[opendtv] (more on) HDTV and the Resolving Power of the Eye
- From: Tom Barry <trbarry@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Fri, 14 Jan 2005 09:17:38 -0500
An interesting article by ABC'c Randy Hoffner. It has some graphics so
you will have to use the link but I've excerpted a few bits below.
Some will of course disagree since it talks again about viewing HDTV at
a 3 screen heights viewing distance. ;-)
see:
http://www.tvtechnology.com/features/Tech-Corner/Hoffner_features.shtml
- Tom
---- small excerpts follow ----
Technology Corner: Randy Hoffner
HDTV and the Resolving Power of the Eye
A principal advantage of HD over SD video is the ability to view larger
images from closer distances.
The design goal for HDTV was to facilitate viewing at a distance of
three picture heights from the screen as opposed to the five- to
seven-picture-height design goal for NTSC. What is the relationship
between resolving power, screen resolution and viewing distance from the
screen?
.
.
.
What does all this mean with respect to HDTV and SDTV pictures? Consider
display sizes, with a 20/20 viewer located about 11.3 feet (3,438 mm)
from the screen. The design goal for HDTV was to facilitate viewing at a
distance of three picture heights, so if three picture heights is 3,438
mm, then one picture height is 1,146 mm, or about 45 inches (3.76 feet).
Since the HDTV aspect ratio is 16:9, the width of the image is 2,037 mm,
or about 80 inches (6.7 feet).
.
.
.
We calculate that the horizontal viewing field is about 32.6 degrees. If
we plug screen height in place of screen width, we calculate that the
vertical viewing field is about 18.8 degrees. Obviously, the viewer's
eyes must move around to see the entire picture at maximum resolving power.
720p & 1080i
Now, let's calculate the distance between scanning line centers and
pixel centers (the line and pixel pitches) for the two ATSC HD scanning
formats for these screens.
To remove the factor of CRT dot pitch, we'll assume that we are
projecting these pictures.
For 1,080 lines, dividing 1,146/ 1,080 gives us a distance of 1.06 mm
between line centers; for 1,920 horizontal samples, 2,037/1,080 gives us
a distance of 1.06 mm between horizontal pixel centers.
In this case, a 20/20 viewer who can resolve lines 1 mm apart will just
be able to see both the line structure and the horizontal pixel
structure from a distance of three picture heights.
For 1,280 x 720, the distance between line centers and pixel centers is
about 1.59 mm, in which case the line and pixel structure would be
somewhat more visible.
...
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