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 Come Dancing in 3D

During last Saturday’s show, the presenters of Strictly Come Dancing made much of the fact that the final will be broadcast in 3D. A look at the comments on the website for the show reveals that quite a lot of viewers are rather confused about whether or not they’ll be able to watch in high definition, so here’s my attempt at clarifying what’s happening (based on my own post on that blog).

I don’t have a 3D set. Will I still be able to watch Strictly in High Definition?

Yes. The normal BBC One HD broadcast will still be happening, so you won’t miss out. The 3D broadcast is on a completely different channel.

I don’t have HD. Can I still watch in 3D?

Only if you go to one of the cinemas that’s showing 3D, or if you buy a 3D receiver (like a Freeeview or Sky HD box) and install it before the weekend, and have a 3D TV. Because 3D needs two pictures – one for each eye – it’s not possible to transmit 3D without using an HD channel, and still maintain good picture quality.

I don’t understand! How can it be in 2D and 3D at the same time?

I think a lot of confusion here comes down to the BBC referring to “The BBC HD Channel” when they’re talking about 3D. There are actually two high definition channels from the BBC; one is BBC One HD, which shows whatever is on BBC One, but in high definition, and the other is just called BBC HD, or “the BBC HD Channel”, and carries a selection of programmes from other channels including, next weekend, the 3D version of Strictly Come Dancing.

So what’s on, where?

Here’s the quick summary:

BBC One, standard definition: The ordinary version of Strictly Come Dancing; find this on Freeview 1, Sky 101, Virgin 101, Freesat 101, or on analogue if you’ve not yet switched over.

BBC One HD: The high definition version of Strictly Come Dancing, in old fashioned 2D. Just like every week. Find this on Freeview 50, Freesat or Virgin 108, Sky 143.

BBC HD: The 3D, high definition version of Strictly. Find this on Freeview 54, Virgin 187, Freesat 109, Sky 169.

What do I need to watch in 3D?

You’ll need a 3D television and access to the BBC HD channel; try the channel numbers I’ve listed above, and see if you get BBC HD. If you do, and your TV is 3D, then put on your glasses, tune in on Saturday night, and you’re all set – your TV should automatically recognise the 3D transmission. If it doesn’t, press the 3D button on the remote, and if you have to pick a ’3D format’, choose ‘Side by side.’

Many 3D televisions will have a built in receiver for Freeview HD; if that’s not available in your area, you’ll need either Sky, Virgin or Freesat, and an HD receiver for one of those services.

Will the 3D broadcast be exactly the same as the normal one?

It’s likely to be slightly different, with some different camera angles – different cameras are used for 3D, and some sorts of shots will work better in one format than another. So, expect there to be some differences betweeen the two.

Will I be able to record the 3D show?

Yes. If you have a digital TV recorder, like Sky+, Freesat+ or Freeview+ HD, then just record the show on the BBC HD channel (see the numbers above), and you’ll be able to watch the 3D broadcast later, just as if you were watching it live.

Why isn’t it in a cinema near me?

Lots of the comments on the BBC site seem to suggest this is poor planning by the BBC, but it’s important to remember a couple of things. First, broadcasting in 3D is still pretty experimental, and secondly, not all cinemas are set up for live 3D, even if they can show feature films in 3D.

Feature films are sent out on hard disks, or downloaded to the cinema in advance, and then shown many times. The Strictly final will be live, and so can only be shown in cinemas that have a suitable link to receive live shows and broadcast them directly to the screen, which not all will be set up to do.

Ultimately, whether or not a cinema is set up for that will depend on the company that owns it, not on the BBC, who can’t pay to install equipment in someone else’s commercial premises. If your local cinema isn’t showing the final, then tell them you’re disappointed. It won’t make a difference this week, but they might realise that there is a market for future events to be shown that way. If you don’t tell them, or just complain about the BBC, your local cinema isn’t going to know they’re missing out on business by not investing in the right equipment.

How does the 3D system work?

For those interested in the technical side, the 3D broadcast will use a system called ‘side by side’. If you tune in to the BBC HD channel and don’t have a 3D set, what you’ll actually see is that the screen has two almost identical pictures, squeezed into the frame. A 3D TV recognises this, and shows one at a time, zoomed to fill the whole screen, and controls the timing with the 3D glasses so that the left eye sees the pictures on the left, and the right eye sees the ones on the right.

One of the consequences of this is that each picture actually has only has as much information as the normal HD picture, because it has to fit into half the screen; you probably won’t notice a massive difference, but that’s why 3D is always used with HD channels – if you tried it on a standard definition one, you’d really notice the difference.

Wimbledon goes 3D

The Mens’ and Ladies’ Finals of this year’s Wimbledon Championships will be the UK’s first free to air 3D television broadcasts.

The BBC has announced that live 3D coverage of the finals will be provided on Saturday 2nd and Sunday 3rd July, on all the digital TV platforms that carry the BBC HD services. The finals will be broadcast in standard definition on BBC One, in high definition on BBC One HD, and in 3D on the BBC HD channel.

There will also be live 3D screenings in some cinemas around the UK, and I daresay a lot of shopping centres will be getting in extra sets of specs, in the hope of shifting plenty of kit on the day.

The BBC press release puts this firmly in the “editorial experiment” category, so it’s premature to expect any regular service at the moment; this is a test, and also a way to mark the 125th anniversary of the Championships as well.

How it works

The release is a bit short on the technical detail, about which I’ll see if I can find more information (Update: ‘side by side’ format is confirmed; see below). But since no special receivers are required, then my gut feeling is that they’ll be using the ‘side by side’ format to broadcast.

In ‘side by side’ 3D, a single frame actually has two images encoded in it, one for the left and one for the right eye. So it looks a bit weird to an ordinary receiver, but you can still make sense of it, as it’s just a picture, albeit with each half almost the same. A 3D TV, however, will spot the format and zoom each half to full screen, and ensure the left image goes to the left eye and the right image to the right eye, via whatever 3D screen technology it actually uses. The downside is that horizontally, instead of 1920 (or 1440) pixels, since there are two images, each eye only receives half that resolution.

It’s possible to broadcast in other ways, but unlikely, I think.

For example, ‘frame sequential’ 3D requires sending a full HD frame for the left eye, and then one for the right eye. This is Panasonic’s preferred format, because each frame is full HD resolution, but of course that means a lot more bandwidth; you at least have to double the frame rate as you need twice as many images. I suspect, at rates that a receiver understands, this wouldn’t actually break most 2D sets, but you’d get a nasty flickering. It’s really best suited to delivery systems like Blu-Ray.

And, since Freeview HD can only go up to 25p as a frame rate, it’s not possible to use frame sequential, on that platform anyway – you’d effectively be getting only 12.5 frames per second. And I think it’s unlikely the BBC will be using different 3D formats for different platforms, too.

The other way of sending 3D, ‘2D plus depth’ is, unless I’m a bit behind, not an entirely finished standard; it involves sending information about the details for one eye, plus that needed to make the image for the other. Ultimately, some variation of this may be used to create a ‘service compatible’ standard, which is one where someone with a 2D set can tune in and see a decent 2D picture, while a person with a 3D one will see the extra dimension – which will avoid the need to have a separate broadcast stream, as the BBC are doing this year.

So, based on what I know now, I’d say it’s most likely going to be a side by side broadcast; I’ll try to check this when I can, though there wasn’t time between reading the release and the end of the embargo.

Definitely a worthy experiment, I think, and it’ll be interesting to see what people make of it. And how many people moan that three channels were taken up with the same programme.

Update: This BBC blog post confirms that they will indeed be using the side by side format, and increasing the resolution of the stream to the full 1920 pixels.