BBC HD

There's a cost saving.

At the moment the BBC have to schedule and playout 3 channels. This costs significant amounts of money - and there cane be costs involved in showing content on BBC HD when it isn't simulcast with the original channel's showing.

BBC One HD, BBC One SD, BBC Two SD, BBC HD all have separate playout areas (BBC One SD will soon be a downconvert of BBC HD) and BBC One, BBC Two and BBC HD all have separate schedules, and thus require independent scheduling (which needs people to do it).

If you move to just having BBC One HD and BBC Two HD (with BBC One SD and BBC Two SD being straight downconverts) you've saved having to schedule and playout an entire channel.

Increasingly BBC Two HD content is being originated in HD - and as the second most popular BBC TV channel it makes sense to simulcast this. BBC Three and BBC Four content will increasingly be repeated on BBC One HD or BBC Two HD until capacity can be found to simulcast them in HD as well I guess?

2x? How did you make that calculation?

720x576 = 414720
1440x1080 = 1555200

That means an HD frame/field has 3.75 x as many "pixels" (or samples) as an SD frame/field.

2x on the width in pixels (720x2=1440), less than 2x on the height in pixels. So scaled by just less than 2x in both directions. No more (and sometimes half) the temporal res. So since HDTVs will generally be bigger than SDTVs, and look more detailed things will jump from frame to frame more.

Yes - I know. If you were delivering 1440x576 that would be twice the resolution of 720x576. If you were delivering 720x1152 that would be twice the resolution of 720x576. If you delivered 1440x1152 that would be 4 x the resolution of 720x576 better. 1440x1080 is just under this, so is 3.75 x the resolution of 720x576.

If you go from 360x288 to 720x576 - is that 2x better or 4x better?

I would say 4 x - as you've doubled both horizontal and vertical resolution, and are delivering 4x more picture information (crudely 4 x as many pixels)

You can, of course, argue about what better means... It's a subjective word.

Many HD equipment manufacturers describe HD as 6 x better than SD. This is because they are usually comparing 1920x1080 with 720x480 (the US SD standard)



If you're referring to the 25p/50i encoder switching, then it will only have half the temporal resolution if the encoder gets things wrong and encodes 50i sourced content as 25p. If it encodes 25p sourced content as 25p, then the temporal resolution is identical to if it was encoded as 50i.

The 25p encoder mode should only be used when encoding 25p sourced content - so that's effectively a moot point isn't it?



Yes. But what's that got to do with an encoder that encodes 25p sourced content at 25p and 50i sourced content at 50i? It's just avoiding encoding 25p content as 50i - as that is a less efficient way of encoding 25p content and wastes bandwith which could be used to carry more spatial detail.

While I think that the encoder switching between 25p and 50i on GOP boundaries could get it wrong (eg. by wrongly encoding 50Hz content as 25Hz), and it can only switch on a GOP boundary, what I was more talking about was the 2x or 3.75x better, where increasing the spatial resolution of the format & content, and screen size, without also increasing the temporal resolution can make the temporal aspect of the bigger/more detailed picture/video format look worse than a smaller display showing a lower spatial resolution picture.

eg. doubling the width in pixels doubles the amount the object jumps in pixels in each frame/field if it's moving across the screen (assuming the video is of the same content). At 25Hz (which is often used on BBC HD) it will jump 4x the number of pixels as SD 50Hz video content. So it can make the motion aspect look worse*.

Also increasing the number of pixels x2 for both X & Y doesn't necessarily mean you will see that times more detail, it depends on how they shot it, what filters they've used in shooting and in post, and motion blur, bitrates etc. And the number of colours is the same eg. 8 bit (even though the colour space is a bit different).

*Which is why the NRK are wanting to have 120Hz for SHV.

Eh? You say "which is often used on BBC HD" as if the same content would be shown at 50i in an SD channel, which is never the case. Whether you watch in SD or HD, any motion will cover the same percentage of your view - and the same number of actual screen pixels, since the SD picture will be upscaled either by the TV or the box.

You can't really compare an episode of Eastenders with one of Blue Planet.

David

Yep - a very valid concern on shows like Top Gear and The One Show where 25p and 50i content are mixed - and unless a GOP boundary is triggered by the encoder change (rather than waiting for a fixed length GOP to end) you can have frames of incorrect content.

Similarly I've not watched close enough - but a 50i show containing a large LED screen with 25p content in it - and a singer small in frame being shot 50i could confuse the encoder?



Yep - better is subjective. I was just questioning the 2x vs 4x.

You're dead right that the higher the spatial resolution, the more of an issue motion blur becomes, and thus limits of low temporal resolution become an issue. (Interesting that Peter Jackson and James Cameron are both planning to use 48Hz rather than 24Hz for upcoming productions)

Size of screen also depends on viewing distance I guess. SD content on a very small screen can look very nice and sharp, and HD content on a huge screen can look terrible. I wouldn't describe the SD on a small screen as 'better' though.



I think you have to be careful what you are comparing.

If you are comparing the same content on SD and HD outlets - then the temporal resolution should be the same on both. 25Hz progressive capture is used on BBC HD, but then these shows are also shown on BBC Two, Three or Four with 25Hz motion...

The 4 x jump is presumably only the case for diagonal motion though - linear horizontal or vertical motion is only 2 x the pixel jump?

I think you have to be careful when introducing temporal resolution into the SD / HD comparison. It's a valid discussion to have - but it depends on a lot more than numbers.



Yes - but you can say the same for SD when showing old archive. You have to have a baseline, or a 'maximum', resolution you can achieve. Doesn't mean that the content carried meets these, but it does give you something to work with.



Do you mean NHK (Nippon Hôsô Kyôkai of Japan) - or NRK (Norsk rikskringkasting of Norway)? Suspect you mean NHK.

It's interesting that they are shifting toward 120Hz. I saw SHV at the same IBC I saw the BBC demonstrating 25p, 50p and 100p (and some 300p slowed down to 100p) to demonstrate the motion blur issues that are increasingly significant as spatial resolution exists.

I think at the end of the day we're kind of agreeing. I just don't agree that describing 1440x1080 as "2x better" than 720x576 is particularly accurate unless you specifically quantify "better" via some objective means to back it up.

Though Peter Jackson (or his cinematographer) is using 48Hz (47.96, with 270 degree shutter) now for shooting The Hobbit film(s) currently in production, James Cameron is still deciding on what higher frame rate to use (mostly talking about 48 or 60).

Yes, I meant them not NRK


For that I was comparing 25Hz temporal res HD (eg. a lot of BBC HD shows) with 50Hz temporal res SD content/broadcasts (eg. live SD TV). So because the HD example is half the temporal res, it jumps twice as much. Because the picture is twice the number of pixels in width, it's doubled again, so on horizontal motion it would jump 4x as much (in pixels).

Comparing:
watching SD content on an SDTV (or a HDTV) at the maximum viewing distance where you can fully resolve the full SD resolution to:
watching HD content on a HDTV at the maximum viewing distance where you can fully resolve the full HD resolution.

By upgrading to HDTV and HD content, and by having a bigger screen size/sitting at a distance where you can get the full benefit from the increase in spatial resolution of HD content, even at the same temporal resolution you've increased the amount objects jump per frame/field relative to your eyes.

Though they're different types of content, you can compare their spatial and temporal resolutions, etc.

Sky i think said 5x better but I actually think its only 4, but you are the expert noggin.

I don't think you can compare 50i SD with 25p HD - that's comparing apples with oranges.

The same temporal resolution is used to broadcast the same content on SD and HD. Very little BBC HD content isn't also broadcast on an SD outlet (Jazz HD shorts and some film shorts spring to mind as BBC HD only) - so I think it's flawed to compare 50i SD with 25p SD.

You have to compare the same content broadcast on SD and HD outlets - doing otherwise is flawed surely?

However if you want to compare 50i SD with 25p SD then you should consider the full motion issues, which will mean that at full motion the SD resolution would be equivalent to 720x288 at 50Hz, whilst at 25p you'll have a 1440x1080 image at 25Hz (albeit potentially with motion blur - though that will depend on shuttering).

Sky will be comparing 1920x1080 with 720x576 (UK SD resolution) - which is 5 x
US broadcasters compare 1920x1080 with 720x480 (US SD resolution) - which is 6 x

The BBC should be comparing 1440x1080 with 720x576 (UK SD resolution) - which is 3.75 x
However you could also compare 1440x1080 with the roughly accepted 4:3 SD analogue PAL resolution (roughly 544x576 resolution) - which is 5 x

Both comparisons assume the same temporal resolution at both resolutions. Neo appears to be arguing that HD is more likely to be 25p/24p than SD which is more likely to be 50i/60i. I'd counter argue that the same content is broadcast on both outlets as a simulcast. Increasingly 50i SD content is sourced 50i HD, and 25p SD content is sourced 25p HD.