Timothy Miller wrote:
On 8/8/06, Dieter <[EMAIL PROTECTED]> wrote:
Since then, I have remembered a problem with using a TV as a temporary
console. A NTSC TV takes 720x480, but most TVs can't really resolve
that, and 80x25 text is likely to be unreadable. IIRC, early personal
computers that used TVs as displays only output 40 chars or so per line.
I agree that this is a problem, but what system is going to handle
this? Probably very little since the original IBM PC will handle
40x25. Most things are going to expect 80 columns, and few will
likely handle more than 80 either. I bet the BIOS boot screens expect
very specific properties and won't handle anything else.
This (40x25) is specified as a VGA character mode so although the BIOS
boot screen isn't going to handle it, it should be possible to select it
to run a program to configure the video card. Perhaps we should supply
a utility to set up the video card which runs in 40 column character
mode. This could run under DOS but we would have to supply a free DOS
and SCO is now PNG (Persona Non Grata) but FreeDOS is still alive. Can
Linux run the console in VGA/VESA mode 1? Or, we could supply a program
that ran on bare iron.
Note that mode 1 only has 350 or 400 vertical lines in the VGA emulation
modes and the default vertical is 70 Hz with 900 total lines. How would
our TV out handle this? OTOH, real CGA character mode (which is in the
IBM VGA spec) will work on a TV since it is non-interlaced with 200
visible lines and 262 total lines at vertical 60 Hz .
I assume PAL and SECAM TVs have the same problem, just with slightly
different numbers.
Of course there are also lots of computer monitors that don't really
resolve what they claim to.
I think we're going to have to try to address this later, if at all.
OGD1 will be the basis for our products for a number of months, so
nothing is etched in stone (or silicon).
What's worse about TV in general is that, with a 3.57MHz subcarrier
for color, you cannot get more than 160 columns of color on the
display (based on 640, assuming 1:1 from the viewable 480 scanlines,
with a horizontal total of 704). Moreover, although the luma is
analog, most TVs blur the heck out of it, bringing the usable
resolution down to something between 160 and 320. So, basically, your
resolution on the display is roughly half the width of a character.
The only recourse we have is to design a really clever font, and even
then, it'll still be just about useless.
Actually, TVs manufactures have found ways around this. The last Sony
my family purchased some years ago claimed a horizontal resolution of
over 1000 lines. At the time I thought this was odd since it was
considerably more than the theoretical limit of +/- 360. IIUC, they
achieve this by using a comb filter rather than a LP filter to remove
the color subcarier from the luminance signal. If using the S-Video
output, this wouldn't be an issue.
--
JRT
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