[opendtv] Re: Why Europe should choose 720P for HDTV
- From: "Alan Roberts" <roberts.mugswell@xxxxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Sun, 12 Dec 2004 14:35:50 -0000
The filters were designed before pixellated displays were common. There was
always the assumption that the image would have the "defocusing" effect of
the crt spot. I believe that this is why compression artefacts look worse on
pixellated displays, they have no post-filter. A simple layer of light
diffuser would fix it. The big JVC projectors effectively do that, and the
pictures look better.
----- Original Message -----
From: "Tom Barry" <trbarry@xxxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Sent: Saturday, December 11, 2004 5:39 PM
Subject: [opendtv] Re: Why Europe should choose 720P for HDTV
> Alan -
>
> But does that 42% amount account for the fact that fixed pixel displays
> may be effectively tiny little squares with sharply defined edges? Or
> is it intend for some CRT spot beam with approx Gaussian distribution?
>
> Or does it matter?
>
> BTW, It's been pointed out here on the list that my concerns about this
> possible non-problem could easily be fixed anyway by simple blurring the
> fixed pixels a bit as displayed. This could even more easily be done
> optically in projectors like DLP's and I suspect many digital movie
> theaters would look slightly better when very slightly out of focus if
> the owners were not so proud of their ability to display non-Nyquist
> graphics images (or concerned they hadn't waited for higher rez).
>
> Comments?
>
> - Tom
>
>
> Alan Roberts wrote:
>
> > The Nyquist limit is exactly 50% of the sampling frequency, you have to
> > filter to lower that this to get smooth motion. Typically, digital
systems
> > have analogue filters to prevent frequencies above 0.45 of sampling
> > frequencies getting into the system. The ITU recommended filters for 601
and
> > 709 sampling are both flat to 42%, then dive steeply to -12dB at 50%,
> > allowing some aliases to creep in. Conventional wisdom is that this is a
> > good idea, I don't go along with it fully because those aliases cause
havoc
> > in compressors, even at low levels.
> >
> > So, you need 20 pixels to show 18 lines or 9 cycles. The filtering
refuses
> > to allow such frequencies to start/stop suddenly, so you get confusion
at
> > the ends of a block of such frequencies, but it all looks fine once the
edge
> > effects have passed. It's interesting to work out the actual pixel
pattern
> > for such frequencies, and to see what happens when it all moves, I've
done
> > so and the results are fascinating, it explains why aliases are such a
bad
> > idea.
> >
> > ----- Original Message -----
> > From: "Tom Barry" <trbarry@xxxxxxxxxxx>
> > To: <opendtv@xxxxxxxxxxxxx>
> > Sent: Saturday, December 11, 2004 4:32 PM
> > Subject: [opendtv] Re: Why Europe should choose 720P for HDTV
> >
> >
> >
> >>Mark -
> >>
> >>How much do you have to filter so that an image can move smoothly across
> >>or down a fixed pixel display 1/2 pixel at a time without aperture
> >>effect artifacts. Is it just to the Nyquist limit, or more?
> >>
> >
> >
> >
> >
> >
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