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.
This eliminates a lot of high frequency material that needs to be processed and transmitted. So it is rather counterintuitive to go from eight bits to ten bits, to go from a smaller word to a larger word, and then end up with a smaller amount of information. That is the beauty of data truncation or algorithms.
This algorithm uses a special ten bit sequence to minimize the zero to one transitions. So now we understand TMDS and all of the benefits it provides, including providing up to a D4k level of transmission. Let us now take a look at how this hardware communication is played out. The block diagram you see below could represent, for instance, the output of a Blu-ray DVD player and the input of an LCD panel in your living room.
It could also represent the output of a codec, a medical imaging device or other such device with an HDMI output, and the HDMI input on something like a projector or LCD panel in a digital signage installation. What you see here should make you feel pretty good because it is very familiar. This is the same Red, Green and Blue that you have been used to seeing all along. So you see that we are really not in foreign territory here. These things are very familiar to those of us who have been working with analog audio and video for a while.
This is where things start to get really interesting. That is what tells your computer to switch from x to x to present on a particular projector.
That process is highly automated. In the digital world it is a little bit more sophisticated. In the digital environment we move from Display Data Channel DDC , which is a digital communications protocol between a display and a source that allows these devices to understand at what resolution they can operate. EDID, now on version 1. The screenshot you see to the right is really kind of interesting. It might be a little bit small, but this shows you that the DDC channel EDID information is not simply transmitting the resolution the device can operate on.
Rather, it is actually transmitting the serial number, the build date, the firmware date, the manufacturing number, the manufacturing identification number, the maximum resolution, the color depth, and a table of all the resolutions that the device can resolve.
So there is a lot of information being transmitted there which makes these monitors compatible with a number of digital devices. Here in lies the very first problem that we experienced. However, what we really had was a failure to properly write EDID information on all of these devices.
In the early days manufacturers actually had something called "plug fests". These were events where display manufacturers brought their displays and source manufacturers brought their sources. They would get together and start plugging these devices together and would take notes until they found where the incompatibilities existed. What we discovered was that a lot of the incompatibilities had to do with the EDID information being improperly coded resulting in the devices being unable to talk to one another.
I'm very happy to say that most of these issues now reside only in legacy equipment, so the only time you are really going to experience any type of EDID information issues is if you are trying to incorporate devices that are four to six years old into a contemporary installation. Most EDID issues have been resolved and, in fact almost every digital device, whether it's an LCD panel, plasma panel, DLP panel or Blu-ray player, have firmware that constantly updates the EDID information to make sure these devices are compatible with all contemporary technologies.
Now, I would like to point out one other thing about the block diagram on the previous page. In the digital world audio is embedded into the Red, Green and Blue digital video signal. This is electronically combined and electronically separated at both ends, so it is part of the video signal. Moreover, audio truly is encompassed within the TMDS environment.
It was just never implemented at the time, and it took HDMI to get us there. Now that we understand transition minimization, we understand that transition minimized differential signaling is a way of transporting digital data between a source and a display. We also understand that there is a line of information called DDCthat allows us to transmit EDID, or extended definition ID data, so that devices can understand what languages they will speak, and at which resolutions they will work.
In the late s and the early part of the 21st century, digital rights management became a factor in all of this. Hollywood, of course, wanted to make sure that we could not make a copy of Titanic for our personal use. So they came up with the concept of digital rights management. There is a great quote attributed to G. Gordon Liddy that is very appropriate when discussing this concept — "Obviously crime pays or there would be no crime. In short, digital rights management has been more successful at causing headaches than it has at protecting digital content.
Let's take a look at what they did when they brought digital rights management into this mix. They implemented what is called HDCP, or high bandwidth digital content protection. It is designed to prevent the copying of digital videoand audio content. Since this is computer data, or high speed data, they could not use something like macro vision, which is an analog fluctuation of the signal, they had to come up with something different.
They actually looked to something that was created for encryption by the CIA in the latter part of the 20th century. It utilizes what is called Blom's Scheme. Blom's Scheme is a symmetric threshold key exchange protocol used in cryptography to be able to encode all of these messages and here is what it does: Part of the EDID structure, the reason it has totell you the serial numbers and all of this other information, is that every display has an associated bit key code and every source has an associated bit key code.
These key codes are built into the devices and are exchanged in order to create an opportunity for the devices to communicate. The DVD player would send the key to the monitor, which would send the key to the DVD player, and the two of them would magically erupt in a third momentary key that would allow those two devices to communicate for that length of time when the two keys had been exchanged.
One source is connected to one display and then HDCP authentication, or key exchange and validation, takes place and allows an image to be displayed. This created a tremendous amount of problems ten years ago.
At that time when we really did not understand how the code needed to be written so that this protocol would allow a single source to authenticate more than one display. So being able to take the output of a code, the output of a DVD player, the output of a digital signage device on HDMI and have multiple televisions was problematic. That is the one too many.
Another thing we did not have was something called authenticate forever. A lot of money has been spent not only eliminating problems with authenticate forever , but also on eliminating the problems of latency that are a byproduct of Blom's Scheme.
This occurs when you are on video input number one and you change to video input number two, yet it takes a second or two to get an image.
This is much slower than in the analog world. We also see this when we change channels on digital televisions. It often times takes what seems to be a very long period of time to lock in on that channel and resolve an image to display.
Authentication of thisBlom's Schemeis what is really happening, enabling these devices to talk to one another. A lot of money has been spent by companies like Crestron and Extron, making products that will perform key authentication management or EDID emulation that will prevent this constant reauthorization.
It was about eight years ago they created the code calledauthenticate forever. This is similar to the electronic equivalent of having kids in the back seat of your traveling for your family vacation.
They ask that constant and repetitive question — "Are we there yet? Can we exchange this key? Can we talk? If you go from video two and back again, it refreshes that Blom's Scheme key exchange between the two devices.
This is just a small piece of information regarding the latencies and the difficulties associated with it. Now we understand that we had to create a way to send digital data, and we had to truncate that data by using transition minimization. We also wanted to place it in a different environment where it was going over twisted pair, and we had to provide EDID so these devices could talk to each other.
Then we had to add high definition content in order for it to become a commercial reality. That is when an actual technology that would allow us to transmit digital video was created. This is something that only emerged into the market around and then in with DVI-D. In fact, as late as there were only 25 companies that manufactured HDMI enabled devices. Today, it is pervasive. Every device, whether it is commercial or consumer, is HDMI compliant. So what was the first product created once TMDS was perfected?
Well, that would be DVI-D. DVI-D is a really neat way of connecting video. First, it has a robust locking connector. It looks a lot like VGA, with those torque screws that provide a secure connection that will not come loose. However, that is a bit of ared herring. Cables do not normally fall out all by themselves. We'll discuss this in more detail when we get to HDMI. There were several different flavors of DVI-D. The acquisition of Tripp Lite transforms Eaton's distributed IT infrastructure and connectivity equipment capabilities.
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We're always available to help with questions, including product selection, sizing, installation and product customization. Tripp Lite W. What the TMDS algorithm is attempting to accomplish with all this manipulation is to create a "perfect" byte of information for transmission; or at least as close to perfect as possible. The theoretical ideal byte would look like , followed by another byte that is This creates data that is both transition minimized and DC balanced.
The first stage of this process is the transition minimization. This is done by comparing the bits in the byte to the first bit and determining if a logical XOR or logical XNOR operation would make the byte have the least number of transitions. The second stage of this process is the DC balancing, which means that the entire byte may or may not be inverted, to balance it with the byte before it. Whether or not these two operations are accomplished is transmitted to the receiving piece of equipment, or more accurately to the algorithm in that equipment by the two control bits that have been added to the end of the byte.
All right, so what does all that complicated computer mumbo-jumbo mean to you, they guy who's trying to hook up his home entertainment system?
Really, all it means is that TMDS is an incredibly complicated system, which works automatically in the background, to insure that you get the crispest, clearest image, without any noise, snow, static lines or other transmission gremlins showing up on your screen. While I won't go as far as to say that there's no possible way that those transmission gremlins can get into your signal and cause the occasional white or black pixel, where it should be blue; the chance of any of those gremlins messing your football game, movie or favorite sitcom up are drastically minimized.
This doesn't mean that you can't have any problems whatsoever. You can still spend a fortune on the latest LED backlit LCD or plasma TV, and Blue-Ray player and even buy gold-plated wires, but if you don't have a good signal, you won't have a good image. A crummy antenna is still a crummy antenna. If you're expecting a good image, make sure you have a good signal, whether from cable, satellite or broadcast.
Let's Look at the Connectivity Requirements When we're talking about digital video signals, we've got to realize that we're talking about massive amounts of data being moved around. Okay, so what? HDCP is an acronym for "High-bandwidth Digital Content Protection" which is an encryption process embedded in source material used to protect digital material from being perfectly copied. Intercoms for Drive-up Customer Service.
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