At CES last month, we saw various products touting newly-developed wired and wireless USB interfaces for PC displays. Using existing networking protocols and proprietary software-compression techniques, the interfaces are said to enable the transmission of graphically-rich content between a single device and multiple displays. The bottom line: if you are using a desktop PC to connect to multiple monitors, this is a nice idea, but if you a laptop with a monitor output, you can already easily drive a second display.

Aldo Cugnini
Analyst

According to DisplayLink, a chip manufacturer now providing one solution to various product manufacturers, their network display technology is, "the first to provide the high image quality and performance of VGA/DVI connections over USB and Wireless USB." They go on to say that, "performance includes instantaneous mouse and keyboard movements, output resolutions up to 1680 x 1050 and 16.7 million [24-bits] colors, as well as smooth DVD video playback."

The way this works is through a combination of a high-performance "Hardware Rendering Engine" and "Virtual Graphics Card" software. Basically, the PC runs a display driver that incorporates a realtime video compression encoder. This compressed stream then connects, via the USB interface, to a high-speed video-decoder chipset, resident either in a small outboard adapter, or built into a display. The graphics are compressed using "a unique DisplayLink adaptive technology that automatically balances compression methods based on the content, available CPU power, and USB bandwidth." The user installs drivers to the PC, and then simply connects the display (or even multiple displays) to the PC’s USB port(s). The drivers supplied can operate the new display in either "mirror" or "extend" modes. In the first, the new display simply repeats the PC desktop. In the "extend" mode, the display can form an additional desktop area for the user.

So how well does it work? To find out, a sample unit was evaluated for more than a week. The needed drivers were installed on a PC, and a DisplayLink USB-to-DVI adapter was connected to a second display running at 1024×768 resolution. During that time, the unit was operated in its different configurations, and applied to an "average" daily spread of PC applications including email, word processing, spreadsheets, Internet browsing, and video playback. Our summary judgment: the extended desktop space is a marvelous convenience - the single display is just too confining when multiple windows are used, and the added space will spoil you. With two full-sized displays, it’s convenient to leave a window maximized on one, and use the other for managing multiple windows. We can also attest to the setup and operation being as simple as advertised.

The interface does have its limitations, however. I had a few occasional problems with the drivers when working with video. If you are running Windows XP, you may not be able to mirror video or drag video to the extended desktop for all video applications, unless hardware acceleration is turned off. (I’m told this is not the case with Vista.) Browser lock-up also happened (although very infrequently), but we should expect bugs like this to be cleared up with driver updates. As for the video quality, no blocking or quantization was discernable - supporting DisplayLink’s claims of a lossless compression algorithm. Wired USB can deliver 480 Mb/s at a maximum cable length of 5 meters, and Wireless USB supports 480 Mbit/s at distances up to 3 meters, and 110 Mbit/s at up to 10 meters. At these rates, a compression system similar to M-JPEG should have just enough bandwidth to run at lossless or near-lossless performance.

Of more practical impact, however, is that the drivers must perform a real-time video encoding, often at more than 60 frames per second, and this means there will be an impact on the PC’s CPU utilization. And since video playback always involves software decoding of the source video, both of these processes must run simultaneously when viewing video on the additional display. As an example, playing a 480p24 video in Windows Media Player on my PC’s main display consumes an average of about 50% CPU bandwidth on my 1.8GHz AMD 3500+ processor. When this video is shown on the extended display, the combined processes peg the CPU at 100%, resulting in dropped frames. The problem becomes compounded with added displays.

Of course, this performance will vary widely, depending on the particular PC, and you may not need more than "YouTube" performance on the extended display. (Keep in mind that the wireless version could have applications for TV viewing, using dedicated hardware, and that won’t have any software limitations. Competing wireless TV technologies are already on the market.)

If you are suing a laptop with a second monitor, this new connectivity does not have much value. But if you are using a desktop PC, adding a second monitor requires a second graphics card, so using the USB port is a meaningful value proposition. Since this is the configuration I tried, the initial impression stands: once you work with the extended desktop, there’s no going back to a single, crowded display. If you’ve got the budget and the (physical) desktop space, you’ll wonder how you lived before without it.

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