XFree86 Video Timings HOWTO : Basic Things to Know about your Display and Adapter
Previous: How Video Displays Work
Next: The monitor's video bandwidth:

## 4. Basic Things to Know about your Display and Adapter

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

- your monitor's horizontal and vertical sync frequency options
- your video adapter's driving clock frequency, or "dot clock"
- your monitor's bandwidth

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.

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

> `

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!

### 4.1. The monitor's video bandwidth:

### 4.2. What these control:

XFree86 Video Timings HOWTO : Basic Things to Know about your Display and Adapter
Previous: How Video Displays Work
Next: The monitor's video bandwidth: