[opendtv] Re: Interlace Artifacts

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Tue, 11 Jan 2005 09:57:37 -0500

At 8:56 PM -0800 1/10/05, John Willkie wrote:
>I agree with the sentiment, in general.  I was even going to voice it here,
>until I recognized that by doing so, I could reignite the "is 720 hdtv or
>better than 1080i argument."



Let's set the record straight for once and for all.

The actual resolution - i.e. the number of pixels/samples in the 
raster - is related to ONLY one thing:

The ability to display a sharp TV picture on a screen of a given 
size, at a defined viewing distance.

Interlace versus progressive sampling is a separate issue.

Progressive sampling will ALWAYS produce superior image quality 
relative to interlaced sampling at a given pixel clock rate - e.g 
720@60P (~55Msamples/sec) versus 1080@30i (~62 Msamples/sec).

The ONLY argument that favors interlace - one that is now irrelevant 
because of the use of digital compression techniques - is that you 
can sacrifice some spatial resolution for improved temporal 
resolution. Interlace was a useful compromise in an analog world 
where there was not enough bandwidth to deliver additional picture 
details (spatial, temporal and color). With digital compression 
techniques interlaced sampling actually results in reduced image 
quality relative to the same raster size sampled progressively - e.g. 
720 x 480 @60p obviously delivers higher image quality that 720 x 
480@30i. More important, the higher quality image can be delivered 
using the same bandwidth or less than the interlaced version.

The threshold where we theoretically move from EDTV to HDTV is 
somewhat arbitrary, based in part on the visual acuity of the 
observer. The REAL measure of HDTV is the size of the image presented 
to the viewer. I think we can all agree that a progressive display 
will look sharp  at some "undefined" viewing distance to any human 

Thus need for higher raster resolutions is DIRECTLY related to the 
size of the screen. As we increase the field of view that is covered 
by a display we need more samples to maintain the same perception of 
sharpness. We've been through these numbers many times, but 720P is 
more than adequate for screens up to approximately 100 inch diagonal, 
while 1080P is needed for large screens.

So the real question is where does EDTV end and HDTV begin?

I saw stories coming out of CES stating that HD displays are now 
available in sizes from 13" to more than 100." This is absurd. As we 
have discussed recently, smaller screens may have high resolution, 
but they CANNOT deliver the HDTV viewing experience - i.e. covering 
approximately 30 degrees of the field of vision with a sharp picture; 
you can sit 18 inches from a 13" display and see all the details in 
an HD image, but these viewing conditions do not stimulate the 
induction effect, where you feel like you are "there." To do that you 
need a BIG screen - 50 inch is barely adequate.

Bottom line, if the screen is less that 36-40 inch it cannot deliver 
the HDTV viewing experience.

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