Video card capabilities

[richard at nezumi.plus.com]

On 2003-06-20 10:51:38 +0000, Csaba Henk wrote:
> In article <20030619095825.713c28cb.spyro at f2s.com>, Ian Molton wrote:
> > 
> > the ramdac on the card may not be able to do 1024x768 though.
> 
> How can I find it out?
> 
With difficulty. Most isa cards have some random ramdac chip chosen
according to the phase of the moon. The driver has to guess which one
it is, and choose some upper limit for its frequency. On modern cards,
much of the guesswork is reduced because the ramdac is build into the
big expensive chip, and there is a reasonably consistent way to find
the maximum frequency.

Your best hope is to feed the serial numbers on the chips into google.
Look for the maximum frequency or dot-clock. On modern cards, the
ramdac can be set to a continuous range of frequencies. The chances
are, you can choose from about 16 fixed frequencies on an ISA card.


> What's the difference between refresh, sync, and freq? xvidtune speaks of
> vsync, svidtune speaks of Vfreq, you speak about vertrefresh... which is the
> thingy which is shown as "V: 85.5 Hz" in my monitor's OCR menu?
> 
1) The graphics card starts by outputting some pixels, up to the limit
   of the horizontal resolution.
2) It outputs the background colour (black unless you know how to
   change it) for some more pixels.
3) The graphics card activates the horizontal sync connection to the
   monitor, to make it move the electon beam to the start if the
   next line. This stays active for a certain number of pixels.
4) The graphics card deactivates the horizontal sync for a certain
   number of pixels.
5) The above is repeated for each line, up to the vertical resolution.
6) Next come a few blank lines, followed by some blank lines with the
   vertical sync active, and some more blank lines without the vertical
   sync.
7) The whole thing starts again.

The number of complete frames drawn each second is the vertical refresh
rate, and is called all sorts of things that match /.*[vV].*/ and
is probably between 50 and 150 Hz.

To make life more confusing, lets include interlace at this point.
When you enable interlace, you have an odd total number of scan lines,
and the vertical sync is activated twice on each frame. The graphics
card outputs the even scan lines on one half frame, and the odd scan
lines on the other. This can reduce the perceived flicker without
requiring enoumous dot clocks and horizontal frequencies. If the image
being displayed has a high contrast between successive scan lines
(lines on different half frames) the flicker becomes appalling.

The next useful figure is the horizontal scan rate. This is probably
between 15kHz and 150kHz. If you monitor is as old as your graphics
card, it may well support only a few slow scan rates. Setting the
horizontal scan rate is always fun on an ancient monitor. Be sure to
have a fire extinguisher ready. Modern monitors are far more forgiving,
and will ALMOST certainly just shut down if you give them a scan rate
they do not like.

Backporch and front porch refer to the times on either side of the
sync pulses when no pixels are being drawn.

The last magic figure is the dot clock. It is the rate at which
pixels are drawn, and is probably beteen 4 and 400 MHz.

To make life fun, some programs like to be given a figure in pico
seconds instead of a dot clock frequency, or a number of pixels.
Some like separate figures for the number of scanlines that make
up the picture, and the times before, during and after the vsync
signal. Others want running total. The same applies to the
horizontal timings.

If you were not confued enough already, turn on double scan. Causes
the graphics card to output each line twice so you can use a low
resolution on an expensive monitor without black lines between each
scan line.

The sync pulses can be positive or negative, and this used to give
the monitor a clue about what frequencies were being used.


When ISA was new and shiny, and 8MHz 16 bit bus was considered
fast. That will almost get you 1024x768 in two colours, but is a
bit slow for four colours even with interlace. 800x600 is an
impressive resoltion for an ISA video card.


> Only svga is accessible on that box (its a 486 with video card on board, so
> I can test the card only on that machine; I have svgalib there). Should I
> use tinyX (whatever it's called now)? Has tinyX Xconfigure? As I know, a
> bunch of settings are hardcoded into tinyX servers.
> 
With a lot of effort you might get X to run (crawl) on a 486. If you
want to do something with it, leave it in text mode. (SVGATextMode
might let you get some more charaters on the screen).


Richard
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