Video Output Performance
Finally, in order to evaluate the dock’s video output capabilities, we first rounded up two 1080p monitors, one which would accept HDMI input, and the other which used VGA/DVI-D. We loaded our favorite open source short film, Big Buck Bunny, to see how the dock would handle output to one or both monitors versus the notebook’s native video adapter (which by contrast was capable of just single-monitor output when HDMI is involved).
Before we get to our results, it’s worth reviewing what the StarTech USB3SDOCKHDV dock claims to be capable of handling. Specifically, it promises dual-monitor output (thanks to the included DisplayLink DL-3900 chip, which supports DL 3.0) via either HDMI + DVI-D or HDMI + VGA combinations (but not by VGA + DVI-D). The maximum supported resolution for each output is as follows:
- VGA: 2048 x 1152
- DVI-D: 1920×1200
- HDMI: 1920×1080
Once installed, the dock appears in Device Manager like this:
We first connected a single external monitor directly to the notebook and extended the Desktop to the second screen. We then opened Big Buck Bunny in 1080p (on YouTube) and stretched the window so that it was full-screen (1920×1080) on the external monitor. We monitored CPU usage and found an average of around 28% (26% with some occasional spikes to be precise):
Using this as our baseline, we then duplicated this setup leveraging the dock for the video output via HDMI to the external monitor. The same output resolution was used (1920×1080). We monitored CPU usage and found an overall rise of around 10% on average:
Nevertheless, video playback was smooth overall. Given the CPU constraints of our low-voltage test system, we could not render two 1080p videos on two external monitors concurrently under any circumstances (except when mirroring, as this essentially only requires decoding of one 1080p video). However, using 480p resolution video stretched to full screen over 1080p output resolution should theoretically achieve the same results. We did this and found—once again—an increase in overall CPU consumption of around another 8 to 10% (for a total of 18 to 20% higher on our test model when using the dock over DisplayLink 3.0). We were able to discern the effective difference by first playing the videos side-by-side on the notebook’s built-in LCD panel, and then subsequently playing them on the two external monitors.
There is an option in the DisplayLink driver software to Optimize for Video; what this actually does is reduces the effective resolution of the video output to the external monitor, which in turn lowers the overall quality of the output. Text and other sharp elements take on an obviously lower-resolution appearance while this setting is in place. This is particularly useful for weaker PCs outputting 1080p video via the dock to an external monitor, but it only works on Windows Vista and later. It’s not recommended unless absolutely necessary, however, thanks to the quality hit you’ll take.
We also measured the input lag to the monitor using the flatpanels.dk Input Lag Test and came away with a result of around 50 ms total lag. This is around 20 ms higher than the ideal range for docking stations, and while most people won’t be bothered by it, it is noticeable.
Overall, while full-screen dual-monitor video output will quickly bring most mobile systems to their knees due to the commingling of video decoding efforts and the dock’s video compression overhead, it’s extremely unlikely that these circumstances would ever be encountered in reality (how often do you need to watch two videos on two different external monitors while still using your PC for other things?). The more realistic scenario of simply extending the Windows desktop to two external monitors with typical activity taking place (and perhaps the occasional video) posed no such problems for our admittedly new Lenovo T440s test unit. However, even older systems featuring Ivy Bridge and perhaps even Sandy Bridge-era CPUs (the oldest systems likely to feature USB 3.0 ports) should be able to manage triple-monitor in a pinch via the StarTech dock, provided video activity is held to a reasonable level. Just be aware that your battery will take a notable hit and there should be no surprises.