3DTV Broadcasting Will Converge Around MVC
Although most 3DTV broadcasts today are using Frame Compatible mode, where two separate HD pictures are fused into a single, 3D-compatible HDTV video stream, compression vendor Envivio is looking ahead to alternatives that could provide a higher quality viewing experience. At NAB the company was demonstrating full-resolution, 1080p, stereoscopic 3D content using its current-generation 4Caster C4 Three Screens video encoder and outlining the suitability of its flexible software architecture for accommodating changes brought about by new standards such as the MVC (Multiview Video Coding) extension to the MPEG-4 AVC standard.
That demo provided two 1080p HD pictures, with one view running at 8Mbps and the ‘Delta’, MVC specific stream running at around 4Mbps – meaning the total bandwidth requirement for what apparently amounts to a full 3DTV experience was 12Mbps. Although Frame Compatible is widely viewed as good enough for today’s requirements, discussions about ‘second generation’ 3D delivery technology are well underway.
According to Envivio, a stereoscopic 1080p 3DTV broadcast in MVC takes 1.5 times more bandwidth than a single stream of HDTV in 1080p. So that means there is a considerable efficiency saving if MVC delivers the same quality ‘full 3DTV’ experience as you would get from two completely separate left and right streams of 1080p HD.
When you compare MVC to Frame Compatible, the benefits are even more obvious, it seems. Boris Felts, VP of Marketing at Envivio, explains: “MVC carries both the 3D and the 2D compatible view and MVC has twice the resolution. If live video was to be broadcast in both 2D and also 3D Frame Compatible, we would have approximately 8 Mbps for the 2D stream and 12 Mbps for the 3D stream, but with only half resolution, making a total of 20 Mbps. On the other hand, MVC would carry the 2D and 3D versions in a single stream at 12 Mbps, in full resolution.”
MVC combines a 2D view with additional ‘Delta’ information that can be used to create the 3D effect and this means the same stream can be used to deliver 2D television (where set-top boxes ignore the additional information) and 3D television. And the set-top boxes capable of handling MVC decoding are on their way, according to Envivio. Felts comments: “Chipsets are available. We expect to obtain an STB reference design shortly for our lab and we anticipate that you will see the first [MVC capable] STB available before the end of the year.”
From an encoding perspective, Envivio says you need more processing power (CPU cycles) for MVC than is necessary for single HD - between 1.5 to two times the power, in fact. “However, we were able to do our MVC demonstration using our current generation encoder,” Felts confirms, adding that the latest, field deployed versions of the 4Caster C4 video encoder can support MVC.
“Even though MVC relies on AVC, you still have to develop a new algorithm, or more precisely extend the current AVC algorithm,” he adds. “The benefit of creating a new algorithm is that there are some additional compression improvements that can be realized by taking advantage of the high redundancy between the left and right views.”
According to Envivio, there is no quality benefit from delivering 3DTV as two separate streams of 1080p (one for each eye), even if there is plentiful bandwidth – which might be the case on FTTH. “With MVC we are delivering full 1080p to each eye already. All you are doing [by delivering two 1080p streams] is eating bandwidth using two discreet streams – there is no quality gained,” Felts says. “One important issue is that in order to do that, you would need a set-top box with dual HD decoding. MVC can deliver the same thing using a single decoder.”
Envivio believes that Frame Compatible can be good for launching 3DTV because it keeps the initial costs down and will enable more content providers and service providers to deliver more content - which is critical to generating consumer interest. How long Frame Compatible dominates remains to be seen. “It will really depend on the viewer adoption of 3D services,” says Boris Felts. “If the pick-up is low, this might stay for a long time. If the pick-up is high, it’s likely to be replaced by ‘service compatible’ MVC much more quickly as providers seek to win the quality battle.”
The company expects an eventual migration towards MVC. “Long-term, once that initial momentum is created, the shift to MVC will ensure the quality of experience required for long-term success,” it says. “With chipsets capable of MVC decoding available in Blu-ray DVD players, we would see the industry converging towards the same approach and using MVC. Remember the chipset manufacturers are the same for Blu-ray players and STBs.”
By John Moulding, Videonet
