 We'll spend some time on signals now to represent data as as communication signals. We've just spent some time on transmission media Some examples of wired media and some of the concepts when we use wireless media like antennas path loss There are a few slides in the in the previous lecture on transmission media towards the end that we skipped over about examples of satellite and other wireless systems, so they were just examples which I think we've mentioned and you probably know many examples of wireless systems already, so That's why we skipped over them quite quickly. The main thing from the previous topic from yesterday and the week before was about antenna, antenna gain, path loss and those relationships that allow us to determine distance, receive power and so on. Today, we'll introduce signal encoding techniques. How do we some different ways in which we take our data that we want to communicate and encode it as a signal? Up until now we've used simple examples like if my data is a sequence of zeros and ones then one scheme what we've said is if we have a bit what a bit one Send a high signal a bit zero send a low signal. That's a signal encoding scheme. It's a way to Generate a signal based upon the data we want to communicate, but there are others, so we'll go through some others today because of limited time, I'll just introduce a selection of schemes today and We'll not finish all hopefully the first two set and some of the things we may skip over and return to after the midterm if we need to explain any further, but we'll just introduce some of the schemes and we split it into four groups Because we distinguish between the analog and digital data Digital data anything made up of zeros and ones So text files images encoded in binary That's our digital data analog data is like audio video analog video other Measurements measured data that is a continuously varying data over time Remember the difference between analog and digital digital is discrete values analog continuously varying So depending upon the data we want to communicate we use different schemes and The signals that we transmit this data with will also distinguish between analog and digital digital signals will be discrete pulses of some power level Typically some discrete voltage pulse, so we send Some voltage say plus five volts for some period of time That's a high level and then for another period of time. We send say minus five volts and We'll use this as a digital signal an analog signal is one which will continuously vary over time and How do we change the shape of our analog signal? What are the three main things that will change the shape of an analog signal three basics of a sine wave amplitude Think of our signals. We've looked at we can change the shape by modifying the amplitude the height the frequency and the phase of The signal and we'll see those come into play when we look at Sending data using analog signals When we have digital signals think we just have a pulse one level or another level with analog signals We've got think of your sine wave So let's go today. Hopefully we'll do the first clue. Maybe we'll jump to the last one if we have time This is general about signal encoding techniques We'll mention this as we go after we go through a few examples This will some of the terms will introduce as we go through examples will be described on this slide Before we go to the specific ones generally These pictures show the different approaches depending on whether we're using digital or analog signals. So the top picture is a Using a digital signal. So the middle component is what's sent across the communication system So we start with some data Analog or digital data and then we encode that data into a digital signal Which is a sequence of pulses We send that digital signal across our communication system The receiver has a decoder that takes those received pulses the different levels and converts it back to Analog or digital data? That's when we're using a digital signal to communicate our data Irrespective of the type of data we encode it into a signal and decode it at the receiver and The plot here is an example of a digital signal X of t is the middle part The other approach is when we use an analog signal This is a plot in the frequency domain, but we can also have plots in the time domain here we take our data digital or analog and modify some continuously varying signal to transmit that between transmitter and receiver and The way we modify it is what we call modulation we take a We take a analog signal and We modify either the frequency amplitude or phase of that analog signal According to what the data is and that process will call modulation So at the transmitter we take our data and we use a modulator We send an analog signal and at the receiver We take the input analog signal and convert it back to the data and that process is called demodulation a Device that does both of these modulates and demodulate is called. What a device that the short name for a device that does both of them is a modem a Modulator and a demodulator. That's what a modem does like your Maybe your ADSL modem your cable modem or your old dial up internet access modem It took data as input and generated an analog signal as output sent across the telephone line and The receiving modem has a demodulator in it and receives an analog signal and converts that back to data so the general Device that does both of them is a modem and what's the name of the device that does? the transmission and reception of a digital signal anyone You've heard of it. Maybe but maybe not in What do you expect? Here we combine modulator and demodulator to get a modem and Coder and decoder. Well encoder another word is just coding So called a codec Some COD EC a device that can encode and decode is called a codec and if you deal with Audio and video and converting between different formats. Sometimes you'll come across this word of a codec your mobile phone When you talk You create an analog your voice is analog data Your voice is continuously varying data your mobile phone has an encoder that takes that input audio and converts it into digital and Sends a digital signal to the mobile phone tower So your mobile phone has an encoder and a decoder or a codec to do that We'll see some other examples of them as we go through and even in the next lecture Different reasons for different using different techniques, but let's go straight to some of the techniques We want to send digital data zeros and ones as digital signals so the digital signals some sequence of Discrete voltage pulses say plus five volts minus five volts or plus positive and negative some voltage Any questions before we continue? Okay, ready for your exam There are always questions about these signaling schemes in the exam Not too hard though We send a discreet voltage pulse any other questions Otherwise, let's keep the noise down for this last lecture Any questions before we move on Okay, we send discreet voltage pulses. We call each pulse a signal element You may have seen this term up come up before when we talk about signaling and Our data our zeros and ones are encoded into signal elements and The example we've seen is we've used for example. Well, we've used the opposite of this binary one data bit one we encode as say a low level a low voltage say negative one volts and Binary zero as a high level say plus one volts or it could be the opposite But we map our digital data to one of the levels of the the digital signal and that signal is Transmitted for a period of time and that represents a single signal element The rate at which we can send bits is called the data rate bits per second But we'll also talk about the signaling rate Which is the rate at which we send signal elements Sometimes it's the same as the data rate. Sometimes it's not It's sometimes called board the signal elements per second. So we'll see some examples of that There are different schemes for encoding bits to digital signals This is an example of one scheme Almost that the obvious one or maybe the opposite of the obvious one Here are some examples of others and we're going to go through all but the last two This describes the schemes and we use some examples to illustrate them It says what is the mapping for our bit zero or one to the particular level of our digital signal? And the top one non return to zero level Says when we have a bit zero we transmit a signal for some duration at a high level and A bit one at a low level. That's it That's one of the signal and coding schemes and then we'll see some of the others and and compare them Let's draw some we have a sequence of bits eight bits. I want to send So that's my digital data To send it. I need to send a digital signal. So I encode it and let's use the the first scheme which is called Non return to zero level non return to zero in that Our signal will be either positive or negative. It would not be zero volts Will not return to zero volts. We will see the level When we come to the next one, there's an alternative of this one invert and it's quite simple we If we go back to our description Want to transmit a bit zero send a signal at the high level bit one low level So the signal becomes bit zero high for some duration And we'll talk about the duration a moment then we have a bit one to send so we change the level to low Then we have a bit zero to send So we go up to high Transmit the signal for that duration Then another bit zero. So it's high again Bit one second bit one Zero There's our digital signal which represents our digital data using non return to zero level NRZ level What? Voltage do we use it doesn't matter Just that that one's negative one's positive What is the duration that we hold at a particular level? Well that is based upon the signaling rate This is a signal element and the duration of each signal element is the same Okay, each signal element has the same duration if the signal element was one millisecond What's our signaling rate? Signal element duration is one millisecond. What's our signaling rate? Anyone if the duration in This example is one millisecond Then we can say the signaling rate we have 1,000 signal elements per second signal elements SE Just to be clear So we don't confuse with bits in this case There are 1,000 signal elements per second if each signal element has a duration of one millisecond This is just an example the signal element duration may be a different value. What's our data rate in this case? How many bits per second? 1,000 bits per second In this scheme each Each signal element represents a single bit so 1,000 signal elements per second is the same as sending 1,000 bits per second In most the examples we'll see that's the case, but in some it's not always the case The signaling rate and the data rate don't have to be the same We could have a single signal element representing more than one bit So if the signal element represented two bits There'd be 2,000 bits per second Or we could have a signal element that represents half a bit that is two signal elements per bit So it's no long it's not necessarily one to one in this case Any questions on non return to zero level? The one I just drew is on in your handouts So it's in the the next slide on their handouts except it's not all bits. I've got eight bits here This example in their handouts is the same first eight bits, but there are a few more bits So you don't have to draw it again. It's it's the top one Another scheme non return to zero inverted or inverted on ones When we have a bit zero There's no transition That is we don't change the level When we have a bit one we change the level From positive to negative or from negative to positive depending upon the previous level Same sequence of bits. Let's try it with Remember the sequence of bits zero one zero zero Maybe I'll write them again just so I Can move Let's assume we start at the low level Come back to that in a moment When we have a bit one so we just transmit the bit zero when we have a bit one descend we invert Which make a transition When we have a bit zero to send we don't invert we keep the same level So I have a zero to send so keep the same level Another zero to send same level Now I have a bit one to send. Let's invert The next bit is a bit one so we invert and then two zeros so this is NRZ inverted two different schemes Transmitting the same sequence of bits using a different digital signal We will not compare them yet We'll just go through how they work and then later we'll talk about some of why would we have different ones? Why don't we just use the first one? Well, there are some differences Signalling rate and the data rate are the same in this case Bipolar AMI now we have to send bit zero we have Zero volts no line signal so send it zero volts bit one will alternate between Positive and negative level for each subsequent bit one in the previous example I assume that we start at the low level the first bit at the low level if we started at the high level here You would have seen the inverse and we upside down because It depends upon the previous bit when we have a bit one We change based upon what the level was before so we need to start at some level Let's do bipolar AMI bit zero bit zero as zero volts. Yes Yeah So all right in the non return to zero inverted We need to assume that the previous bit was some level Okay, so let's say we start assume we start at the negative level That means the next one will change to positive But if I assume the opposite I assumed we started at the positive level The zero would be positive and then the one would go down to negative So it depends on where you start so you need to make some assumption about where you start So the same if you start with a bit one let's assume we start with a negative level and Their first bit is a bit one that will change us immediately up to the positive level okay, so Make some assumption is where you start For a real system it will be described which level to start it there when you start the sequence In an exam or a quiz unless I say otherwise you can assume either If you did it the other way you'd have the inverted signal same with by polar AMI Let's assume the previous The previous bit one was negative There is no previous bit one here, but let's assume it was negative when we have a bit zero we send at zero volts bit one a bit one we go to positive Bit zero back to zero second bit zero at zero The next bit one goes down to negative the next bit one up to positive and then zero zero We alternate when we have bit once positive negative positive negative and just keep doing that as we have bit once The the level is sometimes referred to as a mark so a bit one as a mark so alternate mark inversion we invert Alternating once This assumed that the previous bit was a negative If you made the assumption the opposite one that the previous bit one was positive then you'd Invert this whole picture That is if it was a one here Then the first one would be negative because it's relative to what happened before So this one does return to zero and that has some consequence later when we look at some of the characteristics of these skinks Let's do another one actually will not draw it pseudo ternary Bipolar was with a bit zero send at zero volts with a bit one Alternate between positive and negative pseudo ternary is really the opposite Where the bit one send zero volts with a bit zero alternate between positive negative? I will not draw that But you see with pseudo ternary bit one We have negative sorry bit one. We have a zero bit one Sorry bit zero we alternate between positive negative positive negative positive negative Like the opposite of the bipolar. Am I let's try Manchester encoding Manchester encoding when we have a bit zero to send in the middle of our Interval each signal element what goes for a particular duration in the middle of that duration We'll make a transition from high to low And when we have a bit one in the middle of that duration we'll make a transition from low to high So Manchester encoding see if we can draw that one. Let's repeat our bits so I can remember Same sequence of bits Let's try Manchester We're going to make a transition from high to low Let's go back to our description From high to low when we have a bit zero and from low to high when we have a bit one You get this one right bit zero So now our think of this is the same duration as before but in the middle we make a transition So bit zero we go from high to low bit one will go from low to high we're already The bit one starts at this point. We're already at the low level. So we maintain that and then in the middle We go up to high That's the bit one Bit zero will go from high to low. We're already at high Next bit is a zero. We need to go from high to low So at the start of the transit at the start of the bit interval we need to go up So here we go to high and now the bit zero High to low in the middle So every bit zero must start at high and go down to low So in this case since we were at low beforehand we make a transition at the start of the interval first bit one We're at low we go up to high in the middle Next bit one We'll need to go down at the start and then in the middle go up again Here we're there So we have Manchester encoding every Every signal element has a transition in the middle This is used in in LANs so when you're sending data between Computers using the LAN cable You've got digital data zeros and ones from your computer and it sends digital signals using Manchester encoding. It's very common questions on Manchester how it works you'll be happy to know in in an exam I Will not ask you to remember all of these in exam at least remember the first two and our Z Level and our Z inverted the others. I would describe if I say here's a sequence of bits Create the digital signal if you use differential Manchester then in the exam I would describe this or similar Okay, so you need to better interpret what this means and then draw the signal differential Manchester Similar, but here. There's always a transition transition in the middle when we have a bit zero There's a transition at the start as well with a bit one. There's no transition at the start So let's try that one as our last game to draw if we have a bit zero We'll have a transition at the start of the interval. Let's assume we start high We have a bit zero to send so we must make a transition and every bit has a transition in the middle So that's the first bit zero assuming we started at high Bit zero change at the start then in the middle change again and that leaves us at high Now we have a bit one and the rule there is don't change at the start just change in the middle So we maintain the level for half of the interval and then make a change bit zero change at the start and In the middle change at the start as well as in the middle Another bit zero change at the start as well as the middle a Bit one Don't change at the start. So keep it the same but in the middle change a bit one Don't change at the start keep it the same in the middle change Zero we change and another zero slightly different from our previous Manchester we're changing in The middle for every bit and also at the start for bit one a bit zero differential Manchester There are some other schemes We'll give some examples of where they used maybe we can now We'll come back to this just some examples Where we're here Non-return to zeros used so they used in different technologies. So USB RS 232 serial cables use Non-return to zero and non-return to zero invert on ones Manchester encoding is used in lands So ethernet wired lands Some of the ones that we haven't covered But they're listed on that the previous side of the BA ZS Based on bipolar AMI are used in some wide area network technologies Here's a signal. What's the data that you received if you use non-return to zero level? You receive this signal If you're using NRZ level the basic one the first one what data did you receive? Zero zero one zero zero one one zero okay easy Non-return to zero invert on ones What's the first bit? So the other non return to zero invert on ones non reserve wrong non return to zero I Means when you have a bit one the signal inverts it changes the level So note at the start there's an inversion So the first bit must be a bit one so the first bit one zero One it inverts again it inverts one stays the same zero one Easy non return to zero level What's the sequence of bits you receive? How many bits did I receive? non return to zero level well, let's see At the start it looks so easy this would be Zero how many zeros some say three some say four Yeah, you need to look closely and this is an important point the receiver receives the signal So it's important for the receiver to know the exact duration of each signal element if there's a grid you can work it out, but if It's four in this case But the receiver if it knows the duration the receiver must know exactly where the next signal element starts Because if you don't know where the next signal element starts, then you don't know if you've changed from The old zero to the new zero you wouldn't know how many zeros were here Well, they know the duration, but the problem in practice is sometimes the clocks of the receivers are not synchronized My transmitter is sending a signal where each signal element lasts for one millisecond But the clock of the receiver is such that it thinks it's one millisecond, but it's actually say 1.5 milliseconds clocks of devices at very high are not may not be so accurate at very small time frames so the receiver may have a problem if it Cannot synchronize its clock in this case. It may think this is actually three zeros or Four zeros or five zeros the clocks must be synchronized very well to do that So that's actually a problem in this scheme Is it three four or five zeros? Well, we can check and I can The way I created it it was four zeros But from a receiver's perspective it may not know that exact timing and therefore it could think this was three Zeros and then a one and that would result in a bit error so One of the differences between the different encoding schemes is that some have problems if there's a long sequence of bits of the same The same bit in this case a long sequence of zeros We get the signal at the same level. It's just constant and it's hard for the receiver to know where does the next bit change So it's hard to synchronize and that may result in errors if you use Say Manchester encoding If there is a long sequence of zeros with Manchester encoding thousand zeros in a row There will always be a change for every bit because with Manchester encoding we change We change in the middle all the time So even if there's a long sequence of zeros with Manchester encoding It's easy for the receiver to know when the bit net big next bit starts because it sees a change Must be a new bit So synchronization is not such a problem with Manchester and differential Manchester encoding. That's a good thing about them But with some of the others if we have a long sequence of bits They become hard for the receiver to work out what bits are being received We're not going to go too much detail into the trade-offs. We'll just mention them just to say the fact that Some are better than others in different manners. One of them is synchronization The better they can synchronize the less chance of errors Let's draw a bipolar AMI signal that you receive if you work out the data Bipolar AMI Try and work out the data received Anyone what what's the first bit received? so far You're the receiver you receive the blue signal you need to work as it arrives work out the bits received Before you work it out any questions Ask me and then we can answer to everyone Later why not now? We've got time to answer questions Okay, so if there's no questions, what's the first bit? Zero second bit Okay, so work out as you receive the signal. What are the bits? We'll keep going Again from the start and what happened here error Here this scheme because we expect alternating positive and negative It has inbuilt error detection This is the signal received But we know that the transmitter would never send this signal Because the transmitter follows the rules that we either send zero volts for a bit zero or Alternate between positive and negative for bit ones So we have a positive a negative the next one we expect should be a positive because the transmitter would Transmitter positive, but if we receive zero something's gone wrong between the receiver and the transmitter so in this case we say It's got error detection at this point Some error has been detected What was the error? Well depends upon the system Maybe there was some error in the in the transmission system and the levels were not quite right But this is an advantage of some schemes They're designed such that the receiver can automatically detect errors It may not be able to fix those errors, but at least detect them. We have more. I think that's all for those examples Bipolar AMI has some inbuilt error detection non return to zero Does not Even if it's at the wrong level we don't know that at the receiver Let's compare just quickly some of the differences between them and just very quickly on this So which encoding scheme is best? Well, it depends. There are different factors When we transmit these signals they occupy some bandwidth in our transmission system and Some occupy more than others The more bandwidth you occupy the worse we want to use a small bandwidth to send the same amount of data a Picture that tries to capture that is that this is the frequency plot or the The frequency plot of the different encoding schemes all compared to each other the wider the plot the worse It is because it uses more bandwidth so non return to zero a bandwidth If you look at the width here about one Manchester and differential Manchester is slightly wider bandwidth These others bipolar AMI pseudo ternary and some modifications of them B8 zs HDB 3 have the best In terms of bandwidth they the narrowest a concentration of the frequencies So there's difference in terms of the bandwidth occupied There's difference in terms of synchronization some the receiver can synchronize based upon the signal received others They cannot and if you cannot synchronize you may get bit errors when you have a continuous sequence of one level Some have inbuilt error detection some don't Some work better in the presence of interference So we'll not discuss why but when a designer chooses a scheme they need to consider all of these factors Some are more complex For example the Manchester encoding and differential Manchester There's always a change in the signal level Which means our transmitter must change the level quite often compared to the others You see look at the number of changes for eight or whatever 12 bits Compared to the others Means our device must must be more complex to be able to change the levels much faster So these are more complex and maybe more costly to implement any questions about digital data using digital signals That's about all we want to say You'll see in past exams Here's here's a signal. Tell me the data or here's the data draw the signal for any of these schemes again I Will not I will not describe the first two to you but any of the others I would describe in the exam I would say this is the definition of how it works Not so hard. There are some extensions of bipolar. Am I these two B8 zs HDB3 That's slight extensions that with bipolar am I when you have a long sequence of zeros again We have the constant zero volt signal That's bad because of synchronization and bad because of sending at zero volts is is not good for the receiver. It's So these schemes when we have a Sequence of zeros of a particular length They change it with some special sequence To avoid this long sequence of zeros. That's all You don't need to understand how they change but they're just modifications of bipolar am I to Overcome this problem of having a long sequence of zeros causing us the same signal There are different modifications there They're called multi-level binary schemes We actually had three levels positive zero and negative This one's even easier We have bits to send But now we have a continuously varying analog signal to transmit There are three basic techniques and this captures them and explains them quite well We have a sequence of bits Ignore the digital signal. Just look at the sequence of bits zero zero one one zero one So on that's the digital data. We want to send but we need to send an analog signal Well, we know how can we change the shape of an analog signal vary the amplitude the frequency or the phase? So the first approach It's called amplitude shift keying is to say when we have a bit zero Send at one amplitude in this case in this example the amplitude is zero and When we have a bit one Transmit an analog signal in this case a sine wave at a different amplitude So Plus one for example So think this flat line is a sine wave with zero amplitude, which is just zero signal no line signal So change the amplitude depending upon the bit So when you receive this signal again, we have a signal element duration If the signal is zero volts for this duration Then that means the amplitude is zero. That means the bit received was zero if the signal amplitude is Non-zero in this case a plus one Then it corresponds to bit one being received So shift the amplitude of the analog signal based upon the bit that we want to send The amplitude doesn't have to be zero in this example at zero and a non-zero value. It could be Plus one and plus two Okay to two non-zero values The next one instead of changing the amplitude change the frequency Frequency shift keying the B means binary, which just is there's two different levels of frequency binary for two It's the same here. There are two levels of amplitude. It's just commonly called BFSK When we have a bit zero send a signal at one frequency Here we see the frequency. There's one repetition in our time interval When we have a bit one change the frequency in this example, that's a higher frequency. There are two repetitions in the interval This is again just an example. The concept is change the frequency depending upon the bit to be transmitted this Low frequency for bit zero high frequency for bit one And of course we have phase shift keying Bit zero in this case we send a phase of pi you see we start going down then up in our sine wave When we have a bit one we send at the normal phase a phase of zero so we get the normal go up first and then down Frequency is the same for each signal element. The amplitude is the same But the phase differs depending upon bit zero of bit one Amplitude shift keying phase shift keying Amplitude shift keying frequency shift keying phase shift keying B means binary just being two levels any questions about sending digital data using analog signals I Told you this was easy Again given a signal you should be able to answer. What was the data received? We're not limited to these three We can combine them in different ways so the hint here binary frequency shift keying Means there are two frequencies We can have more than two frequencies. I tried to draw up before What's the data received? Which first? Okay first is it ASK, FSK or PSK Listen to my question. Is it ASK, FSK or PSK? So the amplitude is the same so it's not ASK the phase is the same It's frequency shift keying and you should see maybe That some components have different frequencies What's the data? Received you receive this signal. It's not BFSK in this case Okay, BFSK means binary frequency shift keying meaning two different frequencies This has four different frequencies only three are shown in this case Okay, so this is a trick that is it doesn't have to be two different frequencies and More importantly, you must know the mapping of the frequency to the bits It's not always low frequency bit zero high frequency bit one. It could be the opposite So you need to know the mapping the scheme in this case. I chose this scheme If I want to transmit bits zero zero Two bits I send a signal for particular duration of some frequency F If I want to send the bits zero one, I send it double the frequency to F Higher frequency bits one zero three F one one four F That was the scheme that the transmitter uses and the receiver uses that to decode what's received and The hint in the picture is that these these grid or these Vertical lines show the the duration of each signal element so here is the reception of the signal and You may be able to guess But if you look at the frequency, this is two repetitions in one signal duration This is four. This is one and this is two There's no example of three repetitions. So This is the lowest frequency Zero zero is received at this point This is two times the frequency. So zero one is received in this duration. It's four times the lowest one So one one received Let's write that down. So Just be aware that we don't we're not limited to just two frequencies and That you need to know the encoding scheme to know the exact mapping from the the signal elements The analog signal to the bits the digital data in this case the way I created it was this was Double the original frequency to F So this would be zero one for this this duration and Then we measure the signal for the same duration here The receiver measures it and they see it's four times the original frequency Whatever the frequency was where those one Hertz one mega Hertz or whatever So that will be one one being received Here it's one times the original frequency. So zero zero and two times Zero one So here think of this is one signal element We transmit an analog signal for the fixed duration At a particular frequency, but it represents two bits In this case if we had eight different frequencies, we'd have three bits per signal element Remember because we need to be able to represent any sequence of bits We need to be a power of two the number of different levels We can do the same with phase shift keying instead of two phases. We can have four phases sixteen phases two hundred and fifty six phases and It's common for both frequency shift keying and phase shift keying to have multiple levels more than two Not so common with amplitude shift keying any questions on what I'll call this is FSK with four levels or four FSK sometimes it's written that instead of binary FSK is four levels Even QFSK another common scheme Think about what the signal will be in this case if we have Sequence of bits to send here we combine Amplitude shift keying with phase shift keying and The name for that is called QAM quadrature amplitude modulation You don't need to remember that but we'll see it come up in many examples in real systems But it uses both and the scheme I've defined in this example is that if we want to send bits 00 I will set the amplitude to be one and The phase P to zero For bits zero one keep the amplitude at one and change the phase to pi So in our sine wave a phase of zero we go up and then first in a phase of pi we go down first So upside down sine wave For bits one zero we have an amplitude of two and a phase of zero and Amplitude of two and phase of pi for bits one one So again, we have four combinations So they don't have to be combinations of the same shifting The previous one was four combinations of frequency. This is four combinations combining two different amplitudes and two different phases Can I draw it? What's the first one look like well bit one zero is what we want to transmit so we transmit with an amplitude of two I may need a grid and a phase of zero so Say this is an amplitude of plus two here and a phase of zero mean it's the normal shape sine wave one one Amplitude of two phase of pi so we'll go down first Zero zero amplitude of one phase of zero so the normal shape sine wave but half the amplitude one one is Amplitude of two phase of pi So we just have enough combinations to represent any sequence of two bits the receiver. They measure the signal They will determine the phase the amplitude and determine the bits received this Combination of amplitude shift keying and phase shift keying is very common and real communication systems called QAM and They're not limited to just four levels. There's 16 QAM 256 QAM and even higher the more levels The more bits we can send per signal element here. We have two bits per signal element the faster we can send bits But we've said it before the more levels the more chance of errors because there's a smaller difference between levels So the receiver must be able to distinguish between what levels received in the presence of noise any question on the combination of ASK and PSK Yep, in this case. It's just an example. I chose of how to combine that is this scheme Was designed by me Okay, I chose to map zero zero to amplitude one phase of zero But someone else may design it and say Amplitude to phase of pi equals zero zero. It doesn't have to be this combination So that scheme is part of the designer's choice When you buy a product that uses this it will be part of the standard that says every device must follow a particular scheme So you must know that to be able to map the bits to the signal The values of the amplitude and the phase are not so important in this case. They just need to be different Now in practice how much they're different by to may impact on performance Especially phase a phase of zero and a phase of one is very similar Therefore you try and set the phase to be completely different. Where are they used? So just go back. What do we skip? This is what modems do modem sends an analog signal carrying your computer digital data so Shift keying it's called shift keying the SK is called shift keying is What a modem does? We can only send analog signals, but we want to send digital data So we use shift keying to convert that digital data to an analog signal We saw the three basic techniques ASK PSK and FSK, but you can combine them as well in more complex ways now. I Think that's about all we want to say about those Apart from give some examples of where they used amplitude shift keying. So they have different characteristics Amplitude shift keying tends to be quite inefficient in terms of occupying the bandwidth Therefore is only used when you have very low data rates Or you have a very large bandwidth like optical filer Frequency shift keying is used in telephone systems Coaxial cable So receiving cable TV and Some radio systems You can have more than two frequencies so m goes from two to four as we saw and can go up higher data rate But higher chance of errors as the number of levels go up phase shift keying commonly used in wireless systems and Combined with amplitude shift keying. It's called quadrature amplitude modulation QAM Especially used in ADSL and some wireless systems. So your ADSL modem It takes your computer digital data and sends an analog signal And how does it map the bits to the analog signal using a combination of amplitude and phase shift keying? And I think that's all we want to say some some other examples of where they used mobile phones Wi-Fi cable modems DSL satellite broadcasts use PSK and PSK with ASK and it questions on Digital data with analog signals one more ten minutes to go We'll skip to the last one. It's the easy At least introduce it Here we have analog data like someone speaking some voice some audio Or some music and we want to send it as analog signals and we just very quickly introduce the concepts We can do it quickly because you know them already Amplitude modulation AM AM radio What we do We have the middle the middle plot here think is the data. That's the analog data. We want to send Say it represents someone's voice as they're speaking The top plot is what's called a carrier frequency. We want to send the analog data But at a different frequency Than what the analog data is based upon so voice Voice when you speak ranges from about three three hundred hertz up to about three kilohertz So the frequency of human voice is less than about three or four kilohertz But we want to transmit it say wirelessly from a radio station to your car am radio and To transmit the signal a long distance. We don't transmit it at around four kilohertz. We transmit am radio about one megahertz So it's effectively shifting the frequency of the analog data to a much higher frequency So that the signal can travel a long distance and so that we can have small antennas So the carrier frequency is the one that we want to send our signal out So for example for am radio This may be I don't know am radio channel may be seven nine hundred and fifty kilohertz That's called the carrier frequency. This represents the the analog voice or music an Amplitude modulation we modify the carrier based upon the Input data and we can see that resulting signal that we transmit We see the amplitude of our carrier changes according to the input data This is the basics of amplitude modulation This is what's transmitted the frequency is around the same frequency as the carrier But the amplitude goes up and down as the receiver receives this signal it measures the amplitude and Maps that back to the received data the voice or music frequency modulation FM radio Same concept FM radio. What's the channel? FM radio channel one oh one oh seven points something one oh seven point five mega megahertz. So the carrier frequency is 107.5 megahertz But the the data is the music and the people speaking. That's the middle plot here Which is around several kilohertz So with frequency modulation we change the carrier frequency slightly So it's not exactly one oh seven point five megahertz. It varies over time as The input data in this example as it's high here. We get a slightly higher At slightly lower frequency It's not as changing as much and as we get to the negative input data We get a very or much higher frequency compared to the carrier frequency so this is the The signal that's transmitted if we use FM The receiver receives this it over time it records the signal it measures the frequency of that signal If the frequency is Lower slightly lower than the carrier then it means that the amplitude is positive here If the frequency is slightly higher than the carrier then it means the amplitude is negative So they get this frequency modulated signal to determine the received analog data and There's phase modulation, but it looks very similar to frequency modulation Okay, that we change the phase As we have a change in the input data Where are they used or am FM radio? They're the best examples, but used in other communication systems as well and that's all we'll say about those ones Just be aware that AM FM and PM in terms of phase modulation are about how do Change what's called some carrier frequency Based upon the input analog data and generate an output analog signal so AM FM PM about analog data sending as analog signals and We have some carrier frequency which is changed depending upon whether using amplitude Frequency of phase modulation You will not need to interpret or extract data from plots. That's too hard for us Not like digital data The third one will cover after the midterm So just to summarize in preparation for the exam for this topic Know what is AM PM and FM with respect to? analog data and analog signals ASK FSK PSK With respect to digital data and analog signals and at least remember NRZ level NRZ invert with respect to digital data and digital signals and be able to Use the descriptions of the other schemes where necessary. You don't need to remember them. We will stop there