[opendtv] Re: Why Europe should choose 720P for HDTV
- From: Tom Barry <trbarry@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sat, 11 Dec 2004 12:39:50 -0500
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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