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White Paper Technology An Introduction to TMDS: Understanding DVI-D, HDMI and DisplayPort Signals About the author I really like this opening quote: Joseph D. Cornwall, CTS, ISF-C, DSCE, ROI Business Development Manager “For a list of all the ways technology has failed to improve the quality of life, please press three.” Cables To Go -Alice Kahn Joe Cornwall has worked in the commercial AV Number three is certainly something we would have been pressing a lot had we tried to incorporate HDMI into our industry for more than two decades. He’s held projects from the beginning of HDMI’s introduction back in 2000 or 2001. In fact, what prompted me to write this management and technical positions with Sony, particular paper was hearing people make statements like “We will not use HDMI in our installations or in our designs, General Instrument and Motorola Broadband Communications Sector. Cornwall was a founding we will only use DVI-D.” I thought this was kind of unusual because the two really are the same thing. After member of C-Big, the C-Band Industry Group and conducting a little bit of research, I created this overview to give you a better idea of what is going on with digital served as a member of the Board of Directors for video. the Satellite Broadcasting and Communications Association (SBCA) from 1999 through 2003. Today Joe Cornwall holds the position of Business In this paper, I will explain: Development Manager for Lastar, where he’s responsible for promotion, support and growth of • The Nature of TMDS a full complement of connectivity solutions, • What Makes TMDS so Unique? including the award-winning RapidRun line of products. Cornwall holds CTS, DSCE, ISF-C and • TMDS Applied ROI industry certifications and is a graduate of • Digital Rights Management Massasoit State College and the University of • TMDS, Audio and Control Cincinnati. An Introduction to TMDS: White Paper Technology Understanding DVI-D, HDMI and DisplayPort Signals Why TMDS? This little quote from Silicon Image really says it: “DVI is the accepted standard for transferring serially uncompressed data at high speeds between a PC host and digital display such as an LCD monitor. DVI enables a video signal to be transferred from a PC source to a digital display in its native digital form, simplifying the way PCs communicate with displays and improving display image quality. “ -Digital Visual Interface and TMDS Extensions White Paper by Silicon Image, Oct. 2004- Serially uncompressed data at high speeds – this is the critical concept of this paper. Video is no longer video; video is now high speed digital data and it is nearly indistinguishable from anything that happens between two computer devices. Except in the instance where we are going between computer-generated video content and a fixed pixel display. That was the real key here – the advent of fixed pixel displays such as plasma LCD and DLP projectors, monitors and laptops that are themselves digital devices. We want to eliminate those complex analog-to-digital and digital-to-analog conversions that exist in the digital environment as long as possible, and leverage the native capabilities of that digital environment. We know that years ago we were capable of supporting up to 1080p in the analog environment, and there are a number of devices that have done that. But the only solution that allows us to go beyond 1920x1080, including D4K (3440x2400) and beyond, is TMDS. So if we are going to start looking at very high resolution images, such as what you would see in your local movie theater, we need to move into this environment. Another byproduct of moving into TMDS is better color. Unlike NTSC and analog where we had very low color resolution, a lot of people seem to forget that even with component video when we talk about s-video and composite video, color resolution rarely exceeded about 180 lines of resolution of actual color. InfactNTSC wasoftenreferredtoas“Never The Same Color” because it was so difficult to get accurate color capability. In the TMDS environment, our base color palette is a 16 million true color resolution and can even go beyond that if the project involves simulators or if there is a need to have a virtual reality. We can now leverage xvYCC, also known as Deep Color, and extend this up to more than a billion colors, which truly does encompass every color the human eye can perceive. What is TMDS? TMDS stands for Transition Minimized Differential Signaling. It sounds like a mouthful when you first hear it. TMDS was developed by Silicon Image Inc., a member of the Digital Display Working Group, as a method for transmitting high speed digital data. It incorporates a very unique and very clever algorithm that reduces electromagnetic interference (EMI) and enables the clock recovery at prodigious distances, up to 100ft at 1920x1200. It also enables high skew tolerance on cables that are really complex, and based on their original design, should not be able to produce video images from one end to the other. It does all of thiswitha very high level of confidence. TMDS is a lot like RGBHV, and much like the analog world we live in today, in that it uses four channels: Red, Green, Blue and Clock. So, if someone said to you, “I have four coaxial cables instead of five so since I can’t use RGBHV to connect my video source to my projector or my video source to my display, what can I use?” Well, you would probably respond that they could use SRGB, where we composite the horizontal and vertical sync and then multiplex them on a single cable. That is exactly what is happening on TMDS. So now you can begin to see that we are not in a foreign land. This looks very familiar – Red, Green, Blue and Clock – and it is a two stage process. This algorithm converts the input of an 8-bit video word into a 10-bit video word. By doing that, it does something very counterintuitive; it makes the signal smaller. 2 An Introduction to TMDS: White Paper Technology Understanding DVI-D, HDMI and DisplayPort Signals Transition Minimization That is very unusual. Why would it do that? How does it do that? How did these computer guys get so clever? TMDS signaling uses twisted pair, hence the term “differential” in TMDS. Twisted pair means we are going to have common mode noise reduction, or interference rejection. This means we are going to have a higher head room. It also operates at current modal logic, which tells us that we’re talking about something that is operating under 5 volts. In fact, the way that HDMI operates between the transmitter and the receiver is to operate at a 5 volt handshake, while 3.3 volts is the current mode logic where the actual datais transferred. There are three twisted pairs for Red, Green and Blue, plus a fourth twisted pair for sync and, once again, that unique 8-bit to 10-bit conversion capability. So, what does that mean? That is the transition minimized part of Transition Minimized Differential Signaling. Here is what happens. Pretend the image below is the image you see on your computer screen right now. You see a black screen image with white letters on it. We know that each section of that screen is describedby a digital word. Black, also known in digital as zero IRE, is described by an 8-bit digital word that is 00000000. As we get to the letter “W” in the word “What” in the headline, it changes to white. The signal goes to 100 IRE, or full saturation, full white, where all three colors are at maximum output. This is indicated by a digital word of 11111111. Every time you transition from a digital zero to a digital one, or transition from one bit to the next, it is described by an electrical square wave. Square wave, as you may remember from your engineering classes, is a fundamental sine wave plus all of the odd harmonics, all of the very high frequencies. This means that when we have to make eight transitions we have a tremendous amount of high frequency going through this cable. It becomes very difficult for us to be able to encompass all of this bandwidth on any kind of a practical transmission. So what our computer brethren did was to take a look at this and come up with a better solution. Most of what happens in video happens in shades of gray, but even as we look at the other colors we realize that those colors are described by shades of gray going through a red, green or blue filter, then combining to actually make the color. All of these variations happen in the four middle bits. So the computer guys said, “What if we take the four middle bits and where there are all ones we inverted them and made them all zeroes and then added a one to the very end? That way we eliminate a lot of these transitions, allowing us to carry less high frequency material.” They then went one step further and said, “What if the least significant bit and the most significant bit are ones, and how about we invert those and make those zeroes and then add another bit to the end? By going from eight bits to ten bits, what they have actually done is minimize the transitions from zero to one so that the maximum amount of transitions being applied is five rather than eight.