BBC details of the Streetdance 3D broadcast

Regular readers will recall that at the start of the month, I wrote about the broadcast of Streetdance 3D on the BBC HD channel, and speculated about the method that was being used to provide support for 2D viewers.

In a blog post today, Ant Miller at BBC R&D reveals how it was done.

More 3D on BBC HD – Streetdance

If you thought that the final of Strictly Come Dancing in 3D was all you were going to get from the BBC over the holidays, you might have missed an unexpected treat on New Year’s Day, in the form of Streetdance 3D, on the BBC HD channel.

I watched this mostly because it features dance dreamboat Richard Winsor, whom I’ve seen several times at Sadler’s Wells, but also because I was a little curious about it being broadcast in 3D. It was only a few days ago I posted here that it would be inconceivable that 3D broadcasts wouldn’t, for the time being at least, be accompanied by a 2D simulcast for those without a 3D set. Yet the film was scheduled only on the BBC HD channel, so what was happening?

The clue, it turned out, was in the red button. Switching to the channel while the film was on showed the familiar double image of a side-by-side HD broadcast, where the picture is split down the middle, with an image for each eye on one side of the screen.

Repeated on each side was a red button icon labelled ‘Watch in 2D’, and pressing that switched to the 2D version. So far, nothing unusual there. And my first thought was that it was a simulcast, perhaps using one of the interactive streams that wasn’t needed for anything else.

Except that the 2D version looked better quality than standard definition, and I don’t think there’d be the capacity, even on a day when there’s no sports, to manage that. Another give-away was that there wasn’t any break in either picture or sound when switching between the two. Normally, if the box has to tune to another stream, there’ll be at least a momentary break in both.

Technical tricks

So, how was it done? My guess – I’m waiting for the BBC to confirm, but I’m 99% certain – is that it was all done with MHEG, the interactive ‘red button’ service. This was evident when the application was still available during the following programme.

Pressing Red simply turned on or off one of the useful features that MHEG provides, which is video scaling. Taking advantage of the side-by-side format, the application simply took the left side of the screen, and zoomed it to fill the whole screen; obviously there’s a small loss of horizontal resolution in doing that, compared to broadcasting a full screen HD picture, but it still looked better than SD, and meant there was no glitch on switching formats.

How do I know it was the left hand side? ‘A little Later’ was on after the film, and the application was still live for at least a part of that, with the icon appearing twice on screen. Pressing Red during that zoomed the left side of the screen, losing the right.

So, I’d say that on the evidence so far, it looks like it’s perfectly possible to provide a 2D/3D simulcast without using any extra bandwidth. The downsides are that resolution isn’t quite as high as for HD, and 2D viewers have to take a positive action to see the ‘normal’ version, unlike the Strictly simulcast, where they just tuned to a 2D version.

You probably wouldn’t want to use this for a BBC One programme, because it would end up on the standard def channel version of the channel (unless that can be fed separately from the HD version), and a lot of older set top boxes don’t have an up to date MHEG engine which can handle the video scaling, so wouldn’t be able to switch to 2D; all HD boxes should have the necessary software, as long as they are FreeviewHD certified.

Incompatible equipment

That means, incidentally, that users of the 3View box probably wouldn’t have been able to select 2D, as it doesn’t have MHEG built in, and the same would be true of people watching with a generic satellite receiver, rather than a Sky or Freesat branded one.

So, this is also potentially interesting from that point of view; whatever equipment you have, even a generic satellite or HD terrestrial tuner, you would have been able to watch in 3D. But only equipment compliant with one of the UK’s platforms – FreeviewHD, Freesat or Sky – would have given 2D viewers the opportunity to select that version of the broadcast.

This sort of thing is not going to happen very often, but nevertheless it raises interesting questions about exactly what equipment should be supported by the BBC, and might well be seen by some as pushing people towards platform-compliant equipment.

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.