Why is DC-125 not recommended for HD video?


#1

Why is it that the DC-125 is not recommended for HD video? The bandwidth of USB 2.0 (480 Mbps) should be enough to handle 1920 x 1080 HD. Will this limitation be removed with USB 3.0?


#2

Hi Chris,

It’s a good question - thanks for asking.

Without compression 1920 x 1080 x 24bpp x 30 fps is around 1500 Mbps - at least 3 times the theoretical max of USB 2.0 (480Mbps). And many things eat into the theoretical max, so really it’s more like 8-10 times over in practical, real-world situations. DisplayLink’s chips and software do a great job with lossless pixel and lossy frame compression, but in short with USB 2.0 you’re always pushing the bus to get up to 1920x1080 HD video. Some users will be happy with the result, some won’t - and we want to protect users who are sensitive about video quality from being disappointed with the product.

It’s for all those reasons that we don’t recommend 1920x1080 motion video playback on any USB 2.0 product (even while some other products with the same technology do claim it).

By contrast, web and desktop apps (with much lower fps demands) are fine, and that’s why our UGA-2K-A adapter (up to 2048x1152) is our best selling USB graphics adapter - because people are using these to scale out to 3+ monitors for normal web and applications - and for that kind of scenario being able to plug in additional monitors at will with USB is wonderful.

Beyond the general background, with the DC-125 in particular, there’s a very specific limit – the DL-125 chip in the docking client is limited to 1280x1024/1440x900. It simply can’t set a higher mode. On a higher-res monitor (like 1920x1080), Windows will find the best mode that fits within the capabilities of both the monitor and the DC-125.

Using the DL-125 chip in our DC-125 dock has a bunch of benefits: lower cost, a dock that can be powered only by USB, more consistent performance across all usage scenarios (because the lower resolutions don’t push the USB bus as hard), and much better scalability as a USB thin client (where many DC-125s are in use on one machine). It comes at the sacrifice of running lower resolutions on higher resolution monitors. But overall, it makes this an elegantly simple and lower cost product which fits the demands its designed for.

And, last, you’re right - USB 3.0 will be a nice jump in performance, where 1920x1080 will be no problem (almost without any compression). Those products are coming, likely late this year.

A lot of background, but it’s a common set of questions, so I hope it’s useful.

Thanks for asking!
Bernie


#3