The 3DTV frame-compatible structures of “Side-by-side” and “Top-bottom” fit the left and right images within a shared video frame and could be employed by over-the-air broadcasting using the existing MPEG-2 transport and bandwidth that are currently used for DTV, similar to how cable, satellite and IPTV implemented their 3DTV services over the past months using their existing MPEG-2/MPEG-4 transmission infrastructure. The difference: broadcasters do not have the luxury of using a separate channel for 3DTV as they did.
Is there any other format that would perform the double function without impacting the image quality of either 2D or 3D and still fit in the allotted channel space?
The answer is yes: The service-compatible format, also known as 2D-plus-Delta, but let us first cover some of the factors broadcasters are facing.
The Original Image Resolution cannot be Restored
While the images for both eyes are transmitted together in the same video frame their resolution is halved vertically or horizontally. Current active-shutter 3DTVs on the market can accept the frame-compatible 3D formats and upconvert the images into full frames pairs, restoring the 50% of pixels that were lost in the broadcast by using the TV’s video processing interpolation features.
The pairs of full video frames are then alternately displayed in a time-sequential manner in sync with the active-shutter glasses so each eye can only see its corresponding image.
While pixel interpolation may restore pixels in number to complete an image to the best the video processor can do, it would not be able to restore exactly how those pixels were “originally” because they were already lost on the transmission. With that in mind the quality of that TV circuitry is important to obtain the best visual result.
The passive polarized glasses method of 3DTVs (like most local theaters) shows both halved images as they are, interleaving their video lines on a single video frame before displaying it. The polarized glasses allow each eye to only see the lines intended for that eye. There are no flickering issues with those glasses.
The half resolution of the images transmitted within the top-bottom format is maintained when shown on a passive display, but the side-by-side format looses another 25% of original resolution, for a total of 75% loss from the original image pair, when shown on a passive display.
Hard Pressed Broadcasters
As a viewer, one could naturally dislike the loss of resolution for 3D, but broadcasters are put in a bind for two reasons:
A) They are not able to broadcast the 3D images with the quality of Blu-ray 3D, with full resolution per eye, because the format requires higher bandwidth than the channel allocation they have for HDTV. So the broadcaster would have to use a more efficient method, one could be halving the resolution of the images using the frame-compatible format so they can fit within the channel space.
B) The other reason, in addition to the above, is that their channel space would be wholly occupied with just the 3D version of a program even when using the “frame-compatible” 3D format, leaving no room for the simultaneous broadcasting of the 2D version for legacy HDTVs.
Therefore a broadcaster has to make hard decisions:
1) Which version to broadcast to viewers if the channel is used only for that version (and leave the TV viewers of the other version with no image), or
2) Broadcast both versions but digitally compress them further to share the same bandwidth, which can potentially create issues of image quality for both formats.
At least 50% of US households already have DTVs (in excess of 180 million). By the end of 2012 that number may grow another 80 million to a total of 260 million DTVs. On the other hand, just a couple of million 3DTVs were sold since mid 2010 when 3DTV was introduced, those TV models were from the higher-end DTV lines.
Depending who estimates the expected sales of the 3D category on DTVs, another 10 million (of the overall 80) may be sold by the end of 2012, after two+ years of market availability of 3DTV. Judging by some manufacturer announcements the 3D feature within DTV models will grow its share as it is gradually implemented in smaller sizes and mid level models.
In summary, when looking at the relatively small proportion of 3D as a TV feature within the whole DTV inventory expected by the end of 2012, the prospective of leaving dark so many 2D TVs to broadcast a 3D program for so few 3DTVs makes option 1 unrealistic. Therefore, overcompressing or butchering resolution to make 2D/3D signals fit in the single 6 MHz channel (option 2) seems inevitable. Since 1998 those that appreciated image quality got accustomed to endure that scenario.
HDTV quality has been gradually penalized over the past decade to allow the multi-casting of parallel SD channels and to accommodate the new mobile/portable DTV service. To make matters worse, the FCC proposed a plan to recover part of the spectrum from broadcasters that do not use their whole channel allocation (and “voluntarily” agree to return part of it for a price). Once the spectrum is recovered it will be reassigned for other services within the country’s broadband plan. 3DTV is not a priority within that plan.
Is the US Alone in this Dilemma?
The broadcasters claimed that the service-compatible format more efficiently transmits 2D and 3D together because the Delta difference between the left and right images is the only signal added to the base 2D full resolution image (“2D-plus-Delta”), rather than transmitting two individual L/R half-resolution images as the DVB A-154 frame-compatible format does.
According to the comments issued on some articles, the European broadcasters declared that in addition to being more efficient, the base 2D signal within the transmitted 3D broadcast is full resolution and can be tuned and be fully backward compatible with legacy and future HDTVs.
On June 7, 2010, the ATSC has formed a planning team to work on the 3D broadcast format to be adopted in the U.S. The chair of that planning team is Mr. Craig Todd, Dolby Laboratories, ATSC Board Member. The scope of the team is “Considering broadcaster requirements, report on the likely benefits and limitations of a standard for the advanced television systems committee, inc. terrestrial broadcast delivery of 3DTV.”
Just recently an interim report from that ATSC team was released after 6 months of work. The report addressed a) the human vision health issues of prolonged 3DTV viewing, a much needed analysis that still requires further research, b) broadcasting recommendations for safe levels of image depth and speed changes of depth to avoid viewing discomfort, and c) several scenarios to transmit the 2D and 3D signals over existing infrastructures, with benefits and limitations, and a comparison table at the end.
It is interesting to note that the interim report was not known when I drafted my article and the scenarios follow the general idea and alternatives I analyzed above, including the potential impact to image quality of both 2D and 3D signals due to the needed compression to fit the available bandwidth.
More interesting was that the 2D-plus-Delta format favored by the European broadcasters (and not adopted as 3DTV standard on phase 1 of the European’s 3DTV implementation) was not even mentioned as an alternative, but rather the 2D-plus-Depth format, which uses a depth--map technique over the 2D base image to convey the depth of the 3D image.
Next Article: Which format is the ideal for the U.S.?
Posted by Rodolfo La Maestra, March 26, 2011 10:25 AM
About Rodolfo La Maestra
Rodolfo La Maestra is the Senior Technical Director of UHDTV Magazine and HDTV Magazine and participated in the HDTV vision since the late 1980's. In the late 1990's, he began tracking and reviewing HDTV consumer equipment, and authored the annual HDTV Technology Review report, tutorials, and educative articles for HDTV Magazine, DVDetc and HDTVetc magazines, Veritas et Visus Newsletter, Display Search, and served as technical consultant/editor for the "Reference Guide" and the "HDTV Glossary of Terms" for HDTVetc and HDTV Magazines. In 2004, he began recording a weekly HDTV technology program for MD Cable television, which by 2006 reached the rating of second most viewed.
Rodolfo's background encompasses Electronic Engineering, Computer Science, and Audio and Video Electronics, with over 4,700 hours of professional training, a BS in Computer and Information Systems, and thirty+ professional and post-graduate certifications, some from MIT, American, and George Washington Universities. Rodolfo was also Computer Science professor in five institutions between 1966-1973 in Argentina, regarding IBM, Burroughs, and Honeywell mainframe computers. After 38 years of computer systems career, Rodolfo retired in 2003 as Chief of Systems Development from the Inter-American Development Bank directing sixty+ software-development computer professionals, supporting member countries in north/central/south America.