It’s Harsher On Plasma TVs
When we first got our hands on the Leo Bodnar device, we were surprised when we obtained (nearly) the same 48ms figure from a Panasonic ST50 PDP (plasma display panel) and a new Panasonic ET60 LED LCD (both running in their fastest Game mode). From our experience of playing a decent amount of first-person shooter games online, the Panasonic ST50 is a total joy to play on compared to the LCD. The former feels considerably smoother than the latter, but both are returning basically the same figure.
(...) An LCD-based display updates the screen from top to bottom, one line at a time, which means that a player’s brain cannot make sense of a part of the image until it has been completely rendered. The LCD’s top-to-bottom addressing can be seen with the Leo Bodnar lag tester: measuring the top patch tends to give a lower number than measuring the centre patch from our tests. However, on a PDP, the result is always the same on both patches.
Because plasma displays work by flashing the screen several times just to draw one video frame, on a PDP, an intermediate image doesn’t look half-drawn in the same way that it would on an LCD. Instead, it would have very low gradation (and brightness). In theory, this means that the player has a better chance of seeing the entire gameplay screen, albeit not at full quality, since the subfield drive throws out different steps of the dynamic range quickly just to draw one fully-gradated frame.
This is the key difference. On the LCD, obviously our eyes can’t make sense of parts of the frame which haven’t been drawn yet (parts of the frame are either fully rendered or not), but on the plasma, we get extra temporal precision in the feedback loop, since we can see rough versions of the frames before they’re even fully drawn. And, in a fast-paced game, our brain doesn’t care if it’s seeing incomplete images – it should still be able to make out rough outlines and shapes.
The incomplete frames don’t necessarily even have to be coherent to our eyes. Even if we can detect the screen responding to our finger movements at all, it should be enough to make the game feel much more responsive.
In isolation, and for slow-paced games, this is all basically moot. But in a first-person shooter (even one which only runs at 30 frames per second) or racing game, etc, where the entire screen is moving and split-second decisions count, we think the PDP’s subfield drive helps tremendously in making the gameplay feel smooth. After all, in reality, playing fast-paced games is a continuous feedback loop between the player and the screen.
How does this explain why plasma televisions that feel much more responsive are shortchanged by the Leo Bodnar input lag tester which returns a higher figure? Well, we surmised that the flashing white bars need to hit a specific brightness threshold before they can be picked up by the device’s photosensor for lag time calculation: if you decrease or increase the on-screen luminance using the TV’s [Contrast] or [Backlight] control, the Leo Bodnar’s lag number should rise or drop correspondingly.
A plasma’s subframe, while not bright enough to trigger the photosensor, can readily be perceived by us in the sensorial feedback loop, thus accounting for the discrepancy between the displayed input lag figure and the actual responsiveness of a PDP.
Ironically, the older stopwatch/camera method – though inconsistent – is capable of capturing subframes before they’re fully drawn (since it’s not limited by any luminance threshold, and the shutter speed is much higher than the panel refresh rate), and so more accurately reflects how responsive a PDP is. This is the reason why we continue to run both tests on most HDTVs we review despite the photo method being such a labour-intensive process.