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Why not have a short sticky that tells us what screen we need for HIGH DEFENITION?

Discussion in 'Televisions' started by gbmitie, Sep 5, 2005.

  1. gbmitie

    gbmitie
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    I, like many others, am waiting for HD to start but do I know what screen type I need to buy? Do I heck! I have read about pixels, scalers, interlacing, downscaling, upscaling, native resolution and so on. My life has been geared around the KISS principle so I would like a sticky that tells us what type of display we need to view HD.

    From what I can gleam, at the moment we need 2 displays.

    One for 720p. and an other for 1080i each display to have the native resolution that supports each format otherwise it is not pure HD. Anything else is a compromise.

    gbmitey
     
  2. Stephen Neal

    Stephen Neal
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    Hmm - the problem is there is no simple answer. Different people prefer different display types. Some love LCD, others love plasma. Some love DLP, others can't stand them. There isn't a single answer.

    As for the 720 vs 1080 issue - it isn't quite as clear cut as you might at first think.

    There aren't many HD Ready displays (if any) that will display a 1080i source in 1080i. Most convert to 1080p or 720p - and can't cope with 1080i. (AIUI only the ALiS Plasmas can display interlaced video - they do it in 1024i by cropping 28 lines at the top and bottom of frame. CRTs can also display 1080i natively - but I don't think there are any HD Ready CRTs available in the UK)

    Hate to add to the confusion - but a 1080i interlaced video signal doesn't have a full 1080 lines of resolution - the interlacing process reduces the vertical resolution to around that of an 800p system - so the difference between 720p and 800p vertically isn't that much.

    And also - there is more to picture quality than resolution...

    Sadly this area is "early adopter" territory still - and sadly there are no easy answers to it all yet. In time some specific technologies may win out...
     
  3. gbmitie

    gbmitie
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    thanks for great reply. I am hoping the World Cup will be in HD next June and I just want to get a display that will last a few years!!

    I now know the KISS principle will not apply to HD for sometime to come. I would have a 50hz 36" display any day but cant lift the blighters.

    cheers

    gb--
     
  4. paolo999

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    I'm curious to know what you mean there. Do you mean that a 1080i camera is effectively "de-tuned" (optically / CCD / otherwise) to prevent it rendering a discrete horizontal pixel, i.e. prevent it from being a picture buzz nightmare, hence an approximate performance of 800 lines? (I'm assuming here that our hypothetical camera has a 1080x[something] CCD. Whether such a thing exists or not I don't know.)
     
  5. Stephen Neal

    Stephen Neal
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    1080i signals should be vertically filtered somewhere in the production process to reduce high-vertical frequency information. The reason for this is that HF detail is only present in every other field, and thus produces flicker at the frame NOT field rate. In 50Hz land this means very fine vertical detail flickers at 25Hz if not correctly filtered. You see this on poorly filtered PC graphics used in low-cost broadcast applications.

    CCD cameras used for interlaced TV do this filtering "in camera". A 1080i CDD camera, as you would expect, (*) has a 1080 line sensor, and this captures a progressive 1080 line image at the field rate. However rather than throw away half of the lines in the frame, and clock out every other line to create a field, pairs of frame lines are averaged to create the field. To increase the resulting resolution, however, a line off-set is used between fields, so that the averaging process is altered. This also acts as a vertical filter - reducing interlace flicker.

    So if you imagine a frame being converted to interlaced representation - field 1 would contain an average of lines 1&2, an average of lines 3&4 etc. whilst field 2 would contain an average of lines 2&3, and average of lines 4&5. (The actual frame would be refreshed between fields 1 and 2 though - so the content of the lines would be different between fields if the scene moved)

    Film scanning performs this filtering using alternative methods.

    (*) 1080i images can also be captured on other types of CCD. For example Philips have a clever CCD that has 4320 lines. Different combinations of these lines are averaged and used to create a 1080p frame, 1080i fields, a 720p frame and 576i and 480i fields. (4320 is a neat number divisible by both 1080/540 and 720)
     
  6. paolo999

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    Cheers Stephen - Interesting insight about the in camera thing. I'd guessed that had to be happening conceptually, but wondered whether it was simply a product of optical "spill" between the CCD sensors or whether there was something more active going on.

    My guess is that there's a bias to the averaging, otherwise a still picture will be exactly half res. So odd field line 1 is, say, 75% of frame line 1 and 25% of frame line 2. The even field line 1 is the reverse

    Make sense?
     

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