[opendtv] Re: Math of oversampling - a simple comparison
- From: Jeroen Stessen <jeroen.stessen@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Thu, 28 Apr 2005 16:50:06 +0200
I have sent this mail to Craig, Tom and Bob:
(the files will probably soon appear on www.trbarry.com)
Hello,
At Craig's request:
> If Tom and Jeroen have time to play, I would like to see a down/up
> sample pass through 1024 x 576P.
Here are the results: (2 files deleted)
file <Leno_1080_576_1080p.jpg> was created 1920x1088 -> 1024x576p
-> 1920x1088. It is still not quite as sharp as we had hoped !
Then I have also made file <Leno_1080_720_1080p.jpg> :
1920x1088 -> 1280x720p -> 1920x1088. Now that is finally almost
as sharp as the original. It appears that we need 1280x720 pixels,
that 1024x576 is just too little for this image.
(But I am watching this on a 20" CRT in 1600x1200 resolution
from 1 foot distance, so that is not fair even for HDTV !)
> My educated guess is that it will be nearly impossible to see
> the difference from the original.
Well, you seem to have guessed wrong, but why ?
This morning I have analysed my down- and up-sampling filters,
and I have determined that the cascade of the two filters has a
-3 dB point at 73% of the Nyquist limit of the down-sampled image.
So even if you apply this filter at the original resolution, it
is still a not neglegible low-pass filter. Unfortunately that is
the price to pay for low-aliasing scaling, I can't do much better.
Maybe that is why I need a 25% higher sample rate to get the sort
of sharpness that you educatedly guessed.
Regards,
-- Jeroen
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