A Closer Look at Active vs Passive 3D Flat Panels

Last Friday, video guru Joe Kane visited Grayscale Studio, where Tom Norton and I conduct most of our display reviews, to show us his latest test patterns, which are designed for 3D displays. The images were generated by a VideoForge test-pattern generator from Audio Video Foundry and sent to an Accell HDMI switcher/splitter, which fed two flat panels—a Samsung UN55D8000 with active glasses and LG 55LW5600, which uses passive glasses. (Interestingly, the Accell switcher/splitter can pass 3D from the VideoForge, but not from a 3D Blu-ray player.) The results of these tests were very interesting, to say the least.

Most of Kane's 3D patterns are what he calls "3D flat," which means the left and right images coincide exactly—in other words, the 3D images have no depth, and everything appears to be in the plane of the screen. Why make 3D test patterns like this? Because, Kane says, such images will look the best that a 3D display can manage, and you don't need to wear glasses to use them. A few of the patterns do have some elements behind and in front of the screen plane, and all have "L" and "R" indicators that are isolated for the left and right eyes, respectively, so you can make sure the display and glasses are oriented properly.

Kane also emphasized that a good 3D display must first be a good 2D display—after all, a 3D image is nothing more than two 2D images, so the display must exhibit good 2D performance if it has any hope of doing 3D well. This is another reason to use 3D-flat test patterns to evaluate a 3D display's performance.

As you might know already, passive 3D flat panels use what's called a film pattern retarder (FPR) filter on the screen to oppositely polarize alternating lines in the image—the left eye sees the odd-numbered lines, while the right eye sees the even-numbered lines. In the photo above, we held polarized glasses against the top of the LG's screen. You can clearly see that the first row of pixels are visible through the left lens and not visible through the right lens, and that alternating rows are visible to each eye with a black line between them. This demonstrates that each eye sees only 540 lines of vertical resolution on an FPR-based 3D flat panel.

Makers of such sets claim that, while each eye sees only 540 lines, the brain combines the two images into one, resulting in a total of 1080 lines. However, Kane disputes this, saying that the two images must be kept completely isolated from each other or we would see no 3D effect at all, and if each eye sees 540 lines, that's the total effective resolution.

In addition, this is why thin, black horizontal lines are visible when viewing an FPR-based 3D flat panel within a certain distance from the screen. And this proves that the brain does not completely combine the two images into one—if it did, the black lines wouldn't be visible at any distance. Of course, you can sit far enough away to render those lines invisible, but that would also reduce the amount of detail you see and cause the 3D effect to be less immersive.

LG claims that its FPR panels are actually displaying all 1080 lines of information for each eye, but not at the same time. In the first 120th of a second, the odd lines of the left image are displayed in the odd lines on the screen and the odd lines of the right image are displayed in the even lines on the screen. Then, in the next 120th of a second, the even lines of the left image are displayed in the odd lines on the screen and the even lines of the right image are displayed in the even lines on the screen. (For more on this, see my discussion here.) This is very similar to interlacing, which has its own problems.

However, this puts some lines in the wrong place—even lines from the image in odd lines on the screen and vice versa—which would cause a slight but noticeable vertical judder. And even though all 1080 lines of information for each eye are being displayed, each eye sees only 540 lines of resolution at any given instant. Not only that, objects behind or in front of the screen plane will exhibit interlace artifacts.

To clearly reveal the judder effect, Kane made two copies of a high-res still photo, deleted the odd lines from one and the even lines from the other as seen in the close-up above, and precisely overlapped them, making a 3D flat image. Before visiting the studio, Kane had displayed this photo on other LG FPR sets and didn't see any vertical judder, but we did see a bit of judder on our set. The other sets had the latest firmware available to consumers, but our set has even more recent firmware that is not yet generally available. Kane concluded that previous versions aren't trying to do the interlacing trick, but our version is.

To reduce the appearance of vertical judder, Kane says the TV filters the incoming video signal to remove the highest frequencies in the vertical direction, thus reducing vertical resolution. This is clearly evident in the high-frequency horizontal and vertical bursts in Kane's 3D flat patterns, in which vertical and horizontal lines of single pixels alternate between white and black. As you can see in the photo above, the horizontal burst is fully resolved on the LG, but the vertical burst is nearly invisible.

By contrast, as seen in the photo above, the vertical burst is fully resolved on the Samsung set, which uses active glasses and therefore presents full resolution to each eye.

Another revealing pattern called "pixel phase" combines the horizontal and vertical bursts into a checkerboard of white and black pixels. On the Samsung, this checkerboard is clearly visible.

On the LG, high-frequency filtering renders the pixel-phase pattern almost solid gray.

Kane's 3D test patterns will be available first for the VideoForge, but he also intends to put them on a Blu-ray disc at some point. When? He doesn't know, but I'll be sure to let you know as soon as he tells me.

Of course, consumers don't watch test patterns—they watch real content, which often obscures some of the problems I've been talking about. And I must say that lots of 3D content I've seen on passive panels looks sharper than I would expect with the vertical resolution of only 540 lines. On the other hand, the 3D effect doesn't quite "jell" as completely as it does on active displays—it feels a bit discombobulated. Also, I still see thin, black horizontal lines on FPR flat panels, especially in large single-color areas and text, that are not visible on active sets. And FPR sets exhibit lots of crosstalk when viewed from much above or below the central axis, so anyone watching from the floor won't have a satisfying experience.

All in all, it was a very illuminating afternoon at Grayscale Studio, strengthening my conviction that active 3D flat panels produce a better 3D image than passive FPR sets. Yes, active glasses are more expensive (though they have dramatically dropped in price with Samsung's announcement of $50 models), they are heavier and clunkier than passive glasses, and they require periodic battery replacement or recharging. But if you're like me, you want the best possible image quality, and with 3D flat panels, that means active-shutter glasses.

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COMMENTS
Jarod's picture

Very interesting patterns by wiz Joe Kane. Confirms my theory that active is superior to passive in display panels from my viewing experience as well. Great article!

Audio_Geek_00's picture

I look at the hoops that the electronics have to do in order to give us a compelling 3d image to look at and it is quite amazing. I am still not on the 3d bandwagon as I still feel that there is degradation to the image I have spent a lot of $$ and effort to achieve with my current setup, even with an active system. I want to like it. But my pocketbook and my eyes have a different opinion.

MojoLA's picture

I've seen the active and passive sets in action and am surprised that the esteemed Joe Kane and his colleagues point to only the price of glasses as the downside of active sets.

Active glasses cut brightness a significant amount more than passive does, and with that brightness drop comes severely affected color rendition and shadow detail. When I take the shutter glasses on and off to compare "with and without" pictures, it really is a night and day difference. The drop in overall image quality with active glasses is simply something I couldn't live with to get 3D.

And there is the issue of the flicker itself. While most people (myself included) don't actually "see" the flicker, it's an effect that the brain still perceives and results in a much faster instance of viewing fatigue. A good analogy is the way you can walk into a room and just "know" a TV is on; you can't see or hear it, but there is some sort of high frequency that can be "felt" and when the TV gets turned off, you suddenly feel a sense of relief.

I think the same is true of shutter glasses - I don't see the flicker per se, but when I take the glasses off after even a short viewng session, I can "feel" a sense of relief - that simply doesn't happen when I remove passive glasses. I think most reports of headaches and the discomfort that some experience from watching 3D is most likely a result of "flicker fatigue."

And the price of glasses is only one issue - variety is the other. There are already at least half a dozen companies offering high-end aftermarket polarized glasses, ranging in price from $25 to $150 for the high-fashion Oakleys (this is in addition to the dozens of pairs already available from specialist 3D vendors before the recent popularity boost).

The main reason I bring this up is twofold - firstly, it means it will be far easier to find glasses you find comfortable and good looking in the passive realm - but the other is in finding optically neutral specs.

Almost every pair of glasses - active or passive - I've worn introduce a small amount of green tint to the image; it's not huge, but I don't want ANY external device messing with the tint of my ISF-calibrated set :-) After just a little hunting around, I was able to finally find color-neutral passive specs for about $20.

There may very well be color-neutral shutter glasses out there, but good luck finding them! In addition, if you already have active glasses for your 3D TV, it's unlikely that you're going to spend the money to replace them.

Ultimately, I think passive 3D TV offers far more advantages over active. Despite what I've read about even the small degree of ghosting reviewers have mentioned, neither I nor the several people I know who have bought the LG television have spotted ANY ghosting under normal viewing conditions. When it comes to the infamous vertical resolution drop, I agree with your findings - on paper it seems pretty harsh, but in practice you really don't notice! My speculation on this is that high def simply has so much picture information you'd need to really slash pixels before the image actually began to look truly degraded.

It may have taken a while, but I'm very happy to see manufacturers bring passive technology to the home; to my eyes the picture is superior to active and it's the ONLY way you're going to afford to have glasses on hand for all your friends and in-laws :-)

MintyFresh's picture

Apparently MojoLA just bought a Passive TV. It wasn't anything personal Mojo, you just bought a low performance 3D TV.

MG79's picture

80% of people who try both prefer passive. There is way too many benefits on passives side to argue.

Passive 3D offers:
Best overall color (in any light) / Brightest overall images (in any light) / Greater depth / Sharper images (debatable due to split pixel count) / No Flicker / Viewable 3D in any light w/o flicker / Less cross talk (debatable due to ghosting during above or below eye-level viewing) / Wider horizontal viewing angle (superior to vertical viewing angle when it comes to amount of viewers) / Ability to tilt your head / Lighter glasses / Cheaper glasses (freebies at the theater!) / No recharging / No worry of breaking expensive glasses / Sharing a 3D experience with more people (due to cheaper glasses and wider viewing angle) / No eye strain, no head aches, no nausea (due to flicker and heavy eye wear) / Longer more comfortable 3D viewing time (due to no recharging, no nausea, no flicker to give you a head aches or strain your eyes, lighter glasses, and being able to tilt your head) / Easy of use (no syncing, replacing batteries, recharging, making sure glasses are on) / Passive TV to passive TV glasses compatibility / Passive glasses to theater compatibility / More style choices of glasses / LAST BUT NOT LEAST, Full Screen Split Screen for gaming (the ability to do this without buying a new TV) Click the link to see ---> http://www.youtube.com/watch?v=CVJcVPvjUJo

Active LCD/LED 3D offers:
Higher resolution 1080 image in both eyes (debatable due to more cross talk and flicker) / Higher vertical viewing angle (this benefits people who sit below eye level of the TV w/o tilting the TV down towards the viewers) / Ability to sit closer to the TV (during 3D viewing) / No Jaggies (caused by splitting pixel count) / Better at displaying lower resolution 3D from cable or satellite providers then passive.

Active 3D Plasma offers:
Highest overall resolution (then both passive and active LED) / Better color then active LCD/LED during 3D viewing / Brighter then active LCD/LED / Greater depth then active LED (equal to passive depending on the models tested) / High vertical viewing angle (equal to active LED) / Ability to sit closer to the TV (during 3D viewing) / No Jaggies / Better at displaying lower resolution 3D from cable or satellite then passive.

I'm am no expert, I just gathered as much information that I could from personal observation (going to Best buy, Sears, and Conn's to see for myself). Also reading countless articles and forums to get an overall understanding of both active and passive TVs.

There are (Strengths and Weaknesses) to both. For me (((Passive))) takes the cake when it comes to "my" overall expense, comfort, convenience, and viewing experience. Active Plasma does seem to be better then active LED, but power consumption may be a deterrent for some people. However, people have different preferences and may hold one strength or weakness as more or less important to them. For example if your planning on watching a lot cable or satellite 3D channels that produces only HD in 720p then active may be a better choice for you. Passive 3D is great for people who just want to watch full HD 1080i Blu-ray movies because the splitting of the pixels is far less noticeable. Passive also appeals to gamers who are interested in dual play (full-screen split screen). As of right now all passive TVs can do (full screen split screen) while the only active TV that can is the 24" PS3 TV. Well actually all 3D TVs can do this but to make this work on your current active system you would have to tinker with your expensive active shutter glasses, possibly breaking them.

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