[opendtv] Re: The Math on Screen sizes
- From: Tom Barry <trbarry@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Fri, 04 Jun 2004 19:27:56 -0400
Most CRT based HDTV displays are capable of displaying 1080 lines, in
an interlaced fashion, though if you want the proper 16:9 aspect ratio
on a 4:3 HDTV display the CEA has decided it can still officially be
called an HDTV set if it displays a 16:9 area using only 810 of the
1080 lines. However most newer CRT based 4:3 HD displays can instead
use a squeeze function to place all 1080 lines in a 16:9 area if desired.
An NTSC analog TV only displays about 480, interlaced.
And as already discussed there are nuances of effective resolution,
grill aperture, and spot beam size which detract for the stated numbers.
But any HD display still displays a LOT more resolution than an NTSC
480i display.
- Tom
John Willkie wrote:
> So, ONCE AGAIN, what EXACTLY is the difference between the screen resolution
> of an NTSC set (440 lines?) and an 4:3 "HDTV" set capable of 810 lines?
>
> John Willkie
>
> P.S. Don't think for one second that I didn't notice you offering that one
> of those 4:3 sets was "HDTV" and now you are saying that somebody else said
> it. Me, I'm responsible for my words. Even erroroneous and bad ones.
>
> -----Original Message-----
> From: opendtv-bounce@xxxxxxxxxxxxx
> [mailto:opendtv-bounce@xxxxxxxxxxxxx]On Behalf Of Craig Birkmaier
> Sent: Friday, June 04, 2004 5:40 AM
> To: opendtv@xxxxxxxxxxxxx
> Subject: [opendtv] Re: The Math on Screen sizes
>
>
> 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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