3D: Getting to Know the Glasses
Here's a closer look at the technologies behind the notorious eye wear needed to enjoy 3D experiences.
Last week I promised we’d be covering 3D content available to home users today. Other than PC gaming through NVidia’s 3D Vision system and a handful of 3D Blu-ray spec demos trickling in, there isn’t any. So that covers that, at least for now.
So let’s dive a little deeper in to 3D glasses and how they work. Just like any emerging AV technology there are a plethora of specifications and systems, few of which are directly compatible. The 3D Blu-Ray spec and HDMI 1.4 are going a long way to unify, as are displays supporting multiple specs, but there’s still a lot to learn.
The bulk of modern home 3D technologies will rely on active shutter glasses. The glasses themselves all basically work the same way, using liquid crystal shutters to allow only one eye to see what’s on the screen at any given time. Two things, however, can differ substantially:
Frame Progression Type
While they work the same, what they are allowing each eye to see can differ greatly. Here are some examples.
Alternating lines of resolution are drawn in sequence. Lines 1, 3, 5 ... are drawn while the right eye is exposed, then lines 2, 4, 6 ... are drawn as the left eye is exposed. Interlacing is one of the easiest methods in terms of hardware requirements, but limits resolution to half the display’s capability (since only half the image is displayed to each eye). Popular in older 3D gaming, requires interlaced capable display such as a CRT monitor.
Page flipping alternates the frame, left and right, in sync with the shutter of the glasses. This overcomes the resolution deficiencies of the interlacing method, but requires high refresh rates to minimize flicker and motion artifacts. The new 3D Blu-Ray spec is based on a modified version of page flipping called frame stacking.
Similar to interlacing, a checkerboard pattern is displayed in sync with the shutter of the glasses. The same resolution limits exist, but scan lines and other interlacing artifacts are minimized. This is the method used in “3D-Ready” DLP TVs offered by Mitsubishi and Samsung. An external adapter is required for use with 3D Blu-Ray spec devices.
Since active shutter glasses require the shutter and display be in sync, they also require some sort of signal be sent to the glasses to keep them in sync. Current displays and gaming technologies use one of two methods:
The active shutter glasses are paired with the display via Bluetooth RF frequency, similar to a wireless ear piece with your cellphone, but the only signals present are sent from the display to the glasses for sync of the shutters. Range can be limited, but in practice the range usually exceeds the likely viewing distance by a safe margin.
Instead of radio frequencies, an IR emitter on or connected to the display emits an infrared signal that is detected by the glasses and syncs the shutter with the display. Range is greater than Bluetooth, but in rare instances, certain lighting conditions can interfere with the IR signal.
While most direct-view displays (Plasma, LCD, Laser) coming out will rely on active shutter glasses, front projection systems are able to more easily incorporate passive glasses using light polarization. The active shutter is moved from in front of the eye to in front of the lens. Since there’s only one lens, an extra mechanism is required to block one eye at a time. Instead of completely blocking one frame at a time, the polarizing lens polarizes the light (and in turn the image) in alternating angles. The passive glasses then use polarized filters at different angles for each eye, so that the combined polarized effect is each eye seeing the proper image in sync and creating the 3D effect. This method is far less practical for direct-view displays since the entire screen would need to be polarized in alternating sequence.
Now that you know the deep details of the glasses and how they work, we’ll move on to more details of the new HDMI v1.4 and v1.4a and how they relate to the transmission of 3D video from device to device. See you next week.
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