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720 -> 768 upscaling: Is it really an issue?

Discussion in 'LCD & LED LCD TVs' started by nodabble, Jun 10, 2005.

  1. nodabble

    nodabble
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    I am hoping to buy a 32" LCD TV within the next few months and I have noticed that most manufacturers offer sets with a native resolution of 1366x768 (probably as a result of the PC market). I am also aware that in the not-so-distant-future Sky will start transmitting hi-def broadcasts whose native resolution is 1280x720. Despite the different numbers, both resolutions are 16:9 in ratio.

    So, with HDTV broadcasts in mind, should I opt for an LCD TV with a native resolution of 1280x720 or 1366x768?

    I appreciate that LCD panels with a res of 1366x768 will not display a HD picture with 1:1 pixel mapping, but what I would like to know is whether this is really a problem or not? I have read a thread in this forum (initiated by benwong) where it is implied that a 1280x720 image enlarged (or 'upscaled') to fit on a 1366x768 will suffer a loss in image quality. Fine... I can accept that some interpolation is required to fill the extra pixels but what I would really like to know is whether this is noticable or not?

    From your average viewing distance in your living room, can you tell whether a 1280x720 video has been upscaled to 1366x768 or not? Is it really a big deal?
     
  2. NicolasB

    NicolasB
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    It's not altogether a fair analogy, but try running a TFT computer monitor at something other than its native resolution, and see how happy you are with the result.
     
  3. David Mackenzie

    David Mackenzie
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    But also remember that some of the best LCD TVs will have better scalers built in to take this into account.
     
  4. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    You will probably find that those 720P displays may actually zoom in and rescale the image anyway as they have to deal with dodgy broadcasters and will apply overscanning.

    IE the active picture content in a broadcast is inside the 1280x720 designated area so without overscan you'd get black bars and all sorts of nasties if it wasn't implimented.....I may be wrong of course. If the display has a 1:1 or pixel for pixel aspect ration option then that would solve the overscan issue of course.

    Gordon
     
  5. nodabble

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    NicolasB: I don't think it is a fair anology... With a computer monitor you are sat very close to it and can easily see the blurrieness caused by the image scaling. (Having said that, there are people I work with who are quite happily using their 17" 1280x1024 monitors at a res of 1024x768. It disturbs me!)With, say, a 32" TV you will be sat at least 7ft away from it.

    I would love to hear from anyone who has bothered to set up a 1280x720 TV and a 1366x768 TV next to each other and compared the quality of a hi-def source.

    Can you really notice the difference???
     
  6. redtom

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    Bit of a non-expert here, but will overscanning not become a thing of the past - seeing as computer monitors don't overscan, do LCD tvs do the same?
     
  7. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    No it wont.

    PC desktops are not the same as broadcast video. If a set is designed to accept a video resolution then there is likely going to have to be overscanning.

    Get 20 different PC's running XP with 20 different graphics cards and make them all ouput XGA desktop. They are all likely to use VESA timings and your monitor will display the full desktop correctly.

    If you remove all overscan from a TV and go look at 5 different Sky channels you'll discover that they probably do not all have the same amount of active picture content or even that the actve picture content is centred in the middle of the possible image area good ones to check are Bloomberg (worst ever) Sky News, BBCNEWS24, The Box......,,,. TV's have to deal with this....broadcast video is not like a PC. Plasma and LCD TV's apply overscanning to incoming video signals specifically to address this. You can even go have a read of the thread about the Sony HS50 LCD projector and how if you feed it a 720P HDMI signal (its native resolution) it applies overscan stopping pixel for pixel playback.

    Gordon
     
  8. nodabble

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    Gordon - are you implying that HD broadcasts may not be dispayed at their native resolution even on 1280x720 panels?
     
  9. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    What I'm saying is that currently we have a system that has 625 interlaced lines of which we see 576. 720P has 750 lines of which we see 720. Not all broadcasters actually use all the 576 active lines available to them for active picture content and sometimes the timing of when the image has to start and stop on each line is different between broadcasters (ie it shifts left to right). I see no reason why this is likely to change when they swap to 1080i and 720P broadcasting.....unless someone in the broadcast industry wants to tell us why I could be wrong....

    So I guess I am implying that you may not be seeing native res 720P on a 720P display. That certainly appears to be the case with the HS50 I mentioned.....but the more interesting question is whether you'd notice the difference if it was pixel for pixel.

    Gordon
     
  10. Rob1698

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    Overscanning is not dodgy, it is part of the spec.
    If anything, a broadcaster that does not consider overscanning is dodgy.

    Sure it would not be necessary to have overscanning on an LCD, but as the HDTV standard specifies it, an LCD TV will normally cut off some borders and scale the remainder of the picture to the entire screen. When the native panel resolution would be 1280x720 this would of course be a little funny, but to comply with the standard a TV should still do it.
     
  11. nodabble

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    I see. It would appear, then, that the native resolution of the panel is perhaps not paramount when it comes to hi-def picture quality. As Lyris mentioned, it's probably more to do with how well the built-in scaler performs.
     
  12. Rob1698

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    For normal TV content, scaling is not a problem. A scaler does not do "let's see we need to enlarge the picture to 1.5 times the size so let's display every second pixel twice". That may have happened in the past (especially on computer displays), but it is no longer done today.
    This would not work when scaling by small fractions anyway (like when scaling from 720 or 720-overscan to 768)....

    What a modern scaler does (conceptually) is to build a picture from the pixels that it receives, filtering it at half the sample frequency so that the harsh steps between consequtive pixels are removed, and then sampling th resulting picture at the rate required. This is an operation similar to converting a piece of digital music to a different sample rate.
    As long as the conversion is UP (to a higher resolution), this should not result in loss of quality. Of course, when you convert DOWN (e.g. from 1080 lines to a 768 screen), quality is lost. But that can only be solved by using a higher-def screen.

    When you look at a computer screen, the situation is a little different. When very small fonts are displayed, single-pixel details are present that cannot be accurately scaled this way because they do not obey the rule that the sampling frequency (the pixel distance, in this case) must be at least twice the highest frequency present in the signal. So, the filtering causes some loss and the result appears muddy. On a TV this situation does not occur.
     

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