What are the advantages of Side-by-Side versus Top-and-Bottom 3D Formats? (Part 2)

In my previous post, I pointed out that CableLabs, a standards and research body funded by the cable industry, recently released a specification for stereo-3D formatting for CATV systems:


We covered the intricacies and pitfalls of using TaB (top-and-bottom) format for interlaced sources such as 1080i. As previously mentioned Cablelabs has a couple of figures that visually explain these two alternatives:

Why not keep it simple and use Side-by-Side for all formats then?

To recap, Side-by-Side (SbS) with 720p60 becomes 640×720@60hz per eye. Top-and-bottom (TaB) becomes 720×360@60hz per eye. Both of these are almost standard definition TV resolution. In some cases, the resolution looks closer to VHS than DVD.

So why does CableLabs issue the verdict that ALL 720p 3D CATV should be transmitted TaB? At least 640×720 (SbS) is a nice anamorphic match to the resolution tradeoff of CinemaScope.

As if to add to the confusion, Comcast and several other MSOs broadcast the Masters golf torunament in 1080i SbS format, which was produced by ESPN — who shot the earlier test, and the event itself in 720p60.

How To Uncross Our Eyes Over This

All of these contradictions regarding 720p60 3D transmission seem extremely confusing at first glance — to sort it out one has to peel off all of the layers of the onion, one by one.

The first clue is that CableLabs’ document does not cover the format used in shooting, for initial backhaul to the Comcast Media Center (or other earth station for distribution), nor other intermediate transmissions — just for the CATV system itself, which is to say from the headend to the home.

The second clue is that all of these formats can be converted from one to another; with varying degrees of success and loss of resolution and temporal motion from scenario to scenario.  We saw a pernicious example of this in the previous installment of this series.

The third clue is that almost every North American sports broadcaster: ESPN, Fox Sports, to name a couple, settled on 720p60 as their primary (only) HDTV standard long ago, before 1080p60 began to proliferate. The reasons why this was done are beyond the scope of discussion in this post, but with the exception of TNT Sports, all sports programming in North America is originated in 720p60.

Fourth, in many venues, for many events, we also have to contend with the fact that the camera, switching, tape and replay, and other portions of a program prior to broadcast or uplink termination may also be shot in a different format. Example: a baseball game may be shot with 1080i, shuttled around the broadcast compound, before being converted to 720p60 for uplink.

Fifth, some content does not originate from live events but rather is cinematic content. North American cinematic content is shot at 24fps — sometimes, the legacy content had already been stepped up to 59.94i for broadcast with older analog equipment, sometimes film-to-video transfers of content are available at the original 24fps.

Finally, how the content is actually separated out to YOUR eyes as a viewer by your 3DTV impacts this a great deal. With the latest 3DTV displays, the signal for both eyes is separate, but the actual hardware 3DTV combines them. Various 3DTV displays separate the left and right eye differently, with different mechanisms, advanced engineering, and results. Some use shutter glasses, others use passive glasses (like you get in RealD theaters, like Gucci is producing now)…

Transmission is only half the problem

And this is the key.  To really sort all of this out, we have to analyze the 3D TV display market… and only if we make a very surprising conclusion, then Cablelabs’ push for 720p60 TaB begins to make sense.

This surprising conclusion is for the next installment, when we trace a 720P Stereo3D broadcast from origination to a viewer’s passive-glasses xPol 3DTV set.


~ by opticalflow on November 3, 2010.

One Response to “What are the advantages of Side-by-Side versus Top-and-Bottom 3D Formats? (Part 2)”

  1. […] the first installment we covered how interlaced formats affect 3D broadcasts.  In the second post we covered some of the technical reasons behind using side-by-side (SbS) and top-and-bottom (TaB). […]

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