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ALis plasma screens and 720p

Discussion in 'TVs' started by shashi, Sep 17, 2005.

  1. shashi

    shashi
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    hi all
    Great forum.
    I have been looking at a philips 42pf9966/10 which as oppossed to the 42pf9966/12 can display 720p as well as 1080i.
    My question is :Is this 720p true 720p or is it downscaled to fit the screen?

    I have read that it is turned into 576p.Quite a few firms make Alis plasmas philips, hitachi etc. Can these screens show 720p or is the signal altered?

    thanks very much
     
  2. Quickbeam

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    Both panels have a resolution of 1024 x 1024, but since ALiS panels use a form of interlacing only 1024 x 512 pixels are illuminated at any one time.

    So yes 720p is downconverted, to 1024 x 512p.
     
  3. Joe Fernand

    Joe Fernand
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    shashi

    Keep in mind you need to ensure any screen you purchase can Display 720P and 1080i signals at 50Hz via HDMI or DVI (with HDCP) along with YUV.

    There are a lot of screens that are compatible with 720P and 1080i at 60Hz only!

    Look out for the 'HD Ready' logo - that's the best guide to at least getting signal compatibility with the HD signals were due to receive in Europe.

    If your looking at Plasma Technology then every Plasma Display or PlasmaTV you care to look at has to 're-process' a 720P or 1080i signal - none of them are a perfect match for either signal.

    The ALiS Panels raise pretty heated debate about how they work, what they Display and if they tend to produce more artefacts than non ALiS panels; do a quick search and I'm sure you'll find plenty of 'interesting' reading.

    Best regards

    Joe
     
  4. paolo999

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    That logic doesn't automatically follow - I think you are guessing. It can be done either way. Upscale to 1024i or downscale to 512p then line double.

    From what I can deduct from the manual of a 5200H for example, it upscales.
     
  5. NicolasB

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    ALIS panels are inherently interlaced. Each row of pixels can be updated no more than 30 times a second under any circumstances. If you feed it 50 or 60 distinct frames per second, it's impossible for it to display them without downscaling to half the screen's vertical resolution and interlacing.
     
  6. paolo999

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    Interlacing does not mean displays have half the vertical resolution. This is a common misconception when people discuss ALiS panels.

    Downscaling then line doubling is a way to scale, but it is a poor way and not the only way. I'd need to get some graph paper out to explain this easily - it's tricky to use words to show you how a 50 frame per second progressive picture converts to 50 fields per second without halving the vertical resolution.

    But trust me on this, this method has been around for decades - I spent many years advising broadcasters on equipment that did this specific form of conversion.
     
  7. lukemh

    lukemh
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    But nothing can beat true pixel for pixel reproduction I suggest you look at new 1920 x 1080 42" TFT's now available
     
  8. Stephen Neal

    Stephen Neal
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    Yes - and then you get 512p resolution on fast moving information (rather than 720p resolution), but on static and slow moving information you get 1024i resolution, which should be enough to carry the information carried in a 720p source (vertically)

    A more significant limitation may be that the 1024 horizontal resolution is significantly lower than the maximum 1920 in a 1080i source, and a bit lower than the 1280 in a 720p source.

    lukemh - most ALiS panels DO display 1080i sources vertically with a 1:1 line mapping. They crop the top and bottom 28 lines (total 56 lines), and just display the middle 1024 lines as 1024i - so no vertical scaling. I believe this may be because the original Japanese HD standard broadcast in analogue MUSE and recorded using HiVision gear was nearer 1030-1050i than 1080i ? (It was tweaked to 1080i when the US launched their HD services, and the Japanese digital HD services are now also 1080i)
     
  9. Quickbeam

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    ALiS panels certainly have more than 512 lines of resolution.

    When the rules of interlacing are applied, a 1080i broadcast when cropped to 1024i will have close to 1024 lines of resolution on a still image, and between 512 and 1024 lines of resolution on a moving image.

    720p is non-interlaced. The full frame resolution is 720 lines, and this has to be scaled to fit into a 1024 x 512 field. Surely this involves downscaling? Even if you scale up to 1024 lines, you would still have to drop every other line to fit the interlaced structure of the panel.
     
  10. Stephen Neal

    Stephen Neal
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    That is not quite accurate.

    Broadcast 1080i sources don't come close to the full 1080 line vertical resolution many assume to be possible on static images.

    This is because 1080i sources are vertically pre-filtered to avoid interline interlace twitter.

    In the case of video camera sources this is done "in camera" by the line offset + line averaging process that converts the progressively captured CCD image frame to the fields in an interlaced video signal. In telecine transfers of film and 24p/25p/30p sources this is done as part of the interlacing process by vertically filtering using other means.

    A 1080i video source will have almost no vertical resolution above the 800-ish line level.

    Sure - if you use a PC to generate a 1080i signal you could create a signal with close to 1080 lines of vertical detail, however if displayed on an interlaced display the level of interlace flicker on the >800 line vertical detail will be objectionable. This is because fine detail is only present in one of the two fields, so flickers at frame, not field rate. You see this effect on poor quality computer generated graphics on SD and HD broadcasts.

    No.

    There are two ways of converting 720p to 1024i.

    1. Convert the 720p to 512p by scaling down, and effectively ignoring interlace. This will deliver a picture with the same 512p resolution on both moving and static information, but will not fully exploit the resolution available in a 1024i display.

    2. Scale the 720p to 1024p by scaling up, and then interlace this to 1024i. This will preserve as much of the sources 720p vertical resolution as possible on static and slow moving video information (and film sources which have no intraframe movement) and only drop to 512 line resolution on faster video source movement.

    In any interlaced system - the worst case scenario is that it has the properties of a progressive system based on the field resolution.

    Thus 576/50i SD behaves like 288/50p on fast motion, and 1080/50i behaves like 540/50p on similar sources. However on static and slow moving sequences (and film sources) 1080/50i behaves more like 800ish/25p.

    This trade-off between motion capture and vertical resolution is the whole point of interlacing to reduce bandwith and increase refresh rates.

    It is important to remember that scaling can use quite sophisticated re-sampling techniques, and doesn't require a simple 2:1, 3:2 etc. relationship - though it is easier to do cheaply if this is the case.
     

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