How to config your vedio subsystem for X386 (was Re: X386 1.1)
Chin Fang
fangchin at leland.Stanford.EDU
Thu Mar 7 11:49:48 AEST 1991
In article <1991Mar6.213907.28719 at maverick.ksu.ksu.edu> rrsum at matt.ksu.ksu.edu (Rick Summerhill) writes:
>I am currently using Tom Roell's X386 1.1 with ISC 2.2.1 and have some
>questions:
>
>1. I get errors from Xlib about too many clients. I have 32 pty's and
> 32 streams ports, so that is not the problem. Anyone know why?
I don't have ISC, can't help you on that. But...
>2. Does anyone have a good 1024x768 configuration for a Sigma Legend card.
> My monitor will go to 66 Mhz. Can 1152x900 be used in non-interlaced
> mode with this monitor configuration?
OK. I think it's time to sweep away some confusing terminologies introduced
by marketing people at monitor manufacturers. Monitors NEVER support Mhz
which is by definition, million cycles by second. Your monitor, if multi-scan-
ing, will support khz in it's horizontal sync frequnencies, which by definition
is kilo (or thousand) cycle per second. In the vertical direction, the sync
frequency is always in the order of 10 hz (eg, 15 hz ~ 75 hz, like NEC 4D).
However, if analog type like NEC 2A, only descrete frequencies are supported.
Not a continuous range. So be careful!
NOTHING in the world of monitors will support MILLION hertz range support sync
frequency! It's your vedio adapter which MAY support such a driving frequency!
(see below) Furthermore, it's up to your software driver to see if such a
driving frequency can be utilized or not!
When we talk about display, it's always NECESSARY to consider three things
together:
(1) your monitor's sync frequency ranges for both horizontal and vertical
(2) your vedio adapter's driving frequency bandwidth (from crystals for ex.)
(3) your software's vedio hardware device driver.
Anyway, I regard this frequently advertised usuage as bad as, or even worse
than using Mhz to denote data transfer rate in hard disk ads.
Hz, named after the German who did quite a lot ground work in EE theory,
SHOULD be used to describe oscillatory phenomenon with fixed frequencies.
Any other usuage is INCORRECT!
How to determine a good resolution for your monitor? Please read the following
Definitions 1. screen refresh rate => it's the DRIVING clock frequency divided
by the product of horizontal frame length and vertical frame
length.
Elaborations: Q. what is driving clock frequency?
A. it's the oscillatory frequency of the crystal(on your
vedio board) employed by your graphics software's vedio
driver. For instance, if your driver uses 36 Mhz out of
25 28 0 40 36 40 45 58
32 36 31 35 50 48 33 65 (all in Mhz, for Sigma Legend)
the the driving frequency is 36Mhz. Nothing else.
This is the frequency used by the driver to determine
how frequent to update (thus refresh) screen image.
Q. What's frame length for horizontal and vertical directions?
A. It's the number of clock tics (measured in terms of the
driving timing) for your monitor's electron gun to shoot
a beam of electrons from left side of the tube to the right
side and back. Similarly, for vertical direction, from
bottom to top and back to bottom.
Q. Why this has anything to do with resolution?
A. because it's basics in highschool physics. So you MUST
understand it! (to answer your son's questions). Your
desired resolution in fact should be called the portion
of frame length that electron beam creates visible image
to your eyes! Any image is created with your eyes's
retention and the fast moving electron beam sweeping thru
your monitor. At any instance, there is in fact ONE dot
hitting the screen, but due to your eyes retention, you
see a block of image.
Q. So what does this have anything to do with screen refresh
rate?
A. By definition, one hertz (hz) is one cycle per second.
So, if your horizontal frame length takes x tics, ver. frame
length takes y tics, then to sweep thru the entire screen,
a rectangular area, takes x times y tics. Since your
driving frequency provides say N tics per second by
definition, then obviously your monitor's electron gun(s)
can shoot a dot accross the screen and sweep it from left
to right and back and from bottom to top and back, which
takes total xy tics, N/xy times. This IS your screen's
refresh rate! Because that's how many times your screen
can be updated thus REFRESHED per second!
Q. Why I have to know this B.S.?
A. Because anytime you use a refresh rate lower than 60 Hz
(in layman's words, sixty times every second), you are
trying to demage your eyes! So that you won't be enjoy
gif pictures as you may like in the future!!
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Q. So how do I get a desired resolution?
A. Simple! Jut look at your monitor's data sheet, normally
part of your users' manual. Make sure it's type, ie.
analoy or multiscaning. The later is much flexible if
not better. THE FOLLOWING DISCUSSION DOES NOT APPLY TO
ANALOG TYPE MONITOR!!
Then, use your horizontal max sync frequency, say 55khz
try out the clock frequencies listed or detected by
clock.exe. As an example, say for Sigma Legend, there is
a 65 Mhz clock oscillatory driving frequency available.
And from above assumption, your monitor can sync up to
55 Khz in horizontal. To increase max REFRESH rate and
at the same time get non-interlaceness, do the following
Obviously, if your clock cycles only N times per second,
and if your monitor electron beam syncs at x times per
second, the using the definition of frame length above,
you can only have so many horizontal frames per second:
N/x, in our sample, it would be 65Mhz/55khz=1181 times.
But due to VGA's hardware restrictions, you can only have
multiple of eight number of frames per second in the
horizontal direction. So round it off -> 1176.
Take 80 percent of this clock tics, or 944 tics for your
viewing image. This is a rule of thumb! Don't ask why!
Take 75% of 944 as your vertical tics for viewable image,
you have 708 tics. A rule of thumb is 1.05 times of tics
should be the vertical frame length ->743 tics. Here I
implicitly assumed that you like the Golden ratio.
So your screen refresh rate is 65Mhz/1176*743=74.4 hz!
THIS IS EXCELLENT! Don't settle on anthing less!
The image at this update rate (or screen refresh rate)
WILL LOOK STEADY EVEN TO VEDIO CAMERAS!
And you got 944x708 to boot. Not bad at all! You
can even improve it further to put it into almost 80 Hz
by using the fact that your monitor in horz. direction
can sync at lower frequency then 55 khz. Please take
advantage of it. All this is simple arithematics and
Simple facts about oscilloscopes. No black magic at all!
Memory requirment: 944x708/1024~653 K vedio RAM. So if
you have one meg, you have extra for virtual terminal
switching. See, this is good compromise!
But MAKE SURE that your monitor electron guns can sync
up to 80 Hz vertical. (NEC 4d CANN'T for instance. It
goes only up to 75 Hz in vertical)
There is NO reason whatsover why you have to use 640x480
800x600, or even 1024x768. X386 driver lets you config
your hardware with quite a lot freedom. It usually takes
two to three minutes to come up the right one.
The KEY is high refresh rate with reasonable viewing area.
NOT Hig Res at the price of flickerness!
Q. But how about interlace/non-interlace?
A. The key word in vedio watching is NON-FLICKER! The point
is that non-interlace is just part of the game! With non
interlace alone BUT low screen refresh rate, your eyes will
suffer badly!!!! Interlace-ness just worsen the flickering
at the same refresh rate. If you can manage to get high
enough refresh rate, say 90 Hz for interlace display, you
WILL NOT feel any flicker!! (But I doubt this is feasible)
As a side, that's why I HATE the display of IBM/RS6000 even
with it's blazingly fast TI340X0 assisted graphics!
So, CONCLUSION => NON-INTERLACENESS != NON-FLICKER!
[question deleted, I don't have ISC]
Please direct your comments to the above article to the addressee below:
Chin Fang
Mechanical Engineering Department
Stanford University
fangchin at leland.stanford.edu
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