3D-TV and Bandwidth - Part 2
Recently, a reader posted a question to this column’s Jan. 18 translation on 3D-TV and bandwidth.
“If HDTV is 6x the bandwidth for satellite transport via MPEG-2,” wrote the optimistically-named ‘3D 2010,’ “what is the rule-of-thumb bandwidth necessary for HD 3D?”
First, a distinction: That “6x” number signifies the uncompressed bit rate for HDTV. This matters especially to that link between the set-top and the TV, known as HDMI, which also talks in uncompressed digital.
Another baseline: Let’s say 3D-TV is a feature layer on top of a 720P or 1080i HDTV (because it is), and that it expects incoming pictures to be delivered at 60 frames per second.
To send a true, full-resolution, stereoscopic image to that 1080i or 720p HD display would indeed use twice as many bits, over the uncompressed (HDMI) interface. Why: Because the TV expects 60 frames per second, yet you’re sending two images – one for each eye.
So, to keep it at the same resolution and frame rate, uncompressed, it’d take two times as many bits to do 3D-TV. If there’s a rule of thumb, 3D-2010, that’s pretty close.
However. That third dimension in 3D is depth. For your brain to perceive depth, extra processing is required – which makes resolution anomalies in the original two dimensions less perceptible.
In other words, your brain is so busy extrapolating depth, it likely doesn’t notice the lower resolution on the streams representing each eye.
That’s why you hear people on the bandwidth side of the chain use terminology like “frame compatibility,” and you hear makers of 3D-TVs (and some content owners) use terms like “full resolution 3D.”
Frame compatibility crams the left and right eye images into one frame, each at a lower resolution. It fits into the same space as 2D, needs no special transport handling, and theoretically can’t be resolved as lower resolution to the human eye — because the addition of depth perception blunts a hyper-critical focus on each frame’s resolution.
Here’s how 3D-TV will likely emerge as a cable service, at least in the beginning: On-demand, through the VOD or switched digital video (SDV) passageways, to a set-top outfitted with any updated requirements for 3D-TV.
Like so: You want 3D, Customer Bob? Here’s a new box. Here’s the menu of 3D titles. Knock yourself out.
Longer term, watch for two developments: Another extension of video compression, called Multi-View Coding (MVC), to further squish HD and 3D signals down into a more transport-friendly size, and faster versions of what HDMI does to move uncompressed 3D signals between set-tops and 3D-TVs, at 60 frames per second.
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By Jens Koerner