[opendtv] Re: The Math on Screen sizes

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Fri, 11 Jun 2004 08:42:58 -0400

At 9:48 PM -0700 6/10/04, John Willkie wrote:
>Using those figures, I get similar results: 38.55 inch line width,  or a 44
>inch diagonal measure 16:9 screen, and a 48 inch or so 4:3 screen.  Of
>course, I haven't accounted for having more than one dot of each color per
>pixel, nor the actual pixel configuration.
>
>The issue, then, is how many sets have such closely spaced phosphors, and
>how many sets have monospaced pixels.  Neither appears to be universal.

Phosphor spacing is a non issue, unless the phosphors for the 
primaries share the same real estate, as is the case for direct view 
CRTs with shadow masks.  If you place an RGB phosphor pattern on the 
screen, you are in essence defining the limits of resolution - i.e. 
quantizing the display; it does not matter if there is a shadow mask 
or not.

Using separate CRTs for each primary eliminates this issue. The 
entire face of each tube can be covered with the same phosphors. When 
this happens the limits of resolution are defined by the size of the 
spot beam, and the bandwidth of the amplifiers that carry the source 
signals.It is important to note that you still have the issue of spot 
size elongation near the edges of the CRT. Unfortunately, with 
projection CRTs you ALSO run into another problem...brightness.

In order to get reasonable brightness levels - especially in rear 
projections applications - you need to maximize the beam current, and 
this in turn usually means a large spot size. If you follow the 
instructions that Joe Kane recommends for maximizing the performance 
of CRT-based projection systems, the first thing you do is to back 
off on the brightness and contrast to control the spot size so that 
you can maximize resolution and eliminate the problems with blooming. 
This of course means that you end up with a picture that is less 
bright, with less contrast; but you can deal with this by watching 
the display in a darkened room...like a movie theater.

These issues all go away when you move into the world of micro 
displays where there are "lithographed" pixels. These display have 
near perfect geometry (except for any distortions introduced by 
lenses and mirrors), and the brightness/contrast is -in most cases - 
significantly improved.

One other note. The desire to have more than one screen sample per 
source pixel is an oversampling issue. As we have seen with image 
capture there are significant benefits to over sampling during 
acquisition. The same is also true for displays. More samples help to 
eliminate the perception of individual pixels, and with analog 
sources to provide finer quantization steps as the levels change.

But there are limits here too; the limits of human perception. When 
you reach the point that the observer cannot resolve individual 
pixels, additional resolution is mostly wasted. Just step a few feet 
away from your computer display and you will see a real world example.

Regards
Craig
 
 
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