2D compatible 3D broadcasts
The Digital Service Development group led by Phil Layton in BBC R&D was involved in the previous trial of 3D at the Wimbledon Tennis Championships this year and also the recently broadcast Strictly Come Dancing Grand Final. In this post Dr Peter Cherriman outlines the work to produce 2D compatible 3D broadcasts for Freeview and Freesat receivers.
The BBC has been trialling 3D broadcasting over the last year. While 3D content is an interesting development in television, it creates a number of challenging problems for existing 2D receivers in the market.
In order to allow the introduction of 3D broadcasting with the minimum changes to existing systems and receivers, the DVB broadcasting specifications allow 3D content to be broadcast as HD video. The stereoscopic video is merged into a single HD frame by anamorphically compressing the view for each eye into half of the HD frame. This 'frame compatible' video can then be transmitted on a separate 3D channel or broadcast as part of a 2D HD channel.
Launching a separate 3D channel does reduce some of the legacy 2D receiver issues. However, such a channel would require a significant amount of 3D content and the cost of finding additional capacity. The BBC has broadcast its 3D trials on an existing 2D channel, the BBC HD channel.
Our trial 3D broadcasts started with the Wimbledon Singles Finals and then the Strictly Come Dancing Final before Christmas. These programmes were all broadcast in 3D on the BBC HD channel. Viewers without 3D televisions were able to watch a simulcast in 2D on BBC One or BBC One HD.
However, scheduling a 2D simulcast of a 3D broadcast will not always be possible. We have been looking for solutions to allow viewers with a existing HD receiver and a 2D television to watch the 3D content in 2D. It is possible to generate a 2D view from the frame compatible 3D content, by showing the 2D viewers a stretched version of either the left or right eye view.
With the current broadcasting standards this 2D view results in a loss of resolution, to something less than HD resolution but higher than standard definition.
The 2D view could be created by new functionality in receivers or by using some low-level H.264 signalling. After testing the various options, we found that none worked successfully with all HD receivers already in the market. However, we did find a solution using "Red button" (MHEG) technology that is present in all FreeviewHD and FreesatHD receivers. This solution uses the cropping and video scaling features in the interactive MHEG engine. We originally trialled this technology on FreeviewHD receivers during our 3D Wimbledon broadcasts. We have yet to investigate whether a similar interactive application approach is possible on other platforms.
On new year's day we broadcast the film Streetdance3D, in 3D, on the BBC HD channel. This film had previously been shown in 2D on BBC One and BBC One HD. On this occasion we weren't able simulcast it in 2D on another channel. It was therefore the perfect opportunity to trial this "Red button" technology on both FreeviewHD and FreesatHD receivers. The MHEG application allowed viewers to select between a 2D and 3D view by pressing the Red Button.
Figure 1: Screen capture of MHEG application on a 2D display. The red button toggles between a 2D and 3D view. A) shows the 2D view and B) shows the 3D view.
The "3D view" passed the video straight through, so viewers would see the normal side-by-side pictures (see Figure 1B), until either their 3D TV detected the video was 3D and switched to 3D mode or they manually switched their TV into its 3D display mode.
Figure 2: Screen capture of MHEG application showing its 3D view on a 3D TV. This photo represents what a viewer without glasses would see.
The application's "2D view" used the MHEG video scaling feature to take the left half of the screen and stretch it horizontally by a factor of two in order to fill the whole screen. This effectively cropped out the right half of the screen giving 2D viewers the same picture that people with a 3D television saw in their left eye when wearing 3D glasses.
When the MHEG application was in the "3D view" viewers may have noticed that two red buttons appeared (see Figure 1B). This was to ensure that the red button icon could be seen by both eyes when the television was in 3D mode and therefore did not flicker. The two red buttons were placed so they overlapped in the TV's 3D mode (see Figure 2).
The application was designed to hide the red button icon(s) after 30 seconds, once viewers had made their choice to watch in 2D or 3D.
The MHEG application initially started in the "3D view", but it is possible to configure the MHEG application to start in the "2D view". The reason we started the application in the "3D view" was that the startup time of the MHEG application varied among receivers. If the application had initialised into a "2D view", some viewers would have seen the 3D side-by-side video for a few seconds before the MHEG application loaded and changed to the "2D view". There was also a small chance a 3D television could have detected the side-by-side video as 3D during this small time window, and switched into a 3D mode, just as the MHEG application switched to 2D resulting in a undesirable and confusing display (see Figure 3).
Figure 3: Screen capture of MHEG application in 2D mode on a 3D TV in 3D mode.
In summary, this MHEG application allowed viewers with a FreeviewHD or FreesatHD receivers without a 3D television to watch the film in 2D. The resultant 2D picture was not HD resolution, but was still much better than a standard definition picture. Using this technology may enable the BBC to broadcast 3D content, without the complications of scheduling a 2D simulcast or requiring an extra channel. However there are also some production issues with deriving 2D content from 3D, which also need to be considered.
The BBC is working with standard bodies and in cross-industry forums to help standardise the requirements related to 3D content in both current and future receivers.