[opendtv] Re: 625 video quality is good enough....
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Wed, 20 Oct 2004 09:59:23 -0400
At 9:02 AM +0200 10/20/04, jeroen.stessen@xxxxxxxxxxx wrote:
>
>Not true. If such display accepts 24 Hz refresh at all (and some
>do) then it is only because internally the frames are repeated 2
>or 3 times to generate a more standard refresh rate again.
That is an internal decision for the manufacturer. It may be easier
for them to keep the refresh the same by repeating frames. But it is
not necessary on these display as they are constantly illuminated.
What is the difference between sending the same image to the display
once and leaving it up for 1/24th second, versus three times leaving
the same images up for 1/72nd second each. As far as the view can
see, the result is identical.
>LCDs need a high enough refresh rate because the voltage on each
>pixel cell must be inverted every frame. Since the result for each
>polarity is not equal, this causes some flicker at half the frame
>rate. This can only be masked by a high enough refresh rate.
>(Because of this problem you can actually see interlace-like line
>flicker on some -progressive- LCD projection displays.)
Again, this is a design issue. If what you are saying is true, and I
have no reason to doubt you, then the manufacturer can double or
triple shutter like a cinema projector. Hopefully, they will take
proper care not to get too lazy and make everything conform to 50Hz
or 59.94Hz, requiring a 4% speedup or 3:2 pulldown.
>
>PDPs and DLPs work internally with sub-fields or bit-planes.
>In some fields pixels will be on, in other fields they will be off.
>Especially the large-weight sub-fields will cause frame flicker.
>Again this can only be masked by a high enough refresh rate.
>E.g. for 60 Hz input the MSB fields may not be repeated, for 50 Hz
>input some MSB fields may be produced twice, and for 24 or 25 Hz
>input (if supported at all) the MSB fields may be produced 3 times.
Again, this is a design decision. Clearly the product can be designed
to do it properly.
>
>The end result may be marginally better (e.g. quantisation noise)
>than if the video source had applied 2x or 3x frame repetition,
>but for motion portrayal the result will be largely identical.
Agreed.
>
>And any way you want to render it, 24 Hz motion portrayal sucks !
Yup. Hopefully the cinema gurus will recognize the opportunity to use
some of the advantages of E-cinema to improve their craft. The 24
frame look is something that they like, and they work hard to control
camera movement and zooms to prevent the perception of motion
discontinuity. But they now have the option of using higher frame
rates for scenes that need better motion portrayal. If you refresh
the projector at 72P you can use 24P (triple shuttered), 36P (double
shuttered) and 72P for really outstanding motion portrayal.
>
>> The only displays I have seen that deals adequately with interlaced
>> HD are the ALIS plasma panels.
>
>Surely you did not mean to exclude all those CRT projectors ?
Well, I was talking about "constant illumination" displays, so this
leaves CRTs out. But now that you mention it, I have never seen a CRT
projector that equals a DLP projector, whether or not the original
source was interlaced.
I distinctly remember one of the first HD demos after the U.S. DTV
standard was approved. It was a Space Shuttle launch in 1080i, and it
was shown at a SMPTE conference in Pasadena. They could not get the
big theater venue (with CRT projection) working in time for the live
event, so we watched it on a variety of smaller direct view and RP
displays. In general the pictures looked pretty good. By that evening
they got the theater working and we got to watch the same footage on
a big screen. What we saw were significant compression artifacts and
significant interlace twitter, which were largely masked on the
smaller displays.
Regards
Craig
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