Help a dummy understand please! ( freeview )

Discussion in 'Freeview & YouView' started by bongpig, Jun 17, 2004.

  1. bongpig

    bongpig
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    hi all,

    I have some questions regards freeview reception.

    I was under the impression initially that digital is either picked up, or not. I thought that if you have reception, then the image quality is fixed.

    Now, whilst watching the footie, I notice the image swing from being very clear, with no artifacts around the players, to heavily compressed, with the players smearing into the grass. It tends to swing every 10 seconds or so.

    This is not the usual picture breaking up, or sound pauses, just what looks like a change in compression. The same as taking an mpeg file on my computer, compressing it to half the size, and then switching between the two.

    Can somebody clear this up for me? Once you actually pick up freeview, is there then further levels of quality depending on reception? In other words, is it possible to get the system working so Im always seeing the low compressed ( better ) image, or is this simply the way freeview works?
     
  2. philipb

    philipb
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    I think what you're seeing is not a signal quality thing - its due to the bitrate of the incoming broadcast.

    Was it ITV? Their footie broadcasts are rubbish, Freeview or Sky. BBC is usually better, and Sky sports is usually best. Nothing you can do about it, although sometimes turning off any signal/picture processing done by your TV may help.
     
  3. bongpig

    bongpig
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    Thanks,
    But wouldnt a bitrate issue give me a constant image, or does the bitrate fluctuate?
    What causes the bitrate to go up and down as it does here?
     
  4. LV426

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    From what you describe, it certainly sounds like a compression issue, not a reception one.

    It WILL vary in its effects, because the content of the image (or more particularly, the amount of change occurring in it) is always changing.

    The more change, the greater amount of data (given an unlimited ceiling) is needed to provide artefact-free reproduction. There is a ceiling so it becomes, the more change, the greater the probability of artefacts becoming visible.

    It's not even as simple as that. The compression techniques used mean that short periods of lots of change, can be handled (i.e. allowed more data) at the expense of a nearby part which has little. In other words, data throughput can be borrowed from one sequence of frames, to "pay for" the next or previous.

    The cyclical nature of what you are seeing may be an artefact of this process.
     
  5. bongpig

    bongpig
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    cheers nigel,

    that sounds about right.
    So, there is some kind of buffering going on then? Have i got that right?
    Where a low detail sequence will pass on its unused bandwidth to another sequence and give me what seems like a small period of better quality?
     
  6. LV426

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    Kind of....but it's done at the broadcast end i.e. when they actually generate the digital bitstream. At your end, the receiver receives data at a variable rate - the rate at which it is sent.
     
  7. Kevo

    Kevo
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    Broadcasters also have to pay for the amount of bitrate they use.

    Hence on ITV (or any commercial channel) that the adverts are always cystal clear (for the paying customer), artefact free yet the progamme they are part of maybe very low bitrate quality.
     

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