[opendtv] Re: Math of oversampling - a simple comparison
- From: Tom Barry <trbarry@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Sat, 23 Apr 2005 11:22:42 -0400
Bob Miller wrote:
> If a broadcaster uses a 1080p camera and broadcast as 480P and on the
> reception end it is upconverted to 720P or stays 480P I understand that
> because of oversampling both the 720P or 480P image would be better than
> otherwise would be expected. What I would like to do is quantify this
> value. How would you compare a straight thru 720P broadcast to one such
> as that above. Would the 1080P>480P>720P route be 75% of the quality of
> the 720P>720P>720P route? Would 1080P>480P>480P be 110% of 480P>480P>480P?
Bob -
In practice I guess it is only necessary to beat or maybe match
the competition. And currently most HDTV does not really contain
that much detail. I have included a link to a captured image from
the Pilot of the Medium TV series. The LEFT half of that image
was left untouched.
But on the RIGHT half of the image I filtered out all frequencies
that would correspond to a spatial resolution higher than a 1/4
rez of 960x544, using my DctFilter plugin for Avisynth
(www.trbarry.com/Readme_DctFilter.txt). That filter does a
discrete cosine transform and then (in this case) zeros out all
coefficients except for the 4x4 square in the top left of the 8x8
matrix. Then an inverse transform back to pixels.
Thus the resulting image could theoretically be encoded at only
960x544 without any more loss of detail. Of course in practice
that would introduce additional scaling and compression artifacts.
I did not do that.
So, without further ado, see for yourself at:
www.trbarry.com/Medium_1080p_compare_qtr_rez.jpg
It is not hard to compete with the detail of most HD these days.
Most of the extra pixels are sadly just being wasted anyway.
- Tom
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