This article applies to:
Duplicating your monitor without a splitter is not recommended for timing purposes. To ensure timing accuracy, the participant needs to be viewing Display Index 1 (i.e., the primary display).
When relying on the cloning/mirroring display mode in Windows, determining which display is considered primary may not be consistent between computer configurations. Also, vertical blank synchronization is not accurate between multiple displays. The reason is because when using a machine that has multiple display outputs, the current technology is that each of the outputs has its own pixel clock. These pixel clocks are configured that 99% of the time they do not communicate and the primary display's "refresh request" is given precedence in the cycle. Because of this, multiple-output video cards should not be used in clone mode if display synchronization and timing is a concern. Sometimes, these concerns manifest as visual anomalies, such as display tearing on a secondary monitor.
It is possible to share the same video output to two screens with display synchronization with the use of a DVI splitter. The reason this is possible while the multiple outputs or not is related to the pixel clock. With a single display output, only one-pixel clock is used, and the display is pushed out to the splitter and therefore the monitors at the same time. However, working with a DVI splitter requires a few things to be considered so synchronization is possible.
First, both monitors must be able to display at a refresh rate intended by the video card. This means that a video card pushing out a 120Hz video signal cannot be split to a monitor capable of 60Hz and a monitor capable of 120Hz. A video card can push a 60hz signal to both of those monitors without issue (the 120Hz monitor double samples here).
Second, the video card must be capable of pushing out the correct refresh rate. This means that a video card only capable of 60Hz is not able to push out 120Hz video to two 120Hz capable monitors.
Lastly, (and likely the most overlooked one if you are using higher refresh rates and/or larger resolution monitors) the splitter must be of high enough quality to push out enough bandwidth from the video card to the two monitors. Lower quality splitters are fine for 1024x768@60Hz on two monitors. However, if you are looking at displaying 1920x1080@120Hz, you likely need a higher-quality, powered DVI splitter as lower quality ones do not have the necessary bandwidth to push through that amount of information. In general, it is best to use a powered splitter.
NOTE: PST does not recommend a specific video splitter. If your graphics card supports dual link DVI, lower quality splitters should work.
While: 1. the primary monitor designation may remain the same over time, and 2. these factors allow for possible vertical blank synchronization, a splitter does not always guarantee either of these two outcomes. You need to run timing tests on your own setup. Examples of display timing tests can be found in TIMING: Verifying your Clock and Timing in E-Prime .
If vertical blank synchronization is a concern, such as in multi-subject experiments, you need to run a timing test for both monitors at once. If vertical blank synchronization is not a concern, such as when you only need to follow along as the subject progresses through the experiment run, you still need to run periodic timing tests on the subject monitor, as driver updates and reboots can change which monitor's pixel clock has precedence. All the factors that can impact these outcomes vary across setups and can change without warning, so we recommend you test with some regularity over the course of a given study.
If you use a laptop, all-in-one or Windows tablet, you are often unable to: 1. split the video signal going to the built-in display, and 2. designate any other monitor as the primary monitor, as long as the built-in display is also still in use. To display the same visual content at the same time with any expectation of good timing (before you account for the other factors), you therefore need to use an available display output port to connect a splitter, then connect that to two external monitors. If the laptop, all-in-one or Windows tablet has dual graphics, you also often need to prevent graphics switching scenarios, as we describe in ERROR: Graphics Switching may Lead to Erratic Behavior . Given these and other additional considerations, PST recommends desktops over laptops, all-in-ones or Windows tablets for multi-monitor setups.
NOTE: Most of this article has revolved around the use of DVI for your display source. This is what PST has tested in-house because of the work necessary for the implementation in the Hyperion system. With DisplayPort, it may be easier to synchronize than DVI because of the internal functioning of the technology, but PST has not tested this functionality. DisplayPort splitters may be capable of Single-Stream Transport (SST) mode and/or Multi-Stream Transport (MST) mode. At least some MST splitters create an additional Display in Windows, and therefore still rely on Windows cloned/duplicate/mirrored mode. Further investigations into these technologies are pending, but for now we recommend SST splitters to avoid this reported scenario. If necessary, HDMI should work as well and is preferred over VGA. With the digital signals (e.g., DisplayPort and HDMI), you can convert to DVI and use a DVI splitter. VGA uses an analog signal and is not recommended if possible. VGA may suffer from artifacts and need the clock and phase manually adjusted in the monitor settings.