HDMI 21 TMDS Crash Course ENMU EET 457 Presentation

hello everyone my name is Daniel Tucker and I'm here with an independent research topic I'm presenting for electronic communications to Eastern New Mexico University et 4:57 I'm going to try to keep this pretty short so it's going to be more of a crash a crash course in HDMI and TMDs and the topics I'll be covering today are gonna be what HDMI is and I don't want to insult anyone because most of you probably have used HDMI in your own configurations at home it's a pretty common standard now but I do want to kind of just do a quick introduction to it for those of you that haven't and then to facilitate some of the concepts I'll be presenting during the slides so I'll be going over a quick introduction to HDMI and then after that I'm going to get into the information types and channels that pipe data through on an HDMI cable and then I'm gonna get into the cable types and construction methods so kind of break down the transmission line there and then I'm gonna get into the limitations of that transmission line so where the where the standard and where the implementation of the standard kind of falls short and then I'm gonna get into some of the example test equipment that might be used to test hdmi cables transmitters and receivers and because HDMI is largely based on transition and minimize differential signaling or TMDs I'm gonna have to get into what that is and how that works because that's the bulk of the data throughput on an HDMI cable is teamed yes so we'll get into what that type of signaling is and I'll show you a transmission and receiving block diagram using TMDs and then I'm going to get into the encoding the TMDs encoding on the more modern HDMI standards uses scrambling with linear feedback shift registers and a generating polynomial this produces stability and helps with error detection and correction and that sort of thing and they were able to actually use that generating polynomial and the concept of linear feedback shift registers to greatly enhance the data throughput of an HDMI cable and then I'm going to get into some example implementations of HDMI with electronic components in their applications so without further ado let me just introduce you to HDMI most of you probably already used it before like I said but it is called the high-definition Multimedia Interface he's first launched in 2002 and has seen many revisions since then it is the most currently released specification is 2.1 now that goes up to a data throughput of 48 gigabits per second on the team des quad pair so there's not many devices yet that are leveraging this newest standard most devices currently on the market are revision 2.0 it is HDMI is a proprietary audio/video interface although it does pipe more than just audio and video through on some of its channels and it is used a transmit uncompressed video data and compressed or uncompressed digital audio data and HDMI is a digital replacement for earlier analog video standards it is built around the EIA CEA 861 multimedia standards it's not too important that you know what this is but it does come up if you ever end up working in communications industry or high speed information exchange some sort of job like that you will probably encounter this standard it defines video formats in the wave format waveforms for those video formats defines the transport of compressed and uncompressed linear pulse code modulation audio and defines the transport of auxiliary data it also implements the VSA extended display identification data which is a mechanism that devices can exchange information to each other and have awareness of each other's capabilities so that a video or audio signal that the display isn't capable of reproducing won't actually be sent something do anything with its what HDMI cable looks like and you all probably already know that you might not have known as to that to work with these as a manufacturer and implement HDMI and a compliant way you have to become a certified HDMI adopter and so this involves paying licensing fees and royalties on per units manufactured and of course sending a sample size of your production off for compliance testing and so this applies to cables this applies to the transmitters and the receivers and having a program in place that regulates the compliance of HDMI furthers the technology so there's kind of a money flow there for that and of course it helps consumers ensure that they're getting what they pay for and the current standard allows the currently most recently released set of HDMI specifications 2.1 allows for resolutions of up to 10 K 8 K and 10 K at 60 Hertz HDR and a lot of other optional features that manufacturers can implement if they choose to do so and of course with these with the evolution of the different standards from HDMI we have been seeing constant increases in the data throughput back at 1.4 was 10 point 2 gigabits per second the most widely and currently available standard that is implemented in devices on the market right now is that 18 gigabits per second HDMI 2.0 and right around the corner we're gonna start seeing some devices with really high speed chipsets system-on-a-chip SEF pga is that sort of thing that can pipe out data at 48 gigabits per second with HDMI 2.1 alright so you probably have seen the connectors but you may or may not have known just how many pairs of wires are being routed through that HDMI cable and what the purpose is of those different channels are and I'm gonna break these down a few more slides deeper down here in the presentation just so just hang in there for that but I just kind of wanted to show you the inside of the cable before I start talking about what type of information is transmitted over HDMI so you can see there's a lot going down there and like I said I'll break that down I just wanted to show you that really quickly here before talking about the types of data pipe through with HDMI number one is the pixel color data that's basically your video data and then you're gonna have your audio data all of those are going to be in a digital format to my knowledge HDMI does not transmit any analog sound whatsoever but there are many many digital formats pulse-code modulation and a bunch of proprietary formats like DTS and Dolby the two leading competitors in this digital audio delivery format they also produce the sound for theatres as well as home theaters and internet streaming services and that sort of thing and the third type of data pipe through HDMI is going to be your timing information in your auxilary data such as consumer electronics control so those are the three types of data that are piped through on an HDMI cable and these are the five channels that it uses to accomplish that and I'm gonna break down each one of these channels briefly here so TMDs and perhaps the most important one where the largest amount of data is being piped through transition minimize differential signaling this interleaves audio video and auxiliary data and it has different periods where you can that it does this with you can think about it like TDM or time division multiplexing because during certain periods video data is packets are being piped through and during other periods audio data packets are being piped through and control information as well so it's kind of dividing up the time and sending through data for each of those types of things at different times during a given sequence and that's their schema for how to get all that data through on just 40 MDS lines DDC so this is a display data channel it is a simple communication channel based on I squared C and it's basically to allow for some awareness from the display and the source for compatibility it's basically a table of hex codes that'll get exchanged over serial clock and serial data lines so that the devices are aware of each other's capabilities Audio Return Channel is so that audio can be sent down back in the opposite direction instead of audio coming from the the generation source to the sink like the display can come from the display to the source so if you have an ABR and your home theater setup you can actually use those smart apps on your TV and you have the sound piped back from your TV to the ABR HEC is an optional feature is called the HDMI Ethernet channel uses a single wire with a ground and has a hundred megabits per second bi-directional link CEC allows for the user to control up to 10 enabled devices connected through HDMI so these are the five channels present in an HDMI cable some of these are optional not every cable may have a dedicated wire for the CEC HEC or Audio Return Channel that does depend on the manufacturer and the revision of HDMI to which they are manufacturing for now one of these I do want to break down a little bit further here for you because it is important in the amount of data that's being piped through the concept of differential signaling here what differential signaling is it's pretty simple compared to your single ended transmissions as you see in the diagram at the right you've got your logic level low high and the transition from low to high and the transition from high to low over and over and over again representing your zeros and ones differential signaling has two pairs that share a ground and what they do is they're equal and opposite of each other at all times and that doesn't it kind of seems pointless at first glance but there are a lot of Vantage's with sending your ones and zeros this way one of the primary advantages is that they cancel their own noise because the waveforms are equal and opposite of each other and those wires and cables and on PCBs for traces they're routed close to each other in fact in cables they're a twisted pair so they cancel each other's EMI and they are also less susceptible to external EMI they also allow for lower voltages in systems as well and there's a little noise diagram on how this how these differential pairs work and how they cancel noise further on in the presentation I will do a comparison to single ended signals and these differential signals here and I'll make that picture a little bit bigger and easier for you to see but just know that differential signaling allows for a lot more data throughput because of their almost indifference to noise they're very very good about not only canceling their own EMI but also being less susceptible to external EMI ok so back to the cable here I promised I'd break this down and let's get into that the first thing I want to point out is that with a good cable design and a good manufacturer they are triple shielded and not every manufacturer makes these cables out of the same high quality materials this one here I think that we're seeing is a silver ten conductors with foil and it's got three layers of foil it's got your mess he'll ding right before the PVC jacket for the whole cable and then it's got a foil layer inside of that and each of your TMDs pairs have gold foil on them and those are called your TMDs channels and again what I want to bring up that little noise diagram here because not only do you have these pairs you're seeing the brown and white bread and white green and white blue and white that not only do they have the advantages of being a differential pair for noise cancellation but they've also got that shielding and lots of it so this cable is designed for a lot of data throughput with good signal integrity so these are an unbalanced line we've talked about these in our higher level communication courses they're en mu they're a twisted pair one of these pairs here on screen are obviously twisted but just know that the ones inside the gold floral gold foil are also twisted pairs they just happen to also have a ground in there they have that dedicated ground and of course they have their own foil now this is TMDs channel 0 and again I just wanted to reiterate that this is a transition minimize differential signal coming out of these pairs so as you can see right there each one of these goat gold foil pairs are producing a signal just like this for all the data they pipe through it is this type of signal coming out so that's TMDs channel 0 1 & 2 and if you haven't guessed already during the period of time when video data is being transmitted channel 0 1 & 2 are transmitting your RGB red green and blue pixel values and then of course during the data island periods for the sequence of HDMI the model in which it works it's also going to pipe audio data through there it has that fourth TMDs clock channel other uses of some of these wires here again some of these are optional but I'm going to go ahead and label these as if a manufacturer decided to fully implement every feature you've got your 5 volts in your Cee Cee C which is the consumer electronics control your 5 volts is used to detect when the cable is plugged into another device that is called the hot plug detect so the HDMI cable actually does send out a 5 volts DC constant signal level and then one of these other wires is a return for that level and what that what happens with that is when you get that return detected that means the cables been plugged in and it can begin exchanging the display identification data on the serial clock and serial data lanes which is an I squared C D DC channel and then I'll bring those down here so we don't get too crowded there that's your D DC Ethernet and cec return ground that's your hot plug detect that was the one I was telling you about the 5 volts is returned on and then of course that purple one might be the HDMI Ethernet channel now it is worth noting that no one manufacturer of cable or manufacturer who terminates these cables with connectors they are not consistent with color coding so other than the gold foil pairs there I could have easily mixed up the color coding there and any one of those could be flipped around from manufacturer to another but I did my best to probably pick what was likely um so that's what you've got on your different channels coming through remove that over and just show you to remind you what type of data which type of data is coming out where so on your channel zero one and two you've got your interleaved video and audio coming out of those out of your serial clock and data lines that's the DBC channel you've got your display identification data and of course you got the clock Channel I've got that asterisk there on the clock channel because in the newest revision they have found a lot of freed up overhead on that clock channel and have found ways to pipe even more data in through that clock channel so that fourth TMDs pair has been maximized in the newest standards so that it's not just using clock timing information but it's also transmitting some other type of information so this right here is a an example of some test equipment for HDMI cables transmitters and receivers this device can be used inline it's basically a handheld scanner and that is the exact one you see there in the hard case what that does is you can plug up to an HDMI cable and detect some of the data and and rates of the the types of data and the rates for those data that are coming through and identify those and see if they're coming through like they're supposed to be so this would be very handy for an installer or troubleshooter who works on these types of systems in a commercial environment or maybe even as a home installer for home theaters and stuff like that so like I said the various information types of an HDMI signal can be seen in the photo this is a Mira do Frisco pair scanner and one thing not shown however on this scanner is the edid I did not dig through the documentation for these so there may be a screen where you can pull up the edid but the edid is the extended display identification data this is where the source and the sink or the the device that's generating the video signal and the device such as the TV that is or projector that is playing it is called a sink those devices can exchange information to each other about their capabilities so that the right type of signal is being sent through and that's just a table of hex codes basically Flags identifying what types of audio it's capable of reproducing what its maximum resolution in frame rate that it's capable reproducing whether or not it's HD are compatible and whether or not it is HDCP compliant I'll get into HDCP at the very end of this slideshow it's kind of outside of the scope of this presentation all right so HDMI cable limitations so the limitations of the transmission line so one of my favorite manufacturers of HDMI cables because I've been toying around with this stuff for many years is a company called blue jeans cable I'll cut them down the references here so they make very high quality cable they work with Belden to get perfectly ideally matched lengths of twist twisted pairs for those TMDs channels and there's no signal skew because of that also outside a little bit outside of the scope of this but one thing they did say about HDMI is that it's a fragile interface not least of which because the difficulties of manufacturing twisted pairs fine tolerances and you know the extraordinary bandwidth requirements of the HDMI signal so the primary limiting factor for HDMI cables is cable length 50 foot is generally considered to be the maximum reliable length but what I have found in practice is that that is actually much shorter as soon as you start pushing up that resolution in frame light if you're doing above 1080p or if you're getting up in the 4k territory of 60 Hertz you're gonna have to use the shortest cable possible to avoid a bitrate error I'm going to talk about bitrate error here in just a moment several methodologies such as improved cable design and characteristics and active repeaters error correction and fiber optic encoders and decoders which basically replaced the cable all together have been employed to reduce the error rate of longer cable runs the fiber optic replacement four HDMI cables is a very neat topic it basically in code at the transmit side and decode at the receiver side and they replaced the copper and more silver tend or whatever you have you know conductor type cable whatever you have there it's replaced entirely by fiber optic and as long as the encoders and decoders are fast enough for the data throughput you're trying to achieve it is an excellent solution and you can make remarkably long runs with zero practically zero a bitrate error but that's outside of the scope of this and just want to talk about the actual cables that are most commonly used in everyday applications in the home and retail environment okay so like I said cabling is the primary limiting factor there are no numbers on these charts because it varies greatly from one cable manufacturer to another I could take two cables that are supposedly compliant with the same HDMI standard and it is anyone's guess whether or not they're actually certified there's a lot of people cheating out there and they're using the HDMI logo even though they're not a certified HDMI adopter and they have not sent out their hardware or their cables for compliance testing so it's pretty difficult without some some high-end test equipment and a lot of time to dump into it to find out if the cable really should have been certified where if it ever was certified in the first place but in general even with the high quality cables the longer the length the more degraded the picture quality will will become now notice this isn't a linear drop-off in picture quality there's a bit of a cliff there and that's an interesting topic all on its own I'll try to get into a little bit more of that but not a whole lot so what you get is an increased error rate as the cable length increases and it's pretty slow at first and steady until it reaches that cliff right there and the bit error rate or the BER which we've also discussed in our course dramatically increases the closer you approach that theoretical cliff which we said most people agree upon is about fifty feet but in reality with frame rates and resolutions that is gonna be a lot less than 50 feet you're gonna want to stick to 6 meters or less you know probably 3 to 6 foot cable if you can keep it under that you're you're doing yourself a favor because you won't have to mess with just trouble displaying your content or your audio so what those differential pairs look like on an oscilloscope is often referred to as an eye pattern it's what TMDs signals look like also it is what low voltage differential signals look like another type of differential signaling so you've got your many many traces at the ones and zeros overlapping each other that resemble an eye shape so as a signal further degrades or as the amplitude is lost over a really long wrong long run you'll get what's called a closed eye and there's digital signal processing techniques to open that I back up and clean up that signal Andry transmit it so those are called an active repeater or if it's not a repeater it might just be on the receiving end to help clean up that signal and try to recover it this is a block diagram for a TMDs transmitter and receiver here in this case it's showing you 8-bit red green and blue video data being piped through but keep in mind that HDMI also utilizes these channels to pipe through audio data and the video island or in the data island periods which are the periods outside of the active video area but you get the idea there the triangles on the left are a buffer one but one is just directly connected one is an inverted buffer and so those signal levels are always equal and opposite to each other and on the other end the triangles on the right you just have op amps which are cancelling out that noise and returning those differential pairs back into a single pair and the fourth one the white triangles at the bottom are the same thing it just happens to be the clock Channel speaking of TMDs I talked about this a little bit at the beginning we have linear feedback shift registers they're used for each of the t MDS data on the encoding or source side and one linear feedback shift register is used for each of the T MDS data channels for decoding on the sink side now each bit in the diagram below is represented as a D flip-flop respectively from the least significant bit on the left to the most significant bit on the right and that's essentially just a shift register a bunch of D flip-flops and there you can see the generating polynomial for T MDS transmission and an hdmi system and how this did come out of the hdmi specification version 2.0 so this is for educational use only a little disclaimer I've gotta throw up in there you do have to be an adopter to even get the specification for HDMI and so here's another way to view that data stream coming out of those differential pairs the dark green you see there is what's transporting the pixel color data or the active video data and that dark blue surrounded by the two red bars there is where you're seeing the auxiliary data being transported in the audio and it looks like man that's only a little bit of space for audio compared to a lot of space for video but in reality you do need a lot more data for video than you do for audio and here's how they accomplish that so if you're here's another way to visualize that data that's coming through and what you've got is a couple of porches what they used to call in the old CRT world is the front porch back porch and you're blanking periods or you're blanking intervals those are the areas above the screen and to the left of the screen where data other than just the active video lines can be transmitted and that's where you're able to get for every single frame so one frame which in a 60 frames per second setup this amount of data is being transmitted 60 frames per second so this data on-screen here's 60 times in one second that's plenty of room for audio so the overall frequency at which the frame is presented to the sync device is fast enough that there's plenty of for high-definition audio packets to be delivered in between the delivery of pixel color information I told you I would talk a little bit more about differential signaling here and I wanted to compare the then had the effects of noise on a single-ended signal versus a differential signal on the left you can see that single-ended signal there with logic high logic low and you can see noise and how those two combine and it dramatically affects a single-ended signal especially in high-speed the faster it is the shorter those periods are going to be and the more catastrophic the effects of noise are but as you can see in the diagram here with differential signaling the noise effects are canceled out do the nature of the op amp that's used to recover the differential signal at the other end so you have a buffer and then an inverting buffer our not gay if you will that is causing those two pairs to always be equal and opposite of each other and then at the other end you've got that op amp that is amazing and cancelling out the noise for recovering that signal at the other end and there's that diagram again just pulling up a little bit it's interesting here that it does point out these differential signals are used in low voltage differential signals ECL Peco rs-422 and so on but they are used here in transition minimize differential signaling and just like the name implies the transitions from high to low and low to high are minimized in an attempt to get the highest data throughput possible on these HDMI cables the ones I said that I were using are HDMI 2.0 and there are 18 gigabits per second and that is that is quite a bit of data throughput so the transition time for these signals to go high to low have to be as absolutely minimized as possible in order to achieve such a data throughput and without further adue or getting close to the end of the presentation here I just wanted to show how HDMI and a TMDs system might be on a circuit card or in a new design this one here you're seeing is older it is an evaluation board from digi-key which uses a Texas Instruments TFP 410 which was using the DVI 1.0 standard which is nearly identical to the earliest version of HDMI it is a transmitter and so this chip is a little bit dated but it is a good example of TMDs baked on to an ASIC or an application-specific integrated circuit not reprogrammable at the field level just really cheap and can get the job done at the speeds required but this one in particular only supported video and because this was at the time that HDMI was still a brand new specification so here's a little bit of a newer device this one here you might find in the receiving end of so I can actually inside the TV is what's receiving your video signal and just converting it into RGB values plus audio and this one I think is probably multiplexed this is a great company analog devices despite the name of the company this is not an analog device in any way shape or form it is purely digital but it is the adb 76 23 it's a multiplexed receiver and what this does is decode the TMDs transmission lines and converts that into raw unencrypted RGB values with a refresh rate for your screen and of course it separates out those audio packets and recombines those back into a continuous audio stream now there's a block diagram on here I don't expect you to be able to see that it's probably too small on the screen you're viewing it on but I one thing I did want to point out on the block diagram that in the block diagram came from the datasheet but I wanted to point out one concept about HDMI that I haven't mentioned yet is the HDCP and that is the encryption that is used to ensure people cannot rip off content or at least not easily rip off content like recording blu-rays and recording shows often definitely and that sort of thing and so what that is is it's a just a Content protection protocol that has keys that are stored on the IC generally speaking only the manufacturer of the IC who works closely with the HDMI corporation is privileged to have access to these keys when they bake in their chips on the newest revision they probably even release a new set of keys with every new HDMI specification that comes out in the past those HDMI keys have been or those HDCP keys have been leaked and people were able to create some pretty cool pirate hardware because of those leaked keys but they try to stay on top of it and release new revisions to prevent that from happening in the future the reason why I'm bringing this up is because if you were to DIY or build on your own outside of working for somebody already in the industry you would not be able to create a compliant device without first becoming an HDMI adopter without which also includes becoming an adopter of hdcp so you would not be able to make a device that is fully compatible with all the frame rates resolutions and stuff like HDR that come on modern delivery formats like those HDR blu-rays and those you know 4k content delivery stuff so you couldn't do it by yourself you would probably have to be somewhere in the industry or at least become an HDMI adopter if you're a sole proprietorship or something like that but I would be remiss without talking about without mentioning HDCP if I'm talking about HDMI so moving on this is how this is a soft and hard IP core and what that means is is if you're working in an fpga development environment you can download an entire intellectual property core that is that a collection of the files with hardware description with the description of an hdmi system like that block diagram we saw here basically coated in a hardware description language for the purpose of loading onto an FPGA so that you can work with transition minimized differential signaling for FPGA or for HDMI and the soft IP core is the reason why they have these split up into two IPs here is I think the soft one could probably be used for DisplayPort as well and then the hard one as where as you finalized those signaling types for HDMI and you might swap that hard IP there out for a different one if you were going to work with DisplayPort or some other video transport and so that's an example of a block diagram there that would be the IP course that would be implemented onto an FPGA and that's essentially all I've got for this topic it was like I said it was gonna be a crash course in HDMI and TMDs I had a lot of fun researching the topic and there's still more I'm looking into if you have any questions for me or you have any feedback I would love to hear about it if you're seeing this on youtube just drop me a comment below or just email me if you've got my university email address one of the thank you guys for checking out my presentation and I hope you enjoyed it thank you very much and have a great day

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