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Broadcasting in 3-D

Video in 3-D was all over last year’s NAB show; many booths were displaying the monitors required to view it and the equipment to create it. At this year’s CES, many more manufacturers showed off their 3-D wares and new partnerships to deliver 3-D were announced, including the introduction of a joint 3-D cable channel by Sony, Imax and Discovery Channel that could launch as early as the end of 2010.

A 44in 3-D flat-screen monitor was introduced by Viso for less than $2000, and the Blu-ray Disc Association has adopted a standard to put 3-D onto Blu-ray Discs. Also, ESPN and DirecTV plan to launch a 3-D channel this year. A firmware upgrade to all DirecTV boxes will enable reception of the 3-D signal, and viewers will only need an unmodified HD monitor and 3-D glasses.

How does it work?

Viewing 3-D video works the same way as 3-D in the theaters. There are two different images displayed: one for the left eye and one for the right eye. Offsets between the two images create a 3-D effect, or virtual 3-D. Special glasses must be worn to direct the correct images to the correct eye, and that is where the differences come in — how the glasses work and what is displayed.

Some DVD movies have used colored lenses in the 3-D glasses to filter the images, which is a method called anaglyph. This was the same method used back in the 1950s movie theaters when 3-D was first introduced. This method uses red and cyan colored lenses to separate the two images (broadband spectral filters). The advantage of this system is its simplicity; it uses only one video channel, can use any color video monitor to display the images and the glasses are cheap. The downside is lack of or limited color image reproduction. Today, there have been some major improvements to this system to correct problems with color reproduction. (See Figure 1.)

Today, the 3-D you see in movie theaters is displayed using polarizing filters on both the projector lens and the glasses worn by the viewer. Each of the two images is projected through a polarizing filter, and when the polarized light reaches the viewer, the polarized lenses in the glasses direct the specific image to the correct eye. Some systems use horizontal and vertical polarization, while others use circular polarization to separate the left and right images.

A newer system splits up the red, green and blue light into separate bands in the light spectrum. The left image uses one set of red, green and blue wavelengths, and the right uses a separate set of wavelengths. Each lens of the glasses filters the opposite set of wavelengths (narrowband spectral filters), which allows the left and right eyes to see separate full-color images to create virtual 3-D. The advantage is that the move theater screen does not need to be a silver screen, which is the only type of projection screen that maintains the polarization of the light reflected from it.

3-D TVs

Currently, there are several basic approaches to displaying 3-D on video monitors; the first uses polarizing filters, and the others involve LCD shutters.

The first method uses a polarizing filter that covers the video screen. These are strips of polarizing filters arrayed horizontally across the screen. They alternate the polarization, with each strip covering just one horizontal row of pixels. When the video is displayed, the odd lines carry the left video signal, and the even lines carry the right video signal. When the viewer watches the video, the polarized glasses direct the odd video lines to the left eye and the even lines to the right eye. If circular polarization is used, the image retains its 3-D effect even when viewers tilt their heads. (See Figure 2.)

Some manufacturers call this a “half HD” 3-D system, because each eye is only seeing half of the full HD image. The advantage is that only a single HD channel is required to transmit the 3-D HD signal, and the viewing glasses are inexpensive. The downside is that the monitor has to be fitted with the polarizing filters on its screen, which increases its price significantly. Viewing normal 2-D video, without the glasses, on these monitors is not affected by the polarizing filters.

A second method uses what are called active-shutter glasses. In this case, the frame rate is increased to 120Hz. By alternating the left and right images frame-by-frame, a full HD image (frame) is displayed to each eye. To prevent both eyes from seeing both left and right images, the viewer wears active-shutter LCD glasses. These glasses control the LCDs in both lenses to completely shut off light to the left and then the right eyes. To synchronize the shutters to the displayed video, an infrared signal is transmitted from the monitor to the glasses. The glasses are battery operated and must be recharged. (See Figure 3.)

This method is called “full 3-D HD” my some manufacturers. The advantage to this system is that, depending on the frame rate used, an unmodified HD monitor can be used to watch 3-D content. The downside is that the glasses can cost from $50 to $60 and must be recharged. Also, the time the shutter is open determines the amount of light that the eye sees and, thus, the brightness of the picture; the faster the frame rate, the darker the picture.

A variation that combines the two methods uses an LCD to change the light polarity of the entire screen as the left and right images are displayed. Then the polarized passive glasses will display a full frame to each eye

One more method is used by some digital light processing (DLP) manufacturers. It involves creating a checkerboard pattern of the two images. The projector alternates between the two images, first displaying the left image (think black squares on a checkerboard) and then the right image (the white squares). Although the display method is different, this system also uses the active-shutter glasses. The method works best with DLP displays, but the signal must be fed to in the checkerboard format, which only comes from certain computer graphic boards or a special converter box. (See Figure 4.)

Because the first TVs to use 3-D are large screens (about 50in), it would appear that having full HD frames for both eyes will be needed to get the full 3-D effect; on the other hand, some people have complained about the shutter effect caused by the active glasses.

Standards

SMPTE began working on standards for 3-D transmission back in 2008 with its 3-D Home Display Formats Task Force. This task force announced its recommendations, but not the specifications, at NAB2009. These include a mastering standard based on 1920 x 1080 pixel resolution at 60fps and per eye. The plan requires support for backward-compatibility with 2-D images and content including hybrid products such as Blu-ray Discs that can support 2-D and 3-D displays.

Blu-ray has standardized on a 3-D format for the delivery to the home. The Blu-ray 3D specification uses the Multiview Video Coding (MVC) codec, an extension to the ITU-T H.264 Advanced Video Coding (AVC) codec currently supported by all Blu-ray players. The MPEG4-MVC codec compresses both left and right eye views into an optimized single picture, with the differences between the left and right views carried in a metadata package. This metadata typical creates a 50 percent overhead compared to the normal 2-D Blu-ray data rate. It will provide full 1080p resolution backward-compatibility with current 2-D Blu-ray Disc players.

Transmission

Japan’s BS 11 cable channel has been experimentally transmitting 3-D TV four times daily since 2006. There have been several TV shows that used the anaglyph (red and cyan glasses) for special parts of the program, but none have tried to use 3-D for the complete show.

As of today, there are three ways in which to transmit a 3-D HD signal. The first uses two complete HD channels. It’s simple, but it takes up twice the bandwidth, and keeping the channels synchronized can be difficult.

Another way is with time compression, where the left and right signals are squeezed to fit in one frame. This results in either one picture on top of the other in one frame (over/under) or the two images side-by-side within one frame. The outcome is a loss of one-half the resolution either vertically or horizontally, but only one HD channel is required to transmit the signal. The side-by-side method has been chosen by many cable companies to be their method of transmitting 3-D HD. (See Figure 5.)

The last method is the same one mentioned above in the Blu-ray specification, in which a single 2-D-compatible image is created out of the left and right images, and the differences between the left and right signals are transmitted in a separate metadata package that is about half the size of the main signal. A distinct advantage to this system is that it can deliver two full-bandwidth HD signals to the viewer with a 25 percent reduction in data rate usage, compared to using two full HD channels. (See Figure 6.)

Conclusion

This year likely marks the beginning of 3-D TV, as attested to by the excitement at this year’s CES and the announcements by many manufacturers of released or upcoming 3-D products. Whether broadcasters will join the 3-D fray is yet to be seen. With three to eight programs per channel and mobile DTV, all of which require a channel’s limited bandwidth, is there any room left for 3-D HD?