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ALiS: How fast does it alternate?

Discussion in 'TVs' started by paolo999, Aug 20, 2005.

  1. paolo999

    paolo999
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    Is it matched to the signal e.g. 50 or 60 ?
    Is it fixed?
    Is it a 'higher' rate? e.g. 100Hz? Higher rate variable, e.g. 100/120Hz?
     
  2. Stephen Neal

    Stephen Neal
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    Be interesting to know.

    When I last investigated the underlying technology of plasma displays, it was clear that they didn't run at the source refresh rate, instead they run at a much higher rate, using sub-fields to creata grey scale using PWM-style selective illumination, combined with some dithering to increase the perceived grey-scale and reduce banding.

    I think if you look at a plasma whilst moving your head, say chewing, you see the sub-fields split into posterised bands. (A Panasonic plasma in our office does this very noticably)

    ISTR that the BBC R&D / Delphi plasma that took an SDI feed and converted it entirely digitally for plasma display, improved over the original electronics by increasing the number of sub-fields per source frame, reducing the amount of dithering required and thus reducing the apparent noisiness of the image.

    This was a couple of years ago - but I assume plasmas still use the same system to generate grey scales?


    *** EDIT ***

    Googling found this about Panasonic plasmas :

    What's a subfield, you ask? So did I. I learned that during each frame of the video signal (that is, within each 1/60 of a second), a plasma panel flashes the image 10 times or more, and each of these images is called a subfield. In addition, each subfield lasts twice as long as the previous one—a "powers of 2" progression. The panel's gray-scale performance depends on which subfields are turned on when.

    http://ultimateavmag.com/news/061605Panasonic/

    An NHK paper on HD plasmas also confirmed this.


    *** EDIT 2 ***

    http://magazine.fujitsu.com/us/vol35-2/paper12.pdf

    Seems to confirm that ALiS panels DO display interlaced images natively though.
     
  3. paolo999

    paolo999
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    My guess is that the sub fields behave exactly like that. So the field is "built up" from the successive PWM until it is "full"... then the other field is built up similarly (the previous field now gone), with the field rate being that of the source signal (or, possibly, which would be nasty for us PAL users, at a locked 60Hz, but my guess is that would look too awful to be saleable.)

    It could be done differently, e.g. fields built at 100Hz or some other 50Hz multiple, but given the high bandwidth already needed to achieve the sub field, my guess is that they don't do this - and besides, if they really were engineering it to be the best 1080i device, it would defeat the object because they'd be back to being a (pseudo) progressive scan.

    Now what we need is a friendly retailer with a 5xxx series, and, say a PV500 - wire them up in parallel with a 1080i source, mask out the surrounds and do a blind test. :)
     
  4. Stephen Neal

    Stephen Neal
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    Yep - and also feed them a 720/50p source and repeat!
     

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