Digital Transmission Primer with Ward Silver
Posted by Kirk Harnack [TWiRT] on Oct 24, 2014 11:31:00 AM
I’ve often said, “This here digital’s the coming thing.” We all know it’s here now, and here to stay. Perhaps we’d be well-served to understand some techniques for transmitting digital information over radio frequencies (RF). Ward Silver, N0AX, is a teacher, a contributing editor to the American Radio Relay League (ARRL), and author of the book, “Ham Radio for Dummies,” as well as many others. We’re getting a lesson from Ward on Quadrature Amplitude Modulation (QAM) as well as I & Q signals. Symbol constellations, predictive coding - we’re diving right in.
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Kirk: This Week in Radio Tech, episode 231, is brought to you by the Axia Element 2.0 IP audio console, protected by an industry leading five year warranty. By Lawo, maker of the new crystalCLEAR virtual radio console. CrystalCLEAR is the radio console with a multi-touch touchscreen interface. And, by the Telos Z/IP ONE IP Codec, the IP Codec that drops jaws, not audio.
H. Ward Silver is a teacher, a contributing editor to the Radio Relay League, and the author of the book Ham Radio for Dummies as well as other books. We're getting a lesson from Ward on quadrature amplitude modulation as well as I and Q symbols, symbol consolations, predictive coding - we are diving right in. Join us.
Hey. Welcome in to This Week in Radio Tech. Yes, it's time to talk about radio technology. That is everything from the microphone to the light bulb at the top of the tower, and now just so many more things that may not even involve the tower. I'm Kirk Harnack, your host. This is episode number 231. We're glad to have you along.
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On the show today, we have one of our reliable co-hosts, the best-dressed engineer in radio, reporting from Manhattan. Ladies and gentlemen, it's Chris Tobin. Hey Chris. Welcome in.
Chris: Hello Kirk. Yes, I'm in Manhattan. I'm in a home business operation, a friend of mine, doing some engineering stuff. I thought I'd quickly get out onto the balcony, set up a light kit and camera, and go for it. So far it's working.
Kirk: What kind of light kit are you using there? Is it a real light kit or is it a floor lamp from the house?
Chris: It is a CFL flood lamp that I purchased at a local lighting store. The color temperature is 5000 Kelvin. It works just right with this webcam and I've been using it for all the live shots.
Kirk: What do the neighbors think? Does it bother neighbors? Is it that bright?
Chris: No, it's focused.
Chris: It's not like a typical TV standup that I'm accustomed to doing with other folks and being behind the camera. This is definitely very subtle.
Kirk: Cool. Our guest on the show, I'm really excited to have this gentleman on because I didn't know our guest's name until maybe a couple months ago. I'm reading through the latest QST magazine that crossed my desk. In fact, I'm probably eating my cereal in the morning, looking through QST magazine, and there is an article about digital transmission and how digital transmission actually works with RF to get information from here to there.
This is something that we deal with, but I don't really know exactly how this stuff works. Our guest is Ward Silver. Ward is call sign November, Zero, Alpha, X-ray - N0AX. Ward, welcome in to This Week in Radio Tech. We're glad you're here.
Ward: Hi. I'm glad to be here and I'm happy that you read the article in QST. It seemed to certainly hit a chord.
Kirk: It sure did. I looked you up on the Internet and found your email and somehow got in touch with you. Thanks for responding and thanks for agreeing to be on our show. This is a show where we have fun. We talk amongst engineers. We tell stories. But if we can have a lot of teachable moments, that's what we're really after. We want to teach other engineers what the experts know about various subjects.
We've had shows on the rigors of amplitude modulation and frequency modulation, studio techniques, how-to shows on how to maintain this or that. We had a show where I took a video camera in and showed how we change a blower motor on a 10 kilowatt FM transmitter. There are just all kinds of things that we do shows on. We've had guests from every walk of life.
You may be familiar with the C. Crane Company. Bob Crane, who makes those consumer radios that are so popular, we just had Bob on a couple of weeks ago. We're glad to have you on to talk to us about this digital mode. By the way, I want to mention, Ward I'm sure will show us or maybe pop it up now. Ward, you're the author of a book that ought to be instant Christmas wish list right here.
Ward: This book?
Ward: Ham Radio for Dummies.
Kirk: Ham Radio for Dummies.
Ward: Second edition.
Kirk: I don't know that any Ham Radio guy would be insulted to be called a dummy if he got a book called Ham Radio for Dummies. I certainly wouldn't be.
Ward: Everybody is a dummy about something. It's such a big hobby. It used to be you could read this little magazine that was this big. It was about that thick. If you read the whole thing, you could tell everything there was about ham radio that month. But now, it's so broad, so deep, nobody knows everything about it all.
Kirk: You're exactly right. A few years, I thought ham radio was kind of dying out, yet I look at the membership numbers and it is not dying out. There are ham radio operators around the world and it's just getting more popular. All these modes that we have, ham radio doesn't necessarily mean one thing.
I'm getting ahead of myself. Let's hit up one of our sponsors for some money right now. Our show is brought to you by three sponsors: the folks at Axia, also the folks at Telos, and at Lawo. Our first sponsor on this show is going to be from Axia.
I'm really familiar with Axia because it's part of the company that's called the Telos Alliance and they're my employer, so I want to thank them first of all for sponsoring This Week in Radio Tech. We want to tell you a little bit about the console, the audio console that is the real flagship of the Axia lineup.
You go to so many broadcasters around the world. It's close to 5000 studios now that have an Axia Element console in them. This is the big work horse. I'm not sure if Andrew is going to find a picture to throw up on there, but the Axia Element console, if you look at one you've seen one, but you know what? You can order this console in literally about 14,000 different iterations.
It's variable in size. You can choose the size of frame you want. You can have a double frame if you like, or even four frames if you like. You can put any number of faders in there that you want from two faders up to 40 faders. You can have a 40 fader console, if you like. There are accessory modules that can go into the console. If you want a telephone controller to control your phone system, bam. Just put it right there in the console. It is right there available for the operator to use quickly and easily.
Intercom buttons. You can quickly intercom any other intercom station around your facility, even the soft intercoms-that's just an intercom that's running on a PC. You can have hard buttons that we call film cap buttons. You can have smart switches that have a text label that can be changed on that. It can change instant by instant if you want it to.
So many options, so many ways to order the console. You could spend a day just figuring out what options you want to go in one of these Axia Element consoles. I've installed a bunch of these all around, especially at trade shows if you want to count those. We do build real radio stations at trade shows, or real studios anyway.
The Axia Element console connects to either the Axia power station, and that's a big, honking piece of gear that goes in the rack. It's got the power supply for the console, the power supply for the DSP engine, the mixing engine. It also powers the audio input and output that's built into the back of the power station. And, it powers the Ethernet switch, the gigabit Ethernet switch that's built into the back of the power station.
You put the Axia Element console and all its different permutations, the ways you can order it, match that with an Axia power station and you have got a rock solid audio console for any audio studio, any radio, even television. These are being used in television stations actually in different countries as well as in the US. I've seen some. Check it out on the web, if you would.
I'm so excited about this console. You can't go wrong because there are thousands of them on the air right now - the Axia Element console. Go to axiaaudio.com and look for Element. Element 2.0 console system is what you'll find it under. Thanks for Axia for sponsoring This Week in Radio Tech. Remember, you can't go wrong. Axia connects to so many other brands and systems using audio over IP on Livewire.
Hey, we're on episode 231 of This Week in Radio Tech. Chris Tobin, the best dressed engineer in radio, is with us and our guest is Ward Silver. Let's jump right into it. Chris, you might want to get ready with some follow up questions here.
Ward, digital transmission. I want to understand this. Where would you start me out in understanding how we can transmit stuff that recovers as digital ones and zeros as opposed to amplitude modulation, frequency modulation, single side band modulation, phase modulation. How do we get ones and zeros onto this analog carrier and make it come out the other end? Where do you start me out at?
Ward: Okay. Let's see. When I was a datacom engineer, I used to have a little card that I kept in my pocket all the time that I'd take out and I'd say, "Well, it depends." It's what was on the card, to any possible question that someone would come up and ask. When I talk to people that are experienced with RF, and I used to be a broadcast engineer.
I spent eight years in broadcasting back in the 70s and early 80s in FM and AM and even television. I watched for interesting shows, technical shows about how to get a giant tube with two handles out of a finger stock using an engine hoist, or something like that. That was high tech back then.
Anyway, back in the day, it was AM and it was FM. On the ham stations you had single side band. That was it. It was analog modulation. You would take an oscillator and you would mess with its output in some predictable way so that you wound up with something you could un-mess with on the other end and recover a single channel of information as an analog wave form.
The only digital data back then was radio teletype. Then we had some limited teletype over radio modes like AMTOR and GTOR. That was typical of the commercial systems as well.
Things progressed in the late 1970s. 1979, 1980, the FCC decided to allow amateurs to use codes and ASCII. That was a standard encoding mechanism for data. Suddenly, amateurs were off to the races using the same sophisticated protocols that the commercial and military guys were using. We came up with an adaptation of the X.25 standard. That became AX.25 for amateur X.25 and packet radio.
Packet radio then spawned all sorts of interesting bulletin board systems. There was a lot of overlap between the amateur world and the commercial, wireless data group. For example, deep inside your CDMA phone is a media access control protocol called MACA, M-A-C-A, for multiple access collision avoidance. That was originally developed for packet radio. It wound up becoming part of Wi-Fi and CDMA and other things because the same guys were working in both areas.
We have transmitters, we have audio, and we have this code. Then in the early 90s, another step was taken. You can invent your own mode and use it as long as you publish the details of it so somebody else can use it. That sort of blew the doors wide off. Now there are dozens of digital modes that are being used in amateur radio, some of which leak over into commercial. A lot of the commercial stuff has leaked over into amateur.
There is a lot of confusion on the part of amateurs trying to understand digital, just like broadcast engineers are starting to try and understand digital. How do you do this? How does it actually work? You can get very technical very quickly. It's a very technical field with a lot of layers.
You may have heard the team OSI seven-layer model where it talks about all the different steps in this process, that you go from your application music player or your email program and then that transfers data here. Then there's another layer and another layer. At some point, things fly out over a wire or a transmitter and then they bubble back up through another layer on the other side until they come out your headphones or show up on your computer screen.
If we jumped into it at that level of complexity, it would take several shows to deal with. It's also a big drink of water for anybody who is just trying to get started. The way I present it is, let's talk about a pipe. Basically, we have three sections of this pipe.
One is the actual pipe. If you tuned a radio across a signal and you listened to it, you would hear what's going in across the pipe. That's called the air link, A-I-R. The air link, that's what's actually being transmitted. You might hear tones. You might here this modem sound that sounds like [this]. That's a phase shift modulation. You may hear lots of different things, but all of those are carrying encoded data as some kind of modification of a signal frequency, phase, or amplitude.
The pipe connects on each end to these little machines that are called protocols. Protocols manage how the data gets into and out of the pipe and how the data is packaged so that the corresponding machine on the other end can look at what's coming out of its pipe and make sense out of it. The protocols job is to translate what's going on in the air link down here at the bottom, or the physical layer, up to what's happening at the top, which is your music player or your email program for two particular examples.
They're basically taking data and then they package it up and they stuff it into the pipe. A modulator in the pipe turns it into an RS signal that goes out over the air. The same process goes the other way on the other side. A demodulator receives a signal and it translates the characteristics of the signal into ones and zeros. That it hands out to the protocol in the middle and says, "Here are ones and zeros, protocol. Figure it out."
The protocol looks at the sequence of ones and zeros, turns it into characters, unpackages them, and then hands them farther up to whatever program is running up at the top. It might be an MP3 file. It might be an audio stream. It might be a binary file, like I'm sending a file back and forth. Or, it might be an email. It might be anything. The protocol really doesn't care. It just wants data. It handles the data going in and out of the pipe.
You've got three levels. You've got the pipe down at the bottom. That's your air link. You can listen to it with a radio. You've got your protocol, which is a little machine for getting data and in and out of the pipe in a predictable and reliable way. Then up at the top you've got your application, which is whatever program that you're trying to send or receive data from.
That's basically the structure in which you can start to understand these things. Depending on where your interests are, either professional or personal, you can focus at any level of that three level stack. If you're interested in the RF part of it, you stay with the pipe. If you're a protocol guy, then you stay with the little machines and maybe you're a user, so you stay up at the top level.
That's basically how I approach the problem. How about I be quiet for a minute and we'll let the [inaudible 0:18:15].
Kirk: I've got to chew on that for just a minute. Hey, Chris Tobin, you and I have had a number of conversations over the years about your use of various digital pieces of equipment. I think you've even used transmitters and receivers intended for television work, ENG work, to transmit digital audio from here to there. Tell me about how you got interested in that and relate what you can about how they worked to this discussion about how this modulation takes place.
Chris: Gee, that's a good one.
Kirk: I think the letters COFDM come into play.
Chris: Yeah, I'm trying to think of the best way to put it. When I was playing with the COFDM and various other versions of the TDMA, CDMA that's out there, the TV guys I was working with, we did some video links in Washington DC. We were learning and discovering the same thing with Moto Turbo, which is a LAN mobile version of a digital format. We discovered that the less direct line of sight you had with the transmitter to receiver, the more robust the signal was for recovering.
Kirk: That's pretty counter intuitive. That's what COFDM does for you?
Chris: That's what got everybody confused. The group of us were amateur operators and broadcast engineers. We're all sitting there on the mall in Washington going, "Let me get this straight. I am now pointing the antenna at a building. It's receiving this signal and it's, we'll just say full DFQ. Full quieting, pure video, everything is what it's supposed to be.
Now when I point it towards you at the building you're in with the antenna in the window, you're having macro blocking, drop outs, and it just doesn't work." They're like, "Yes." This is about five or six years ago. We were like, "Wow, this is really interesting."
As we move forward to today, and over the last couple of years I've been using the same technology for radio and doing all kinds of audio stuff. Now it's becoming commonplace. Your Wi-Fi is a form of the OFDM thinking.
Kirk: Oh. Wi-Fi is probably a good example because broadcast engineers, we certainly touch that quite a bit. We also touch the modulation techniques used in HD radio. Ward, I recall a conversation I had some years ago with an engineer who was working on the iBiquity HD Radio system. He was telling me about advances that they had made. I want to say the term he was using was "bits per hertz" - so, the number of bits you could get per hertz of bandwidth, perhaps it was. Can you help me understand that concept of counting, or getting efficient with bits for a given bandwidth?
Ward: There is a fundamental rule. It's called Shannon's Law. It says you can get x number of bits in a bandwidth of a certain width in hertz. As I recall, it takes two hertz to get one bit per second through it, or something like that. It's a two to one relationship. Each bit that you send through, it could just be a zero or a one. If you don't know anything about the signal and you're just listening for zero or one.
Imagine yourself looking at a guy with a semaphore flag far away through binoculars. You want to know if he's waving the flag or he's not waving the flag. That's a zero or a one. Basically, that's what you're doing in a raw data environment. Your pipeline is listening for that zero or one coming through. If you have a very simple encoding or protocol, that's basically your limit.
That puts some pretty severe limits on how much data you can get through this pipe. By using more and more sophisticated codes in the presence of noise and all the other horrible things that happen to our signals as they go along, you can get closer and closer and closer to the theoretical maximum. What you do is you add error correction information. You add predictive coding. You do all sorts of interesting things to make the data transmission process more robust.
Each type of signal, like HD TV or HD radio or anything else, has its own method of encoding and packing data and assembling it so that it's transmitted with the most bits per unit of bandwidth, yet still preserves some required limit of fidelity and resistance to error. It's like Chris was talking about the idea that he would have a full quieting, very strong signal at the other end, but it would still break up, pixelate, and just basically be unusable probably because of some kind of multi-path where you have two very strong paths coming from the transmitter and then coming back to the receiver with a slight phase difference between them so that they interfere.
Even though the signals might be very strong, they're interfering with each other. That distorts the signal so badly that the encoding can't recover from it. By pointing the antenna off in some other direction, you find a way of getting the signal from hereto there by only one path where there is not some competing path that's trying to interfere with it. Even though that's counter intuitive, what the receiver hears is only one clean signal even if it goes by another path.
That's a little bit off the subject, but basically by improving the robustness and the fidelity of the signal, you can use more and more sophisticated codes to help recover from the distortions and fadings and all the other things, noise, that the signal encounters on its way. Each time you have a signaling event, each time the guy with the flag waves it or doesn't wave it, that's called a symbol. The number of symbols that you can pump through your protocol and pipe tell you how much data you can pack into the bandwidth of the signal.
The symbols are what the coding machine produces and you wind up with... I have a copy of that article you were talking about here. You can see...
Kirk: That's it, yes. Yes.
Ward: This little diagram. There we go. Okay. We'll get it that way. That particular coding has four different states, each one of the corners here: this one, this one, this one, and this one. I would not make a very good weather man. I'm not doing this very well.
What it can send is four different symbols through the pipe: this one, this one, this one, and this one. The guy with the semaphore flag can wave his flag four different ways. The system at the other end tries to detect which one of those symbols has come through the pipe. The lines on this drawing here show the machine at the other end trying to decide which one of those signals has come through.
Any kind of noise or distortion shows up as the variations in those lines as it goes around this diagram. If I sent you a sequence of these four symbols, that's basically a drawing showing how the machine on the other end is trying to recover each one of those symbols. If I can make each of those symbols apply to more than one digital bit at a time, then I can send more bits per symbol than just one.
When you hear a modem, we all remember the old training sequence where the modem starts out with a couple of tones, and then it has another couple of tones, and then there's this back and forth between the two modems. Then it goes like that. That's a training sequence.
Basically, the two modems are talking to each other saying, "Can you one bit per symbol?" The other one says, "Yeah, I can do one bit per symbol." So the other one says, "How about two bits per symbol?" "Okay, I can do two." "How about four? Can you do four?" And then it says, "Yeah, I can do four." They build up that way until one of them says, "No, I can't go any farther."
They settle at this particular set of encoding in the number of bits per symbol. They agree on that and then they start exchanging data back and forth. For a particular channel in a particular type of modulation, there are limits about how many bits you can pack in per symbol. The modems or the systems on each end, the protocol engines, will negotiate back and forth to see how many bits that is. Then they'll apply the appropriate coding.
The number of bits per hertz is not fixed. Although, for broadcasts where you don't have a negotiation between the receiver, which just receives, and the transmitter, which just transmits, you only have basically one symbol rate at a fixed number of bits per symbol. But when you're talking about a communications system going back and forth where both have intelligence, you can negotiate the number of bits per symbol. The number of bits per symbol than ultimately determines your bit rate and whether you've got enough bandwidth there with all the other...
What am I trying to say here? With all the other constraints that you've placed on the system. If you have a certain level of error rate that you can tolerate, then the system will tell you, "I can get full CD quality, 44.1 Kbits per second audio." Or Maybe I'm doing 192 kilobyte per second digitization or something like that based on their ability to negotiate between each end of the link.
In broadcast where you're just shoving data into a transmitter, out it goes and the receivers never communicate with the transmitter. You basically have to pick one symbol rate and stay with it. In that case, the broadcast industry has done modeling to say within our expected coverage area and the expected quality of receiver, what is our expected maximum symbol rate? We're going to stick with that. That'll satisfy 90 or 95% of our expected customers. That's how it works.
I'm going to stop again and let you get a word in edgewise, Kirk. Go ahead.
Kirk: I'm sure Chris Tobin would have a comment or question, but I've got a quick one. Actually, it's a quick question. The answer may not be quick. I want to understand generally this rule or this axiom. What is the relationship between efficiency and robustness in a digital system?
I've been led to believe that if you did some kind of modulation that's, say, 64-QAM or 128-QAM, that's a whole bunch of possible symbol locations, but that system itself is kind of fragile. It becomes very difficult to determine, is my signal ending up here or here or here? And maybe we should explain a bit about what QAM is. Quadrature amplitude modulation is what that means. I'm curious to understand this relationship between efficiency and robustness.
Ward: All right, I'll make this a very quick answer. I'm going to go back to this diagram here. This is called a constellation diagram. Each one of the four points are the points of the constellation. There are four. This might be quadrature amplitude modulation. It might be quadrature phase shift modulation. Basically, the system moves around between those four points. That's all you get - four different symbols.
You can see from this diagram what happens when there is noise or distortion. The errors start to build up and it's harder for the system to determine whether the system is here at this point right here in the corner, or is it down here, or is it over here? It's hard to tell. The faster you move, the harder it is to decide. Some of these systems, they don't have four states. They might have 16. There are systems that use up to 256 different points in the constellation. There are even more, but I think the biggest one that you'll encounter commercially is 256 states in the constellation.
The machine on the other end has to decide which of those 256 different states the data link is in. If you have noise or distortion or you're moving very fast, all three of those things make it harder to decide at the other end. If it's harder to decide, you're more susceptible to error. You have to balance speed against ability to decide. Do you want to be fast or do you want to be error free?
You basically study your communication link and you decide on a combination of speed and error rate that you can live with and then you stay with it. The more different states you have or the faster you move between them, the less robust your system is. Does that answer your question?
Kirk: Yeah, I guess it does. It follows along with what I was thinking about it. Before we get through our conversation, I want to also see if I can understand. I've read about the things we just talked about - QAM and 4 and 16. You mentioned the 256 and even higher. I kind of understand that, where the RF carrier is at a certain amplitude in a certain phase at a certain time and that represents a symbol.
I would then like to see, are there other forms of digital modulation besides what I understand as QAM? You mentioned quadrature phase modulation, I believe. Are there other things as well? Are there other ways to modulate a carrier to convey digital information besides my little understanding of QAM?
Ward: I would say the big two are amplitude modulation and phase shift modulation and combinations of the two. They are picked for their various either economic considerations. Quadrature amplitude modulation is fairly inexpensive both to generate and to receive. And then the more sophisticated types of modulation cost more, but they're more tolerant of errors in the pipe. Depending on your application, you pick certain types of modulation.
Beyond that, there are certain characteristics of the way that you move from symbol to symbol such as shaping the pulses. There are different types of ways that you change between the different states that make it more robust when you're moving very fast. All of those things add complexity and they're all more expensive. It's a budgeting kind of thing.I would say, back in the old days, you just had a frequency shift keying where you had your two tones, and the signal would jump back and forth between one tone or another. Radio teletype worked that way and you could send that kind of signal through the voice input to a transmitter. But now, by having the two carriers, basically they're called I and Q. One is in phase and one is 90 degrees out of phase.
Basically for quadrature amplitude modulation, you're turning them on and off. You have zero zero state. You have the one zero state. You have the zero one state. And you have the one one state. There are your four symbols. Basically, the system is going like this and turning them on or off and moving around in that four point diagram. That's the simplest way to think of it.
You can also do that with phase shift and you also do it with combinations of things. As you can see, it gets really complicated really quick. Back to you.
Kirk: Hey, Chris Tobin, I've got a question, but I think it's going to be a long answer. I'll tell you what. Chris Tobin, do you have a question you'd like to pass along or a comment on right now?
Chris: The only thing I'll comment on, I have no questions at the moment because the answers have been made clear, is that the Shannon's Law which came out in the 40s, he's a Bell Labs engineer, was based on work done by Nyquist and Hartley back in the 20s to give you an idea of the history that we're talking about.
Here is technology we're using today, digital, to do all kinds of great stuff that was developed and thought of and somewhat ahead of its time. It was done back in the 20s and 40s because back in that time, the phone company was trying to find a better way to communicate over wire, over copper. That's what these guys came up with. That's the background I wanted to throw in there just for a little historical moment.
Kirk: That's interesting.
Chris: As far as quadrature and trying to figure out which quam and phase shift, I think of it as three dimensional chess, or if you're a Dr. Who fan, it's a TARDIS. Basically, you're playing with time, space, relationship, and everything else. You have 90 degrees out of phase, 180, it depends on where it's at and all of these other things. It just gets really wild when you start trying to figure out, as you were saying, the link budget and figure out what's more robust, what's more efficient, what's more economical. It's fascinating when you start getting into it.
Kirk: I do have some follow up questions about quadrature and I and Q. Maybe it's just my mental block, but this is something that I need a little bit more explanation on. And then after I get that clear, we're going to let Ward loose on whatever else he wants to teach us.
Our show is This Week in Radio Tech, by the way. It's episode number 231. Our guest is Ward Silver. Ward is the author of Ham Radio for Dummies and he's a contributing editor at the ARRL. You'll see his articles in QST magazine and other ARRL publications. Our show is brought to you in part by the folks at Lawo. Lawo is a console manufacturer from Germany. They have some absolutely gorgeous consoles.
Usually, people think of Lawo of being really large consoles, and they certainly make those, but they also make smaller consoles that are appropriate for radio stations. One of them that people are really talking about is the crystalCLEAR. They're calling it the virtual radio mixing console because there isn't a hardware console in front of you. It is a multi-touch touchscreen provided by a high speed computer running Windows 8. The application just takes over, fills up the whole screen. Your talent, your disc jockeys, never see Windows.
What they do see is an absolutely gorgeous audio console virtually. It's on their screen. Because the console is defined in software, that means that buttons can do fabulous things. You can touch a button and have it open up totally contextual options that are related only to what you're doing at that point with that fader. What that means is if you've got a microphone fader and you touch the Options button with this, you don't have to be given a whole menu of stuff that's not related. You're just given the options that are available and appropriate for that fader in that use at that time.
It is multi-touch, so that means you can move several faders up and down at the same time. You can turn faders on and off, touch option buttons, have your hand on the monitor fader so you can adjust your headphone settings or your speaker settings. At the same time, they'll work with your on air faders.
The console consists of two parts. One part goes in the rack. That's where all your audio input and output goes. There are mike inputs, analog inputs, AES digital inputs, as well as outputs too, analog and AES digital outputs. There is also the options for audio over IP. It follows the RAVENNA standard. The RAVENNA standard for AoIP includes the newly-about one year now-approved AES 67 AoIP standard. That's something that you may be very interested in for connecting studios together.
The other part of the console is the part that's in front of you, the operator. That's this multi-touch touchscreen interface. Up to ten touches at once, it'll totally follow all of them. That's how you work the console. It's just that easy. The console features precision stereo PPM meters, integrated pre-fader level-or queue, as we all it in the states-three stereo mixing groups: program one, program two, and a record mixing group.
And there's even a panic button that can clear changes you've made to a current scene. If you accidentally, "Whoops! I didn't mean to do that." You push the button, they call it a panic button, and you're back to normal operation. It deletes whatever it is you just changed. It's like an undo button.
Twenty-four sources can come into the mix engine. Eight can be simultaneously active in your mixing platform on your console because it's an eight fader mixer. Power supply redundancy is available. Of course, it has GPIO for on air lamps and muting speakers.
If you would, check it out-you'll be very interested in this-at the Lawo website. Lawo, L-A-W-O. It's pronounced Lawo, but we spell it L-A-W-O, dot com. Look for the radio consoles and you'll find the crystalCLEAR console. Thanks for Lawo for sponsoring a part of This Week in Radio Tech.
It's episode 231. Ward Silver is our guest and our cohost, Chris Tobin, is along with us. Chris is live in Manhattan on a lovely evening there, just gorgeous outside. It looks like a great night to have dinner on the sidewalk instead of inside the restaurant.
Chris: Actually, yeah. On my way in, actually I can see from here looking down onto the street, onto the avenue, there are two restaurants that have outdoor seating. They're very busy. Actually, it's very comfortable. This spring-like weather today is gorgeous and it's clear skies. It's great.
Kirk: I wish I could tell you I'd be right over, but I can't do that. Our guest is Ward Silver. Ward, okay, at least twice in our conversation, once before the show and once during the show, you have mentioned quadrature and you've mentioned these letters I and Q. I almost feel like Sesame Street. Today's show about quadratures is brought to you by the letters I and Q.
Ward, I totally understand a carrier. I understand amplitude modulation and how it creates side bands when you do that. I totally get frequency modulation. You move the carrier up and down. You get, theoretically, an infinite number of side bands up and down. What I don't get is this I and Q and what's so special about 90 degrees in relation to these carriers. Help me understand this concept.
Ward: Okay. You understand the basic carrier, single carrier analog modulation or amplitude modulation. This simplest is Morse code. You're just turning it on and off. Amplitude modulation where you take a single carrier and you vary its amplitude to carry the signal. You can also add frequency modulation where you have a single carrier and you move its frequency back and forth.
IQ quadrature modulation uses two carriers. It takes one carrier and it splits it into two paths: one where there's no phase shift, zero, and that's the in phase carrier or component; the other is shifted 90 degrees. That is called the quadrature, 90 degrees. The reason 90 degrees is special is because mathematically, the two sinusoids, the two sine waves that are coming out, if one is 90 degrees shifted from the other, mathematically they're called orthogonal. They're easy to separate in a demodulator, for example.
If I take these two components, one in phase and one out of phase, and I shoot them through the pipe, it's easy for me to recover the two signals at the other end with a simple mixing circuit or a mixing algorithm if I'm using a software defined radio, or DSP digital signal processing, and I can get two completely independent streams of data back out of the system.
The in phase gets modulated with ones and zeros, and the quadrature gets modulated with ones and zeros. They get combined in the transmitter. Off they go. They come out the other end. I mix them down or I apply my mathematics in the microprocessor and I get out my ones and zeros on either side. Basically, by using this 90 degree phase shift and certain mathematical properties of the sinusoids, I basically have a much more robust way of looking at the information coming than if I only have one carrier and I'm only varying the amplitude up and down or the frequency back and forth.
Kirk: I think I'm starting to get it. At least I'm starting to get a little bit of an idea here. There's one thing that I'm still fuzzy on, the I and the Q signals themselves. Are they signals that are turning on and off? Are they going up and down in frequency? What am I looking for as a characteristic of what they are doing?
Ward: Let's talk about quadrature amplitude modulation first. Quadrature from having the two different signals, the in phase and the Q, and I turn them on and I turn them off. They can either be a zero or a one. Those are the four points of this diagram right there. For example, this right over here in this corner might be both I and Q off. Zero applied to both of them. This one on the other side of the square-it's hard to move this back and forth-might be both of them on, or one one. In this case, one of them is off, and in this case the other one is off.
Basically, the simplest IQ modulation there is QAM. I'm just turning these signals on and off. At the other end, I get back my ones and zeros just from watching the two components turn on and off. I can also do that with phase. I can do a wide variety of things with those two signals. The simplest one is quadrature amplitude modulation where I'm simply turning the two components on and off.
Kirk: If I were to really understand and learn about the I and the Q signals and the balance modulators and the 90 degree phase shift, if I really got that concept down, what's the next thing I need to move to? In other words, are there other forms of encoding transmission that are also popular? If I learn this, it really covers a lot of technology?
Ward: I would stay simple. Start with QAM. The next thing you need to worry about is, what kinds of coding and error detection and error correction can I apply to this data stream to increase the quality of the data I get out at the other end? While you were on the commercial break, I looked up the exact form of Shannon Hartley information theorem. It's in this handbook, the ARRL handbook, that I edit.
It's in the form of an equation where the capacity of the channel is equal to the bandwidth that you have times the log base 2, not base 10, of 1 plus the signal to noise ratio. That's in bits per second. It depends on your bandwidth and it depends on the signal to noise ratio. Then you can also see this little graph up here. As the signal to noise ratio gets better and better and better, you can get closer and closer and closer to the theoretical maximum.
How do you make the signal to noise ratio better? You can make your transmitter stronger. You can turn up the juice. That works. That's a perfectly legitimate way of increasing your signal to noise ratio.
You can also do that in what's called coding space. If I knew that there was only a certain sequence in that constellation diagram, if I knew it was down at this corner and I was always going to go this way, for example, it would make it a lot easier for me, if I knew I was always going to move counterclockwise or clockwise around the square there, it would make it a lot easier for my receiving machine to decide which corner of the square that my air link had arrived at, and then generate the appropriate combination of zeros and ones.
This is called, for lack of being very precise, and I'm sure some of the really advanced modulator guys are out there going, "It's imprecise. It's over simplified. It's not really that way." Basically, go talk to them. They'll tell you a week at a time. You'll get the full meal there. Basically, predictive coding in one of the most common forms, and popular forms, is called the Viterbi, V-I-T-E-R-B-I, in coding for the guy that invented it.
It says, "I'm going to take the data that you give me, whether it's email, audio, or whatever, and I'm going to look at that data. I'm going to generate a sequence of things that I'm going to transmit, a sequence of symbols that I'm going to transmit. But, we're all going to know that there's a certain sequence that we're going to take through this constellation pattern."
Maybe I have four points. Maybe I have eight points. Maybe I have 16 points. Whatever. There is a certain set of rules that I'm going to apply about how I move around within that constellation. I know that if I'm here, for example, I'm only going to go over here somewhere. If my receiver says, "I think he's down here or he's over here," then I know that's wrong. That's an error. By using these codes, I can effectively increase my signal to noise ratio. When you increase your signal to noise ratio, you can do it by increasing the signal, or you can do it by reducing the noise.
What is noise? Noise is decoding errors. If I reduce the number of possibilities that my receiver has to sort through to make a decision, then I can effectively reduce the noise and raise my signal to noise level. By using these predictive codings, I can raise the number of bits per second that I can pack through my limited amount of bandwidth.
What do you do if you detect an error? The next layer up the stack, the next part of what your protocol machine has to do is it has to decide. When you pick a protocol, is the protocol only able to detect and error? If so, it says, "Can you please send me that data again?" You get what's called an ARQ or an ACK/NAK where you have acknowledge and not acknowledge.
It will say, "I didn't get that last one. Send it again." I'm going to send it and I'm going to send it until it goes, "Okay, I've got it. ACK. Go onto the next one." That's an ACK/NAK or ARQ protocol. But I can also, if I have enough channel capacity, if I have enough signal to noise ratio, I can pack in some redundant data so that the system on the other end can recover from a certain level of errors that occurred during the transmission as measured by the bit error rate, which is the number of errors in bits transmitted per second. The higher the bit error rate, the harder it is for me to recover this information.
By using this forward error correction and sending the redundant information along with the data, at the other end my machine can go, "I missed this bit. I missed that bit. But, I have this little table over here and I can use this table to fill in. These bits that I missed must be a zero over here and a one over here."
That's grossly oversimplifying it, but basically, if I've got enough computing power on each end, I can use this predictive coding and these error correction techniques to recover from a certain number of errors. What's happening when you see, for example, on an HD TV signal when it starts to block or pixelate is that the machine on the other end is getting so many errors that it cannot recover the real information.
It can't fill in. It can't guess. It's got too many errors to recover from, so it just throws up blocks or blue screen, or at some point you see the picture break up entirely because the machine on the other end is getting so many errors that it has overwhelmed the error coding. It throws up its hands and it says, "That's it. Signal is low. You're out."
These codes are very powerful. They can maintain a real high quality signal recovery until all of a sudden they can't, so they just fall off the edge of the table. With analog, those of you that remember, in the old days-let's see, where's my hand over here? You would drive around and listen to FM. It would get kind of noisier and noisier and noisier and noisier, but you could still hear The Beatles or whatever it was. At some point, it was too scratchy and you changed the channel. It was kind of a graceful degradation.
With digital, it's basically just all the way it's really good and then all of a sudden it's really not and so it just falls off the table there. What you need to do is after you understand the very basics of the modulation start moving up to see how the data is encoded, and then the error management structure of the protocol, and then how does the protocol actually package the data?
Some protocols pack audio and video together, plus control, plus information, meta information about the channel. They identify the station and various other things. At some point, the protocol take that apart, takes all that other stuff away, and it generates the audio signal or the video signal that goes to your monitor, your earphones.
Start simple. Understand one simple set of modulations and then trace that all the way up to where it shows up at the top as music or picture or email or whatever. And then you'll be well equipped to change and say, "I can't get my signal through this fast enough. I need a more robust communication link." Then you can start looking at different types of modulation down at the bottom to see the effect. You'll understand how the effect of that modulation affects everything all the way up to what's called your application level, or the software that you're running on the top.
Back to you.
Kirk: Wow. I'm going to have to ask Chris Tobin if he's got any last questions because we're about out of time, I'm told. That's about as much as I can possibly digest. I've got so many more questions, but I've got a feeling they'd take three more hours to answer. Chris, what do you have for us?
Chris: First of all, with the Viterbi bit error rate, for those of you in the audience who remember the Star Guide satellite delivery system for network radio, the Viterbi bit rate was two thirds. That was something you had to remember on the receivers. That was when I managed uplinks. The earth stations, we had to remember all of these little details.
As far as quadrature modulation, remember the compatible version of that was called C-QUAM. You remember AM stereo. AM stereo was a signal that had a quadrature of 90 degrees and an in phase. When those two didn't properly match or decode, what did you get? We got that platform motion effect as I'm moving left to right in the camera right now.
Those are the things I'm adding in. Just for giggles and grins, I have here a Motorola app code 25 compatible radio, which is another form of digital communication to throw into the mix of things we're talking about.
Kirk: Those of you who have heard my story I've told many times in public gatherings: this here digital is the coming thing. It's everywhere. Ward, I wish we had another two hours, but we don't. I appreciate so much the time that you've taken to come talk to us about the digital transmission and modulation this evening. Thank you.
Ward: You invited me, thanks. I had to over simplify things so dramatically, but go out and start at the bottom. Work your way up. I you've got a ham license, you can tinker with this stuff. You can put your probes down in there and see it go, or just talk to the experts at your station or your facility and ask them for textbooks and references and dig through it. It's worth it. It's a really amazing technology. There are a lot of layers to it. It's very complicated. But yeah, that digital stuff, it's a coming thing, you bet.
Kirk: We're not ready to go just yet. I do want to tell our listeners and viewers about our third sponsor, and that's Telos and the Z/IP ONE IP Broadcast Codec. It's the codec that drops jaws, not audio. There are literally hundreds of radio station using the Z/IP ONE codec every day for studio transmitter links. And there are hundreds and hundreds more that are using them on an as needed basis for remote broadcasts, for symphony concerts, for ball games, for remote broadcasts from the Ford dealer, or from the county fair.
The Z/IP One IP Broadcast Codec is really the Swiss army knife of codes. It hooks up of course to an Internet connection. It can be wireless. It can be Wi-Fi. You can hook it up to something like a cradle point travel router and get yourself some 3G or 4G signal in foreign countries when, for example, the wired Internet had a big firewall on it that couldn't be penetrated like the one in China.
We demonstrated it with the iPhone in a Wi-Fi hotspot and it worked great. We use the Z/IP One IP Broadcast Codec for, as I said, studio transmitter links. All over the state of Florida just happens to be one place where several broadcasters are doing this with links from the studio to the transmitter site. Redundant links as well.
Even the Florida Public Radio emergency network, connecting 15 public radio stations in Florida altogether to provide basically play by play warnings for hurricanes and other severe weather. They're all connected together by the Z/IP One IP codec. It does all the codecs you're going to want: AAC, high efficiency AAC, and all that, linear PCM. It does MPEG 2, so it's compatible with older devices. It's just one codec after another. G722 in fact, and even G711. You can turn it into a SIP device and have it make a phone call or answer a phone call over a SIP protocol.
It works with Lucy Live and Lucy Live Light. It's just in use all over the place. Our radio stations use it and so do hundreds and hundreds more. Check it out, if you would. Go to the website telos-systems.com. Look for the Z/IP One. Z/IP stands for Zephyr IP. You're all familiar with the ubiquitous Zephyr ISDN codec. This is the IP update to that codec. It's the Z/IP One. Thanks for very much to Telos Systems for sponsoring This Week in Radio Tech.
Ward, thanks again for being with us on This Week in Radio Tech. I guess, for more information you said we just barely scratched the surface of some of these subjects. Ham Radio for Dummies, your book, will cover some of this and the ARRL handbook that you've shown on the screen there, the one for 2015. My goodness, that is packed full of deep information, isn't it?
Ward: Yeah. If you're used to buying professional literature, you'll be established that it only costs 50 bucks. 1300 pages of good, practical, wireless information at all levels. If you're not a ham radio operator, here's my little ham radio assistant here. If you're not a ham radio operator, ham radio is the most powerful communications service available to the private citizen in the world. If you would like to take your expertise and experiment with technology or use it in public service or just enjoy operating with it, you can definitely do a lot worse than ham radio. It's a going thing. It's a lot of fun.
Kirk: Ward, thanks again for being with us and thanks for taking the time to write that book, Ham Radio for Dummies. I need to update my ARRL handbook as well as get Ham Radio for Dummies. I am ordering both right after the show. I really am. I'm going to make it a present to myself.
Chris Tobin, thank you for being with us from Manhattan. I appreciate your contributions.
Chris: You're welcome. For those who are not ham operators, definitely look into it. I can assure you that I have enjoyed some great tinkering and experimentation with some repeater systems and antenna design and signal propagation to figure out a few things. I've applied those to the days when RPs were popular. I can assure you that it's paid off handsomely. With the digital stuff, you can do ever more. It's definitely worth a look.
Kirk: It sure is. Thanks a lot, everyone. We're going to go. Our show has been brought to you by the folks at Axia with the Element console, at Lawo with the crystalCLEAR virtual console, the crystalCLEAR fusion, and also from the folks at Telos and the Telos Z/IP One, like the one right behind me here. I use it in my stations.
We'll see you next week on This Week in Radio Tech. Bye-bye everybody.
Topics: Digital Transmission
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