pete@Octopus.COM (Pete Holzmann) (01/11/90)
I notice some people asking the same question I've asked over the last 6+
months... how to get a television signal out of a PC. Here's a review of
what I've found (so far)...
1) I haven't seen any EGA->NTSC methods. I recall a rumor of one, but never
have been able to find it.
2) I picked up a USVideo Recordable VGA card many months ago. To start at
the end- after several returns to the factory for improvements,
tuning and BIOS upgrades, the factory engineers admitted that their
card is simply not designed to produce a high quality TV signal
when used to display commonly found graphics. Specifically, anything
that is only one pixel wide drives the video signal nuts.
'Nuts' in this case means: colors are shifted (white turns into a
rainbow), the screen 'shimmers' horribly. The result of this is that:
- normal text of any kind can not be viewed reasonably
- line drawings (in my case, maps) look awful. A solid colored
map with country borders one pixel thick has horrible borders.
- Any solid fill that contains isolated contrasting-color pixels
in its fill pattern will shimmer badly.
Along the way, I also discovered that they have various other problems
with their card. Some should be fixable without a redesign (i.e. a
BIOS upgrade to fix the standard-VGA incompatibilities that make it
go to lunch when running various standard software programs and/or
video test programs (like the PCTech Journal system test). Other
problems will take a redesign, I'd guess, like the fact that it
loses sync every time you change video modes (so the screen rolls
once every time you start or stop a graphics program; very annoying!)
3) I picked up a Willow Peripherals VGA-TV card. This produces good quality
video. Period. It is very vga compatible. Period. There are some
limitations still; most of these may be inherent in the VGA->NTSC
process:
- Single pixel wide horizontal lines tend to suffer from slight shimmer
due to NTSC video interlace.
- High contrast edges in the image tend to shimmer a little.
(Both of the above effects can be minimized by reducing the
brightness on the TV, and by using a high quality TV)
- The board puts out very high levels of chroma (i.e. too much color)
using normal software. It would be nice to be able to set the
maximum 'color' level. DOS certainly doesn't let you adjust your
color palette on the fly!
- When in 640x480 mode, the top and bottom portion of the screen are
not visible on any normal TV. Even though the TV signal contains
482 lines, only the center 400 are normally visible unless you
have a card that can go into overscan mode (i.e. change the
pixel aspect ratio). In the long run, this is the most painful
part about doing VGA->NTSC... 640x480 isn't really practical!
Note that text modes are 640x400, so there's no problem there!
4) I've used Targa boards, and seen targa clones (i.e. Everex makes one) in
use. These boards put out 512x400 with a zillion simultaneous colors.
They work, work well, and have lots of extra features (i.e. live video
capture, genlock and overlay built in). They are NOT CGA/Mono/EGA/VGA
compatible. You run a Targa board in *addition* to a normal video board.
CONCLUSION:
- If you are serious about video output from a PC, have the $$, and
don't need VGA compatibility, use a board designed from scratch for
the purpose: Targa or compatible.
- If you want to get there cheaply (I wanted to get large-screen computer
output cheaply), and/or require VGA compatibility, the best I've
seen so far is from Willow peripherals. But be prepared to deal with
the 640x480 problem. (By the way, 320x200 is actually 640x400 doubled,
so it works wonderfully well!)
'Nuff said.
Pete
--
Peter Holzmann, Octopus Enterprises |(if you're a techie Christian & are
19611 La Mar Ct., Cupertino, CA 95014 |interested in helping w/ the Great
UUCP: {hpda,pyramid}!octopus!pete |Commission, email dsa-contact@octopus)
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