A major report commissioned for the regulator Ofcom has recommended the use of the 2D plus Difference method of 3DTV transmission.
The proposal for the system, also known as 2D plus Delta, is made in Beyond HDTV: Implications for Digital Delivery, written by the consultants ZetaCast. Under the method one eye is chosen as the 2D video, which is encoded conventionally, allowing a viewer with only a 2D decoder to view the 2D video normally. When a stereoscopic decoder is used, the difference signal modifies the 2D video to recreate the view from the other eye. The difference signal can be compressed using a standard video encoder. ZetaCast estimates that the total bitrate would be between 1.4 and 1.8 times the 2D video alone.
The report carries significant weight because of an earlier report in which ZetaCast recommended DVB-T2 could be used in combination with a revision of the multiplex layout to launch HD services over the terrestrial system. BSkyB, which has plans to launch a pay-per-view 3D channel during 2010 is proposing the use of Frame compatible, side-by-side AVC.
“Of the ‘2D plus metadata’ methods, ‘2D plus difference’ is the most practical today,” says the report. “2D plus depth has the potential to reduce the required bit-rate further, but the depth map is likely to be difficult to create with any great precision, particularly for real-time events”.
Part of the ZetaCast scenario is that by 2020 a combination of a new compression algorithm and more sophisticated encoder implementations will take the bitrate down to 50% of the levels required today for 1080p/50 content, ie, 6.5 Mbps. The consultancy also assumes there will be no fundamental breakthrough in “3D” display technology before 2020. This leaves the same options for 3D display as today, requiring the use of either polarised or shutter glasses.
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