By David Wood, DVB CM-3DTV group
We are in a race. Walk into many a large TV store across the world and there, enticingly, is a ‘3D’ TV display – just waiting for your credit card. There is a 3DTV Blu-ray standard and 3D Blu-ray players on sale (though alas, it is said, fewer 3D Blu-ray discs on sale than different logos for 3D televisions). There are 3DTV satellite channels on air in the United States and Japan, demo channels on Eutelsat and Astra in Europe, a 3D IPTV channel from Orange in France, the BSkyB 3DTV satellite channel, trials in Australia, and more. The world needs to focus this incredible enthusiasm for 3DTV into a common open standard, particularly for the first pay TV services, so we can protect the investments of both broadcasters and the public. It has to be done carefully and with due diligence. But the reality is that it has to be done quickly. It is being achieved.
It may seem that there is a random assortment of bodies developing overlapping 3DTV standards, but the real situation is that those involved are pieces of a large jigsaw puzzle, that actually fit together quite well.
The standard’s jigsaw
The DVB project is a pre-standardization body developing standards for the broadcast or cablecast of 3DTV. The SMPTE is developing standards for the file format that will be used for the production of 3DTV. The IEC/ISO MPEG community is largely developing compression technology for future 3DTV systems. The ITU-R hopes to prepare worldwide Recommendations for 3DTV from inputs it receives. We should also remember the HDMI consortium of set makers, which has developed the physical interface between a 3DTV set-top box and the display. And we also look to the 3D@home consortium, hoping they will rationalize the (crazy) differences in standards used for active 3D viewing glasses. Not too bad a jigsaw?
Since February 2010, the DVB project has been working on 3DTV standards. First Commercial Requirements are prepared, and then the technology to provide them is found, which is back-checked that it will do that is asked of it. Sub-groups on 3DTV in both the Commercial Module and the Technical Module are in action.
Steering Board approval
At its meeting on July 2, 2010, the DVB Steering Board approved the Commercial Requirements for 3DTV. This represents a major step forward which should lead, all fingers crossed, to a DVB 3DTV specification before the end of 2010.
The 20 Commercial Requirements are tailored to a particular business environment, where you need to provide 3DTV by making use of an existing population of set-top boxes. This is often the case for pay TV operators, who will be the first with their feet in 3DTV waters. This is not the only commercial situation, but it is the one covered by the DVB Commercial requirements now agreed.
The job to be done here is to deliver two images, the Left and Right eye signals, so that they can be seen properly with depth on a 3D display, using special glasses. If, as here, you must use existing HDTV set-top boxes, the signal must appear to the set-top box to be an ‘HDTV’ signal. The set-top box decodes it as a normal HDTV picture, and passes it to the display. The display then unravels the picture to create the Right and Left eye images for display. This is the ‘Frame Compatible’ approach. Viewers will need a new display but not a new set-top box – though possibly an upload may be needed.
The Frame Compatible approach
The Commercial Requirements for this environment, stated simply, are that the baseband signal broadcast must come in, and then be ‘passable’ over the connector between the set-top box and the display. Fortunately, much work on this connector has been done by the HDMI consortium that we can draw on for our broadcast signal.
There is a range of different ways of arranging a Frame Compatible signal, but in essence it is a ‘spatial multiplex’ of the Left and Right images, which together forms an HDTV image frame. The ways of doing so include the ‘Side-by-Side’ approach, where the Left and Right pictures are sub-sampled and anamorphically squashed in width.
They also include the ‘Top and Bottom’ approach where the images are sub-sampled vertically, anamorphically squashed in height, and placed one above the other. When deciding what to allow in a broadcast system we have to take into account that there are four HDTV formats, all in use, and that in different cases the Side by Side or Top and Bottom approach can deliver the higher picture quality. In the CM-3DTV discussions we have tried to make sure that the formats allowed would meet worldwide needs.
The eight formats
There are eight formats listed in the Commercial Requirements, which is actually a small subset of all the possible combinations of field rates, picture rates, scanning algorithms, and lines/screen. They are as follows:
a.) 720p@50Hz Top & Bottom
b.) email@example.com Hz Top & Bottom
c.) 1080i@50Hz Side by Side
d.) firstname.lastname@example.org/60Hz Side by Side
e.) email@example.com/24Hz Top & Bottom
f.) 720p@50Hz Side by Side
g.) firstname.lastname@example.orgHz Side by Side
h.) email@example.com/24Hz Side by Side.
The formats a) to e) above are given in the relevant HDMI specifications as mandatory formats for all displays. The last three are listed in the HDMI Primary formats so they cannot be guaranteed for all 3DTV sets in the short term.
The format being broadcast needs to be signaled to the receiver, and the Requirements call for the signaling to be extendible, to include other formats if needed.
The Requirements also call for a system that will allow exclusively 3DTV programs and mixed programs, but suggests that the switch from one to the other should only take place during program transitions.
Provision should be made for identification of 3D programs.
Subtitling, captioning, audio, and pixels
One of the stickiest areas for television engineers in 3DTV is how to cope with subtitles, captions, and other multimedia items that are meant to be overlaid on the picture, but are not broadcast embedded in the picture. The 3DTV must be able to signal where a subtitle should go with respect to the stereoscopic picture. As an example, it might signal the position of the sub-title in the ‘z’ or depth plane, via the ‘disparity’ between the left and right images. The specification should also provide a mechanism that allows on-screen graphics to be best positioned on the screen. We might normally expect such things to be placed just forward of the most forward object in the scene, but this is not necessarily always the case. Default behavior is also needed where there is no positioning information available.
It should also be possible to signal that a simulcast in 2D of a 3D broadcast is available.
It should be possible to signal pixel arrangements for the two images in the frame, though the HDMI specification may not carry all pixel arrangements.
The signaling that should be broadcast is one of the most important elements, and this must include all the eight formats above, but equally allow expansion. An annex to the requirements asks the DVB Technical Module to investigate a format often termed ‘tile framing’ and to provide guidance on its advantages and disadvantages.
The audio system was the subject of some discussion. In practical terms the only choice, and only sound technology available, is that associated with HDTV today (surround sound etc.) so support for this is included in the requirements.
Finally, we should not forget the requirement that nothing proposed in the signaling or distribution video formats for the 3DTV specification should prevent future expansion to support new encoding technologies.
The first part of the journey
The Requirements can be found in full in the DVB ‘Blue Book’ which is available on the DVB website, together with a number of guidance and informative notes, and a glossary of terms. The DVB Technical Module is now actively working on a specification to meet these requirements. If you want more, try the 3DTV webinar on the DVB website. The recently published 3D production guidelines from BSkyB is worth downloading from its website.
Our hope is that there is now a signal to all those interested in 3DTV that specifications are ‘in hand’. This work should also provide useful input to the other bodies listed in the first paragraph. We believe the die has been cast on ordered and standardized broadcast and cablecast 3DTV for the Frame Compatible community.
David Wood is the Deputy Director, Technology and Development at European Broadcasting Union and Chairman of the DVB CM-3DTV group. CM-3DTV was established at the beginning of 2010 to prepare commercial requirements for 3DTV delivery standards. The group is developing a series of commercial requirements corresponding to identifiable business cases for 3D TV.
Copyright DVB SCENE. Reprinted with kind permission of DVB.