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| HowTo Linux Zone | Linux Zone Home | E-Mail Me | XFree86 Video Timings HOWTO Eric S. Raymond <esr@thyrsus.com>
v3.6, 13 June 1999
How to compose a mode line for your card/monitor combination under
XFree86. The XFree86 distribution now includes good facilities for
configuring most standard combinations; this document is mainly useful
if you are tuning a custom mode line for a high-performance monitor or
very unusual hardware. It may also help you in using kvideogen to
generate mode lines, or xvidtune to tweak a standard mode that is not
quite right for your monitor.
______________________________________________________________________
Table of Contents
1. Disclaimer
2. Introduction
3. Tools for Automatic Computation
4. How Video Displays Work
5. Basic Things to Know about your Display and Adapter
5.1 The monitor sync frequencies
5.2 The monitor's video bandwidth
5.3 The card's dot clock
5.4 What these basic statistics control
6. Interpreting the Basic Specifications
6.1 About Bandwidth
6.2 Sync Frequencies and the Refresh Rate:
7. Tradeoffs in Configuring your System
8. Memory Requirements
9. Computing Frame Sizes
10. Black Magic and Sync Pulses
10.1 Horizontal Sync:
10.2 Vertical Sync:
11. Putting it All Together
12. Overdriving Your Monitor
13. Using Interlaced Modes
14. Questions and Answers
15. Fixing Problems with the Image.
15.1 The image is displaced to the left or right
15.2 The image is displaced up or down
15.3 The image is too large both horizontally and vertically
15.4 The image is too wide (too narrow) horizontally
15.5 The image is too deep (too shallow) vertically
16. Plotting Monitor Capabilities
17. Credits
______________________________________________________________________
1. Disclaimer
You use the material herein SOLELY AT YOUR OWN RISK. It is possible
to harm both your monitor and yourself when driving it outside the
manufacturer's specs. Read ``Overdriving Your Monitor'' for detailed
cautions. Any damage to you or your monitor caused by overdriving it
are your problem.
The most up-to-date version of this HOWTO can be found at the Linux
Documentation Project <http://metalab.unc.edu/LDP> web page.
Please direct comments, criticism, and suggestions for improvement to
esr@snark.thyrsus.com. Please do not send email pleading for a magic
solution to your special monitor problem, as doing so will only burn
up my time and frustrate you -- everything I know about the subject is
already in here.
2. Introduction
The XFree86 server allows users to configure their video subsystem and
thus encourages best use of existing hardware. This tutorial is
intended to help you learn how to generate your own timing numbers to
make optimum use of your video card and monitor.
We'll present a method for getting something that works, and then show
you how you can experiment starting from that base to develop settings
that optimize for your taste.
If you already have a mode that almost works (in particular, if one of
predefined VESA modes gives you a stable display but one that's
displaced right or left, or too small, or too large) you can go
straight to the section on ``Fixing Problems with the Image''. This
will enlighten you on ways to tweak the timing numbers to achieve
particular effects.
Don't assume that you need to get all the way into mode tuning just
because your X comes up with a scrambled display first time after
installation; it may be that most of the factory mode lines are OK and
you just happened to default to one that doesn't fit your hardware.
Instead, cycle through all your installed modes with CTRL-ALT-KP+. If
some of the modes look OK, try commenting out all but a 640x480 and
check that that mode works. If it does then uncomment a couple of
other modes, e.g. an 800x600 and a 1024x768 at a frequency that your
monitor should be able to handle.
3. Tools for Automatic Computation
Starting with XFree86 3.2, XFree86 provides an XF86Setup(1) program
that makes it easy to generate a working monitor mode interactively,
without messing with video timing number directly. So you shouldn't
actually need to calculate a base monitor mode in most cases.
Unfortunately, XF86Setup(1) has some limitations; it only knows about
standard video modes up to 1280x1024. If you have a very high-
performance monitor capable of 1600x1200 or more you will still have
to compute your base monitor mode yourself.
There is a KDE tool called KVideoGen
<http://without.netpedia.net/kvideogen/> that computes modelines from
basic monitor and card statistics. I've experimented with generating
modelines from it, but haven't tried them live. Note that its
horizontal and vertical `refresh rate' parameters are the same as the
sync frequencies HSF and VSF we describe below. The `horizontal sync
pulse' number seems to be a sync pulse width in microseconds, HSP
(with the tool assuming fixed `front porch' HGT1 and `back porch' HGT2
values). If you don't know the `horizontal sync pulse' number it's
safe to use the default.
Recent versions of XFree86 provide a tool called xvidtune(1) which you
will probably find quite useful for testing and tuning monitor modes.
It begins with a gruesome warning about the possible consequences of
mistakes with it. If you pay careful attention to this document and
learn what is behind the pretty numbers in xvidtune's boxes, you will
become able to use xvidtune effectively and with confidence.
If you have xvidtune(1), you'll be able to test new modes on the fly,
without modifying your X configuration files or even rebooting your X
server. Otherwise, XFree86 allows you to hot-key between different
modes defined in Xconfig (see XFree86.man for details). Use this
capabilty to save yourself hassles! When you want to test a new mode,
give it a unique mode label and add it to the end of your hot-key
list. Leave a known-good mode as the default to fall back on if the
test mode doesn't work.
Towards the end of this document, we include a `modeplot' script that
you can use to produce an analog graph of available modes. This is
not directly helpful for generating modelines, but it may help you
better understand the relationships that define them.
4. How Video Displays Work
Knowing how the display works is essential to understanding what
numbers to put in the various fields in the file Xconfig. Those
values are used in the lowest levels of controlling the display by the
XFree86 server.
The display generates a picture from a series of dots. The dots are
arranged from left to right to form lines. The lines are arranged
from top to bottom to form the picture. The dots emit light when they
are struck by the electron beam inside the display. To make the beam
strike each dot for an equal amount of time, the beam is swept across
the display in a constant pattern.
The pattern starts at the top left of the screen, goes across the
screen to the right in a straight line, and stops temporarily on the
right side of the screen. Then the beam is swept back to the left
side of the display, but down one line. The new line is swept from
left to right just as the first line was. This pattern is repeated
until the bottom line on the display has been swept. Then the beam is
moved from the bottom right corner of the display to the top left
corner, and the pattern is started over again.
There is one variation of this scheme known as interlacing: here only
every second line is swept during one half-frame and the others are
filled in in during a second half-frame.
Starting the beam at the top left of the display is called the
beginning of a frame. The frame ends when the beam reaches the the
top left corner again as it comes from the bottom right corner of the
display. A frame is made up of all of the lines the beam traced from
the top of the display to the bottom.
If the electron beam were on all of the time it was sweeping through
the frame, all of the dots on the display would be illuminated. There
would be no black border around the edges of the display. At the
edges of the display the picture would become distorted because the
beam is hard to control there. To reduce the distortion, the dots
around the edges of the display are not illuminated by the beam even
though the beam may be pointing at them. The viewable area of the
display is reduced this way.
Another important thing to understand is what becomes of the beam when
no spot is being painted on the visible area. The time the beam would
have been illuminating the side borders of the display is used for
sweeping the beam back from the right edge to the left and moving the
beam down to the next line. The time the beam would have been
illuminating the top and bottom borders of the display is used for
moving the beam from the bottom-right corner of the display to the
top-left corner.
The adapter card generates the signals which cause the display to turn
on the electron beam at each dot to generate a picture. The card also
controls when the display moves the beam from the right side to the
left and down a line by generating a signal called the horizontal sync
(for synchronization) pulse. One horizontal sync pulse occurs at the
end of every line. The adapter also generates a vertical sync pulse
which signals the display to move the beam to the top-left corner of
the display. A vertical sync pulse is generated near the end of every
frame.
The display requires that there be short time periods both before and
after the horizontal and vertical sync pulses so that the position of
the electron beam can stabilize. If the beam can't stabilize, the
picture will not be steady.
In a later section, we'll come back to these basics with definitions,
formulas and examples to help you use them.
5. Basic Things to Know about your Display and Adapter
There are some fundamental things you need to know before hacking an
Xconfig entry. These are:
· your monitor's horizontal and vertical sync frequency options
· your monitor's bandwidth
· your video adapter's driving clock frequencies, or "dot clocks"
5.1. The monitor sync frequencies
The horizontal sync frequency is just the number of times per second
the monitor can write a horizontal scan line; it is the single most
important statistic about your monitor. The vertical sync frequency
is the number of times per second the monitor can traverse its beam
vertically.
Sync frequencies are usually listed on the specifications page of your
monitor manual. The vertical sync frequency number is typically
calibrated in Hz (cycles per second), the horizontal one in KHz
(kilocycles per second). The usual ranges are between 50 and 150Hz
vertical, and between 31 and 135KHz horizontal.
If you have a multisync monitor, these frequencies will be given as
ranges. Some monitors, especially lower-end ones, have multiple fixed
frequencies. These can be configured too, but your options will be
severely limited by the built-in monitor characteristics. Choose the
highest frequency pair for best resolution. And be careful --- trying
to clock a fixed-frequency monitor at a higher speed than it's
designed for can easily damage it.
Earlier versions of this guide were pretty cavalier about overdriving
multisync monitors, pushing them past their nominal highest vertical
sync frequency in order to get better performance. We have since had
more reasons pointed out to us for caution on this score; we'll cover
those under ``Overdriving Your Monitor'' below.
5.2. The monitor's video bandwidth
Your monitor's video bandwidth should be included on the manual's spec
page. If it's not, look at the monitor's higest rated resolution. As
a rule of thumb, here's how to translate these into bandwidth
estimates (and thus into rough upper bounds for the dot clock you can
use):
640x480 25
800x600 36
1024x768 65
1024x768 interlaced 45
1280x1024 110
1600x1200 185
BTW, there's nothing magic about this table; these numbers are just
the lowest dot clocks per resolution in the standard XFree86 Modes
database (except for the last, which I interpolated). The bandwidth
of your monitor may actually be higher than the minimum needed for its
top resolution, so don't be afraid to try a dot clock a few MHz
higher.
Also note that bandwidth is seldom an issue for dot clocks under 65MHz
or so. With an SVGA card and most hi-res monitors, you can't get
anywhere near the limit of your monitor's video bandwidth. The
following are examples:
Brand Video Bandwidth
---------- ---------------
NEC 4D 75Mhz
Nano 907a 50Mhz
Nano 9080i 60Mhz
Mitsubishi HL6615 110Mhz
Mitsubishi Diamond Scan 100Mhz
IDEK MF-5117 65Mhz
IOCOMM Thinksync-17 CM-7126 136Mhz
HP D1188A 100Mhz
Philips SC-17AS 110Mhz
Swan SW617 85Mhz
Viewsonic 21PS 185Mhz
PanaSync/Pro P21 220Mhz
Even low-end monitors usually aren't terribly bandwidth-constrained
for their rated resolutions. The NEC Multisync II makes a good
example --- it can't even display 800x600 per its spec. It can only
display 800x560. For such low resolutions you don't need high dot
clocks or a lot of bandwidth; probably the best you can do is 32Mhz or
36Mhz, both of them are still not too far from the monitor's rated
video bandwidth of 30Mhz.
At these two driving frequencies, your screen image may not be as
sharp as it should be, but definitely of tolerable quality. Of course
it would be nicer if NEC Multisync II had a video bandwidth higher
than, say, 36Mhz. But this is not critical for common tasks like text
editing, as long as the difference is not so significant as to cause
severe image distortion (your eyes would tell you right away if this
were so).
5.3. The card's dot clock
Your video adapter manual's spec page will usually give you the card's
maximum dot clock (that is, the total number of pixels per second it
can write to the screen).
If you don't have this information, the X server will get it for you.
Recent versions of the X servers all support a --probeonly option that
prints out this information and exits without actually starting up X
or changing the video mode.
If you don't have -probeonly, don't depair. Even if your X locks up
your monitor, it will emit a line of clock and other info to standard
error. If you redirect this to a file, it should be saved even if you
have to reboot to get your console back.
The probe result or startup message should look something like one of
the following examples:
If you're using XFree86:
Xconfig: /usr/X11R6/lib/X11/Xconfig
(**) stands for supplied, (--) stands for probed/default values
(**) Mouse: type: MouseMan, device: /dev/ttyS1, baudrate: 9600
Warning: The directory "/usr/andrew/X11fonts" does not exist.
Entry deleted from font path.
(**) FontPath set to "/usr/lib/X11/fonts/misc/,/usr/lib/X11/fonts/75dpi/"
(--) S3: card type: 386/486 localbus
(--) S3: chipset: 924
---
Chipset -- this is the exact chip type; an early mask of the 86C911
(--) S3: chipset driver: s3_generic
(--) S3: videoram: 1024k
-----
Size of on-board frame-buffer RAM
(**) S3: clocks: 25.00 28.00 40.00 3.00 50.00 77.00 36.00 45.00
(**) S3: clocks: 0.00 0.00 79.00 31.00 94.00 65.00 75.00 71.00
------------------------------------------------------
Possible driving frequencies in MHz
(--) S3: Maximum allowed dot-clock: 110MHz
------
Bandwidth
(**) S3: Mode "1024x768": mode clock = 79.000, clock used = 79.000
(--) S3: Virtual resolution set to 1024x768
(--) S3: Using a banksize of 64k, line width of 1024
(--) S3: Pixmap cache:
(--) S3: Using 2 128-pixel 4 64-pixel and 8 32-pixel slots
(--) S3: Using 8 pages of 768x255 for font caching
If you're using SGCS or X/Inside X:
WGA: 86C911 (mem: 1024k clocks: 25 28 40 3 50 77 36 45 0 0 79 31 94 65 75 71)
--- ------ ----- --------------------------------------------
| | | Possible driving frequencies in MHz
| | +-- Size of on-board frame-buffer RAM
| +-- Chip type
+-- Server type
Note: do this with your machine unloaded (if at all possible).
Because X is an application, its timing loops can collide with disk
activity, rendering the numbers above inaccurate. Do it several times
and watch for the numbers to stabilize; if they don't, start killing
processes until they do. Your mouse daemon process, if you have one,
is particularly likely to trip you up (that's gpm for Linux users,
mousemgr for SVr4 users).
In order to avoid the clock-probe inaccuracy, you should clip out the
clock timings and put them in your Xconfig as the value of the Clocks
property --- this suppresses the timing loop and gives X an exact list
of the clock values it can try. Using the data from the example
above:
wga
Clocks 25 28 40 3 50 77 36 45 0 0 79 31 94 65 75 71
On systems with a highly variable load, this may help you avoid
mysterious X startup failures. It's possible for X to come up, get
its timings wrong due to system load, and then not be able to find a
matching dot clock in its config database --- or find the wrong one!
5.4. What these basic statistics control
The sync frequency ranges of your monitor, together with your video
adapter's dot clock, determine the ultimate resolution that you can
use. But it's up to the driver to tap the potential of your hardware.
A superior hardware combination without an equally competent device
driver is a waste of money. On the other hand, with a versatile
device driver but less capable hardware, you can push the hardware's
envelope a little. This is the design philosophy of XFree86.
You should match the dot clock you use to the monitor's video
bandwidth. There's a lot of give here, though --- some monitors can
run as much as 30% over their nominal bandwidth. The risks here have
to do with exceeding the monitor's rated vertical-sync frequency;
we'll discuss them in detail below.
Knowing the bandwidth will enable you to make more intelligent choices
between possible configurations. It may affect your display's visual
quality (especially sharpness for fine details).
6. Interpreting the Basic Specifications
This section explains what the specifications above mean, and some
other things you'll need to know. First, some definitions. Next to
each in parens is the variable name we'll use for it when doing
calculations
horizontal sync frequency (HSF)
Horizontal scans per second (see above).
vertical sync frequency (VSF)
Vertical scans per second (see above). Mainly important as the
upper limit on your refresh rate.
dot clock (DCF)
More formally, `driving clock frequency'; The frequency of the
crystal or VCO on your adaptor --- the maximum dots-per-second
it can emit.
video bandwidth (VB)
The highest frequency you can feed into your monitor's video
input and still expect to see anything discernible. If your
adaptor produces an alternating on/off pattern, its lowest
frequency is half the DCF, so in theory bandwidth starts making
sense at DCF/2. For tolerately crisp display of fine details in
the video image, however, you don't want it much below your
highest DCF, and preferably higher.
frame length (HFL, VFL)
Horizontal frame length (HFL) is the number of dot-clock ticks
needed for your monitor's electron gun to scan one horizontal
line, including the inactive left and right borders. Vertical
frame length (VFL) is the number of scan lines in the entire
image, including the inactive top and bottom borders.
screen refresh rate (RR)
The number of times per second your screen is repainted (this is
also called "frame rate"). Higher frequencies are better, as
they reduce flicker. 60Hz is good, VESA-standard 72Hz is
better. Compute it as
RR = DCF / (HFL * VFL)
Note that the product in the denominator is not the same as the
monitor's visible resolution, but typically somewhat larger. We'll
get to the details of this below.
The rates for which interlaced modes are usually specified (like
87Hz interlaced) are actually the half-frame rates: an entire
screen seems to have about that flicker frequency for typical
displays, but every single line is refreshed only half as often.
For calculation purposes we reckon an interlaced display at its
full-frame (refresh) rate, i.e. 43.5Hz. The quality of an
interlaced mode is better than that of a non-interlaced mode with
the same full-frame rate, but definitely worse then the non-
interlaced one corresponding to the half-frame rate.
6.1. About Bandwidth
Monitor makers like to advertise high bandwidth because it constrains
the sharpness of intensity and color changes on the screen. A high
bandwidth means smaller visible details.
Your monitor uses electronic signals to present an image to your eyes.
Such signals always come in in wave form once they are converted into
analog form from digitized form. They can be considered as
combinations of many simpler wave forms each one of which has a fixed
frequency, many of them are in the Mhz range, eg, 20Mhz, 40Mhz, or
even 70Mhz. Your monitor video bandwidth is, effectively, the
highest-frequency analog signal it can handle without distortion.
For our purposes, video bandwidth is mainly important as an
approximate cutoff point for the highest dot clock you can use.
6.2. Sync Frequencies and the Refresh Rate:
Each horizontal scan line on the display is just the visible portion
of a frame-length scan. At any instant there is actually only one dot
active on the screen, but with a fast enough refresh rate your eye's
persistence of vision enables you to "see" the whole image.
Here are some pictures to help:
_______________________
| | The horizontal sync frequency
|->->->->->->->->->->-> | is the number of times per
| )| second that the monitor's
|<-----<-----<-----<--- | electron beam can trace
| | a pattern like this
| |
| |
| |
|_______________________|
_______________________
| ^ | The vertical sync frequency
| ^ | | is the number of times per
| | v | second that the monitor's
| ^ | | electron beam can trace
| | | | a pattern like this
| ^ | |
| | v |
| ^ | |
|_______|_v_____________|
Remember that the actual raster scan is a very tight zigzag pattern;
that is, the beam moves left-right and at the same time up-down.
Now we can see how the dot clock and frame size relates to refresh
rate. By definition, one hertz (hz) is one cycle per second. So, if
your horizontal frame length is HFL and your vertical frame length is
VFL, then to cover the entire screen takes (HFL * VFL) ticks. Since
your card emits DCF ticks per second by definition, then obviously
your monitor's electron gun(s) can sweep the screen from left to right
and back and from bottom to top and back DCF / (HFL * VFL) times/sec.
This is your screen's refresh rate, because it's how many times your
screen can be updated (thus refreshed) per second!
You need to understand this concept to design a configuration which
trades off resolution against flicker in whatever way suits your
needs.
For those of you who handle visuals better than text, here is one:
RR VB
| min HSF max HSF |
| | R1 R2 | |
max VSF -+----|------------/----------/---|------+----- max VSF
| |:::::::::::/::::::::::/:::::\ |
| \::::::::::/::::::::::/:::::::\ |
| |::::::::/::::::::::/:::::::::| |
| |:::::::/::::::::::/::::::::::\ |
| \::::::/::::::::::/::::::::::::\ |
| \::::/::::::::::/::::::::::::::| |
| |::/::::::::::/:::::::::::::::| |
| \/::::::::::/:::::::::::::::::\|
| /\:::::::::/:::::::::::::::::::|
| / \:::::::/::::::::::::::::::::|\
| / |:::::/:::::::::::::::::::::| |
| / \::::/::::::::::::::::::::::| \
min VSF -+----/-------\--/-----------------------|--\--- min VSF
| / \/ | \
+--/----------/\------------------------+----\- DCF
R1 R2 \ | \
min HSF | max HSF
VB
This is a generic monitor mode diagram. The x axis of the diagram
shows the clock rate (DCF), the y axis represents the refresh rate
(RR). The filled region of the diagram describes the monitor's
capabilities: every point within this region is a possible video mode.
The lines labeled `R1' and `R2' represent a fixed resolutions (such as
640x480); they are meant to illustrate how one resolution can be
realized by many different combinations of dot clock and refresh rate.
The R2 line would represent a higher resolution than R1.
The top and bottom boundaries of the permitted region are simply
horizontal lines representing the limiting values for the vertical
sync frequency. The video bandwidth is an upper limit to the clock
rate and hence is represented by a vertical line bounding the
capability region on the right.
Under ``Plotting Monitor Capabilities'') you'll find a program that
will help you plot a diagram like this (but much nicer, with X
graphics) for your individual monitor. That section also discusses
the interesting part; the derivation of the boundaries resulting from
the limits on the horizontal sync frequency.
7. Tradeoffs in Configuring your System
Another way to look at the formula we derived above is
DCF = RR * HFL * VFL
That is, your dot clock is fixed. You can use those dots per second
to buy either refresh rate, horizontal resolution, or vertical resolu&SHY;
tion. If one of those increases, one or both of the others must
decrease.
Note, though, that your refresh rate cannot be greater than the
maximum vertical sync frequency of your monitor. Thus, for any given
monitor at a given dot clock, there is a minimum product of frame
lengths below which you can't force it.
In choosing your settings, remember: if you set RR too low, you will
get mugged by screen flicker.
You probably do not want to pull your refresh rate below 60Hz. This
is the flicker rate of fluorescent lights; if you're sensitive to
those, you need to hang with 72Hz, the VESA ergonomic standard.
Flicker is very eye-fatiguing, though human eyes are adaptable and
peoples' tolerance for it varies widely. If you face your monitor at
a 90% viewing angle, are using a dark background and a good
contrasting color for foreground, and stick with low to medium
intensity, you *may* be comfortable at as little as 45Hz.
The acid test is this: open a xterm with pure white back-ground and
black foreground using xterm -bg white -fg black and make it so large
as to cover the entire viewable area. Now turn your monitor's
intensity to 3/4 of its maximum setting, and turn your face away from
the monitor. Try peeking at your monitor sideways (bringing the more
sensitive peripheral-vision cells into play). If you don't sense any
flicker or if you feel the flickering is tolerable, then that refresh
rate is fine with you. Otherwise you better configure a higher
refresh rate, because that semi-invisible flicker is going to fatigue
your eyes like crazy and give you headaches, even if the screen looks
OK to normal vision.
For interlaced modes, the amount of flicker depends on more factors
such as the current vertical resolution and the actual screen
contents. So just experiment. You won't want to go much below about
85Hz half frame rate, though.
So let's say you've picked a minimum acceptable refresh rate. In
choosing your HFL and VFL, you'll have some room for maneuver.
8. Memory Requirements
Available frame-buffer RAM may limit the resolution you can achieve on
color or gray-scale displays. It probably isn't a factor on displays
that have only two colors, white and black with no shades of gray in
between.
For 256-color displays, a byte of video memory is required for each
visible dot to be shown. This byte contains the information that
determines what mix of red, green, and blue is generated for its dot.
To get the amount of memory required, multiply the number of visible
dots per line by the number of visible lines. For a display with a
resolution of 1024x768, this would be 1024 x 768 = 786432, which is
the number of visible dots on the display. This is also, at one byte
per dot, the number of bytes of video memory that will be necessary on
your adapter card.
Thus, your memory requirement will typically be (HR * VR)/1024 Kbytes
of VRAM, rounded up (it would come to 768K exactly in this example).
If you have more memory than strictly required, you'll have extra for
virtual-screen panning.
However, if you only have 512K on board yor video card, then you won't
be able to use this resolution. Even if you have a good monitor,
without enough video RAM, you can't take advantage of your monitor's
potential. On the other hand, if your SVGA has one meg, but your
monitor can display at most 800x600, then high resolution is beyond
your reach anyway (see ``Using Interlaced Modes'' for a possible
remedy).
Don't worry if you have more memory than required; XFree86 will make
use of it by allowing you to scroll your viewable area (see the
Xconfig file documentation on the virtual screen size parameter).
Remember also that a card with 512K bytes of memory really doesn't
have 512,000 bytes installed, it has 512 x 1024 = 524,288 bytes.
If you're running X/Inside using an S3 card, and are willing to live
with 16 colors (4 bits per pixel), you can set depth 4 in Xconfig and
effectively double the resolution your card can handle. S3 cards, for
example, normally do 1024x768x256. You can make them do 1280x1024x16
with depth 4.
9. Computing Frame Sizes
Warning: this method was developed for multisync monitors. It will
probably work with fixed-frequency monitors as well, but no
guarantees!
Start by dividing DCF by your highest available HSF to get a
horizontal frame length.
For example; suppose you have a Sigma Legend SVGA with a 65MHz dot
clock, and your monitor has a 55KHz horizontal scan frequency. The
quantity (DCF / HSF) is then 1181 (65MHz = 65000KHz; 65000/55 = 1181).
Now for our first bit of black magic. You need to round this figure
to the nearest multiple of 8. This has to do with the VGA hardware
controller used by SVGA and S3 cards; it uses an 8-bit register, left-
shifted 3 bits, for what's really an 11-bit quantity. Other card
types such as ATI 8514/A may not have this requirement, but we don't
know and the correction can't hurt. So round the usable horizontal
frame length figure down to 1176.
This figure (DCF / HSF rounded to a multiple of 8) is the minimum HFL
you can use. You can get longer HFLs (and thus, possibly, more
horizontal dots on the screen) by setting the sync pulse to produce a
lower HSF. But you'll pay with a slower and more visible flicker
rate.
As a rule of thumb, 80% of the horizontal frame length is available
for horizontal resolution, the visible part of the horizontal scan
line (this allows, roughly, for borders and sweepback time -- that is,
the time required for the beam to move from the right screen edge to
the left edge of the next raster line). In this example, that's 944
ticks.
Now, to get the normal 4:3 screen aspect ratio, set your vertical
resolution to 3/4ths of the horizontal resolution you just calculated.
For this example, that's 708 ticks. To get your actual VFL, multiply
that by 1.05 to get 743 ticks.
The 4:3 is not technically magic; nothing prevents you from using a
different ratio if that will get the best use out of your screen real
estate. It does make figuring frame height and frame width from the
diagonal size convenient, you just multiply the diagonal by by 0.8 to
get width and 0.6 to get height.
So, HFL=1176 and VFL=743. Dividing 65MHz by the product of the two
gives us a nice, healthy 74.4Hz refresh rate. Excellent! Better than
VESA standard! And you got 944x708 to boot, more than the 800 by 600
you were probably expecting. Not bad at all!
You can even improve the refresh rate further, to almost 76 Hz, by
using the fact that monitors can often sync horizontally at 2khz or so
higher than rated, and by lowering VFL somewhat (that is, taking less
than 75% of 944 in the example above). But before you try this
"overdriving" maneuver, if you do, make sure that your monitor
electron guns can sync up to 76 Hz vertical. (the popular NEC 4D, for
instance, cannot. It goes only up to 75 Hz VSF). (See ``Overdriving
Your Monitor'' for more general discussion of this issue. )
So far, most of this is simple arithmetic and basic facts about raster
displays. Hardly any black magic at all!
10. Black Magic and Sync Pulses
OK, now you've computed HFL/VFL numbers for your chosen dot clock,
found the refresh rate acceptable, and checked that you have enough
VRAM. Now for the real black magic -- you need to know when and where
to place synchronization pulses.
The sync pulses actually control the horizontal and vertical scan
frequencies of the monitor. The HSF and VSF you've pulled off the
spec sheet are nominal, approximate maximum sync frequencies. The
sync pulse in the signal from the adapter card tells the monitor how
fast to actually run.
Recall the two pictures above? Only part of the time required for
raster-scanning a frame is used for displaying viewable image (ie.
your resolution).
10.1. Horizontal Sync:
By previous definition, it takes HFL ticks to trace the a horizontal
scan line. Let's call the visible tick count (your horizontal screen
resolution) HR. Then Obviously, HR < HFL by definition. For
concreteness, let's assume both start at the same instant as shown
below:
|___ __ __ __ __ __ __ __ __ __ __ __ __
|_ _ _ _ _ _ _ _ _ _ _ _ |
|_______________________|_______________|_____
0 ^ ^ unit: ticks
| ^ ^ |
HR | | HFL
| |<----->| |
|<->| HSP |<->|
HGT1 HGT2
Now, we would like to place a sync pulse of length HSP as shown above,
ie, between the end of clock ticks for display data and the end of
clock ticks for the entire frame. Why so? because if we can achieve
this, then your screen image won't shift to the right or to the left.
It will be where it supposed to be on the screen, covering squarely
the monitor's viewable area.
Furthermore, we want about 30 ticks of "guard time" on either side of
the sync pulse. This is represented by HGT1 and HGT2. In a typical
configuration HGT1 != HGT2, but if you're building a configuration
from scratch, you want to start your experimentation with them equal
(that is, with the sync pulse centered).
The symptom of a misplaced sync pulse is that the image is displaced
on the screen, with one border excessively wide and the other side of
the image wrapped around the screen edge, producing a white edge line
and a band of "ghost image" on that side. A way-out-of-place vertical
sync pulse can actually cause the image to roll like a TV with a mis-
adjusted vertical hold (in fact, it's the same phenomenon at work).
If you're lucky, your monitor's sync pulse widths will be documented
on its specification page. If not, here's where the real black magic
starts...
You'll have to do a little trial and error for this part. But most of
the time, we can safely assume that a sync pulse is about 3.5 to 4.0
microsecond in length.
For concretness again, let's take HSP to be 3.8 microseconds (which
btw, is not a bad value to start with when experimenting).
Now, using the 65Mhz clock timing above, we know HSP is equivalent to
247 clock ticks (= 65 * 10**6 * 3.8 * 10^-6) [recall M=10^6,
micro=10^-6]
Some makers like to quote their horizontal framing parameters as
timings rather than dot widths. You may see the following terms:
active time (HAT)
Corresponds to HR, but in milliseconds. HAT * DCF = HR.
blanking time (HBT)
Corresponds to (HFL - HR), but in milliseconds. HBT * DCF =
(HFL - HR).
front porch (HFP)
This is just HGT1.
sync time
This is just HSP.
back porch (HBP)
This is just HGT2.
10.2. Vertical Sync:
Going back to the picture above, how do we place the 247 clock ticks
as shown in the picture?
Using our example, HR is 944 and HFL is 1176. The difference between
the two is 1176 - 944=232 < 247! Obviously we have to do some
adjustment here. What can we do?
The first thing is to raise 1176 to 1184, and lower 944 to 936. Now
the difference = 1184-936= 248. Hmm, closer.
Next, instead using 3.8, we use 3.5 for calculating HSP; then, we have
65*3.5=227. Looks better. But 248 is not much higher than 227. It's
normally necessary to have 30 or so clock ticks between HR and the
start of SP, and the same for the end of SP and HFL. AND they have to
be multiple of eight! Are we stuck?
No. Let's do this, 936 % 8 = 0, (936 + 32) % 8 = 0 too. But 936 + 32
= 968, 968 + 227 = 1195, 1195 + 32 = 1227. Hmm.. this looks not too
bad. But it's not a multiple of 8, so let's round it up to 1232.
But now we have potential trouble, the sync pulse is no longer placed
right in the middle between h and H any more. Happily, using our
calculator we find 1232 - 32 = 1200 is also a multiple of 8 and (1232
- 32) - 968 = 232 corresponding using a sync pulse of 3.57
microseconds long, still reasonable.
In addition, 936/1232 0.76 or 76%, still not far from 80%, so it
should be all right.
Furthermore, using the current horizontal frame length, we basically
ask our monitor to sync at 52.7khz (= 65Mhz/1232) which is within its
capability. No problems.
Using rules of thumb we mentioned before, 936*75%=702, This is our new
vertical resolution. 702 * 1.05 = 737, our new vertical frame length.
Screen refresh rate = 65Mhz/(737*1232)=71.6 Hz. This is still
excellent.
Figuring the vertical sync pulse layout is similar:
|___ __ __ __ __ __ __ __ __ __ __ __ __
|_ _ _ _ _ _ _ _ _ _ _ _ |
|_______________________|_______________|_____
0 VR VFL unit: ticks
^ ^ ^
| | |
|<->|<----->|
VGT VSP
We start the sync pulse just past the end of the vertical display data
ticks. VGT is the vertical guard time required for the sync pulse.
Most monitors are comfortable with a VGT of 0 (no guard time) and
we'll use that in this example. A few need two or three ticks of
guard time, and it usually doesn't hurt to add that.
Returning to the example: since by the defintion of frame length, a
vertical tick is the time for tracing a complete HORIZONTAL frame,
therefore in our example, it is 1232/65Mhz=18.95us.
Experience shows that a vertical sync pulse should be in the range of
50us and 300us. As an example let's use 150us, which translates into
8 vertical clock ticks (150us/18.95us 8).
Some makers like to quote their vertical framing parameters as timings
rather than dot widths. You may see the following terms:
active time (VAT)
Corresponds to VR, but in milliseconds. VAT * VSF = VR.
blanking time (VBT)
Corresponds to (VFL - VR), but in milliseconds. VBT * VSF =
(VFL - VR).
front porch (VFP)
This is just VGT.
sync time
This is just VSP.
back porch (VBP)
This is like a second guard time after the vertical sync pulse.
It is often zero.
11. Putting it All Together
The Xconfig file Table of Video Modes contains lines of numbers, with
each line being a complete specification for one mode of X-server
operation. The fields are grouped into four sections, the name
section, the clock frequency section, the horizontal section, and the
vertical section.
The name section contains one field, the name of the video mode
specified by the rest of the line. This name is referred to on the
"Modes" line of the Graphics Driver Setup section of the Xconfig file.
The name field may be omitted if the name of a previous line is the
same as the current line.
The dot clock section contains only the dot clock (what we've called
DCF) field of the video mode line. The number in this field specifies
what dot clock was used to generate the numbers in the following
sections.
The horizontal section consists of four fields which specify how each
horizontal line on the display is to be generated. The first field of
the section contains the number of dots per line which will be
illuminated to form the picture (what we've called HR). The second
field of the section (SH1) indicates at which dot the horizontal sync
pulse will begin. The third field (SH2) indicates at which dot the
horizontal sync pulse will end. The fourth field specifies the toal
horzontal frame length (HFL).
The vertical section also contains four fields. The first field
contains the number of visible lines which will appear on the display
(VR). The second field (SV1) indicates the line number at which the
vertical sync pulse will begin. The third field (SV2) specifies the
line number at which the vertical sync pulse will end. The fourth
field contains the total vertical frame length (VFL).
Example:
#Modename clock horizontal timing vertical timing
"752x564" 40 752 784 944 1088 564 567 569 611
44.5 752 792 976 1240 564 567 570 600
(Note: stock X11R5 doesn't support fractional dot clocks.)
For Xconfig, all of the numbers just mentioned - the number of
illuminated dots on the line, the number of dots separating the
illuminated dots from the beginning of the sync pulse, the number of
dots representing the duration of the pulse, and the number of dots
after the end of the sync pulse - are added to produce the number of
dots per line. The number of horizontal dots must be evenly divisible
by eight.
Example horizontal numbers: 800 864 1024 1088
This sample line has the number of illuminated dots (800) followed by
the number of the dot when the sync pulse starts (864), followed by
the number of the dot when the sync pulse ends (1024), followed by the
number of the last dot on the horizontal line (1088).
Note again that all of the horizontal numbers (800, 864, 1024, and
1088) are divisible by eight! This is not required of the vertical
numbers.
The number of lines from the top of the display to the bottom form the
frame. The basic timing signal for a frame is the line. A number of
lines will contain the picture. After the last illuminated line has
been displayed, a delay of a number of lines will occur before the
vertical sync pulse is generated. Then the sync pulse will last for a
few lines, and finally the last lines in the frame, the delay required
after the pulse, will be generated. The numbers that specify this
mode of operation are entered in a manner similar to the following
example.
Example vertical numbers: 600 603 609 630
This example indicates that there are 600 visible lines on the
display, that the vertical sync pulse starts with the 603rd line and
ends with the 609th, and that there are 630 total lines being used.
Note that the vertical numbers don't have to be divisible by eight!
Let's return to the example we've been working. According to the
above, all we need to do from now on is to write our result into
Xconfig as follows:
<name> DCF HR SH1 SH2 HFL VR SV1 SV2 VFL
where SH1 is the start tick of the horizontal sync pulse and SH2 is
its end tick; similarly, SV1 is the start tick of the vertical sync
pulse and SV2 is its end tick.
To place these, recall the discussion of black magic and sync pulses
given above. SH1 is the dot that begins the leading edge of the
horiziontal sync pulse; thus, SH1 = HR + HGT1. SH2 is the trailing
edge; thus, SH2 = SH1 + HSP. Similarly, SV1 = VR + VGT (but VGT is
usually zero) and SV2 = SV1 + VSP.
#name clock horizontal timing vertical timing flag
936x702 65 936 968 1200 1232 702 702 710 737
No special flag necessary; this is a non-interlaced mode. Now we are
really done.
12. Overdriving Your Monitor
You should absolutely not try exceeding your monitor's scan rates if
it's a fixed-frequency type. You can smoke your hardware doing this!
There are potentially subtler problems with overdriving a multisync
monitor which you should be aware of.
Having a pixel clock higher than the monitor's maximum bandwidth is
rather harmless, in contrast. (Note: the theoretical limit of
discernable features is reached when the pixel clock reaches double
the monitor's bandwidth. This is a straightforward application of
Nyquist's Theorem: consider the pixels as a spatially distributed
series of samples of the drive signals and you'll see why.)
It's exceeding the rated maximum sync frequencies that's problematic.
Some modern monitors might have protection circuitry that shuts the
monitor down at dangerous scan rates, but don't rely on it. In
particular there are older multisync monitors (like the Multisync II)
which use just one horizontal transformer. These monitors will not
have much protection against overdriving them. While you necessarily
have high voltage regulation circuitry (which can be absent in fixed
frequency monitors), it will not necessarily cover every conceivable
frequency range, especially in cheaper models. This not only implies
more wear on the circuitry, it can also cause the screen phosphors to
age faster, and cause more than the specified radiation (including X-
rays) to be emitted from the monitor.
Another importance of the bandwidth is that the monitor's input
impedance is specified only for that range, and using higher
frequencies can cause reflections probably causing minor screen
interferences, and radio disturbance.
However, the basic problematic magnitude in question here is the slew
rate (the steepness of the video signals) of the video output drivers,
and that is usually independent of the actual pixel frequency, but (if
your board manufacturer cares about such problems) related to the
maximum pixel frequency of the board.
So be careful out there...
13. Using Interlaced Modes
(This section is largely due to David Kastrup
<dak@pool.informatik.rwth-aachen.de>)
At a fixed dot clock, an interlaced display is going to have
considerably less noticable flicker than a non-interlaced display, if
the vertical circuitry of your monitor is able to support it stably.
It is because of this that interlaced modes were invented in the first
place.
Interlaced modes got their bad repute because they are inferior to
their non-interlaced companions at the same vertical scan frequency,
VSF (which is what is usually given in advertisements). But they are
definitely superior at the same horizontal scan rate, and that's where
the decisive limits of your monitor/graphics card usually lie.
At a fixed refresh rate (or half frame rate, or VSF) the interlaced
display will flicker more: a 90Hz interlaced display will be inferior
to a 90Hz non-interlaced display. It will, however, need only half the
video bandwidth and half the horizontal scan rate. If you compared it
to a non-interlaced mode with the same dot clock and the same scan
rates, it would be vastly superior: 45Hz non-interlaced is
intolerable. With 90Hz interlaced, I have worked for years with my
Multisync 3D (at 1024x768) and am very satisfied. I'd guess you'd need
at least a 70Hz non-interlaced display for similar comfort.
You have to watch a few points, though: use interlaced modes only at
high resolutions, so that the alternately lighted lines are close
together. You might want to play with sync pulse widths and positions
to get the most stable line positions. If alternating lines are bright
and dark, interlace will jump at you. I have one application that
chooses such a dot pattern for a menu background (XCept, no other
application I know does that, fortunately). I switch to 800x600 for
using XCept because it really hurts my eyes otherwise.
For the same reason, use at least 100dpi fonts, or other fonts where
horizontal beams are at least two lines thick (for high resolutions,
nothing else will make sense anyhow).
And of course, never use an interlaced mode when your hardware would
support a non-interlaced one with similar refresh rate.
If, however, you find that for some resolution you are pushing either
monitor or graphics card to their upper limits, and getting
dissatisfactorily flickery or outwashed (bandwidth exceeded) display,
you might want to try tackling the same resolution using an interlaced
mode. Of course this is useless if the VSF of your monitor is already
close to its limits.
Design of interlaced modes is easy: do it like a non-interlaced mode.
Just two more considerations are necessary: you need an odd total
number of vertical lines (the last number in your mode line), and when
you specify the "interlace" flag, the actual vertical frame rate for
your monitor doubles. Your monitor needs to support a 90Hz frame rate
if the mode you specified looks like a 45Hz mode apart from the
"Interlace" flag.
As an example, here is my modeline for 1024x768 interlaced: my
Multisync 3D will support up to 90Hz vertical and 38kHz horizontal.
ModeLine "1024x768" 45 1024 1048 1208 1248 768 768 776 807 Interlace
Both limits are pretty much exhausted with this mode. Specifying the
same mode, just without the "Interlace" flag, still is almost at the
limit of the monitor's horizontal capacity (and strictly speaking, a
bit under the lower limit of vertical scan rate), but produces an
intolerably flickery display.
Basic design rules: if you have designed a mode at less than half of
your monitor's vertical capacity, make the vertical total of lines odd
and add the "Interlace" flag. The display's quality should vastly
improve in most cases.
If you have a non-interlaced mode otherwise exhausting your monitor's
specs where the vertical scan rate lies about 30% or more under the
maximum of your monitor, hand-designing an interlaced mode (probably
with somewhat higher resolution) could deliver superior results, but I
won't promise it.
14. Questions and Answers
Q. The example you gave is not a standard screen size, can I use it?
A. Why not? There is NO reason whatsover why you have to use 640x480,
800x600, or even 1024x768. The XFree86 servers let you configure your
hardware with a lot of freedom. It usually takes two to three tries
to come up the right one. The important thing to shoot for is high
refresh rate with reasonable viewing area. not high resolution at the
price of eye-tearing flicker!
Q. It this the only resolution given the 65Mhz dot clock and 55Khz
HSF?
A. Absolutely not! You are encouraged to follow the general procedure
and do some trial-and-error to come up a setting that's really to your
liking. Experimenting with this can be lots of fun. Most settings
may just give you nasty video hash, but in practice a modern multi-
sync monitor is usually not damaged easily. Be sure though, that your
monitor can support the frame rates of your mode before using it for
longer times.
Beware fixed-frequency monitors! This kind of hacking around can
damage them rather quickly. Be sure you use valid refresh rates for
every experiment on them.
Q. You just mentioned two standard resolutions. In Xconfig, there are
many standard resolutions available, can you tell me whether there's
any point in tinkering with timings?
A. Absolutely! Take, for example, the "standard" 640x480 listed in
the current Xconfig. It employes 25Mhz driving frequency, frame
lengths are 800 and 525 => refresh rate 59.5Hz. Not too bad. But
28Mhz is a commonly available driving frequency from many SVGA boards.
If we use it to drive 640x480, following the procedure we discussed
above, you would get frame lengths like 812 (round down to 808) and
505. Now the refresh rate is raised to 68Hz, a quite significant
improvement over the standard one.
Q. Can you summarize what we have discussed so far?
A. In a nutshell:
1. for any fixed driving frequency, raising max resolution incurs the
penalty of lowering refresh rate and thus introducing more flicker.
2. if high resolution is desirable and your monitor supports it, try
to get a SVGA card that provides a matching dot clock or DCF. The
higher, the better!
15. Fixing Problems with the Image.
OK, so you've got your X configuration numbers. You put them in
Xconfig with a test mode label. You fire up X, hot-key to the new
mode, ... and the image doesn't look right. What do you do? Here's a
list of common video image distortions and how to fix them.
(Fixing these minor distortions is where xvidtune(1) really shines.)
You move the image by changing the sync pulse timing. You scale it by
changing the frame length (you need to move the sync pulse to keep it
in the same relative position, otherwise scaling will move the image
as well). Here are some more specific recipes:
The horizontal and vertical positions are independent. That is,
moving the image horizontally doesn't affect placement vertically, or
vice-versa. However, the same is not quite true of scaling. While
changing the horizontal size does nothing to the vertical size or vice
versa, the total change in both may be limited. In particular, if
your image is too large in both dimensions you will probably have to
go to a higher dot clock to fix it. Since this raises the usable
resolution, it is seldom a problem!
15.1. The image is displaced to the left or right
To fix this, move the horizontal sync pulse. That is, increment or
decrement (by a multiple of 8) the middle two numbers of the
horizontal timing section that define the leading and trailing edge of
the horizontal sync pulse.
If the image is shifted left (right border too large, you want to move
the image to the right) decrement the numbers. If the image is
shifted right (left border too large, you want it to move left)
increment the sync pulse.
15.2. The image is displaced up or down
To fix this, move the vertical sync pulse. That is, increment or
decrement the middle two numbers of the vertical timing section that
define the leading and trailing edge of the vertical sync pulse.
If the image is shifted up (lower border too large, you want to move
the image down) decrement the numbers. If the image is shifted down
(top border too large, you want it to move up) increment the numbers.
15.3. The image is too large both horizontally and vertically
Switch to a higher card clock speed. If you have multiple modes in
your clock file, possibly a lower-speed one is being activated by
mistake.
15.4. The image is too wide (too narrow) horizontally
To fix this, increase (decrease) the horizontal frame length. That
is, change the fourth number in the first timing section. To avoid
moving the image, also move the sync pulse (second and third numbers)
half as far, to keep it in the same relative position.
15.5. The image is too deep (too shallow) vertically
To fix this, increase (decrease) the vertical frame length. That is,
change the fourth number in the second timing section. To avoid
moving the image, also move the sync pulse (second and third numbers)
half as far, to keep it in the same relative position.
Any distortion that can't be handled by combining these techniques is
probably evidence of something more basically wrong, like a
calculation mistake or a faster dot clock than the monitor can handle.
Finally, remember that increasing either frame length will decrease
your refresh rate, and vice-versa.
Occasionally you can fix minor distortions by fiddling with the
picture controls on your monitor. The disadvantage is that if you
take your controls too far off the neutral (factory) setting to fix
graphics-mode problems, you may end up with a wacky image in text
mode. It's better to get your modeline right.
16. Plotting Monitor Capabilities
To plot a monitor mode diagram, you'll need the gnuplot package (a
freeware plotting language for UNIX-like operating systems) and the
tool modeplot, a shell/gnuplot script to plot the diagram from your
monitor characteristics, entered as command-line options.
Here is a copy of modeplot:
#!/bin/sh
#
# modeplot -- generate X mode plot of available monitor modes
#
# Do `modeplot -?' to see the control options.
#
# Monitor description. Bandwidth in MHz, horizontal frequencies in kHz
# and vertical frequencies in Hz.
TITLE="Viewsonic 21PS"
BANDWIDTH=185
MINHSF=31
MAXHSF=85
MINVSF=50
MAXVSF=160
ASPECT="4/3"
vesa=72.5 # VESA-recommended minimum refresh rate
while [ "$1" != "" ]
do
case $1 in
-t) TITLE="$2"; shift;;
-b) BANDWIDTH="$2"; shift;;
-h) MINHSF="$2" MAXHSF="$3"; shift; shift;;
-v) MINVSF="$2" MAXVSF="$3"; shift; shift;;
-a) ASPECT="$2"; shift;;
-g) GNUOPTS="$2"; shift;;
-?) cat <<EOF
modeplot control switches:
-t "<description>" name of monitor defaults to "Viewsonic 21PS"
-b <nn> bandwidth in MHz defaults to 185
-h <min> <max> min & max HSF (kHz) defaults to 31 85
-v <min> <max> min & max VSF (Hz) defaults to 50 160
-a <aspect ratio> aspect ratio defaults to 4/3
-g "<options>" pass options to gnuplot
The -b, -h and -v options are required, -a, -t, -g optional. You can
use -g to pass a device type to gnuplot so that (for example) modeplot's
output can be redirected to a printer. See gnuplot(1) for details.
The modeplot tool was created by Eric S. Raymond <esr@thyrsus.com> based on
analysis and scratch code by Martin Lottermoser <Martin.Lottermoser@mch.sni.de>
This is modeplot $Revision: 1.9 $
EOF
exit;;
esac
shift
done
gnuplot $GNUOPTS <<EOF
set title "$TITLE Mode Plot"
# Magic numbers. Unfortunately, the plot is quite sensitive to changes in
# these, and they may fail to represent reality on some monitors. We need
# to fix values to get even an approximation of the mode diagram. These come
# from looking at lots of values in the ModeDB database.
F1 = 1.30 # multiplier to convert horizontal resolution to frame width
F2 = 1.05 # multiplier to convert vertical resolution to frame height
# Function definitions (multiplication by 1.0 forces real-number arithmetic)
ac = (1.0*$ASPECT)*F1/F2
refresh(hsync, dcf) = ac * (hsync**2)/(1.0*dcf)
dotclock(hsync, rr) = ac * (hsync**2)/(1.0*rr)
resolution(hv, dcf) = dcf * (10**6)/(hv * F1 * F2)
# Put labels on the axes
set xlabel 'DCF (MHz)'
set ylabel 'RR (Hz)' 6 # Put it right over the Y axis
# Generate diagram
set grid
set label "VB" at $BANDWIDTH+1, ($MAXVSF + $MINVSF) / 2 left
set arrow from $BANDWIDTH, $MINVSF to $BANDWIDTH, $MAXVSF nohead
set label "max VSF" at 1, $MAXVSF-1.5
set arrow from 0, $MAXVSF to $BANDWIDTH, $MAXVSF nohead
set label "min VSF" at 1, $MINVSF-1.5
set arrow from 0, $MINVSF to $BANDWIDTH, $MINVSF nohead
set label "min HSF" at dotclock($MINHSF, $MAXVSF+17), $MAXVSF + 17 right
set label "max HSF" at dotclock($MAXHSF, $MAXVSF+17), $MAXVSF + 17 right
set label "VESA $vesa" at 1, $vesa-1.5
set arrow from 0, $vesa to $BANDWIDTH, $vesa nohead # style -1
plot [dcf=0:1.1*$BANDWIDTH] [$MINVSF-10:$MAXVSF+20] \
refresh($MINHSF, dcf) notitle with lines 1, \
refresh($MAXHSF, dcf) notitle with lines 1, \
resolution(640*480, dcf) title "640x480 " with points 2, \
resolution(800*600, dcf) title "800x600 " with points 3, \
resolution(1024*768, dcf) title "1024x768 " with points 4, \
resolution(1280*1024, dcf) title "1280x1024" with points 5, \
resolution(1600*1280, dcf) title "1600x1200" with points 6
pause 9999
EOF
Once you know you have modeplot and the gnuplot package in place,
you'll need the following monitor characteristics:
· video bandwidth (VB)
· range of horizontal sync frequency (HSF)
· range of vertical sync frequency (VSF)
The plot program needs to make some simplifying assumptions which are
not necessarily correct. This is the reason why the resulting diagram
is only a rough description. These assumptions are:
1. All resolutions have a single fixed aspect ratio AR = HR/VR.
Standard resolutions have AR = 4/3 or AR = 5/4. The modeplot
programs assumes 4/3 by default, but you can override this.
2. For the modes considered, horizontal and vertical frame lengths are
fixed multiples of horizontal and vertical resolutions,
respectively:
HFL = F1 * HR
VFL = F2 * VR
As a rough guide, take F1 = 1.30 and F2 = 1.05 (see ``'' "Computing
Frame Sizes").
Now take a particular sync frequency, HSF. Given the assumptions just
presented, every value for the clock rate DCF already determines the
refresh rate RR, i.e. for every value of HSF there is a function
RR(DCF). This can be derived as follows.
The refresh rate is equal to the clock rate divided by the product of
the frame sizes:
RR = DCF / (HFL * VFL) (*)
On the other hand, the horizontal frame length is equal to the clock
rate divided by the horizontal sync frequency:
HFL = DCF / HSF (**)
VFL can be reduced to HFL be means of the two assumptions above:
VFL = F2 * VR
= F2 * (HR / AR)
= (F2/F1) * HFL / AR (***)
Inserting (**) and (***) into (*) we obtain:
RR = DCF / ((F2/F1) * HFL**2 / AR)
= (F1/F2) * AR * DCF * (HSF/DCF)**2
= (F1/F2) * AR * HSF**2 / DCF
For fixed HSF, F1, F2 and AR, this is a hyperbola in our diagram.
Drawing two such curves for minimum and maximum horizontal sync
frequencies we have obtained the two remaining boundaries of the
permitted region.
The straight lines crossing the capability region represent particular
resolutions. This is based on (*) and the second assumption:
RR = DCF / (HFL * VFL) = DCF / (F1 * HR * F2 * VR)
By drawing such lines for all resolutions one is interested in, one
can immediately read off the possible relations between resolution,
clock rate and refresh rate of which the monitor is capable. Note that
these lines do not depend on monitor properties, but they do depend on
the second assumption.
The modeplot tool provides you with an easy way to do this. Do
modeplot -? to see its control options. A typical invocation looks
like this:
modeplot -t "Swan SW617" -b 85 -v 50 90 -h 31 58
The -b option specifies video bandwidth; -v and -h set horizontal and
vertical sync frequency ranges.
When reading the output of modeplot, always bear in mind that it gives
only an approximate description. For example, it disregards
limitations on HFL resulting from a minimum required sync pulse width,
and it can only be accurate as far as the assumptions are. It is
therefore no substitute for a detailed calculation (involving some
black magic) as presented in ``Putting it All Together''. However, it
should give you a better feeling for what is possible and which
tradeoffs are involved.
17. Credits
The original ancestor of this document was by Chin Fang
<fangchin@leland.stanford.edu>.
Eric S. Raymond <esr@snark.thyrsus.com> reworked, reorganized, and
massively rewrote Chin Fang's original in an attempt to understand it.
In the process, he merged in most of a different how-to by Bob Crosson
<crosson@cam.nist.gov>.
The material on interlaced modes is largely by David Kastrup
<dak@pool.informatik.rwth-aachen.de>
Martin Lottermoser <Martin.Lottermoser@mch.sni.de> contributed the
idea of using gnuplot to make mode diagrams and did the mathematical
analysis behind modeplot. The distributed modeplot was redesigned and
generalized by ESR from Martin's original gnuplot code for one case.
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