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I’ve been asked more than a few times this year if it is at all possible to transmit 3D over digital terrestrial television broadcasting (DTTB), or what we know simply as “free digital TV.” There seems to be a perception that one must have a Pay TV subscription service (cable, DBS, FiOS, or U-Verse) to access 3D programming.

Believe it or not, carrying 3D over terrestrial broadcast stations is mostly a business decision. Yes, major TV networks like CBS, NBC, Fox, and ABC could start broadcasting programs in 3D right now. And your 3D-enabled TV would be able to process the 3D signals correctly so the programs can be watched with active-shutter glasses. (I’m not going to discuss color anaglyph 3D here, which works over any TC channel, but produces the lowest quality of 3D.)

The ‘catch’ is that the 3D content would have to be delivered in a frame-compatible format, such as 720p/60 top + bottom (like ESPN uses), or 1080i/30 side-by-side (like DirecTV uses). Both of these formats were specifically developed to fit in a standard 6 MHz channel space, using a maximum bit rate of about 19.39 Mb/s. And in fact, broadcasts of 3D content from earlier this year were delivered in the MPEG2 format that is standard for over-the-air digital TV.

The top + bottom format used for 720p/60 frame-compatible broadcasts.

The fact that cable companies and satellite broadcasters are now moving to MPEG4 encoding for 3D carriage shouldn’t be discouraging. MPEG4 (more specifically, H.264 AVC) provides for 50% compression efficiency over MPEG2. But broadcasters can still pipe a pretty good 3D signal into your home using MPEG2, which has also gotten a lot more efficient in the nearly 20 years it’s been around.

Remember that both of the frame-compatible 3D formats sacrifice some image resolution to fit within a standard channel width/bit rate constraint, no matter what service you get 3D from. For top+bottom, your TV receives a combination frame with two 1280×360 images, anamorphically squeezed in the vertical plane. For side-by-side, each frame of video provides a pair of 960×1080 images, anamorphically squeezed in the horizontal plane.

Your 3D TV separates the two frames and reverses the anamorphic squeeze with a stretching process, resulting in full left eye/right eye frames – albeit with somewhat lower resolution. But today’s TVs do a pretty good job of interpolating pixels to correct for de-interlacing and judder, so these half-resolution images don’t look nearly as bad as you might think.

The side-by-side 1080i/30 format used for 1080i/30 broadcasts.

So, what’s holding broadcasters back? For one thing, available bits! DTTB is limited to a maximum bit rate of 19.39 Mb/s, and that leaves just enough room for one full HD channel (15 Mb/s maximum bit rate) and perhaps a standard-definition channel (3 MB/s maximum bit rate) to go along with it. So a broadcaster would have to devote the entire HD bit rate to the 3D program. Jamming a second 720p/60 or 1080i/30 3D program alongside the standard 2D broadcast would not be practical, as image quality on both channels would suffer.

Another possibility would be to transmit a 2D signal (left eye) and carry the right eye signal as a separate program. This would be a similar approach to analog FM stereo broadcasts, where the stereo information is transmitted as a subcarrier, or analog color TV, where the color burst is also carried as a subcarrier.

This technique can be accomplished digitally by transmitting a full-bandwidth 2D signal (left eye) and carrying additional metadata (2D + depth information) required to create the stereoscopic effect. That metadata would add something to the payload, and would rely on the some of the image processing inside the TV.

Now, a broadcaster could carry the Super Bowl in full HD as before (720p or 1080i), yet still enable 3D viewing for TVs equipped to handle the 3D signal. But there’s another ‘catch:’ Your TV would have to recognize the metadata ‘package’ and be able to open it up, rebuild the right eye frames, and sequence them accordingly.

Because it’s not likely that a DTTB station would use its entire bandwidth to carry a 3D broadcast of a big event, the 2D + depth format would make the most sense, just as older black and white TVs could still display a color TV program simply by ignoring the chrominance signals. I don’t know of any consumer TVs that are equipped to handle the 2D + depth format, so some sort of outboard adapter would be required to make this work.

The good news is that such a 3D converter box would not have to be expensive. It would incorporate an ATSC tuner (maybe even a pair of tuners!) and would be equipped to process the DTTB 3D signal into top + bottom or side-by-side formats, using a standard HDMI output connection to the TV. Frankly, such a box ought not to cost much more than $100, and could also be sold as a bundle with one or two pairs of universal active shutter glasses. (Motorola showed a prototype 3D converter box at NAB 2010 for older, non-3D TVs.)

What programs would work in 3D? Aside from football, which I do not believe benefits much from 3D based on my recent experiences, I’d say basketball, hockey, auto racing, Olympics individual events, golf, and tennis. Basically any sports event where the camera can get close enough to realistically create a sense of depth.

Who would be likely to try 3D broadcasts? My guess would be ABC and Fox for starters, given how much sports programming these networks already carry and how many stations they own. ABC, of course, is part of Disney, who also owns ESPN. ABC has the NBA, college football, and the Indianapolis 500, while Fox operates an extensive sports division and covers college and pro football, auto racing, and major league baseball.

NBC might also dip their toes in the water with Olympics coverage, Notre Dame football, NASCAR, horse racing, and Sunday Night Football. I’d see CBS as the last network to try this out, simply because they adhere to a strict ‘no multicast’ policy at all of their owned-and-operated (O&O) CBS and CW stations.

So the answer to the question is “Yes, free TV stations can broadcast 3D programming, and they can broadcast it now.” The catch is, do they want to, and which delivery format would they adopt to make it work?

Posted by Pete Putman, November 15, 2010 9:17 AM

About Pete Putman

Peter Putman is the president of ROAM Consulting L.L.C. His company provides training, marketing communications, and product testing/development services to manufacturers, dealers, and end-users of displays, display interfaces, and related products.

Pete edits and publishes HDTVexpert.com, a Web blog focused on digital TV, HDTV, and display technologies. He is also a columnist for Pro AV magazine, the leading trade publication for commercial AV systems integrators.