LCD vs Plasma: which is the best for 3D TV?
9th Sep 2010 | 09:52
We investigate which is the best 3D TV tech for you
LCD vs Plasma in 3D: what is crosstalk?
Building the perfect TV has always been an impossible task for the bigshots of the electronics world.
While the current batch of flatscreen efforts are brighter, thinner and better than ever, the problem is that there's always room for improvement.
The contrast ratio can always be improved on as well as aspects like colour reproduction, pixel density, resolution, brightness, thickness and bezel size, to name but a few.
And that's not even mentioning the new challenges that inevitably get thrown up as new technologies are invented.
The current headache facing Sony, Panasonic and the like comes in the form of a new phenomenon found in 3D TVs called 'crosstalk'.
How it works
To create the illusion of 3D on a two-dimensional screen, a 3D TV needs to show two separated images, one for each eye.
Current 3D TVs show the two separate images sequentially - so very quickly one after the other. Active shutter 3D glasses are then synchronised by the TV's infrared emitters, and close and open the shutters many times per second, in time with the images.
When the left eye image is shown, the right eye shutter closes and so forth.
But if the two images are not separated perfectly, part of the right eye image will be seen by the left eye and vice versa. And this causes crosstalk - a sort of blurry ghosting effect around some edges in the picture.
Crosstalk can occur if the liquid crystals in an LCD panel do not switch fast enough from bright to dark or vice versa, or if the phosphor cells in plasma panels have an afterglow that lasts too long.
3D glasses can also cause some crosstalk if they're not precisely synchronised with the TV or if they are sensitive to an inclination angle.
At best, crosstalk can be an unwanted distraction, while at its worst it can completely ruin the 3D effect and make a 3D picture unwatchable. And it's the battle to eliminate crosstalk from 3D TV pictures which is currently occupying the minds at the big telly manufacturers.
So if you're thinking of buying a new 3D TV, how do you ensure that you buy the TV with the least amount of this disturbing crosstalk?
Many people believe it comes down to that age-old choice between LCD and plasma. So which telly tech is best for 3D?
LCD vs Plasma in 3D: the case for plasma
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."
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."
LCD vs Plasma in 3D: the case for LCD
LCD vs Plasma in 3D: the case for LCD
There are certainly many good reasons for Panasonic to say that PDP is a fundamentally better technology for 3D than LCD.
However, plasma is not without its flaws.
Brightness has always been a problem for plasma TVs - the panels struggle to produce sufficient amounts of light while also keeping power consumption down and maintaining a thin chassis.
This is an area where LCD TVs have always excelled. And with recent advances and the introduction of Direct LED LCDs - where an array of LEDs behind the LCD panel illuminate the screen instead of an always-on CCFL lamp - the sheer amount of light produced is a factor which gives LCD something of an advantage when it comes to reducing crosstalk.
Danny Tack, director for technical marketing at Philips, and named the 56th most influential person in Tech in T3's Tech100, believes that it's LCD's ability to produce more light that makes it a better technology for 3D.
The Philips approach
"Crosstalk is the new thing which you need to keep under control. The way you can keep it under control is by playing with light and some clever processing," says Tack.
"My opinion is that you can have good quality with plasma, but you get better 3D quality from LCD because there is much more light. There are two key facts which we think makes plasma weaker for 3D than LCD.
"For starters, the light that comes out of a plasma is not polarised. It comes out in all directions. In LCD, it's polarised already coming out of the screen. So in LCD you get 500nit polarised light out, and in plasma you get less than 500nit and it's circular light, it has to go through the glasses which are polarised which means with LCD you don't lose light and with plasma you do.
With plasma's glasses you only get half the light through because you only get light from one direction, you lose 50 per cent brightness. That's a fact."
Tack also believes that contouring is also a big problem for plasma 3D TVs. Contouring - more commonly referred to as posterisation or solarisation - is a phenomenon seen in older plasma screens where a gradient of colour is not produced accurately enough, resulting in the appearance of ugly colour bands with edges that aren't supposed to be there.
"Yes, plasma has done a great job, year-by-year increasing their sub-fields, went up to 10 and got no contouring. They solved the contouring problem almost completely," Tack says, "but going now to 3D, you have two pictures at once. They claim full resolution per eye, but now actually with the sub-fields you need to divide them over two pictures so you get per picture now only five sub-fields. That is going back in time, we know that when you have less sub-fields, you have contouring. So that is a problem that comes back in plasma."
A sub-field is something a plasma panel uses to pulse illuminated cells repeatedly while each frame is being displayed. A 600Hz sub-field drive is able to pulse each cell/pixel ten times per frame, creating smooth motion.
The scanning backlight
Tack also says that while edge-lit LCD TVs have a comparable amount of crosstalk to plasmas, Direct LED TVs can have much less crosstalk as a result of a scanning backlight. A scanning backlight is able to reduce and increase brightness in parts of the screen to manage the crosstalk problem in realtime.
"I would do the same if I was the Panasonic guys, I'd bring up response time. Indeed we all know that certain colours in PDP have a very fast response, but others have a lot longer so a slower response time.
"For me, the key things when it comes to good 3D is you can reduce it to two elements. It's getting the crosstalk under control, and getting sufficient light. On those two elements, with light, LCD is definitely in the advantage because we can produce much more - we can create a huge amount of light and we can use that advantage over plasma by trading it off. How do you trade it off? By scanning, you lose light by that but scanning definitely will improve your crosstalk because you will not put light where the overlap of right and left pictures are and by doing that you can reduce crosstalk."
Tack admits that Philips' new range of 3D TVs do still have a crosstalk problem, but he says he firmly believes it will be eliminated within a few years.
"Yes, that is true, in the first generation of 3D I agree with you there is in every product a bit of crosstalk. But we can go very fast to no crosstalk. I have seen solutions like the goggles, if you can get the goggles' shutter time higher or you can have better technologies in the goggles, you can use that to eliminate crosstalk. Next year we will make a big step forward. Whether it's going to be zero I don't know but we will do our best."
LCD vs Plasma in 3D: the verdict
LCD vs Plasma in 3D: the verdict
With so many factors affecting crosstalk on 3D TVs, it's impossible to find a definitive winner between LCD and plasma.
However, that doesn't mean both are as good as each other. Indeed, editor of AV trade title Home Cinema Digest and regular TechRadar contributor, Jamie Carter, thinks that plasma has the edge over LCD at the moment:
"3D is a very new technology – when did the first generation of any new tech look immaculate right from the off? It's true that the specification for 3D Blu-ray adopted throughout the industry came from Panasonic, so it has had more time to work on reducing crosstalk, but it's still first-gen technology.
"Crosstalk is basically when the two 3D images don't swap quick enough – your right eye is seeing what was only meant for the left eye, and vice versa. Surely this is mainly down to the speed of the panel - and in that regard, plasma is going to have the advantage for now. From what I've seen, I think plasma has the edge over LCD for now in terms of crosstalk – it's barely noticeable on plasma."
Carter agrees with Tack that LCD TV's superior brightness can be an advantage, but when it comes to crosstalk he's not so sure it's a good thing.
"Plasmas are traditionally dimmer than LCD panels," he says, "but on either technology the 3D glasses cuts out a lot of the light. The lack of brightness is arguably safer since crosstalk is most obvious in 3D images that feature a lot of contrast between light and dark colours.
"It's fair to say that only 400Hz LCD TVs can expect to really challenge plasma in terms of reducing crosstalk, especially if 3D gaming becomes mainstream. Panasonic can't afford to sit on its laurels, though – there may be little crosstalk on a 3D plasma, but I've noticed flicker around the edges of moving objects; 3D is still some way from looking 'real'."
Meanwhile, although Panasonic has so far been the only company to release 3D plasma TVs, Carter thinks it could be telling that Samsung and LG have both recently launched 3D plasmas.
"Manufacturers are still testing the water. Until now Samsung and LG have been concentrating on LCD for their 3D sets, but both announced 3D plasmas at IFA – LG's PX990 and Samsung's C680 and C490 plasmas. Is that revealing, and a sign that LCD is being sidelined for 3D? It's more likely a case of sitting on the fence – Samsung, LG and Panasonic are the last three remaining manufacturers with huge plasma TV production plants."
It's not just Carter who believes plasma has the advantage, either. The consensus across the TechRadar team is that LCD has some catching up to do, and in fact we couldn't find any TV journalists at all who would endorse LCD as the superior technology.
So what does this mean if you're looking to buy a new 3D TV? The key thing to remember is that most of the content you're going to be watching will still be 2D. So basing your choice solely on 3D performance would be a mistake.
It's about finding that all-round performance sweet spot, which ultimately comes down to personal preference.
Liked this? Then check out Best LCD TV revealed: our pick of the top TVs
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