Sky 16:9 & 4:3 transmissions are the same stream

Discussion in 'Sky Digital TV Forum' started by Andrew_B, Feb 17, 2003.

  1. Andrew_B

    Andrew_B
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    I've been trying to get to the bottom of whether there is any compromise in picture quality between 16:9 & 4:3 transmissions on Sky Digital. The reason I've been pondering this is that there is a finite amount of bandwidth any broadcaster is prepared to 'spend' on a channel and there are more square inches of picture on a widescreen image than a 4:3 one. If the amount of data used for both images was the same and the vertical height is the same number of lines (PAL), then would the wider image have a lower resolution horizontally?

    Most people seem to be under the impression that two data streams are transmitted, one for each format. This is not the case. I corresponded with a bloke from Sky and he said:

    Q: Are there two transmissions, one each for 4:3 and 16:9?
    A: No, it is the same transmission

    Q: Does the Sky Digibox just 'lop the sides off' a 16:9 image to make it 4:3 if it's instructed to output 4:3 in the Services setup?
    A: No- it re-scales it.

    Q: Is a 16:9 transmission squeezed at the sides or stretched from the top and bottom, or is it a native format?
    A: A 16/9 transmission (like Sky premier WS) is native 16/9. Hence the "WS" in the channel name. To view this in 4/3 then the STB re-scales, otherwise the people would look either too fat or too thin. (Or too long or too short!)

    Q: If it's a native format, the vertical pixel count should be the same as 4:3 PAL, but there should be more pixels horizontally to provide enough resolution for the extra sq inches.
    A: There are the same number of pixels 720x576 in both formats. The pixels size (shape) changes between the two. The signal out of the STB is analogue, and is re-sampled by the plasma screen to whatever the resolution required. Therefore it should all work fine....

    I got the above replies about a year ago and still thought something didn't quite ring true, but given I had exhausted any further avenues of investigation, I gave up.

    Just recently, Sky have been dithering again about commencing widescreen broadcasts and we've been hearing various technical 'excuses', so it's been playing on my mind again. As a result, I conducted a little Sky+ experiment by recording a 16:9 programme then changing the STB to 4:3. Lo and behold, I had a perfect 4:3 image. Switching the Digibox back to 16:9 brought the full size image back. The 4:3 image was just the 16:9 one with the sides lopped off. You can record a 4:3 image on Sky+ and output it as 16:9 too, there is no difference in the amount of space used so it must be the same single data stream.

    How are we supposed to interpret this? If you think about it, the number of horizontal pixels used to represent the portion of the 4:3 image which exists in the middle of the 16:9 image are the same regardless of the chosen output aspect ratio. I am willing to accept that all PAL transmissions are 720x576, so how 'clever' is a Digibox at 'rescaling'? If you follow the theory, true 4:3 transmissions should look better than 'scaled' 16:9 ones because all of the horizontal pixels are used instead of just the middle section. More horizontal picture detail would then be present in the analogue video signal sent to the display device.

    Is there someone here who has an actual qualified understanding of MPEG2 video and whether or not anamorphic 16:9 transmissions are really the same as 4:3 ones, but with different shaped pixels (as indicated above)? Is there anyone else who can convince me that a Sky Digibox can reshape those 16:9 style pixels into 4:3 ones, rather than just lop the sides off the image?

    Sorry if this is a boring waste of time, but hey...
     
  2. rct

    rct
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    You are correct in your understanding:
    Think of it as all images being effectively transmitted as a 4:3 image, widescreen pics have a flag to instuct the TV to vertically "squash" i.e 16:9 has the same pixel count as a real 4:3 image. Where a 4:3 image is necessary from a 16:9 image transmittted like this, the picture is essentially "stretched" horizontally and then the sides removed - horizontal resolution is lost. This is what Sky referer to as scaling. Inevitibly, assuming the same vertical screen height, a 4:3 TV displaying a 4:3 version of a 16:9 transmission, will have a lower quality picture than a 16:9 TV displaying the same image fullscreen.

    I think I've got this right... anyone please feel free to correct me.

    :)
     
  3. StooMonster

    StooMonster
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    I recon they just "lob the sides off" if it's a widescreen transmission.

    AFAIK most 16:9 programme content is created in order to do this. Sadly :blush: I was reading a website that compared widescreen DVDs and BBC/C4 16:9 transmissions of Buffy and Angel versus the Sky One 4:3 transmissions. They had plenty of screen dumps and you can clearly see that 4:3 transmitted version is the centre of the picture with the edges cropped.

    Even sadder, I've read some more about television production and how to film/video programmes this way -- so content works with either 16:9 or 4:3.

    Therefore, as 16:9 programme content is designed to be dual purpose, and works with edges cropped off; I suggest that is exactly what the digibox does when widescreen sources are displayed with 4:3 settings. Therefore losing a significant part of the picture resolution.

    StooMonster
     
  4. cjdshaw

    cjdshaw
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    Widescreen programmes in the UK are all produced "14:9 safe". That means that they are shot 16:9 but are framed such that if you chop "a bit" off the sides, all the important image is still there. If you watch terrestrial analogue broadcasts of widescreen programmes, you will see that they have thin back lines top and bottom. They have also cut a bit off left and right. This produces the safe 14:9 image, a halfway house between 4:3 and 16:9.

    For reasons known only to themselves, most STB's don't output this format but only allow you to choose 16:9 or 4:3. If you choose 4:3 pan and scan, more than intended is chopped off the sides, so you may lose some important parts of the image.

    Quality wise, you are partly correct. A true 4:3 programme is higher quality than a 4:3 cropped version of a widescreen programme. The true 4:3 is the full 720x576 resolution, whereas the cropped version is the middle 540x576 scaled up. Also, a widescreen programme does use the same number of horizontal pixels as a 4:3 one, so the pixels per inch is lower.

    [Is there anyone else who can convince me that a Sky Digibox can reshape those 16:9 style pixels into 4:3 ones, rather than just lop the sides off the image?]
    I have Telewest, not Sky, but it should be able to do either. Setting your output to 4:3 letterbox causes it to reshape the 720x576 image to 720x432 and add black at the top and bottom. Setting it to 4:3 pan and scan makes it lop off the sides as I explained above.

    Hope I made some sense. Feel free to query anything not clear.

    (I work in video and film, so I'm allowed to know this stuff without being too sad)
     
  5. Andrew_B

    Andrew_B
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    Hi Stoo,

    Were you reading that .pdf from, Granada I think, stating that it would not accept material where 'critical action' was outside the 4:3 window? Given that analogue transmissions of 16:9 material are commonly transmitted in 14:9 with small black bars top and bottom anyway, you'd think they would let it go. Watching some of the daytime rubbish in 16:9 with two 'live' presenters, they are obviously still huddled together in the middle of the screen! If you switch the Digibox output to 4:3, it looks really funny. Even if they're not using the extra space, they certainly look more comfortable with a table lamp or potted plant off to one side:laugh: .

    I think I've got it in my head now that both 4:3 & 16:9 PAL transmissions are 720x576 and the pixels are the same size in each. The difference is that in 16:9 transmissions, the pixels are 'intended' to be a bit wider. The point the Sky 'insider' made about the video signal being analogue to the display device is the key, I think. TV pictures aren't like PC signals, so they're not pixel perfect - they are a representation of the available detail in the image, displayed according to the abilities of the display device. ( I'm excluding the use of scalers here ). So, although the source of the image may originally have been 720x576 pixels, this is irrelevant to the display because it's now an analogue PAL video signal with no pixels. I don't know what the theoretical equivalent resolution of your average CRT TV set is, but I'm willing to bet that most would equate to less than 720x576, so those displays would be compromising the available detail in the video signal anyway.

    I'd really like to hear it from a Sky insider that the digibox just chops the sides off, though:D

    Andrew_B
     
  6. Andrew_B

    Andrew_B
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    Thanks for the post cjdshaw. We must have been keyboard bashing at the same time - go to sleep!

    Okay, so if you've got a Digital STB, setting the output to 4:3 will reduce horizontal picture detail on 16:9 transmissions, but native 4:3 stuff has more detail per sq inch than native 16:9. Bummer! Is there an expression 'worst of both worlds'? Or is it 'heads you win, tails I loose'?

    Are you aware of approximately the equivalent resolution of your average CRT set? If the resolution is quite low, for most TV's the difference in detail may be irrelevant between the different formats.

    How do you suggest the set top boxes 'rescale' the various images? My two thoughts are 1. applying some kind of 'video matte' or 2. removing rows/columns of pixels from the image to the desired effect. Are you aware if this 'rescaling' process takes place in the 'digital domain' or with the analogue signal.
     
  7. StooMonster

    StooMonster
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    ...is overscanning.

    Different channels have different overscanning settings, which can also depend on content.

    For example, I set up iScan Ultra and Panny5 50" plasma to show all 720x576 pixels for SkyDigital. However, lots of channels have black vertical borders (some thin, others very wide) on either or both sides; particularly if it's NTSC 4:3 content converted to PAL 4:3. Therefore I had to change the geometry of the display so that there were no borders on all channels; thereby cropping picture detail from some.

    I used to know all this stuff about CRTs, from things I did in the past; but as far as I recall, traditionally regular 4:3 televisions had approximately 540-NTSC and 768-PAL phosphors across (considering each single phosphor to be three RBG ones); although this is probably redundant since many tvs are multi-region compatible, however, I understand 768 wide are sold as premium product in US.

    I recall that NTSC analogue transmissions are 442x330 colour pixels wide and PAL are 461x400ish. Interestingly SECAM is 520 colour pixels wide.

    I could be totally wrong about all the above, and probably am. I have creaking memory. :clown:

    StooMonster
     
  8. cjdshaw

    cjdshaw
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    While the horizontal resolution of an analogue signal is a somewhat vague number, the vertical is not. Vertically, NTSC has 486 lines and PAL has 576. Typically 10% of these are lost to overscan.
    To further complicate matters, many digital video formats (such as MPEG and DV) demand that the image can be broken up into 16x16 pixel blocks. For CCIR PAL (720x576) and square pixel PAL (768x576), this is not problem. But NTSC's vertical resolution is not divisible by 16. So digital NTSC streams drop the top and bottom three rows to make a 480 line image.
    For more fun and games, the colour and luminance actually have different resolutions. Look up 4:2:2 on Google for info.

    Andrew_B: The scaling is almost certainly done in the digital domain. It would be done by mixing scan line together. Simply removing scan lines gives a horribly jaggy effects. Unfortunately, the STB doesn't know if the signal it's getting was sourced from 50Hz interlaced (TV cameras) or 25Hz progressive (film sped up slightly). If it knew it was from a progressive source, it could do much better by scaling the whole frame, rather than the two fields.

    Of course the ultimate answer is, Buy a widescreen TV and stop worrying :lesson:
     
  9. StooMonster

    StooMonster
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    Thanks for that cjdshaw.

    But are the vertical resolutions always fixed for analogue transmissions? I guess they are, I thought they may be different for older/analogue data. Perhaps I was thinking of VCRs?

    Although PAL and NTSC vertical resolutions are defined as you state, AFAIK the video bandwidth of an analogue signal limits the horizontal resolution e.g. 4.2MHz for NTSC and 5.0MHz for PAL.

    Which means that NTSC analogue signal has a luma of 442 and chroma of 377 with YIQ colour model, whereas PAL analogue signal has a luma of 520 and chroma of 461 in YUV (YCbCr) colour model. (Yes I did just look it up ;) )

    Digital domain, well that's a whole different kettle of fish. I think Andrew_B is talking about the horizontal scaling with a digital source i.e. Sky Digital. Which IMO doesn't mean any scan lines would be removed, surely, and therefore no issues with jaggies or such; IMHO the 16:9 wide image has a simple crop applied to it, very easy to do.

    I can understand that there is vertical scaling in analogue tranmission of 16:9 material as 14:9; but wouldn't that be done at the source with some lovely Snell & Wilcox (or somesuch) kit?

    StooMonster
     
  10. cjdshaw

    cjdshaw
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    I'm strictly a digital boy. Don't ask me about bandwidths and the like.
    The simple crop would take the 720x576 digital signal and crop the 540x576 centre area out of it. The trouble is, most digital to analogue converters (D-to-A's) need a strictly compliant digital stream to work from i.e. 720x576. So you need an extra step which scales the image up horizontally in the digital domain. Jaggies are still an issue as a cheap scaler would simply double some columns of pixels, rather than doing a weighted average
    I admit when I talked about scaling down, I was thinking about my Telewest box which can make a letterboxed 4:3 image from a 16:9 anamorphic signal. Can a Sky box do this or are 4:3 TV owners (spit) forced to watch a cropped image?
    Also, while a component feed from a digital source like Sky or DVD may have higher luminance resolution than an analogue one, it has a lower chroma resolution. MPEG-2 has its chroma sampled at 4:2:0 which means the chroma has effectively half the horizontal and vertical resolution of the luma.
    Like I say, hours of fun.
     
  11. rct

    rct
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    We have a choice of either letterboxed or cropped 4:3 as well.

    Far too many people prefer cropped 4:3 out of sheer ignorance.

    I believe some education is in order! :lesson:

    :D
     

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