We were recently contacted by Raymond Wang via our YouTube channel (see pinned post on our LG 34GK950F video review) who has developed a novel method of input lag assessment using basic equipment which many users will have at home. His tool, appropriately named Display Input Lag Tester (download here via Google Drive) allows users to have an LED of choice on their keyboard (num lock, caps lock or scroll lock) light up when they click their mouse button. They can then use a high speed camera (such as the ‘slow motion’ recording feature found on some modern Smartphones) to calculate the time delay between the mouse button being clicked and the screen reacting to this input. In other words, it gives users an indication of the input lag of their display. This can be calculated if the video is reviewed or edited using software that allows the user to see individual frames of the video, which includes the player integrated into Windows 10.

The principle is essentially similar to signal generator based input lag assessment tools, Leo Bodnar’s device being the best known iteration of that. These tools would require specific investment by the user for something that would have no other use to them. Furthermore, they’re usually inflexible in terms of the assessment conditions (e.g. a specific test program run at a certain refresh rate and resolution). This sort of ‘self-contained’ test has some advantages over methods such as SMTT 2.0 or the less reliable ‘stop watch and camera’ method, in particular that it doesn’t require multiple screens to be set up in clone mode. The Display Input Lag Tester tool is not only self-contained, it does not restrict the user in terms of the testing environment. They could run the monitor at any refresh rate it supports, any frame rate and in any specific application. This sort of flexibility is very welcome, so we took the tool for a spin to see whether it could provide reliable measurements that were at all comparable to something like SMTT 2.0.

Our intial testing using a Dell S2719DGF and this tool has been very promising indeed. Using a Samsung Galaxy Note 9 with 960 fps slow motion video capture and a Rocat Arvo gaming keyboard (low latency, caps lock LED lights up with very little delay), we found the tool did indeed offer useful data and if used in an appropriate way provided strikingly similar results to SMTT 2.0. The video below shows three separate tests, which are explained below the video.

The first test has the monitor running at 155Hz with the monitor running Microsoft Paint fullscreen such that a left click closes this view and returns to the application. The delay between the keyboard’s caps lock LED first lighting up (‘zero point’) and the monitor first exhibiting a change on the screen was calculated. This method of assessment minimises the impact of the response time element of input lag (‘element you see’) and gives a better indication of signal delay (‘element you feel’) which is really what most users are interested in when it comes to input lag. Bear in mind that monitors typically refresh from top to bottom and you’re able to see (more or less) when the monitor first starts reacting to the input without having to wait for lower regions of the screen to refresh or the pixel response itself to complete in its entirity. This video just shows one run of the test, but repeating it several times and averaging is advisable. After 10 repeats ~3ms of input lag was calculated which is surprisingly close to what we calculated using SMTT 2.0, with minimal variation between each run. The second test shows the same thing, with the monitor set to 60Hz. ~8ms of input lag was calculated (averaged over 10 runs) which was again consistent with our findings on SMTT 2.0.

The third test is a particularly interesting one as it uses an in-game example. Specifically, Battlefield V run in single player with the monitor set to 155Hz and the frame rate limited to 144Hz. FreeSync is enabled on the monitor. It’s actually connected to an Nvidia GPU and using Adaptive-Sync or ‘G-SYNC compatible’ mode, which is actively being engaged due to the frame rate being below the static refresh rate set (155Hz selected, game running at 144fps, monitor running at 144fps with Adaptive-Sync). The time between the LED first lighting up and the weapon showing signs of life, which in this case includes a huge decal on the wall directly in front of the weapon, was calculated as ~15ms average over 10 runs. Note that this can’t be directly compared to the earlier tests as you’re assessing the centre of the screen, are running a game and subjectively assessing what you believe is the start of a series of animations. However; you can cross-compare by testing using the in-game method and changing specific conditions. For example having Adaptive-Sync (e.g. FreeSync) active vs. the game running with Adaptive-Sync off. You can then establish the input lag penalty of activating FreeSync on the monitor.

We will continue to assess monitors using the Display Input Lag Tester and will cross-compare with SMTT 2.0 for our own records. If we are satisfied with the accuracy of the methdology we will consider incorporating it into our reviews officially. It would be particularly interesting to include a measure of the latency penalty of applying a Variable Refresh Rate (VRR) technology on a monitor, for example.