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Which 3D Format?

Which 3D Format?

It’s possible that all this talk of side by side and over/under 3D formats has started to get you feeling a bit puzzled. After all, the recently agreed Blu-ray 3D standard has plumped for an 'alternating frame' system, hasn’t it?

Indeed it has. And it’s here - in understanding how the next generation of 3D TVs can handle multiple 3D formats - that 3D life circa the first half of 2010 gets seriously confusing. Certainly, a lot more confusing than you might have expected in the wake of a so-called 3D format agreement!

To try and get to the bottom of this multi-format situation, the first thing I have to do is explain how the Blu-ray alternate frame system works, and why it was developed when there were already side by side and over/under 3D alternatives.

Basically, the Blu-ray system presents the slightly off-set left and right 3D images as two completely separate alternating frames, one for each eye - not simultaneously within the same frame, like Sky’s system does. The alternating frames switch from eye to eye so quickly that your brain puts the separate images together to form a single 3D effect. In order for the effect to work, you have to wear special glasses, like you do with Sky’s 3D system. But these glasses are very different, for they contain active shutters that open and close in sync with the alternate frames as they switch from eye to eye.

Because of this difference in the glasses technology, Sky’s side by side 3D system is often talked about as a passive 3D system, while the alternate frame approach is described as an active 3D system.


The reason the AV world has felt the need to come up with the new Active 3D system is simple but important: resolution. After all, if you adopt the side by side or over/under passive 3D system, you’re inevitably not going to be able to retain a Full HD resolution, since effectively two different image frames are having to share the same 'space' and resolution normally occupied by just one. If, on the other hand, you present a full single frame to each eye sequentially, you can present each eye with a Full HD, 1,920 x 1,080 image, retaining the resolution that AV fans hold so dear these days.

The first question all this raises is why Sky didn’t just keep things simple and follow the Full HD Blu-ray standard. The answer has a number of sides.

First, the alternate frame system for Blu-ray is designed to work with 1080p/24 sources, whereas Sky needs to broadcast in 1080i/50. Especially where sport is concerned.

Sky could potentially show 3D programmes in 1080p/50 - the 50Hz rate has to stay - but for now, at least, the broadcasting infrastructure doesn’t exist to support the extremely heavy data streams 1080p/50 broadcasts would require.

Brian Lenz told us that it might just be possible to do 720p alternate frames using the current infrastructure, but this brings us to the second reason why Sky is sticking with the side by side system. For it’s Sky’s view, after extensive testing, that the 1080i 'side by side' 3D system consistently produces the best 3D effect for the sort of generally action-packed material its 3D efforts are focussed on.

Next, adopting the side by side approach means Sky doesn’t have to do anything radical at all to the infrastructure of their content delivery system, be it at the broadcasting or receiver box end.

A final explanation - some people might have used the word justification here, I guess - for Sky opting for the side by side approach is simply that not all of the 3D TVs coming our way are going to use the active, alternate frame 3D system. The LG sets used for Sky’s recent 3D pilot footie match, for instance, use the passive approach.

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