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The ABCs of 3D: Key Terms You Need to Know

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ABCof3d.gif3D TV: Traditional film/television content is viewed in two dimensions (height and width). Three-dimensional content adds a depth dimension that more closely mimics what we see in real life. While a traditional 2D television can display 3D content (viewable with passive 3D glasses), the quality of the image tends to suffer. A 3D TV uses the process of stereoscopy (see below) to produce a higher-quality, more immersive 3D experience. (Stereoscopy should not be confused with holography, which also produces a 3D effect in which the perspective of the object you are viewing changes as you move. In a stereoscopic 3D TV, the perspective is fixed and does not change as you move.)


Stereoscopic 3D (also known as Stereoscopy): To achieve the 3D effect, two images with slightly different perspectives are shown simultaneously; one image is sent to the left eye, the other to the right eye. Our brain puts the two images together to form a three-dimensional image. Stereoscopic 3D requires the use of glasses (either passive or active) that properly filter the signal to send the correct image to each eye. The new lineup of 3D-capable TVs and Blu-ray players employs the stereoscopic 3D method.

Auto-stereoscopic 3D: This method also uses stereoscopic transmission but does not require the use of glasses or other headgear to view the 3D image. There are a number of ways to achieve this; one common method is to use a lenticular screen that directs different images to different parts of the screen, but this has limitations in image resolution and viewing area. Auto-stereoscopic 3D divides the resolution by the number of fixed viewing positions: With two positions, you see half the resolution; with four, you see a quarter of the resolution, etc. Consequently, this 3D method is currently better suited for handheld displays that are designed for a single viewer, such as gaming devices, laptops and smartphones. Some TV manufacturers have shown prototypes of auto-stereoscopic 3D TVs, but TV resolution needs to increase before it is a truly viable option.

Anaglyph Glasses: This is the type of 3D glasses with which most of us are likely familiar--simple, passive glasses that contain a red filter for one eye and (usually) a cyan filter for the other. The left-eye and right-eye images in the stereoscopic 3D signal have been color-tinted, and the color filters in the glasses direct the appropriate image to each eye. As a result, the anaglyph method causes color distortion, among other technical issues.

Polarized Glasses: Also a passive system, these glasses control the type of light that reaches each eye in order to create the 3D effect. The left-eye and right-eye images in the stereoscopic 3D signal contain light that has been polarized differently, and the light filters in the glasses direct the appropriate image to each eye. One method, called Xpol, polarizes the light in a manner that sends alternating lines to each eye, which results in half the resolution. A 1920 x 1080 signal is reproduced as 1920 x 540 for the left eye and 1920 x 540 for the right eye.

Active-Shutter Glasses: The new crop of 3D-capable TVs uses active 3D glasses, as opposed to the passive methods described above. As the 3D TV displays the two images in the stereoscopic signal, the active-shutter glasses rapidly "blink" on and off (they go from transparent to opague) in sync with the signal to ensure that the left eye receives the left-eye signal and the right eye receives the right-eye signal. Active-shutter glasses communicate with the TV via a transmitter or emitter (see below), and they require a power source, usually in the form of a rechargeable battery. At this point, the 3D glasses and 3D TV must come from the same manufacturer; however, we expect that, in the near future, non-proprietary glasses will be available.

Sync Emitter/Transmitter: To communicate with active-shutter glasses, a 3D-capable TV transmits the signal over infrared (IR) or radio-frequency (RF) technology via an emitter that is either attached to or embedded in the TV.

Full HD 3D: In a Full HD 3D signal, each image in the stereoscopic signal has a 1920 x 1080p resolution. Blu-ray 3D offers a Full HD 3D signal, which has a data speed of 6.75 Gbps.

Frame Sequential 3D: The Frame Sequential method for displaying a stereoscopic 3D video signal is to alternately flash the full image for each eye--i.e., the left-eye image for frame 1, followed by the right-eye image for frame 1, followed by the left-eye image for frame 2, etc. The new 3D-capable TVs from Panasonic, Sony, Samsung and LG use this display method. (Note: Just because a TV uses a certain format to display the 3D signal does not mean the TV has to receive incoming signals in that same format. The HDMI 1.4 spec requires that 3D TVs are able to accept multiple 3D formats.)

Checkerboard 3D: The Checkerboard method for displaying a stereoscopic 3D video signal divides the left-eye and right-eye images into grids and then combines elements of each grid into a checkerboard pattern. This is the format accepted by all Mitsubishi 3D-ready DLP rear pros, as well as older 3D-ready DLP and plasma models from Samsung. Most new 3D Blu-ray players will not output this format (the exception is Panasonic's DMP-BDT300 and BDT350); Mitsubishi offers a special converter box that allows for compatibility between a new 3D Blu-ray player and the company's line of 3D-ready TVs.

Over/Under 3D (also known as Top-and-Bottom 3D): The Over/Under method for displaying a stereoscopic 3D video signal embeds the two images--one on top of the other--in the same frame. The Full HD 3D signal output by new 3D Blu-ray players uses an Over/Under format in which two 1920 x 1080 images (plus 45 pixels in between for blanking) are built into one signal that has a 1920 x 2205 resolution.

Side-by-Side 3D: The Side-by-Side method for displaying a stereoscopic 3D video signal embeds both images--side by side, obviously--in the same frame. This is currently the method being used by satellite/cable operators and broadcast providers to transmit a 3D signal, and it requires some loss in resolution to fit both images in the same frame. For instance, the new ESPN 3D channel broadcasts a 720p/60 side-by-side image. The 1280 x 720 frame holds two 640 x 720 images. Because it has the same resolution as the 2D signal, a side-by-side 3D image uses the same bandwidth, which is why it is the desirable choice for satellite/cable operators.

Crosstalk (also known as Ghosting): This effect occurs when information from one image in the stereoscopic 3D signal leaks into the other--for example, when the left-eye image leaks into the right-eye image--which causes a ghosting or double-image effect.

Flicker: The flicker effect occurs when the viewer is able to perceive the opening and closing of the shutter in active 3D glasses. This effect will more likely be visible in 3D TVs with lower refresh rates.

* Thanks to our friend The HD Guru (www.hdguru.com) for his help with this article.

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