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Here Comes Yet Another Digital Interface

Yes, Virginia, there's a second digital successor to the digital successor to VGA jacks. The Digital Visual Interface (DVI), which was developed in the late 1990s for ?smart? connections between computer display cards and monitors, will be replaced by an even smarter interface ? one that can carry both video and audio at high bit rates.

Yes, Virginia, there's a second digital successor to the digital successor to VGA jacks. The Digital Visual Interface (DVI), which was developed in the late 1990s for “smart” connections between computer display cards and monitors, will be replaced by an even smarter interface — one that can carry both video and audio at high bit rates. (But don't blink your eyes because this new interface isn't a “lock” by any means.)

DisplayPort, which was developed by a consortium of companies and the Video Electronics Standards Association (VESA), went into the approval process over a year ago, but has yet to appear on any computer monitors or projectors. It takes DVI to a new level, particularly in terms of bandwidth.

DVI, which comes in two flavors — single-link and dual-link — has a maximum bandwidth of 340 MHz using all 24 pins of the connector (dual-link mode) for transition-minimized differential signaling (TMDS). That means you can display a maximum resolution of 2048x1536 with a 60 Hz refresh, or 1920x1080 pixels with an 85 Hz refresh.

That's nice, but we're already starting to see LCD monitors on the market with much higher resolution on the market, not to mention 4K front projection systems (4096x2160 pixels) with fast refresh rates. Refresh such a display at 60 Hz, and you'll need a lot more system bandwidth to pass all of that data.

In contrast, a 1920x1080 signal progressively scanned at 60 Hz, which seemed like a lot of data seven years ago, only needs about 25 percent of what's required for 4K imaging. So it's pretty obvious that DVI won't cut the mustard for future high-resolution imaging.

How does the High Definition Multimedia Interface (HDMI) enter into the picture? HDMI, developed by Silicon Image, was adapted from DVI-D and included copy protection, but added the ability to carry up to eight channels of digital audio. HDMI 1.1 and 1.2, which use a much smaller multi-pin connector, offer 5 Gb/s or 165 MHz of bandwidth — the same as DVI single-link.

The catch? HDMI was designed to be compatible with digital video standard display rates, such as 480i/25/30, 480p/50/60, 720p/50/60, 1080i/25/30, and 1080p/50/60. (Also 1080p/24, /48, and /72 Hz, for those who want pure multiples of standard 35mm film and digital production rates.) That list doesn't include PC-standard display rates.

To make matters worse, HDMI V1.3, which was just released this past June, now has the capacity to carry up to 10 Gb/s of data (340 MHz) in single-link mode and ostensibly double that rate in a dual-link configuration, opening the way to even higher resolution in consumer HD displays.

Enter DisplayPort, which also offers double the bandwidth of DVI single-link (10 GHz) mode, and can also carry multiple channels of audio. In VESA's August 2005 press release, the organization said: “…DisplayPort enables a common interface approach across both internal connections, such as interfaces within a PC or monitor, and external display connections, including interfaces between a PC and monitor or projector, between a PC and TV, or between a device such as DVD player and TV display....”

Wait a minute. Isn't that last description a job for HDMI? You would certainly think so.

Both HDMI 1.3 and DisplayPort also support enhancements such as Deep Color, which is the ability to show expanded color palettes that go beyond such industry standards as NTSC, PAL, REC.601 (SDTV), and REC.709 (HDTV). If your display is capable of rendering wide color gamuts (if it uses color filters or LEDs), then it could signal the video source to send the expanded color palette information instead.

That, of course, assumes the content was encoded with sufficient color space. Deep Color won't do you any good if the content you're watching was originally transferred and coded to a phosphor-based space like REC.601 and REC.709.

In any event, we seem to be headed down parallel roads with HDMI and DisplayPort. Both systems use a small, multi-input high-bandwidth connector to carry video, audio, EDID info, and other goodies, plus a secure, copy-protected handshake.

That's right. DisplayPort will also incorporate a copy protection system, just like HDMI. And that probably makes sense because they're all digital bits, regardless of the resolution at which they're displayed. Home media servers, which are essentially PCs, would be likely to make use of DisplayPort as opposed to HDMI.

That's all well and good, except another player has entered the picture. A group of manufacturers, including Apple, Samsung, Intel, LG Electronics, and Silicon Image, have now endorsed the Unified Display Interface (UDI) as a replacement for DisplayPort, which hasn't really come to market yet. (Is technology really moving that fast?)

So, are we re-inventing a re-invented wheel? Why do we need UDI if DisplayPort is supposed to do the trick? Well, you and I may not need UDI, but one company — Silicon Image —apparently does.

The reason? It has no stake in DisplayPort, which in theory could do everything HDMI does, and cut out valuable royalties and intellectual property income. The list of companies behind the DisplayPort initiative includes ATI, nVidia, Dell, Genesis, Hewlett-Packard, Tyco, Molex, and Philips.

There are some similarities between UDI and HDMI. The latter has 19 pins (20, counting the shell), while UDI has 19 pins with three extra pins reserved for additional functionality. The outer shell is similar in size and shape, although there's apparently enough of a difference that the two aren't swappable.

In response to complaints that HDMI cables can easily pull out, Silicon Image is looking into a more mechanically secure version of the plug. The folks at VESA already beat the company to it, with a push-to-unlock design on its DisplayPort connector. Presumably, UDI will incorporate a similar feature. The UDI Special Interest Group (SIG) even has a PowerPoint presentation on its website detailing the rosy future of HDMI and UDI, showing both connectors being used to wire up a home theater system with a media player and network.

In essence, what we're looking at is another format war, not unlike the HD-DVD and Blu-ray competition. The question is, which manufacturers will get behind and support which formats? Having Dell, Philips, ATI, Genesis Microchip, and nVidia behind DisplayPort gives that contender serious weight. In contrast, UDI has only Apple, Intel, and Samsung — three unlikely bedfellows to begin with because Samsung makes Windows PCs with Intel processors, and Apple is, well, Apple.

What this really means to you — besides another AV interfacing headache — is that all digital display devices in future systems (and many audio interfaces, too) will be migrating to one of these plug-and-play, two-way secure interconnects.

Gone will be the three-wire, four-wire, and five-wire BNC plugs, composite and S-video cables, and even those 15-pin VGA plugs. Gone too, eventually, will be the DVI connector, which seems like it just arrived only a short while ago. Perhaps those folks with crystal balls back in 1998 could have looked a bit further in the future and saved us a lot of aggravation by working toward one common standard for all displays, instead of this, “wait, I've got a better idea!” approach we've wound up with.

No matter how it plays out, video and audio interface companies stand to make lots of money filling in the blanks until the dust settles. And you'll learn to flinch every time you hear the words, “display standards.” Got aspirin?

Pete Putman is a contributing editor for Pro AV and president of ROAM Consulting, Doylestown, PA. Especially well known for the product testing/development services he provides manufacturers of projectors, monitors, integrated TVs, and display interfaces, he has also authored hundreds of technical articles, reviews, and columns for industry trade and consumer magazines over the last two decades. You can reach him at pete@hdtvexpert.com.



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