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Geometry question

Discussion in 'TVs' started by Zaichik, Oct 1, 2004.

  1. Zaichik

    Zaichik
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    My Panny TX28DTX10 has a slight horizontal bow at the bottom of the screen, which is to be expected; however straight horizontal lines (like banners on the news channels) also curve into the screen at the sides, and the tops of the banners also bow upwards. All three sets that I have had have done this, but the demo one in the shop does not. Is there anything that I can adjust in the service menu to correct this?

    As ever, thanks for your help. :thumbsup:
     
  2. mdinch

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  3. LV426

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    Yes, good luck. This is one reason why I'm inclined to defend LCD and Plasma against CRT, when people (naively) say that CRT is best. In many respects it may well be, but geometry isn't one of them.
     
  4. Zaichik

    Zaichik
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    Thanks guys! :thumbsup:
     
  5. Laurel&Hardy

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    No, geometry is never perfect on CRT, but be honest with yourself. It has to be bad for it to be noticed in normal viewing condition. Unless you spend all day looking at images generated by a pattern generator or DVD test disc you'll never see it.
     
  6. LV426

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    I'd agree that for most picture content minor geometry errors are an irrelevance. However, the circumstances what I'd find it noticeable are more common nowadays, with widescreen TVs, 4x3 images, letterboxed widescreen movies and the tendency of many channels to show horizontal banners (like news tickers) and other graphics (all of which are meant to display precisely straight horizontal or vertical edges and lines).
     
  7. probedb

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    i'd agree, it's the voltage spike problems that annoy the hell out of me on my old panny.

    anything bright on the screen distorts tho whole image horizontally ! new tv time :)
     
  8. zaphody

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    This is interesting: are you talking about the horizontal streaking/Banding, which is discussed here: http://www.avforums.com/frame.html?http://www.avforums.com/forums/showthread.php?t=137785 ???

    ("shiny objects, such as jewellery etc are causing what I can only describe as "streaking" right across the screen, originating from the object itself. This is noticeable at all times, but particularly bad if the background is dark.")


    If yes, please explain to me, what you mean by "voltage spikes".

    Thanks,
    Zap
     
  9. LV426

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    I suspect what is being described here isn't exactly "voltage spike". I'd guess that it's actually poor voltage regulation.

    Simply what happens is that high brightness draws more current from the power supply. The power supply is barely adequate (or actually inadequate) to deliver the current demanded, and so voltages in the set reduce. Reduced voltage applied to the scanning coils around the tube neck result in poorer control of the scanning.

    Result: For any line that contains high levels of brightness, the said line will be longer than it should be. Hence any vertical edges in the image (including those at the edges of a 4x3 centred image) end up with a kink in them, outwards when bright and/or inwards when dark.

    Better (for which, read, more expensive) power supplies are the answer. Unfortunately, in their ever increasing attempts to drop prices and/or increase margins, many manufacturers tend to skimp somewhat.

    Solution: The easiest thing for a user to do to minimise the effect is to reduce their contrast setting. This reduces white brightness, reduces demand on the PSU and hence improves the stability of the image.
     
  10. probedb

    probedb
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    Exactly what the problem is :)

    It's a right bugger, pity you can't upgrade TV PSUs like computer ones ! Interestingly enough I was originally supplied a European model before returning it as it couldn't do Nicam but the European one was fine !
     
  11. LV426

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    ....and it's much more common that TV manufacturers' brochures and the "CRT is best" brigade would have us believe. Pretty well all large CRTs have it to a greater or lesser degree.

    A really good way to see this happening is to watch a channel with a logo and rapidly changing content - music video's are a good example. Study the logo closely and watch it happily bounce around in the corner of the screen as the rest of the image content changes.

    Then try the same on a Plasma or LCD.
     
  12. Laurel&Hardy

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    Oh I agree, that is CRT's biggest failing.

    BUT...Whilst people keep saying 'I'm not paying a grand for that 36" set' for example then what do you expect? A decent PSU for a 36" CRT will cost a packet - just look at the size of the thing! You get what you pay for and the same goes for LCD and Plasma too - the cheap ones look truly horrid. Whist people make demands to push prices down two things are certain:

    1. You will end up with inferior products.
    2. Production gets shipped to the cheapest countries - China for example.

    How to kill manufacturing in the UK stone dead - keep insisting on reduced prices for your electrical goods.
     
  13. LV426

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    It's quite plausible that a (hypothetical) large CRT made to the same cost as an equivalent sized flat panel might have this particular problem, and other poor setup issues like static geometry and convergence fully or almost fully addressed. But, as you say, few would part with the requisite amount of cash. However, I guess that susceptibility to external magnetic fields (colour impurity) is probably much harder to address.
     
  14. red16v

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    A high end professional monitor (Sony for example) incorporates a magnetic shield (mu-metal I think) around the crt to eliminate the effect of external magnetic shields. Whilst it's not rocket science I guess if it were to be applied to large screen domestic CRTs then it would raise the cost of manufacture. regards, yt.
     
  15. LV426

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    It's a shame, really, that, as consumers, were aren't offered the choice of paying a lot for something approaching perfection. Not that I, personally, would be likely to choose a CRT - not even a really, really good one. But I'm sure there are some who would relish such an option.
     

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