There are some fundamental things you need to know before hacking an Xconfig entry. These are:
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.
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:
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. 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:
wga
Clocks 25 28 40 3 50 77 36 45 0 0 79 31 94 65 75 71
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):
640x480 25
800x600 36
1024x768 65
1024x768 interlaced 45
1280x1024 110
1600x1200 185
BTW, there's nothing magic about this table; these numbers are just the lowest dot clocks per resolution in the standard XFree86 Modes database (except for the last, which I interpolated). The bandwidth of your monitor may actually be higher than the minimum needed for its top resolution, so don't be afraid to try a dot clock a few MHz higher.
Also note that bandwidth is seldom an issue for dot clocks under 65MHz or so. With an SVGA card and most hi-res monitors, you can't get anywhere near the limit of your monitor's video bandwidth. The following are examples:
Brand Video Bandwidth
---------- ---------------
NEC 4D 75Mhz
Nano 907a 50Mhz
Nano 9080i 60Mhz
Mitsubishi HL6615 110Mhz
Mitsubishi Diamond Scan 100Mhz
IDEK MF-5117 65Mhz
IOCOMM Thinksync-17 CM-7126 136Mhz
HP D1188A 100Mhz
Philips SC-17AS 110Mhz
Swan SW617 85Mhz
Viewsonic 21PS 185Mhz
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).
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.