USB Monitors? DisplayLink's Technology, Examined

DisplayLink Explored: What's Going On?

USB is ubiquitous. Mice, keyboards, printers, iPhone, webcams, external hard drives, and even external optical drives are available with a USB interface. USB 2.0 (480 Mb/s) is still unquestionably most prevalent, but you're going to see even more exciting, performance-oriented options emerge now that USB 3.0 (5 Gb/s) is being integrated on an increasing number of motherboards and chipsets.

DisplayLink's technology lets you connect a display using the USB 2.0 bus and adapting to one of its native inputs (like DVI, for example), or it lets you send the display signal across USB to a display capable of accepting USB input. Thus, you'll find DisplayLink in adapters, docking stations, USB-based monitors, and projectors, too. But no matter which device you're using, the underlying process is the same:

  1. Encoding video on the host system
  2. Sending data
  3. Decoding data on the USB device

On paper, DisplayLink claims it supports full 32-bit colour at resolutions of up to 2048x1152, but according to the company, USB 2.0 doesn’t provide enough bandwidth to support resolutions above 1024x768. It's currently demoing a version of the tech able to capitalize on USB 3.0, but that's not ready for prime time quite yet. 

As a result of USB 2.0's limitations, video data must be compressed using a proprietary algorithm. Much of this process is undisclosed because it comprises the company's secret sauce. However, we were able to pick the brains of the DisplayLink engineers to get a better idea of what it does under the hood.

The first step of the process, encoding, happens in two stages. First, static video information is stripped from the signal. The idea there is to reduce the amount of data the system sends because transmitting more information costs energy, processing overhead, and precious bandwidth. Then, the video is actually encoded. In practice, DisplayLink divides the screen into sections and only encodes the video data for the sections that change. As a result, moving your mouse across the screen requires far less processing than watching full-screen video.

All of the encoding is done on the host (it's not offloaded to DisplayLink's logic). So, the workload is always the same on every DisplayLink-based device. The only factors that affect the processor workload are resolution and the on-screen activity.

Once the video data is stripped and encoded, it’s sent over USB to the DisplayLink device. This is where the company's hardware decompresses the stream and outputs a standard VGA/DVI signal. According to DisplayLink, all of the technology on the decode side is homegrown, which is another component of the secret sauce the company won't discuss any further.