[opendtv] Re: Math of oversampling

  • From: Mark Schubin <tvmark@xxxxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Fri, 22 Apr 2005 13:20:06 -0400

>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?

The calculable quality is not betterness but potential sharpness.  The 
psychovisual sensation of sharpness is proportional to the square of the 
area under a modulation-transfer-function (MTF) curve, a curve plotting 
contrast ratio against detail fineness.  There are many factors 
contributing to the function (lens, aperture, etc.), but, for the 
purposes of your question, two "all-else-being-equal" (meaning an HD 
lens on an SD camera)  factors will affect the sharpness: the number of 
samples in the imaging chip and the optical low-pass filtering.  Even 
dealing with just the former involves significant math:

I understand that I can't include attachments on this list server, but, 
if anyone wants a diagram of the various contributions to MTF, contact 
me directly, and I'll send it.  It's only about 70 KB.


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