Re: PC cards with real RGB out?

R. Mark Clayton wrote:

Quite a bit of confusion here.

Yes, let's see if we can correct some of that. :D

Older TV standards used to run interlaced, usually at their countries mains frequency.

They still do - the standards, and the SD broadcasts adhering to those standards, that is. But the new 100 Hz CRT-based sets and flat-panel sets may not display the pictures that way.

Although a whole frame was only sent 25 (or 30 in USA) times a
second, the interlacing meant that alternate lines were written every 20mS, and so to appeared stable and not to flicker.

It is important to realize that in interlaced video signal, motion - such as a ball rolling across the view, or a camera panning the scene - advances not only in each "frame" but in each field.

Hence, it is more useful to consider interlaced video, shot with an interlacing video camera, in terms of fields than in terms of "frames". Those "frames" do not really exist!

If you combine two adjacent fields - an odd field and an even field - and try to display them both at the same time - as a single picture - you don't really get "a frame" in the film frame sense, but a weird mash-up of odd and even lines. Adjacent fields do not really "belong" together because they depict two different instances of time. Trying to treat them as if they were a "frame" only leads to what many people call "interlacing artifacts" (even though it is not _interlacing_ that causes them to show up!), such as those depicted here:

<http://neap0litan.net/xvid/decoding/artifacts.jpg>

Here's a web page that explains all this quite nicely:

<http://lurkertech.com/lg/fields/fields.html>

More modern TV remember the screen state an rewrite the whole screen
at 100Hz [progressive] scan, giving a much more stable picture.

They also need to do lots of so-called "deinterlacing" trickery - which they would not need to do at all if they just displayed the picture in interlaced fashion as it was originally intended to be displayed.

The original VGA (640x480) was designed to be compatible with the
number of lines (actually displayed) and aspect ratio of NTSC TV's.

NTSC TVs do not actually display all 480 lines due to overscanning...

<http://scanline.ca/overscan/>

....and the number of active lines in a "proper" NTSC signal was traditionally thought to be a bit higher than 480 (486, as I've understood), but the proliferation of digital video equipment based on 480 lines has made this notion of a "proper" NTSC signal obsolete.

HDTV 1080p is currently 1920x1080 pixels at a frame rate of 24 - 30
frames per second. For a decent picture, even this will need to be
redrawn a number of times per frame (plasma) or held static (probably
easy on LCD) if it is not to flicker.

The technique of scanning the whole picture each time (as on 100Hz
TVs), even if the received [broadcast] signal is only 1080i
(interlaced) will have the effect of deinterlacing it, although there
will be small artefacts compared with 1080p (e.g. off a DVD)

Acquiring images in interlaced format is a weird idea these days because the new flat panel tv sets are not really built to display the images "natively" in interlaced fashion. This means that all kinds of needless trickery must be applied to make progressive video out of the interlaced source, and you never get to see the picture in its full glory. (Some CRT-based HDTV sets _were_ capable of displaying 1080i natively, without needing to resort to deinterlacing, but those were generally only sold in the US and people don't seem to want CRT-based sets any longer, so there's apparently no market for them these days.)

Since the HDTV transition is still only in its infancy in Europe, I think we should should stick with 720p24/25/50/60 and 1080p24/25/50/60, and completely forget about the 1080i format.

--
znark

.

Relevant Pages

  • Re: Why interlaced HDTV?
    ... >To display such a signal with the whole picture changing 50 times per second ... which is why interlace has been used by all broadcasters since the ... >Updating only one field's worth of the picture, or half the number of picture ... which is why there is a move to 1080p/720p 50 FRAME ...
    (uk.tech.digital-tv)
  • Re: 1080i v 1080p...
    ... frame is ready 1/25s later. ... True deinterlacing would attempt to reassemble each pair of fields ... into a single frame, and display the result, so picture content would ... you don't get something for nothing out of interlacing. ...
    (uk.tech.digital-tv)
  • Re: HD-DVD - The Final Death Blow
    ... But human persistence of vision melds that picture into 1080 lines by 1920. ... resolution for a 1080i signal. ... 1080i frame. ... He doesn't seem to understand interlacing and has in his mind ...
    (comp.sys.mac.advocacy)
  • Re: Is there any real advantage to having an HDMI port?
    ... CRT displays use little dots called 'phosphors'. ... the phosphors glow long enough for the entire frame to be glowing at the same time. ... If the picture is interlaced, the even numbered lines will be a bit brighter than the odd numbered lines. ... A "modern" display technology - like LCD, Plasma, or DLP/LCOS - doesn't draw one pixel, or even one line at a time. ...
    (alt.games.video.xbox)
  • Re: Is there any real advantage to having an HDMI port?
    ... the phosphors glow long enough for the entire frame to be ... If the picture is interlaced, ... The entire display changes ... Here's another interesting: LCD & DLP displays ...
    (alt.games.video.xbox)