LCD Display Performance: Input Lag, Response Time, Latency

Want to know a little more about what you might be purchasing when in the market for a new computer monitor? I've written up a quick blurb here of some of the performance problems that have plagued monitors in the past, and what to watch out for when you are purchasing a monitor either for graphics work, gameplay, or general use purposes.

I will mention quite often "panel" references, because quite frankly of the dozens of monitors available today there are in fact only about four major manufacturers of the actual screen itself. It is like the dozens of video card manufacturers that buy the core processing chips and reference board designs from two key companies; ATI and Nvidia.

Here are some initial keynote differences between "Input Lag", "Response Time", and "Latency" for the performance of monitors and other LCD displays:

INPUT LAG

"Input lag is a phenomenon associated with some types of LCD displays, and nearly all types of HDTVs, that refers to latency, or lag measured by the difference between the time a signal is input into a display and the time it is shown by the display.

This lag time has been measured as high as 68ms, or the equivalent of 3-4 frames on a 60 Hz display. Currently, the only TFT panels known to have this phenomenon are so-called overdrive panels. These include S-PVA, S-MVA, and Overdrive-TN panels. S-PVA have been observed to suffer from greater input lag than P-MVA panels, while IPS, S-IPS and AS-IPS panels are not or only minimally affected."

RESPONSE TIME

"Response time is the amount of time a pixel in an LCD monitor takes to go from black to white and back to black again. It is measured in milliseconds (ms). Lower numbers mean faster transitions and therefore fewer visible image artifacts."

LATENCY

"LCD screens with a high response time value often do not give satisfactory experience when viewing fast moving images (They often leave streaks or blur; called Ghosting).

But an LCD screen with high response time AND significant input lag is unsuitable for playing fast paced computer games or performing fast high accuracy operations on the screen (e.g. CAD design) due to the mouse lagging behind. Manufacturers only state the response time of their displays and do not inform customers of the input lag value."

Both of these factors together are often referred to as "Latency".

Until recently these problems were extremely apparent, especially in your typical desktop LCD. They would suffer from either a larger Response Time or a higher Input Lag (but often not both as discussed below).

TODAY'S PANELS

The negative about many of today's panels is the trend towards more complex inputs that requires increased complexity in a panel's circuitry. This is where higher input lag is still apparent in several PVA/MVA panels (which are considered a mid-range performance product) when monitors introduce technologies like DisplayPort.

In positive contrast, most of these panels today have response times well within the acceptable levels of human vision (4ms and below is indiscernible due to the limits of the human eye's nerve impulse speed to the visual cortex).

This is especially true of "TN (Twisted Nematic) panels that offer lower quality overall, more basic "features" such as stand height/swivel adjustments, but at a lower price range as well. Their major advantage beyond price is 5, 4, or even 2ms response times (gray-to-gray).

What I've noticed in the industry recently is a shift towards these "cheaper quality" panels to help continue bringing down the price for the average panel in a series. Unfortunately, manufacturers are thus reducing or even eliminating PVA/MVA, and in some cases the higher-end S-IPS displays from lines that once were prominent mainstream products.

I've also heard about some real quality control issues in the industry recently, some panels that have potential but the monitor manufacturers alter them too much and ruin the overall display. Color banding, back-light non-uniformity, color "stains", poor OCD controls, and even monitors are there physically tilted when placed on a flat desk are just some of the reported problems.

Sometimes I think people like to whine, while sometimes these are truly common problems that are to be of a concern when purchasing a monitor in question. On the flip-side I've also seen some great monitors (if you're willing to pay the price, especially for S-IPS panels).

Those desktop LCDs to watch for extremely high Input Lag are currently the Dell 2408WFP (Rev. A01 just came out which may help) and Samsung 245T (same panel).

Those to watch for TN technology but may not necessarily state TN are monitors that have a lower viewing angle of either 160 or 170 degrees. Those that are 176 "full" viewing angle displays are either PVA/MVA/S-IPS. Reported response times of 2, 4, or 5 can also be indicators of TN technology use.

OUR PERCEPTIONS

I think a lot of this is "subjective" though, as the main monitor I am currently using is a SyncMaster 205BW, and while it is a TN display it really doesn't bother me. Yes, I can notice the common TN issues if I look for them, and a potential lack in color depth compared to my Dell laptop's display, but otherwise it isn't that big of a deal. Your experiences may also differ depending on its use (PC vs. Console, TV vs. Desktop, etc.).

On another personal note I do recall experiencing poor "Input Lag" on my old wireless mice. That was extremely annoying even when I was running typical desktop applications. While I couldn't see the actual lag, I could "feel it", so I know the effect is there in certain cases with these monitors.

What monitors, both for PC and console system uses, do you have and what are you happy or not happy about them? This summer I need at least one good 24" monitor (in the $400-700 price range preferably) and I don't want a high input lag lemon or a risky proposition off of eBay.

No comments: