🚀 go-pugleaf

Hm. Let's try this from the beginning... The Falcon can display graphics in
a number of different resolutions. It can also use a variety of devices to
display on, e.g., SC1224, TV, or VGA monitor. There is a fundamental difference
between a VGA monitor and a TV or SC1224 monitor, though - the VGA monitor
scans twice as fast as either of the other two. (I'm going to just say "TV"
from now on, since the 1224 and TV are functionally equivalent - same scan
rate, same rez.)

Ok. Let's think about how a TV operates - the US NTSC standard puts 525 scan
lines per video frame, but only draws half of those scan lines per field. I.e.,
the TV screen is refreshing at 60 hz, but only one half of the image is drawn
during each refresh. One "field" consists of odd scan lines, the other field
consists of even-numbered lines. This is just how the interlaced TV video
works. So while you have a field rate of 60 hz, you only have complete frames
at 30 hz. Each field consists of 262.5 scan lines. The name "interlace" arises
because the scan lines are not drawn contiguously in each field, instead they
are drawn with a 2:1 interleave....

Of the 262.5 scan lines available in every 60 hz interval, a portion are
pretty much invisible because they occur above or below the visible area of
the CRT's picture tube face. The exact number available will vary from set to
set, so computer-generated displays generally pick a conservative number of
lines to use and center that in the 262.5 that are available. (Remember the
8-bit Ataris? 320x192 display, but you could alter the display list and make
it 320x200 or 320x208 with no trouble, just by removing black lines from the
top and bottom of the display list...) This is the principal behind "overscan".
On the STs the number chosen was 200, even though, by tweaking the vertical
size control on the monitor, you could easily get 250 or so scan lines drawn
onto the screen.

So anyway - with a TV, you can draw 262.5 lines 60 times a second. Or,
using interlace, you can draw 525 lines 30 times a second.

In article <1h0uavINN2tt@golem.wcc.govt.nz> wells_s@kosmos.wcc.govt.nz writes:
>What's the difference between line-doubling & interlace anyway??

As I mentioned before, the VGA standard scans twice as fast as a TV. That
works out to something like 480 lines, 60 times per second. In order to get
a VGA monitor to display a 200-line screen, you need to either slow down the
VGA display, or artificially increase the scan-rate of the video data to match
the VGA scan rate. Since VGA uses a fixed frequency (i.e., incapable of
multisync) the Falcon uses line-doubling. That means that every scan-line of
a 200-line image is sent out twice, at the normal VGA rate.

So, in essence, interlace and line-doubling are completely opposite.
Interlace is a technique to display more lines than are possible to display
on a slow display device. Line-doubling is a technique to display fewer lines
than ordinarily possible on a faster display device.

Please note that this is not the same meaning as "line-doubling" applied to
HDTV, IDTV, etc. (In which case, line-doubling means de-interlacing an
image, to get e.g. 525 lines per screen, 60 times per second...)
>
>I think what Len is possibly trying to get at (correct me if I'm wrong
>Len 8-) is this.
>
>is the VGA mode capable of being displayed on a normal TV/colour monitor, 
>and is this what the overscan makes possible?

There are no "VGA modes." There are various resolutions, each of which can be
displayed on a VGA or TV display (with the help of various special techniques
like interlace or overscan).
>
>(e.g. the VGA 320*240*256 mode) - is this mode just the falcon
>320*200*256 mode with overscan being used on the vertical resolution
>to get the extra 40-pixels?

The 320x240x256 resolution is displayed using line-doubling on the VGA, and
using overscan on the TV. It is not a "VGA mode" since there is no such thing.
>
>So, basically - is it possible to use a normal TV or monitor (colour
>of course) to displayer the 'normal' resolutions, AND the VGA resolutions,
>or is a VGA monitor *required* for the VGA res's??

As I've said many times before, all of the available resolutions can be
displayed on a TV. You can't use a VGA trick (like line-doubling) on a TV,
but then again, you don't *need* it there anyway. On the other hand, not all
resolutions are available on the VGA. (I.e., no 640 wide true-color resolutions
can be displayed on VGA.) [I don't feel like rehashing the reasons for that...
   }-)  ]
-- 
  -- Howard Chu @ Jet Propulsion Laboratory, Pasadena, CA

All true wisdom is conveyed in one-line witticisms.