  XFree86 Video Timings HOWTO
  Eric S. Raymond <esr@thyrsus.com>
  Version 4.2, 28 January 2000

  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

  If you have a relatively new monitor (1996 or later) that supports the
  PnP specification, there is a chance that you use the read-edid
  program to ask the monitor for its stastics and compute a mode line
  for you.  See  <http://altern.org/vii/programs/linux/read-edid/>.

  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 what you could consider to be a
  series of raster 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
  beams inside the display, one for each primary color.  To make the
  beams strike each dot for an equal amount of time, the beams are swept
  across the display in a constant pattern, called a raster.

  We say "what you could consider to be a series of dots" because these
  raster dots don't actually correspond to physical phosphor dots.  The
  physical phosphor dots are much smaller than raster dots -- they have
  to be, otherwise the display would suffer from severe moire-pattern
  effects.  The raster dots are really samples of the analog driver
  signal, and display as a grid of dots only because the peaks and
  valleys in the signal are quite regularly and finely spaced.

  The pattern starts at the top left of the screen, goes across the
  screen to the right in a straight line, moving ever so slightly
  "downhill" (the downhill slope is too small to be perceptible). Then
  the beams are swept back to the left side of the display, starting at
  a new 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 beams are moved from the bottom
  right corner of the display (sweeping back and forth a few times) 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 during a second half-frame.

  Starting the beams at the top left of the display is called the
  beginning of a frame.  The frame ends when the beams reach the the top
  left corner again as they come from the bottom right corner of the
  display.  A frame is made up of all of the lines the beams traced from
  the top of the display to the bottom.


  If the electron beams were on all of the time they were 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 beams are hard to control there.  To reduce the
  distortion, the dots around the edges of the display are not
  illuminated by the beams (because they're turned off) even though the
  beams, if they were turned on, would be pointing at them.  The
  viewable area of the display is reduced this way.

  Another important thing to understand is what becomes of the beams
  when no spot is being painted on the visible area.  The time the beams
  would have been illuminating the side borders of the display is used
  for sweeping the beams back from the right edge to the left. The time
  the beams would have been illuminating the top and bottom borders of
  the display is used for moving the beams 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 beams (according to the desired color) at each dot to
  generate a picture. The card also controls when the display moves the
  beams from the right side back to the left 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 beams 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 beams can stabilize.  If the beams can't stabilize, the
  picture will not be steady.

  For more information, see TV and Monitor Deflection Systems
  <http://fribble.cie.rpi.edu/~repairfaq/REPAIR/F_deflfaq.html>.

  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:


  o  your monitor's horizontal and vertical sync frequency options

  o  your monitor's bandwidth

  o  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 extrapolated).  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-
  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. Keep it above 60Hz.  72Hz is the VESA
  ergonomic standard. 120Hz is the flicker rate of fluorescent lights;
  if you're sensitive to those, you need to keep it above that.

  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 time units (usually microseconds).
        HAT * DCF = HR.


     blanking time (HBT)
        Corresponds to (HFL - HR), but in time units (usually
        microseconds).  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.  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.


  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: 1.11 $
  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:


  o  video bandwidth (VB)

  o  range of horizontal sync frequency (HSF)

  o  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>

  Nicholas Bodley <nbodley@alumni.princeton.edu> corrected and clarified
  the section on how displays work.

  Payne Freret <payne@freret.org> corrected some minor technical errors
  about monitor design.

  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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