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Basic HDTV question re: aspect ratio

Discussion in 'TVs' started by Dune, Jul 4, 2005.

  1. Dune

    Dune
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    Hello,

    I have read a little about HDTV and have taken on board the fact that the minimum spec for a TV to be suitable for HDTV requires it to have 720 pixels vertically (progressive signal).

    But what is the significance of the horizonal dimension of the screen in relation to HDTV?

    I noticed the new Panasonic 37 inch plasma is 1024 x 720.

    This seems somewhat short of what I would expect horizontally which would be 1280 for a 16x9 picture.

    The 1080i version requires a screen of 1920x1080 and I can't say I have come across that sort of specification in a TV.

    How doess a set such as the Pansonic deal with a 16x9 hi def picture? I realise the pixels will be oblong to give the correct aspect ratio but it appears there aren't enough of them in the horizonal dimension to display the picture without some sort of scaling or processing.

    Won't this adversly affect the quality of the picture in some way?

    I have not done an exaustive search for sets that have a full compliment of pixels but do any exist?

    Or am I barking totally up the wrong tree here?

    Dave
     
  2. Stephen Neal

    Stephen Neal
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    Yep - basically the TV is likely to scale the image. This isn't uncommon though as 1:1 displays are not universal.

    1280x720 and 1920x1080 are the two basic HDTV formats - though non-square pixel 1440x1080 is also in use (the widespread HDCam broadcast video tape machines reduce the 1920x1080 input signal to this format when they compress the signal to record it on tape, and 1440 is also broadcast in some parts of Australia I believe)

    Many plasmas and LCDs have resolutions that are not 1280x720 or 1920x1080 (though many DLPs are based on 1280x720 panels) - and scale their input HD video to their panel resolutions - which can be numbers like 1024x1024, 1366x768 etc. (Some 1366x768 don't scale 720p material vertically and include black bars top and bottom and left and right instead)

    Also - even flat panel sets with what appear to be a 1:1 input to output resolution often include a small amount of scaling (though it can be optional) to zoom in a little to the picture to avoid seeing the edges (where often there are distracting bits of junk) - simulating overscan (the function of CRT TVs scanning past the edge of the visible tube area)

    If you are looking for 1920x1080 native displays then there are some - but they aren't cheap. Sharp's Aquos 45" display is one, as is the Sony Qualia 006 70" Rear Projection set (based on an LCOS-type reflective LCD system). There are also some huge, and very expensive, plasmas with this resolution.

    1280x720 displays are far more common - with most HD DLP rear pro sets based around this format natively.

    LCD panels seem to often use 1366x768 or similar (I think because it is the 16:9 version of the 1024x768 4:3 VGA - or XVGA, WVGA, ZVGA or whatever it is called - PC format)

    There are also some computer LCD panels (like those made by Apple) that are sharper than 1920x1080.
     
  3. pjskel

    pjskel
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    Apple's panels are made by Samsung. Apple don't manufacture them themselves. You're referring to the 30" which is 2560x1600 and the 23" which is 1920x1200.
     
  4. StooMonster

    StooMonster
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    Also, many LCDs are 10:9 ratio in both pixels and physical size; whereas plasmas, if not 16:9 in pixels, are always 16:9 in physical size.

    StooMonster...

    ...crying because his 2560x1600 pixel Apple 30" Display developed a vertical line of dead pixels this PM, and needs to go home to Apple for repair/replacement. :(
     
  5. NicolasB

    NicolasB
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    I think you meant 16:10.
     
  6. Dune

    Dune
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    Thanks for the detailed reply. Glad to know I was on the right lines!

    So something like a Sagem DLP with its 1280x720 resolotion will have to do less work than a Panasonic 500 series with 1024x720 (37 inch) or 1024x768 (42 inch) to display a 720p picture?

    Given some DVD players will scale their output do they have the capability to scale to resolutions like 1024x720? I am not sure why you would have the DVD player do the scaling if the TV can do it.

    What about the 1080i stuff? I assume apart from the Sharp you mentioned just about every set going within a reasonable price will be scaling such pictures.

    I am assuming 1080i is supierior quality to 720p. Is that correct? If so why the two standards? In fact despite which is better, why the two standards and will Sky be sticking to just one?

    Dave
     
  7. Stephen Neal

    Stephen Neal
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    All progressive displays (DLP, Plasma and LCDs are all progressive) will need to de-interlace the 1080i stuff, and then most (apart from the 1080 native panels like the Sharp) will need to then scale it.
    That debate runs and runs. 1080i has a higher horizontal resolution than 720p, but being interlaced has reduced vertical resolution on moving images (as when stuff is moving 1080i effectively becomes 540p), and doesn't deliver the full 1080 line resolution you might imagine, even on static images (because fine vetical detail on interlaced displays would flicker, high frequency vertical detail is filtered out in interlaced signals)

    In fact 720p and 1080i have roughly the same vertical resolution - 1080 beats 720 in horizontal resolution - 720 beats 1080 in detail on moving images.

    There are arguments for both - though 1080i is losing some ground in the theoretical argument stakes as there are likely to be very few interlaced displays produced (the only real native interlaced displays are based on CRTs - either direct-view or projectors), and modern digital compression systems like MPEG2 and MPEG4 deliver better compression with progressive sources.

    720p also has fewer samples per second to cope with - so less data to broadcast, so either occupies less bandwith, or should deliver images with fewer compression artefacts in a given bandwith.

    However - a lot of the feedback from the US is that even on 720p displays, 1080i often looks better than 720p - even when carrying interlaced source material (rather than 1080/24p originated stuff with no motion between fields)

    The 1080i standard is based on the original Japanese HiVision system that has been used in production and broadcast since the 80s - with analogue broadcast techniques initially being used. This is also known as the 1152 standard (as it uses 1152 lines to carry 1080 active lines)

    Initially the Japanese were on their own when it came to HD (and in fact their system used between 1035 and 1050 active lines - not 1080) - but they were making real production kit, and making programmes in HD for a long time. (The BBC made a drama in the Japanese standard in the late 80s..)

    The US and Europe initially worked on systems based on their SD systems so proposed 1250/1152 from 625/576 and 1050/960 from 525/480 - all interlaced. However these simple double-resolution systems ceased to make as much sense once digital transmission and compression techniques started to become available.

    In the US there were a number of standards proposed - and eventually rather than picking one - a system with about 18 possible formats was chosen. (Effectively 640x480, 720x480, 1280x720 and 1920x1080 in 24, 30 fps progressive and 60fps interlaced or progressive, and in 16:9 - apart from 640x480 which was 4:3 only, and 720x480 which was either 4:3 or 16:9)

    Thus when the US broadcasters were looking to chose an HD standard - the one format that had a ready made supply of production gear (cameras, vision mixers, VTRs etc.) was the Japanese 1080/60i system. NBC, CBS and PBS went with this format - and were able to start producing HD stuff really quickly.

    There were also arguments though, that with technology moving forward, interlaced scanning was an archaic bandwith reduction system, and much lobbying by some of the US committee members meant that 720/60p was also adopted as a US standard.

    Initially only ABC chose this format (not sure when ESPN did) and were limited to showing 720/24p film transfers (or conversions of shows shot in 1080/60i?) initially - as there was little 720/60p production kit around (There was no other area using it - certainly not Japan)

    Fox launched with 480/60p (though a lot of their shows were 480/60i produced) - and only relatively recently moved to 720/60p.

    Effectively the US is split. However most drama and high-end documentary is shot in 1080/24p - which converts to both 1080/60i and 720/60p quite effectively - so it isn't as big an issue as it could be.

    In the UK and Europe most HD services will operate with receivers that can cope with 1080/50i and 720/50p - Sky's will cope with both. The HD Ready logo for displays stipulates that they cope with both formats on their inputs (though they only have to have a minimum of 720 lines)

    Whether Sky run entirely in 720p or 1080i for their own services is still not clear.

    At the moment many think that Sky Sports will be 720/50p - as progressive delivers better detail on motion. Some think that their film and general channels may be 1080/50i - as most of the source material will be 1080/24p on these, and thus 50p would offer no real benefit over 50i, and 1080 would offer a higher resolution picture horizontally.

    The BBC is currently producing in 1080/25p and 1080/50i - though this doesn't mean it will broadcast in 1080/50i - there is still merit in converting to 720/50p for transmission.

    At least one EBU (European Broadcasting Union) report has suggested that 720/50p has real benefits in Europe - as most displays are likely to be progressive flat panels, not interlaced CRTs...

    Loads of issues - wait and see is still a good motto!
     
  8. Dune

    Dune
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    Thanks once again for the detailed reply. I can certainly see the logic behind 720p being a European standard due to the fact the HD displays will be progressive.

    I would have thought by now given the US and Japanese experience of HD issues such as optimum panel dimensions would have been long decided. I mean by that they would have had experiece of 1024x720 displays and concluded if they were a "good thing" or not in practice compared to 1280x720.

    I notice that to get 1280 on the long side in a plasma you are talking 50 inches such as the Pioneer 505.

    Not only expensive but too big for my house!

    I was thinking of a Pany 37 inch 500 series but am now wondering about waiting to see what the next gen Philips 37 inch LCD will be like as I have this probably irrational notion that 1024 horizotal resolution is going to spoil things in some way!

    Saw a Samsung 40 inch LCD over the weekend but wasn't too impressed as it played a Samsung demo but then it was in Curry's and who knows what the source was. (A Pany 42 inch 500 gave a better picture running the same demo but that was directly from a DVD player).

    Dave
     
  9. StooMonster

    StooMonster
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    :oops:
    Hrm... late night posting, yes 16:10 aspect ratio!

    StooMonster
     
  10. Quickbeam

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    The 16:10 ratio is only really meant for PC monitors though. 15:9 (1280 x 768) is a more common aspect ratio in larger LCDs screens, though these are being phased out in favour of 'true' 16:9 panels.

    Why can't manufacturers settle on 16:9 for both widescreen monitors and TVs? :(
     
  11. cerebros

    cerebros
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    becuase that would be too easy :D
     
  12. Stephen Neal

    Stephen Neal
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    16:10 screens can be VERY useful for displays on systems used to edit 16:9 video - as they allow for a menu below the video!
     
  13. StooMonster

    StooMonster
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    That's right ... the pixel matrix of 1280x768 is a common LCD "widescreen" resolution, but who's to say those pixels are square? Similarly a lot of 42" plasmas are 1024x768 resolution.

    I was highlighting physical dimensions of LCDs which are more commonly 16:10 than they are 16:9 which is common on plasmas; not the pixel resolutions.

    Saying that, things may have changed in the last year since I last checked. ;) But I doubt it as LCDs are also used for computer monitors.

    Or use the Apple 30" display where 16:9 1920x1080 HD is but a little window in the middle of the 16:10 ratio 2560x1600 pixel display. :) And you can fit dozens of menus around the video!

    StooMonster
     
  14. Quickbeam

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    So was I. The aspect ratio of 1280 x 768 LCDs is 15:9, with square pixels. This is why a 30" LCD panel can have the same height as a 32"; the former being a 15:9 1280 x 768 panel, the latter a 16:9 1366 x 768 panel. Many 15:9 panels stretch 16:9 pictures to fill the screen, and the distortion is just enough to be disconcerting.

    So that's why 16:10 panels exist. I knew there had to be a reason.:D
     
  15. StooMonster

    StooMonster
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    Perhaps LCDs always have square pixels ... and LCD televisions with resolutions that are 1280x768 pixels are always 15:9 ratio, whereas 1366x768 are always 16:9 ratio, and 1920x1200 are always 16:10 ratio?

    Whereas plasmas have pixels of different shapes ... but are always physically 16:9 ratio? i.e. Many 42" plasmas typically have 1024x768 pixels which is a 4:3 ratio; yet their rectangular pixels means physical screen sizes are 16:9 ratio. Additionally 1280x768 and 1366x768 plasmas are also 16:9 ratio.

    Interesting, another difference between LCD and plasma technology. :cool:

    StooMonster
     
  16. DanH

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    Ive not seen any LCD TVs use non square pixels yet. Do they exist? Ive always thought it a safe bet to figure out the aspect ratio by looking at the resolutions, but obviously that doesnt work with plasmas.
    Its always baffled me how you get 1280 square pixels displayed correctly on 1024 non square pixels without stretching the image.
     

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