24" LCD Roundup

Input Lag and Response Times

We've heard frequent complaints about input lag on various LCDs, particularly after our last review of the Samsung 245T. We decided it was time to take a closer look at the subject and see if we could come up with a repeatable test. We ended up settling on a test similar to what we were using to show response times, with a few changes.

We run the Wings of Fury benchmark in 3DMark03, with the resolution set to the native LCDs resolution -- in this case 1920x1200. Our test system is an overclocked quad-core Q6600 (3.30 GHz) running two Radeon HD 3870 cards in CrossFire on a Gigabyte GA-X38-DQ6 motherboard. (This is the same system used in our initial testing of 3DMark vantage.) We connect the test LCD and a reference LCD to two outputs from the Radeon 3870 and set the monitors to run in clone mode.

The reference Monitor is an HP LP3065, which we have found to be one of the best LCDs we currently possess in terms of not having input lag. (The lack of a built-in scaler probably has something to do with this.) [Ed: Before you ask, no, I do not have any CRTs around that I can use as a reference monitor, and frankly I don't want any. They're huge, heavy, and require more power, and the best ones were made over five years ago. Sorry - LCDs are where everything is heading. R.I.P. CRT.]

While the benchmark is looping, we snap a bunch of pictures of the two LCDs sitting side-by-side. We set our camera to f/2.2, ISO-400, and a 1/400 sec exposure in order to get a clear snapshot of the on screen action. (Note that these settings have changed from previous articles.) 3DMark03 lists a runtime with a resolution of 10 ms at the bottom of the display, and we can use this to estimate whether a particular LCD has more or less input lag in our reference LCD. We then sort through the images and discard any where the times shown on the LCDs are not clearly legible, until we are left with 10 images for each test LCD. We record the difference in time relative to the HP LP3065 and average the 10 results to come up with an estimated input lag value.

It's important to note that this is merely an estimate -- whatever the reference Monitor happens to be, there are some inherent limitations. For one, LCDs only refresh their display 60 times per second, so any measurement less than approximately 17 ms is not 100% accurate. Second, the two LCDs can have mismatched vertical synchronization, so it's entirely possible to end up with a one frame difference on the time readout purely because of this. That's why we average the results of 10 images, and we are confident that our test procedure can at least show when there is a consistent input lag/internal processing delay.

Here is a summary of our results, followed by a sample image chosen to highlight the pixel response time of the LCDs. Despite what the manufacturers might advertise as their average response time, we found most of the LCDs were equal in this area -- they all show roughly a one frame "lag", which equates to a response time of around 16 ms.

ASUS MK241H

Dell 2407WFP

Dell 2408WFP

Gateway FHD2400

Gateway FPD2485W

HP w2408

LaCie 324

Samsung 245T

Samsung 2493HM

The table tells a clear story: all of the S-PVA panels as we mentioned clearly have more input lag/internal processing lag than all of the TN panels. This is a pretty shocking result, as it indicates that the problem may actually be inherent in S-PVA technology, although there are still panels that do better in this area. The Gateway FPD2485W and Dell 2407WFP both have an input lag just under 20 ms -- or a one frame delay on average. The Samsung 245T has a 27 ms delay on average, indicating it would be one or two frames behind what is actually happening. Worst of all are the Dell 2408WFP in the LaCie 324, which have a two or three frame lag. (This is discounting any other lag that is present between the user and what they are seeing on the display; there's also a slight amount of lag associated with reading input from your mouse/keyboard, processing that input, rendering the resulting image, and sending that to your display.) In contrast, the four 24" TN panel LCDs all more or less match the HP LP3065.

Thus, we have no choice but to conclude that if you are seriously concerned about input lag, you will have to sacrifice viewing angles, color accuracy, and/or overall display quality in order to avoid this on the current 24" offerings. On the other hand, we have played games on all of the test LCDs, and we honestly can't say that we noticed any difference in our overall performance. But we weren't hopped up on Bawls and we don't have a 1337 name like Fatal1ty. Competitive gamers will probably feel differently.