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<article>

<title>XFree86 Video Timings HOWTO
<author>Eric S. Raymond &lt;esr@thyrsus.com&gt;
<date>Version 3.0, 8 Aug 1997

<abstract>
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 xvidtune to tweak a standard mode that is
not quite right for your monitor.
</abstract>

<toc>

<sect>Disclaimer
<p>

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 <ref id="overd" name="Overdriving Your
Monitor"> for detailed cautions. Any damages 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 <url
url="http://sunsite.unc.edu/LDP"
name="Linux Documentation Project"> web page.

Please direct comments, criticism, and suggestions for improvement to
<htmlurl url="mailto:esr@thyrsus.com" name="esr@snark.thyrsus.com">. Please do
<em>not</em> 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.

<sect>Introduction<label id="intro">
<p>

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. 

Starting with XFree86 3.2, XFree86 provides an <bf>XF86Setup</bf>(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, <bf>XF86Setup</bf>(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.

Recent versions of XFree86 provide a tool called <bf>xvidtune</bf>(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 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 <ref id="fixes" name="Fixing Problems with the
Image">.  This will enlighten you on ways to tweak the timing
numbers to achieve particular effects. 

If you have <bf>xvidtune</bf>(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 capability to save
yourself hassles!  When you want to test a new mode, give it a unique
mode label and add it to the <EM>end</EM> of your hot-key list.  Leave a
known-good mode as the default to fall back on if the test mode
doesn't work.  

<sect>How Video Displays Work<label id="video">
<p>

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. 

<sect>Basic Things to Know about your Display and Adapter<label id="basic">
<p>

There are some fundamental things you need to know before hacking an Xconfig
entry.  These are: 

<itemize>
<item>your monitor's horizontal and vertical sync frequency options
<item>your video adapter's driving clock frequency, or "dot clock"
<item>your monitor's bandwidth
</itemize>
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
<ref id="overd" name="Overdriving Your Monitor"> below. 

The card driving clock frequency: 

Your video adapter manual's spec page will usually give you the card's 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.  Even if your X locks up your monitor, it will emit a line of clock and
other info to standard output.  If you redirect this to a file, it should be
saved even if you have to reboot to get your console back.  (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.) 

Your X startup message should look something like one of the following
examples: 

If you're using XFree86: 

<tscreen><verb>
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
</verb></tscreen>

If you're using SGCS or X/Inside X: 

<tscreen><verb>
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
</verb></tscreen>

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.  SVr4 users:
the mousemgr process is particularly likely to mess you up.  

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: 

<tscreen><verb>
wga
	Clocks	25 28 40 3 50 77 36 45 0 0 79 31 94 65 75 71
</verb></tscreen>
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! 

<sect1>The monitor's video bandwidth:
<p>

If you're running XFree86, your server will probe your card and tell you
what your highest-available dot clock is.

Otherwise, your highest available dot clock is approximately 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). 

Your monitor's video bandwidth should be included on the manual's spec page.
If it's not, look at the monitor's highest 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): 

<tscreen><verb>
	640x480			25
	800x600			36
	1024x768		65
	1024x768 interlaced	45
	1280x1024		110
	1600x1200		185
</verb></tscreen>

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: 

<tscreen><verb>
	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
</verb></tscreen>
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). 

<sect1>What these control:
<p>
 
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. 

<sect>Interpreting the Basic Specifications<label id="specs">
<p>

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 

<descrip>
<tag/horizontal sync frequency (HSF)/
	Horizontal scans per second (see above). 

<tag/vertical sync frequency (VSF) /
	Vertical scans per second (see above).  Mainly important as the upper
	limit on your refresh rate. 

<tag/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. 

<tag/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
	tolerably crisp display of fine details in the video image, however,
	you don't want it much below your highest DCF, and preferably higher.

<tag/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,
	<em>including the inactive left and right borders</em>.  Vertical
	frame length (VFL) is the number of scan lines in the
	<em>entire</em> image, including the inactive top and bottom
	borders.

<tag/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
<tscreen><verb>
	RR = DCF / (HFL * VFL)
</verb></tscreen>

	Note that the product in the denominator is <em>not</em> 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.
</descrip>

<sect1>About Bandwidth:
<p>

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, bandwidth is mainly important as an approximate cutoff point
for the highest dot clock you can use. 

<sect1>Sync Frequencies and the Refresh Rate:
<p>

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: 

<code>
     _______________________
    |                       |     The horizontal sync frequency
    |-&gt;-&gt;-&gt;-&gt;-&gt;-&gt;-&gt;-&gt;-&gt;-&gt;-&gt; |     is the number of times per
    |                      )|     second that the monitor's
    |&lt;-----&lt;-----&lt;-----&lt;--- |     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_____________|    
</code>

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 <em>refreshed</em>) 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: 

<code>
        RR                                      VB
         |   min HSF                     max HSF |
         |    |             R1        R2  |      |
max VSF -+----|------------/----------/---|------+----- max VSF
         |    |:::::::::::/::::::::::/:::::\     |
         |    \::::::::::/::::::::::/:::::::\    |
         |     |::::::::/::::::::::/:::::::::|   |
         |     |:::::::/::::::::::/::::::::::\   |
         |     \::::::/::::::::::/::::::::::::\  |
         |      \::::/::::::::::/::::::::::::::| |
         |       |::/::::::::::/:::::::::::::::| |
         |        \/::::::::::/:::::::::::::::::\| 
         |        /\:::::::::/:::::::::::::::::::|
         |       /  \:::::::/::::::::::::::::::::|\
         |      /    |:::::/:::::::::::::::::::::| |
         |     /     \::::/::::::::::::::::::::::| \
min VSF -+----/-------\--/-----------------------|--\--- min VSF
         |   /         \/                        |   \
         +--/----------/\------------------------+----\- DCF
           R1        R2  \                       |     \
                          min HSF                |    max HSF
                                                 VB
</code>

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 <ref id="cplot" name="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. 

<sect>Tradeoffs in Configuring your System<label id="trade">
<p>

Another way to look at the formula we derived above is 

<tscreen><verb>
	DCF = RR * HFL * VFL
</verb></tscreen>
That is, your dot clock is fixed.  You can use those dots per second to buy 
either refresh rate, horizontal resolution, or vertical resolution.  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 <TT>xterm -bg white -fg black</TT> 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. 
 
<sect>Memory Requirements<label id="sizes">
<p>

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 800x600, this would be 800
x 600 = 480,000, 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 are
necessary on your adapter card. 

Thus, your memory requirement will typically be (HR * VR)/1024 Kbytes of VRAM,
rounded up.  If you have more memory than strictly required, you'll have extra
for virtual-screen panning. 
                 
However, if you only have 512K on board, then you can't 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
<ref id="inter" name="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 SGCS X (now called 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. 

<sect>Computing Frame Sizes<label id="frame">
<p>

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
non-Golden-Section 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 <em>sure</em> 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 <ref id="overd" name="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! 

<sect>Black Magic and Sync Pulses<label id="magic">
<p>

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). 

<sect1>Horizontal Sync:
<p>

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: 
<code>
  |___ __ __ __ __ __ __ __ __ __ __ __ __
  |_ _ _ _ _ _ _ _ _ _ _ _                |
  |_______________________|_______________|_____ 
  0                       ^               ^     unit: ticks
                          |   ^       ^   |
                          HR  |       |  HFL
                          |   |&lt;-----&gt;|   |
                          |&lt;-&gt;|  HSP  |&lt;-&gt;|
                          HGT1         HGT2
</code>
                  
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 concreteness 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:
<descrip>
<tag/active time (HAT)/
	Corresponds to HR, but in milliseconds.  HAT * DCF = HR.
<tag/blanking time (HBT)/
	Corresponds to (HFL - HR), but in milliseconds.  HBT * DCF = (HFL -
	HR).
<tag/front porch (HFP)/
	This is just HGT1.
<tag/sync time/
	This is just HSP.
<tag/back porch (HBP)/
	This is just HGT2.
</descrip>

<sect1>Vertical Sync:
<p>

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 micro second 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: 
<code>
   |___ __ __ __ __ __ __ __ __ __ __ __ __
   |_ _ _ _ _ _ _ _ _ _ _ _                |
   |_______________________|_______________|_____ 
   0                      VR              VFL     unit: ticks
                           ^   ^       ^
                           |   |       |
                           |&lt;-&gt;|&lt;-----&gt;|
                            VGT    VSP
</code>

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 definition 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:

<descrip>
<tag/active time (VAT)/
	Corresponds to VR, but in milliseconds.  VAT * VSF = VR.
<tag/blanking time (VBT)/
	Corresponds to (VFL - VR), but in milliseconds.  VBT * VSF = (VFL - VR).
<tag/front porch (VFP)/
	This is just VGT.
<tag/sync time/
	This is just VSP.
<tag/back porch (VBP)/
	This is like a second guard time after the vertical sync pulse.  It
	is often zero.
</descrip>

<sect>Putting it All Together<label id="synth">
<p>

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 indicates
at which dot the horizontal sync pulse will begin.  The third field indicates
at which dot the horizontal sync pulse will end.  The fourth field specifies
the total horizontal 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 indicates the line number at which the vertical sync pulse will begin.
The third field specifies the line number at which the vertical sync pulse will
end.  The fourth field contains the total vertical frame length (VFL). 

Example: 
<tscreen><verb>
     #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
</verb></tscreen>
(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: 
<tscreen><verb>
<name>   DCF     HR  SH1 SH2   HFL   VR  SV1 SV2 VFL
</verb></tscreen>
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. 
<tscreen><verb>
#name    clock   horizontal timing   vertical timing    flag
936x702  65      936 968 1200 1232   702 702 710 737
</verb></tscreen>
No special flag necessary; this is a non-interlaced mode.  Now we are really
done.

<sect>Overdriving Your Monitor<label id="overd">
<p>

You should absolutely <EM>not</EM> 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
discernible 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... 

<sect>Using Interlaced Modes<label id="inter">
<p>

(This section is largely due to David Kastrup
&lt;dak@pool.informatik.rwth-aachen.de&gt) 

At a fixed dot clock, an interlaced display is going to have
considerably less noticeable 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 <EM>refresh rate</EM> (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 <EM>jump</EM> 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
unsatisfactory flickery or washed out (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. 

<tscreen><verb>
ModeLine "1024x768" 45 1024 1048 1208 1248 768 768 776 807 Interlace
</verb></tscreen>

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. 

<sect>Questions and Answers<label id="answe">
<p>

   Q. The example you gave is not a standard screen size, can I use it? 

   A. Why not?  There is NO reason whatsoever 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 <EM>every</EM>
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 employs 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 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: 

<enum>
<item>
for any fixed driving frequency, raising max resolution incurs the penalty
of lowering refresh rate and thus introducing more flicker.
<item>
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!
</enum>

<sect>Fixing Problems with the Image.<label id="fixes">
<p>

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 problems and how
to fix them. 

(Fixing these minor distortions is where <bf>xvidtune</bf>(1) really shines.)

You <em>move</em> the image by changing the sync pulse timing.  You
<em>scale</em> 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! 

<sect1>The image is displaced to the left or right
<p>

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. 

<sect1>The image is displaced up or down
<p>

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. 

<sect1>The image is too large both horizontally and vertically
<p>

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.

<sect1>The image is too wide (too narrow) horizontally
<p>

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. 

<sect1>The image is too deep (too shallow) vertically
<p>

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. 

<sect>Plotting Monitor Capabilities<label id="cplot">
<p>

To plot a monitor mode diagram, you'll need the gnuplot package (a
freeware plotting language for UNIX-like operating systems) and the
tool <TT>modeplot</TT>, a shell/gnuplot script to plot the diagram from your
monitor characteristics, entered as command-line options. 

Here is a copy of modeplot:

<code>
#!/bin/sh
#
# modeplot -- generate X mode plot of available monitor modes
#
# Do `modeplot -?' to see the control options.
#
# ($Id: video-modes.sgml,v 1.2 1997/08/08 15:07:24 esr Exp $)

# 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: 1.2 $
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
</code>

Once you know you have <TT>modeplot</TT> and the gnuplot package in
place, you'll need the following monitor characteristics: 

<itemize>
<item> video bandwidth (VB)
<item> range of horizontal sync frequency (HSF)
<item> range of vertical sync frequency (VSF)
</itemize>

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: 

<enum>
<item> All resolutions have a single fixed aspect ratio AR = HR/VR.
Standard resolutions have AR = 4/3 or AR = 5/4.  The <TT>modeplot</TT>
programs assumes 4/3 by default, but you can override this. 
<item> For the modes considered, horizontal and vertical frame lengths are
fixed multiples of horizontal and vertical resolutions, respectively:

<tscreen><verb>
	HFL = F1 * HR
	VFL = F2 * VR
</verb></tscreen>
</enum>

As a rough guide, take F1 = 1.30 and F2 = 1.05 (see <ref id=frame>
"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: 

<tscreen><verb>
	RR = DCF / (HFL * VFL)		(*)
</verb></tscreen>

On the other hand, the horizontal frame length is equal to the clock rate
divided by the horizontal sync frequency: 

<tscreen><verb>
	HFL = DCF / HSF			(**)
</verb></tscreen>

VFL can be reduced to HFL be means of the two assumptions above: 

<tscreen><verb>
	VFL = F2 * VR
	    = F2 * (HR / AR)
	    = (F2/F1) * HFL / AR	(***)
</verb></tscreen>

Inserting (**) and (***) into (*) we obtain: 

<tscreen><verb>
	RR = DCF / ((F2/F1) * HFL**2 / AR)
	   = (F1/F2) * AR * DCF * (HSF/DCF)**2
	   = (F1/F2) * AR * HSF**2 / DCF
</verb></tscreen>

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: 

<tscreen><verb>
	RR = DCF / (HFL * VFL) = DCF / (F1 * HR * F2 * VR)
</verb></tscreen>

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 <TT>modeplot</TT> tool provides you with an easy way to do this.  Do
<TT>modeplot -?</TT> to see its control options. A typical invocation 
looks like this: 

<tscreen><verb>
	modeplot -t "Swan SW617" -b 85 -v 50 90 -h 31 58
</verb></tscreen>

The -b option specifies video bandwidth; -v and -h set horizontal and
vertical sync frequency ranges. 

When reading the output of <TT>modeplot</TT>, 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 <ref id="synth" name="Putting it All
Together">. However, it should give you a better feeling for what
is possible and which tradeoffs are involved. 

<sect>Credits<label id="credi">
<p>

The original ancestor of this document was by Chin Fang
&lt;fangchin@leland.stanford.edu&gt;.   

Eric S. Raymond &lt;esr@snark.thyrsus.com&gt; 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
&lt;crosson@cam.nist.gov&gt;. 

The material on interlaced modes is largely by David Kastrup
&lt;dak@pool.informatik.rwth-aachen.de&gt; 

Martin Lottermoser &lt;Martin.Lottermoser@mch.sni.de&gt; contributed
the idea of using gnuplot to make mode diagrams and did the
mathematical analysis behind <TT>modeplot</TT>.  The distributed
<TT>modeplot</TT> was redesigned and generalized by ESR from
Martin's original gnuplot code for one case.
<verb>
$XFree86: xc/programs/Xserver/hw/xfree86/doc/sgml/VidModes.sgml,v 3.11.2.2 1998/02/20 23:10:30 dawes Exp $
  
  
  
  
  
$XConsortium: VidModes.sgml /main/7 1996/02/21 17:46:17 kaleb $
</verb>
  
</article>

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