LCD vs Plasma in 3D: the case for plasma
Panasonic is famous for making some of the world's best plasma screens, while also dishing out a fair helping of top-end LCD TVs too.
However, the Japanese giant says it believes plasma to be by far the best tech to use with 3D, and that's why it has limited its 3D output to its VT20 and GT20 plasma TV ranges thus far.
"Obviously we think that plasma is better for 2D so this already implicates that it would be better for 3D as well because for a TV, 3D is much more difficult to handle," Markus Wagenseil, technical marketing manager for Panasonic, tells TechRadar.
"On the TV, [crosstalk] depends on how fast the TV can swap from the left eye image to the right eye image and then back to the left eye image again. It has always been the case that the response time of a plasma is much, much quicker than of an LCD.
"So we're saying that we have a response time of around 0.001 milliseconds, so basically there is no response time, you put a charge on a cell and it will light up.
"We don't say that plasma is completely crosstalk free because if you light up the plasma it needs some time to afterglow. To reduce that we developed a short stroke phosphor for our 3D TVs which has a decay time of only one third of that in a conventional plasma. So with that we reduce the crosstalk from white into black.
"But there is a second type of crosstalk - from black into white - because an LCD always has to twist the crystals whether it needs to switch from black to white or white to black. And from black to white is a much more crucial crosstalk because LCD produces up to 35 per cent crosstalk when the pixels are changing from black to white, while plasma has zero crosstalk in this area."
The role of heat
Wagenseil also highlighted another factor which affects crosstalk in LCD panels, which is heat.
As an LCD TV heats up, the liquid crystals in the panel become less viscous and are thus more agile.
This means a cold panel cannot perform as well as a hot panel, because the liquid crystals cannot twist as quickly, which causes crosstalk issues.
"Another thing you don't have with plasma is that you have no run-in time in terms of heat," says Wagenseil.
"3D LCDs need a run-in time of between 60 and 90 minutes in order to achieve a reasonable quality, but with plasma you can just switch it on and it's maximum quality straight away."
And Panasonic doesn't believe the advantages of plasma over LCD stops there either. Wagenseil says that the glasses used with 3D LCD TVs are also responsible for creating more crosstalk than those compatible with plasma displays.
"The final major implication is with the glasses," says Wagenseil. "LCD is outputting polarised light while plasma outputs unpolarised light. So what do you think happens when someone wearing 3D specs moves their head?
"With unpolarised light nothing happens. Basically you can turn the glasses all you want and they will work in any direction. With an LCD, the glasses will shut down if you angle them to 90 degrees because the polarisation angle of the glasses is then not aligned any more with the polarisation angle that's outputted by the TV.
"Another competitor found that it's a very good idea to put a circular polariser on the glasses and just leave the front polariser on the glasses away. So with that, their one advantage is that they don't lose so much brightness. But there's a serious issue coming with that because only inclining your head by 10 to 15 degrees you already end up with 20 per cent crosstalk by default.
"And if you incline the glasses 90 per cent the closed lens actually lets in double the amount of light than the open eye. So there are lots of implications for LCD which makes it really hard to fight for."
Eliminating crosstalk
All TVs these days come packing mighty image processors, and so 3D TVs use those processors along with some complex algorithms to identify where crosstalk is likely to occur, and to adjust the picture accordingly.
The main picture adjustment that a TV can make to manage the crosstalk issue is to reduce brightness in the parts of the screen where crosstalk is going to occur. By temporarily reducing brightness in these areas, you reduce crosstalk and make what remains as well hidden as possible.
But Wagenseil says that even with powerful processors, LCD TVs are still riddled with crosstalk problems.
"If you think, with an LCD now you have a warm up time of between 60 and 90 minutes plus the panel has different temperatures wherever you look at them because in front of the power driver it's very hot and on the outside areas of an LCD there's not as much temperature. So looking at that, as an LCD manufacturer, how do you adjust your crosstalk compensation to so many variables. It's not going to happen perfectly."
