Technology Brief – DisplayPort

If you are the proud owner of a ThinkPad R500, T500 or an X200 with a media slice, you will have no doubt noticed a new port on your system. It looks like a USB port, but if you tried to plug a USB device into it, you found out that it would not fit. That port is a DisplayPort, and it is a next generation video interface for notebooks and desktops. Anyone who knows PCs knows the venerable VGA port. It's usually colored blue, and is sort of "D" shaped. You use it to connect an external display or projector to your laptop or desktop. Inevitably, you always try to insert the video cable upside down and have to kink a very thick and awkward cord in order to fit it correctly into the socket. Then and only then, you get to deal with two fixed screws molded into the VGA cord in order to lock it into place. While these usually twist easily into place, woe to that person who has tried to unscrew them after some overzealous nut has cranked them down like life support equipment on the space shuttle. Assuming you don't need a torque wrench to get them unstuck from your video card, you're practically guaranteed to remove the metal screw holders from the back of that video card. Then it's up to you to decide what to do next as you hold these two impossibly small and hexagonal shaped screws in your hand. Usability wise, it's a terrible system that should never have been approved as an industry standard. Aside from its physical indignities, it's an analog system in a digital world. Your video card converts its digital signals into an analog signal which is fed over the wires to the electron guns in your CRT. If you happen to be using a projector or an LCD display, the signal is converted BACK into a digital signal and then continues along its way. The signal bandwidth is limited, and therefore the resolution possibilities. It's an outdated technology. However, considering it IS a standard, it will be with us for a long time. The next iteration was DVI, which you may know as a rectangular white connector. This had the advantage of providing digital output for higher resolution displays with higher quality. There are several versions, including DVI-D, DVI-A, DVI-I, and dual link. I'm not going to go into those variations here except to say that DVI was definitely an improvement all around for display quality and versatility. However, one major drawback is that the DVI port is physically huge. This limited its use on laptops as it not only required extra circuitry to implement, but it also meant giving up several other ports in order to make room for it. Thus, most vendors included it only as part of external port replicators or docks. DisplayPort is the evolution of digital video technology, and has been badly needed for a long time. It has significant cost advantages over both VGA and DVI as it requires less pins and wires for the same display resolutions. Logically it is the same as PCI Express, so it is very easy to integrate for "free" into chipsets. An easy way to think about how it works is to think of it as sending "packetized" video much like TCP/IP. This allows for some pretty cool things to happen. One, it can carry more than just video. The specification allows for audio transmission over the same cable. It has a high speed back end channel that can carry non-video data like USB data. This way displays with a built in USB hub would not need a second USB cable to enable that hub. What is even cooler is that this packetized video technology allows for future enhancements like daisy chained monitors. Today if you want quad display support, you need four video out ports on your video card for each display you want to connect. With the next generation of DisplayPort, you could run one cable from your video card to your first display. Your first display could then directly connect to your second display which connects to your third, and so on. There is much less cable clutter this way. Plus, the spec allows cable runs up to 15 meters, and there are no screws in order to connect the DisplayPort to the system. Now of course, backwards compatibility is very important for any standard to succeed. DisplayPort can use an adapter to convert its signal to DVI and, with the right converter, can convert to VGA signals as well. It also easily converts to HDMI. In fact, the two are so similar that many have asked, "Why are you using DisplayPort instead of HDMI on your ThinkPad notebooks?" To put it simply, HDMI is a consumer standard whereas DisplayPort is a commercial standard. Despite some technical details which I'll explain in just a moment, the reason they coexist is due more to plain old greed than anything else. DisplayPort is royalty free while HDMI is a paid, licensed standard. In other words, every time an HDMI port is put on a system or device, that manufacturer has to pay a few cents back to the HDMI consortium in order to be able to use that port. It very quickly adds up to real money once you're talking about millions of systems. Thus, the DisplayPort consortium was formed. In addition to Lenovo, other members of this consortium include our rival PC companies as well as Intel and Microsoft. Other than that, technically there are very good arguments for using DisplayPort instead of HDMI for commercial systems. As I mentioned earlier, DisplayPort is easily implementable in chipsets while HDMI requires extra logic circuits in order to make it work. Also, DisplayPort uses more wires to achieve higher resolutions while HDMI uses higher clock rates for higher resolutions. A VGA resolution display requires one DisplayPort lane or ¼ the number of wires as it would on HDMI. HD video 1920 x 1080 requires only two lanes, or ½ the number of wires as HDMI would need. Fewer wires equals less manufacturing cost and complexity. Since DisplayPort uses packetized video, it allows for future extensions like daisy chaining. HDMI is a 1:1 connection which means that every device needs a physical connection back to the source to work. And, since an adapter can easily convert between HDMI and DisplayPort, it can coexist in a digital home theater world. You'll notice that we haven't eschewed HDMI completely. Our ThinkPad SL series includes HDMI ports instead of DisplayPorts on purpose. Since our SL lineup is designed for small business and is more likely to be connected to a home theater system than a non-SL ThinkPad, the right choice was HDMI for that lineup. Though readers of this blog are just as likely to connect their ThinkPad notebooks to their HD TVs as to high end digital displays, the vast majority of ThinkPad notebooks will never be used this way. They will, however, be connected to external displays. Since PC desktops will be using DisplayPorts as well, it makes it much easier to coexist. International Data Corporation (IDC) is very bullish on DisplayPort adoption over the next few years in the commercial space. You'll very quickly see it take the PC world by storm.