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Is HD TV well encoded?

Discussion in 'Televisions' started by richard plumb, Dec 28, 2004.

  1. richard plumb

    richard plumb
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    I was thinking about the planned launch of HD on Sky, and how bad the current digital SD images often are. Surely this is because of the desire to squeeze too many channels in, and they don't have enough bandwidth.

    If SD stuff is badly encoded, then what hope do we have for HD? Or is the assumption that initial HD will be bought by early adopters (you lot), and therefore they need to keep the quality up?
     
  2. Dutch

    Dutch
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    Sky will probably use the MPEG-4 AVC HP codec to allow them to have twice as many channels they could have had if they'd used MPEG-2. Hopefully they would keep the bitrate up around 10-12 Mbps to keep the early HD adopters happy. I believe the EBU should specify a minimum bitrate broadcasters can use for HD to enable high quality to be maintained so we don't end up with the bitrate-starved SD stuff we get now.

    Steve
     
  3. Stephen Neal

    Stephen Neal
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    Just as SDTV can be well encoded and poorly encoded, as well as run at higher and lower data rates, so can HDTV.

    AIUI Euro1080 runs at around 18Mbs for its video - using MPEG2 at 1080/50i. In the US 1080/60i is running at between 10 and 18Mbs using MPEG2, with 720/60p running at similar levels, though sometimes dropping to as low as 8Mbs. There are other factors other than final bit rate that play a part in determining picture quality though. They include how many MPEG2 decodes and recodes have been in the broadcast chain, how high a data rate previous MPEG2 processes have run at, performance of the MPEG2 encoders used by the broadcasters etc. Final bit rate is only one factor, and comparison of just that isn't that useful as everything else might not be equal.

    At SDTV the BBC run BBC One at around 4.8Mbs MPEG2 on Freeview, whilst other broadcasters drop as low as 2Mbs I believe. For really good quality SD video for broadcast (at 4:2:0 and with a long GOP) you should really be around the 8Mbs level I believe - DVDs max out at around 9.9Mbs AIUI (though can benefit from multiple pass encoding)

    It is likely that Sky and other European HD broadcasters will chose a more efficient codec than MPEG2 though - MPEG4 AVC H 264 or VC1 (aka Windows Media 9?) - to allow lower data rates to be used for a given picture quality, reducing transmission costs.

    I suspect Sky will be quite canny and chose quite a high picture quality initially - they run their Sports Channels pretty high at SD. They may then ramp them down to see if people notice. (The BBC did loads of tests using us as guinea pigs in the late 90s and early 00s on their SD broadcasts ISTR)
     

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