[opendtv] Re: The Math on Screen sizes
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Fri, 4 Jun 2004 08:40:20 -0400
At 6:25 PM -0700 6/3/04, John Willkie wrote:
>Please, please, somebody show me where I have gone wrong.
Nobody ever said anything about 810 samples per line being HD,
although most HD capable display cannot resolve much more horizontal
detail.
The 810 number comes from the CEA. It is a caveat for 4:3 CRT based
RPTVs and relates to the number of active lines of video when a 4:3
HD capable display is showing 16:9 source. In essence, the display
scans 1080 lines (two 540 line fields, spreading the lines over the
entire vertical area of the 4:3 display. The alternative would be to
reduce the vertical drive and scan only the 16:9 portion of the
screen with the full 1080 lines; but many manufacturers experienced
differential burn in using this technique. The CEA agreed to allow
manufacturers to stretch the 1080 lines over the 4:3 screen, Using
only the central 810 lines to display 16:9 HD source. The remaining
lines are typically filled with grey to average out burn-in, although
some manufacturers do allow the unused area to be black.
As for the actual horizontal resolution of CRT based displays, there
are two different issues.
For direct view CRT displays the limiting horizontal resolution is
determined by the shadow mask of the CRT; the number of slits or dot
triads across the width of the screen; The best I have seen in and
consumer HD display of this type is about 850 samples per screen
width. I think that Princeton offered a 720P display with better
horizontal resolution. You don't need a spreadsheet to figure this
out...just count the slits (or look in the specs for the display).
For rear projection CRT displays the situation is very different, as
the tubes used for these sets do not have shadow masks. Thus the
limiting horizontal resolution is determined by the spot size of the
beam and the band pass of the amps that are driving the CRTs. Again,
looking at the specs, you will find that most CRT based RPTVs offer
no more than 850 lines horizontal resolution.
Regards
Craig
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