What causes ghosting in LCDs and which settings or exterior circumstances have an impact on ghosting?
Autor: Johannes Post
06/11/2005, 08:27 CET
Ghosting has two different causes which add up:
1. The inertia of liquid crystals in LCDs causes switching from one color to another to take some time.
2. In CRTs, each pixel flashes for a short amount of time once every vertical cycle. In contrast to that, an LCD's pixel maintains its brightness over the complete period of the cycle. This certain property of LCD results in the so called "Sampling and Hold" effect. It makes smoothly moving objects or pictures appear out of focus when the movement is followed with one's eye. The resulting streaks (motion blur) have nothing to do with the inertia if liquid crystals, which means that even if liquid crystals switched without any delay, there would still be these S&H ghosting effects.
The following list is meant to give an overview on outside influences and the effect that they have on ghosting:
"System configuration", "hardware" (how fast is the computer?)
If a game requires too many resources to run smoothly on the PC, ghosting will be perceived differently than when it would run smoothly. Especially in smooth movements, ghosting can be recognized particularly easy because the eye has less trouble following the movement.
"The utilized graphics port (analog/ digital)"
Once more, we want to point out that the belief "that there is more ghosting in analog mode than in digital" is nothing more than a myth. Whether connected analog or digital, the picture is always transmitted from the graphics card to the monitor in "whole frames". Of course there are differences in the way the content is displayed: The monitor can "interpret" voltages " as brightness values to be different than those that graphics card originally sent out, which means there will differences in brightness, contrast and gamma values of course. Furthermore the image quality in analog mode seems slightly less sharp. But this deviation is nearly unperceivable in high quality monitors. If there is really more streaking visible in analog mode than in digital mode, it is primarily because of the differences in contrast, brightness and gamma and this can usually be correct by making adjustments in the OSD.
"The kind of DVI cable"
There is definitely no influence the cable could have on ghosting. At most it can influence how "secure" the data transmission is, meaning that a kind of "electrical storm of pixel" could potentially be the consequence of a low quality cable.
"Brightness/ contrast" (+ gamma):
These can have a substantial effect on the amount of ghosting, since streaking varies according to the different brightness transitions. This is dependent on the response time gradient of the panel. For the most part ghosting decreases at high contrast settings. Some graphics card drivers (nVidia cards) have the option "digital vibrance control" causing a non-linear increase in contrast. Many users have reported that this has significantly diminished ghosting.
This does not make any impact on the occurrence of streaking. Of course there are quite many graphics cards out there that deliver a cheap analog output signal. This indeed brings the picture out of focus or makes the color appear paler. But with DVI, the graphics card can only become a ghosting determining factor in terms of performance (see above: "system configuration")
Affects the system performance. Additionally there can be differences in configurability. (gamma gradient, "digital vibrance" vSync, and so forth…) But other than that there should be no impact on ghosting.
Please refer to the LCD FAQ for detailed information. vSync is more about smooth animation (tearing, frame drops) than about ghosting and therefore can be interesting for every gamer, of course. vSynch affects the smoothness of the animation and by that its influence is indirect (see above: "system configuration").
"Refresh rate" (=vertical frequency)
This actually makes a certain impact on the amount of ghosting, but it is rather complex. However, one can't generally say that "more is better" or "less is better". Furthermore, it should be considered that many LCDs have an internal, fixed frequency at which the panel is updated. So it wouldn't make sense to use a different refresh rate with these models anyway.
In principle, every LCD has only one ideal resolution. The influence on ghosting can arguably be ascribed to subjectivity of perception for the most part since interpolated resolutions seem somewhat blurred anyway. Maybe ghosting doesn't stand out as much then.
Can not be related to ghosting, but affects performance.
When an image looks "smoother", maybe one can't see the streaks as clearly. Thus anti-aliasing can lead to a very huge performance drops. Users enabling anti-aliasing will definitely obtain frame rates that are way below those with AA disabled.
Affects the perceived contrast/ brightness of the panel and can therefore influence the perception of ghosting in a certain way. From our experiences, ghosting becomes more visible in dark environments.
"Environmental temperature" (heavy impact)
The warmer, the better! Those who want to see some really heavy ghosting, should try and set up his LCD in an ice cold basement!
"Distance to the LCD"
The nearer one is sitting to the LCD, the easier one can recognize them"
"For how long is the LCD in use"
Warmth of the panel is influenced by the hours of operation. (see "environmental temperature")
"Who is sitting in front of the LCD, and how fit does he feel right now?"
The perception of ghosting is a very subjective matter! For the most part, various persons perceive ghosting to a different degree. These differences in perception can be enormous!"