Sky HD signal format question: fixed or switching?

Discussion in 'Sky Digital TV Forum' started by StooMonster, Jun 20, 2005.

  1. StooMonster

    StooMonster
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    How do the DirecTV HD STBs or HDTV broadcasts in USA handle channels that switch between SD and HD content?

    For example, say we had BBC HD and half the programmes are new HD ones and half are old SD ones.

    The HD ones are in 720p50 which is fine, but are the SD ones:
    • completely broadcast in native 576i50
    • partially broadcast in 576p25 where appropriate, and 576i50 for the rest
    • upscaled to 720p50
    Thought: the screen switch modes and setting in first two options, but stay fixed in the third; is this a consideration? Also, in first two one could use a third-party scaler to 720p whereas in third option one can't.

    Second question, if SD is upconverted to 720p50 is this done at STB level so that one can have preference of aspect ratio or at broadcast?

    For example, I like all 4:3 material to be in 4:3 'pillarbox' (with fat black bars down left and right and no distortion), but others prefer no bars and would like a 'stretch' (fills screen but distorted), others may prefer 14:9 presentation (think black bars and cropped top and bottom), some may prefered 'zoom' (meant for 'letterbox' on 16:9 but some people like it for 4:3).

    If this is STB level it's a user preference to set, but if it's broadcast there is no choice.

    The only live HDTV broadcasts I've seen in USA, had adverts pillarboxed (hurrah) but I have no idea if it's broadcast that way or plasma was setup that way.

    StooMonster
     
  2. David Mackenzie

    David Mackenzie
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    From what I remember from most US sites, the SD stuff is upconverted by the STB. And yes, the decoder does the pillarboxing.

    I've no idea if it's a user option, although I get the feeling it won't be. I need to ask your questions to someone over at the US AVS Forum.
     
  3. Quickbeam

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    Are you sure about that? It was my understanding that the upconversion and pillarboxing (if required) are done by the broadcaster, not by the STB.

    If HD channels used format switching between SD and HD they would have a problem if they wanted to show a trail for an HD programme during the credits for an SD one, or vice versa.
     
  4. beeblebrox12

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    All upconversion is done by the broadcaster. No switching of resolution, refresh rate or whatever on the fly. If a channel is 1080i, everything on that channel is always 1080i. Pillarboxed or not, produced in HD or upconverted, commercials are always 1080i too.
    Same applies to 720p channels.
     
  5. StooMonster

    StooMonster
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    I am trying to second guess what Sky HD will be like, and plenty of Americans read this board.

    StooMonster
     
  6. Stephen Neal

    Stephen Neal
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    In the US they run their HD channels at a fixed setting - either 1080/60i or 720/60p. Any 1080/24p, 720/24p or 480/60i material is converted to the destination format - with most 4:3 stuff presented as 12P16 pillarbox, though some broadcasters originally stretched to 16F16, and some still use a Panorama-type non-linear stretch.

    I VERY much doubt the UK broadcasters will attempt to dynamically switch their channels between formats either. Trying to build a decent presentation system - where you can mix between shows, and do those lovely credit shrinks and superimposed trails, is only really feasible with current broadcast presentation kit if you run at a fixed line and field/frame rate. Switching between 1080/50i and 576/50i within a stream would be a nightmare.

    Whether you run separate 576/50i and 1080/50i or 720/50p chains or run a single chain at 1080/50i or 720/50p (with 576/50i upconverted to 1080 or 720) and downconvert permanently from 1080/50i or 720/50p permanently to derive your 576/50i SD networks is more interesting. (The double conversion of SD to and from HD may not be great for SD picture quality?)
     
  7. Quickbeam

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    beeblebrox12, how do you find the picture quality of upconverted SD on HD channels compares with that of native SD channels? My guess is that upconverted SD will look better than native SD because of the HD channel's higher bit rate. Obviously there are scaling issues: for example, if you have a 720p display SD will be upconverted to 1080i on some channels, then downconverted to 720p again.
     
  8. ianh64

    ianh64
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    You may want to check out this BBC whitepaper. It may not be quite what you are asking but is interesting none the less.

    -Ian
     
  9. Stephen Neal

    Stephen Neal
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    You also have to be careful when comparing SD fed directly into a display with SD upconverted to HD and then fed into a display. If you are watching both broadcasts on the same display then there are the following issues :

    1. Is the SD direct feed analogue 4:3 composite, 16:9 digital component etc.?
    2. Is the display displaying SD natively, or upconverting to HD in the receiver.

    Therefore comparing SD direct and SD via HD may also be comparing upconversion by the broadcaster and upconversion in the TV, as well as broadcast artefacts?
     
  10. beeblebrox12

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    Upconverted SD on the HD channel is much better than the equivalent SD channel. For the simple reason that upconversion is done at the broadcaster's studio and the signal is delivered to the STB using the bandwith of an HD channel. Most of the SD channels on my satellite provider have undergone so many compressions and conversions, they can't even be compared to the upconverted SD on the respective HD channels.
    My TV is 720p (DLP), but I don't see any issues with 1080i to 720p conversions. As it's been mentioned many times, most 1080i programmes look better than some programmes from a 720p channel, even on my 720p TV.
    I use only DVI from the STB to the TV for both SD and HD. Composite or S-Video on a DLP TV is a suicide.
    I have also been comparing quite a lot my friends' CRT projection TVs and plasmas, and as far as upconverted HD vs. regular SD channels, results are identical. In general, SD channels on cable and DBS are not so good here (overcompressed), HDTV is the saviour.
     
  11. Quickbeam

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    I was of course referring to SD digital broadcasts - it would be pointless to try to compare analogue to upconverted digital SD delivered at HD bit rates, especially on digital displays which are not forgiving of noisy analogue transmissions. I believe some cable providers carry a mix of analogue and digital channels?

    I suspected that upconverted digital SD would look better than native SD - thanks for confirming that. At least with HD broadcasts there is some quality control - with SD the aim seems to be use the lowest possible bit rate.
     
  12. rpm

    rpm
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    I also have a question on the Sky HD service. My Hitachi 5200 can only show the 1080/50, so do you think i could set the STB to this, which would then upconvert any 720/50 broadcasts - or would i simply not be able to view any broadcasts at 720/50??
     
  13. Tarbat

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    Sky have stated (on their pre-release HDTV website) that "All you need to do is make sure that your TV supports either 720p/50 or 1080i/25". The word "either" would suggest that the STB will upconvert/downconvert between 720p and 1080i.
     

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