Strictly in 3D – bandwidth

I posted a couple of weeks ago about the impending broadcast of Strictly Come Dancing in 3D, and I thought a few readers might be interested to know how it all worked out.

The 3D broadcast on BBC HD was preceded by a screen explaining to viewers how to set their TV to the appropriate ‘side by side’ mode, which should have been fairly simple for most – on the Samsung set I was using, it’s the first option that appeared when I pressed the 3D button on the remote control. And on the whole – at least until my guest sat on the glasses and broke them – it worked pretty well.

But what about bandwidth? Well, I’ve checked the recording sizes and durations for the 2D and 3D broadcasts on Freeview HD, as reported by the Digital Stream box that I’m currently using for HD viewing.

The 3D recording of the results show lasted 1h 4m 19s, and took up 4.5GB of disk space

The 2D recording from BBC One HD lasted 1h 4m 28s, and took up 4.8GB.

The difference in timing is probably a combination of the response time of the recorder, and slight differences between the flags being updated on the two channels; at any rate, nine seconds over an hour is negligible and we can, broadly speaking, consider each recording to be about the same length.

So, that leaves us with 4.5GB for 3D and 4.8GB for 2D, and I think that’s more or less in line with what I’d have expected, though there would need to be a lot more 3D around to be entirely sure.

The bandwidth on Freeview HD is shared between the four channels, and the BBC ones at least are ‘statistically multiplexed’, which means that if one channel is showing fast changing information, compared to something more static on the other, then the channel that needs more bandwidth can get it – think, for example, Match of the Day vs Great Expectations. Match of the Day will very likely get more bandwidth.

But, of course, for Strictly, it was essentially the same programme on two channels, and so the end result was always going to be pretty similar, and for the short to medium term at least, I think it’s pretty much inconceivable that any 3D broadcast won’t also be accompanied by a 2D simulcast, certainly on the BBC.

Why is the 3D broadcast slightly smaller, though? My suspicion here is simply that it will compress a bit better; 3D using the ‘side by side’ format has two almost identical pictures making up each frame – and in doing so, incidentally, halves the horizontal resolution of the 3D picture, compared to the 2D one – and that means more repetition.

Repetition is what compressors look out for, so that they can save space, so my gut feeling – and if you know more about the intricate innards of broadcast compression feel free to comment – is that the side by side format will tend to compress slightly better than a comparable full-frame HD picture.

As to what it looked like? Well, for the bits that I could see, the lovely Harry Judd looked even lovelier in 3D, but like quite a few people, I do find watching 3D a bit wearying, and the standard HD picture was, overall, crisper and a lot easier on the eye.

2 Replies to “Strictly in 3D – bandwidth”

  1. The side by side compression isn’t as good as you would suspect because a motion vector between the equivalent part of the left view and right view would be costly to code and so would not be selected. Blu-ray 3D doesn’t have this issue since motion vector lengths between views are shorter.

    1. Thanks for that – it was just a hunch, and I can see from your email you know a hell of a lot more about encoding than I do.

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