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3D Display Technology: Passive Polarized Vs. Active Shutter

HP 2311 gt 23" Monitor Review: Passive, Polarized 3D On A Budget
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Regardless of the display you use, stereoscopic 3D works by delivering a unique, dedicated image to each eye that represents two different perspectives. Your brain blends them to deliver a sense of depth.

There are really only two commercially-successful 3D display technologies available to PC enthusiasts right now: active shutter systems, which use alternate-frame sequencing to block light to one eye while the other is shown its appropriate image, and passive circularly-polarized systems that let you tilt your head and still maintain left/right separation. The technology HP uses to achieve stereoscopy with its passive display is called film-type patterned retarder, or FPR.

We covered the basics of active versus passive systems in Wall Sized 3D Gaming, Just like The Theatres Do It back in 2010. For the purposes of this article, though, it makes sense to recap the most important differences between passive polarized FPR-based displays and active alternate-frame sequencing-based screens.

Passive 3D (Polarized, Film-Type Patterned Retarder)

This type of display exploits the wave-like properties of light to control what is seen from each eye. It relies on a polarized filter that covers the screen. This filter has two orientations: one for even lines of resolution, and one for odd lines of resolution. Each alternate line displays the output intended for a specific eye. The viewer must wear polarized glasses in order to realize the illusion of 3D; however, the glasses contain no electronic parts—each eye is simply covered by a passive polarized filter. The filter over the right eye blocks out the lines of resolution intended for the left eye, and the filter over the left eye blocks out the lines intended for the right eye. In this way, each eye only receives its intended perspective, even though the output for both eyes are displayed on the same screen at the same time.

This describes the FPR technology used in HP's 2311 gt. Polarized passive displays are also used in a vast majority of movie theatres, but they employ a different method that doesn't halve resolution. If you’re interested, you can read more about that here.

The Benefits Of Passive 3D

Now, let’s consider the implications of FPR-based displays. First, the glasses require no active electronics, so they’re very inexpensive to manufacture and less cumbersome than shutter glasses. They're so inexpensive, in fact, that you’re given a pair every time you go see a move in stereo at the theater. As mentioned, you can even use the glasses from theaters equipped with RealD's technology on HP's 2311 gt.

Second, many folks find polarized displays easier to look at for prolonged periods because they aren't receiving 60 flashes per second from active eye-wear. This seems to predominantly be a case-by-case observation, though, and there are those who aren't bothered by the shutter effect at all.

Also, passive FPR displays allow the user to view the light from the display 100% of the time. This isn't the case with an active shutter-glasses  solution where the aperture is closed half the time in each eye, and brightness is negatively affected.

An FPR-based implementation works at 60 Hz, so a standard HDMI cable has ample bandwidth to enable stereo content. This isn't the case with active systems, which require 120 Hz output to deliver 60 frames to each eye, each second. As a result, using HDMI 1.4, you can only get 60 frames per second at a maximum 720p resolution using shutter-based technology. That doesn't mean you can use any old 3D TV in your living room with a set of polarized glasses. However, as it pertains to polarized monitors like HP's 2311 gt, passive 3D is certainly more accessible. 

Passive 3D's Compromises

So far, polarized stereoscopy sounds pretty awesome, right? Well, there is a significant caveat. Mainly, when it comes to a FPR display, each eye is only treated to every second line of resolution, or, half of the frame. Fine detail is most affected by this, and and small text in a computer game can be difficult to make out. The closer you get to the screen, the more obvious this effect becomes. It is a significant detractor from image quality. As you might imagine, a loss of resolution is more pronounced up in front of a PC monitor than it would be from the couch looking at a 3D-capable television.

In addition, the polarized filter on the screen has to be aimed. And that means there is an optimal viewing angle. If you stray too far from it, severe anomalies like ghosting start cropping up, making the output intolerable.

Active 3D (Shutter, Alternate-Frame Sequencing)

Shutter-based systems that employ alternate-frame sequencing use powered shutter glasses that, as its name suggests, alternately block the output to each eye. With displays operating at 120 Hz, that means each eye gets a smooth 60 frames every second.

An infrared emitter is responsible for sending a signal to the glasses to blank-out every other frame. So, each eye sees a single frame of output and is then covered for the next one, intended for the other eye. At the aforementioned 120 Hz, this happens quickly enough that you don't notice the blanking process. The most noticeable impact is a darker image.

Because the glasses in an active system perform a major role, flipping back and forth, they require power. That means they also need batteries that periodically have to be recharged or replaced. The glasses also host wireless receivers and transparent LCD screens, so they tend to be expensive as well (in the $50 to $150 range). Multiply that out for a family of five, and you're looking at a major variable in the decision whether or not to invest in the technology. Additionally, the loss of light in each eye half of the time is very perceptible. And finally, the glasses are not very elegant, given their active circuitry.

Active displays can deliver true 1080p content to each eye 60 times a second. Unfortunately, the HDMI 1.4 standard doesn’t offer enough throughput to support that quantity of data, though. Consequently, typical 3D televisions are limited to 24 FPS at 1080p. That's fine for a Blu-ray movie, but it's painfully inadequate for smooth gaming. Instead, owners of GeForce graphics cards have to look for 3D Vision-certified monitors. If you have a new-enough Radeon card, an AMD HD3D-certified monitor does the trick, too. AMD's Radeon HD 7000-series cards do support a new "fast HDMI" standard able to serve up 60 FPS at 1080p over HDMI. But with no compatible displays currently available, that feature doesn't figure into today's story.

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