720p DLP's Only Getting 540p out of 1080i Material?

Discussion in 'Displays' started by EricRWem, May 2, 2005.

  1. EricRWem

    EricRWem Screenwriter

    Jun 4, 2004
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    A story linked from slashdot stated that "most" 720p DLP sets out there only are converting one half of the 1080i stream to create a 720p signal.

    From the original article here:

    Any thoughts or opinions?
  2. Stephen Hopkins

    Stephen Hopkins HW Reviewer
    HW Reviewer

    Jul 19, 2002
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    Makes sense to me. 1080i->540p conversion is pretty strait forward and is usually done extremely well. That's why 960x540 lcd panels do a great job displaying 1080i material, often times better than 720p panels. Once 1080p displays become the norm (you're going to see a move in this direction this fall) then the processing is going to have to catch up and then you'll start to see 1080i source material deinterlaced to 1080p. I don't think the 1080i->540p->720p is a big source of picture degredation and can probably be done much better than 1080i->1080p->720p w/out display cost rising greatly due to the processing neccesary.
  3. John S

    John S Producer

    Nov 4, 2003
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    I believe that to be more or less correct....

    The scale of 1080i source to 720p, should be done in the digital relhm. I mean, this does not seem to be an issue at all with external HD tuners at all.

    So once again.. What exactly is the issue here again???
  4. Allan Jayne

    Allan Jayne Cinematographer

    Nov 1, 1998
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    Scaling can only be done in the digital domain.

    Converting 1080i to 540p consists of doing nothing.

    ************************************************** ****
    1080i to 720p

    If the even 540 line fields are upconverted into 720 line frames in a slightly different manner than the odd fields (even without true de-interlacing and not requiring added horsepower), the resulting resolution on a 720 line display can be close to 720 lines vertically.

    If the same formula is used for all fields, the resulting resolution of the subject matter will be no more than 540 lines vertically even though there are 720 scan lines.

    The concepts of "bob" and "weave" also apply. If only 540 lines of the 720 line frame are filled in for each incoming field, the other 180 lines keeping what was left over from the previous frame, this is the weave method which gives the full 720 lines of resolution but with combing artifacts. If all 720 lines are filled in, with interpolation used for 180 of them, this is the bob method without combing but with some softening.

    Video hints:

    Example of bob upscaling (degenerate case with repetition rather than interpolation)

    Odd 540 line 1080i field has lines 1, 3, 5, 7, 9 (first 5 scan lines)
    Upscaled to 720p "odd" frame 1, 3, 3, 5, 7, 9, 9 (first 7 scan lines)

    Even 540 line 1080i field has lines 2, 4, 6, 8, 10
    Upscaled to 720p "even" frame 2, 2, 4, 6, 6, 8, 10 for "different" method, note that when flashed on the screen alternately with the "odd" frame there are seven unique optical blends for the seven scan lines.

    Upscaled to 720p "even" frame 2, 4, 4, 6, 8, 10, 10 for "same" method, note that when flashed on the screen alternately with the "odd" frame, of the seven scan lines only five are unique and there is still optical blending.

    Representatives from DVDO and Lumagen have told me that their products (scalers) do 1080i to 720p using the "different" method, the description they uses was "introducing a half line phase shift to the even field" which causes different scan lines to line up between the source field and the target frame (for odd versus even) and be transposed directly while other source scanlines use interpolation to generate the necessary target scan lines. This would give these products a one-up on the typical HDTV set top box or the typical TV set that treated the 1080i as 540p.

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