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Extended display identification data

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Extended display identification data (EDID) is a data structure provided by a computer display to describe its capabilities to a graphics card. It is what enables a modern personal computer to know what kind of monitor is connected. EDID is defined by a standard published by the Video Electronics Standards Association (VESA). The EDID includes manufacturer name, product type, phosphor or filter type, timings supported by the display, display size, luminance data and (for digital displays only) pixel mapping data.

EDID structure 1.0 was defined in 1994; version 1.1 followed in 1996, then 1.2, and 1.3 in 2000. All these define upwards compatible 128 byte structures. EDID structure 2.0 defines a new 256-byte structure.

The channel for transmitting the EDID from the display to the graphics card is usually the I²C bus. The combination of EDID and I²C is called the Display Data Channel version 2, or DDC2. The 2 distinguishes it from VESA's original DDC, which used a different serial format.

Before DDC and EDID were defined, there was no standard way for a graphics card to know what kind of display device it was connected to. Some VGA connectors in personal computers provided a basic form of identification by connecting one, two or three pins to ground, but this coding was not standardized.

The EDID is often stored in the monitor in a memory device called a serial PROM (programmable read-only memory) or EEPROM (electrically erasable PROM) that is compatible with the I²C bus.

Many software packages can read and display the EDID information, such as read-edid[1] and Powerstrip[2] for Microsoft Windows and XFree86 (which will output the EDID to the log if verbose logging is on (startx -- -logverbose 6)) for Linux and BSD unix. Mac OS X natively reads EDID information (see /var/log/system.log or hold down Cmd-V on startup) and programs such as SwitchResX[3] or DisplayConfigX[4] can display the information as well as use it to define custom resolutions.



[edit] Enhanced EDID (E-EDID)

Enhanced EDID is based on the EDID version 1.3, but offers support of extensions, which enable EDID 2.0 to be supported as an extension. Data fields for preferred timing, range limits, monitor name are required in E-EDID. E-EDID also supports dual GTF, standard timings aspect ratio change.

With the use of extensions, E-EDID string can be lengthened up to 32KiBytes.

[edit] EDID Extensions assigned by VESA

  • Timing Extension (00h)
  • Additional Timing Data Block (CEA EDID Timing Extension) (02h)
  • Video Timing Block Extension (VTB-EXT) (10h)
  • EDID 2.0 Extension (20h)
  • Display Information Extension (DI-EXT) (40h)
  • Localized String Extension (LS-EXT) (50h)
  • Microdisplay Interface Extension (MI-EXT) (60h)
  • Display Transfer Characteristics Data Block (DTCDB) (A7h, AFh, BFh)
  • Block Map (F0h)
  • Display Device Data Block (DDDB) (FFh)
  • Extension defined by monitor manufacturer (FFh): According to LS-EXT, actual contents varies from manufacturer. However, the value is later used by DDDB.

[edit] Limitations

Some graphics card drivers have historically coped poorly with the EDID, using only its standard timing descriptors rather than its Detailed Timing Descriptors (DTDs). Even in cases where the DTDs were read, the drivers are/were still often limited by the standard timing descriptor limitation that the horizontal/vertical resolutions must be evenly divisible by 8. This means that many graphics cards cannot express the native resolutions of the most common wide screen flat panel displays and liquid crystal display televisions. The number of vertical pixels is calculated from the horizontal resolution and the selected aspect ratio. To be fully expressible, the size of wide screen display must thus be a multiple of 16×9 pixels. For 1366×768 pixel Wide XGA panels the nearest resolution expressible in the EDID standard timing descriptor syntax is 1360×765 pixels. Specifying 1368 pixels as the screen width would yield an unnatural screen height of 769.5 pixels.

Many Wide XGA panels do not advertise their native resolution in the standard timing descriptors, instead offering only a resolution of 1280×768. Some panels advertise a resolution only slightly smaller than the native, such as 1360×765. For these panels to be able to show a pixel perfect image, the EDID data must be ignored by the display driver or the driver must correctly interpret the DTD and be able to resolve resolutions whose size is not divisible by 8. Special programs are available to override the standard timing descriptors from EDID data; PowerStrip for Microsoft Windows and SwitchResX for Mac OS X.

[edit] Disabling DDC/CI

On Microsoft Windows (Versions XP and above), there is no software provided option to disable plug and play monitor detection. This causes problems with computer/monitor switching applications and causes computer games to select display resolutions higher than the monitor is physically capable of displaying resulting in a garbled display.

In these circumstances, it may be necessary to remove pin 12 from the monitor VGA cable, to disable plug and play monitor detection. This allows display resolution to be selected manually and not overridden when the display adapter is removed and reinserted or the KVM switch is operated.

[edit] External links

[edit] Extensions

[edit] Software

[edit] References

  1. ^ read-edid software for Linux and Windows
  2. ^ Powerstrip for Windows (Shareware)
  3. ^ SwitchResX for Mac OS X shows EDID and customizes display timings
  4. ^ DisplayConfigX for Mac OS X shows EDID and customizes display timings
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