy.
So this is what one can say. based on these two differences. So when all samples are quantized and encoded there exists more redundant information in the transmitted signal. In DPCM to reduce redundant information and to achieve more compression only the difference in between consecutive samples are transmitted.
So in DPCM we only send difference in between consecutive samples. So we reduce redundant information and and we provide more compression. So this is basic advantage which is there with DPCM.
Like one can observe here and here I have shown two scales. One scale that is there with complete signal and one scale that is there with difference in between consecutive samples. So to explain that if I say I have two samples consecutively I have two samples right. And one sample that is having voltage 4 and other sample that is having voltage 4.1. So in PCM we need more levels.
Why the reason is we need to send information of this level as well as this level. While in case of DPCM we just need to send difference. So when you send it by DPCM you will be finding this difference only.
if I say this is 4 voltage and this is 4.1 voltage this difference that will be 0.1 voltage that is what we need to send. So there will be less number of levels the reason is here there will be more voltage so levels will be more here difference will be having less voltage so levels will be less. So more levels will be there in case of PCM so that requires more number of bits per level. And that one can easily calibrate by having levels L is equals to 2 into Xmax divided by delta.
So as you increase L you will have to increase number of bits per second. So that is what the case which will be there with PCM. But in case of DPCM. You will be decreasing number of levels.
Why? The reason is you just take a difference in between consecutive samples. So difference will always provide less number of levels. So here we provide less number of levels which will provide less number of bits per second.
Here L is number of bits and L is number of level. So here we provide compression of signal by using DPCM while in case of PCM we need to send information of all samples. So which will be?
having more memory requirement and in case of DPCM there will be less memory requirement for encoding. So this is what the basics regarding DPCM. In next session I'll explain you encoder and decoder of DPCM. So which will explain you like see how to have encoding of analog signal by using DPCM and even I'll provide decoder so you will get to know like see how to reconstruct DPCM signal by using DPCM decoder or a DPCM receiver.
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