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

[edit] Limitations

A major limitation of EDID is that it cannot express the native resolutions of the most common wide screen flat panel displays and liquid crystal display televisions. The number of horizontal pixels must be a multiple of 8. The number of vertical pixels is calculated from the horizontal resolution and the selected aspect ratio. To be fully expressable, 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 expressable in the EDID syntax is 1360×765 pixels. Specifying 1368 pixels as the screen width would yeld an unnatural screen hight of 769.5 pixels.

Many Wide XGA panels do not advertise their native resolution, 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. Special programs are available to override the EDID data; PowerStrip for Microsoft Windows and DisplayConfigX for Mac OS X

[edit] EDID 1.1 data format

Byte sequence
00–07: Header information
08–17: Complete serial number
  08–09: Manufacturer ID
  10–11: Product ID Code (little-endian)
  12–15: Serial Number (little-endian)
  16: Week of Manufacture
  17: Year of Manufacture.  Add 1990 to the value for actual year.
18: EDID Version Number
19: EDID Revision Number
20-24: Basic Display Parameters
  20: VIDEO INPUT DEFINITION
    bit 7: 0=analog, 1=digital
    if bit 7 is digital:
      bit 0: 1=DFP 1.x compatible
    if bit 7 is analog:
      bit 6-5: video level
       00=0.7, 0.3, 01=0.714, 0.286, 10=1, .4 11=0.7, 0
      bit 4: blank-to-black setup
      bit 3: separate syncs
      bit 2: composite sync
      bit 1: sync on green
      bit 0: serration vsync
  21: Maximum Horizontal Image Size (in centimeters).
  22: Maximum Vertical Image Size (in centimetres).
  23: Display Gamma.  Divide by 100, then add 1 for actual value.
  24: Power Management and Supported Feature(s):
    bit 7: standby
    bit 6: suspend
    bit 5: active-off/low power
    bit 4-3: display type.
      00=monochrome, 01=RGB colour, 10=non RGB multicolour, 11=undefined
    bit 2: standard colour space
    bit 1: preferred timing mode
    bit 0: default GTF supported
25-34: CHROMA INFO
  25: low significant bits for Red X (bit 7-6), Red Y (bit 5-4), Green X (bit 3-2), Green Y (bit 1-0).
  26: low significant bits for Blue X (bit 7-6), Blue Y (bit 5-4), White X (bit 3-2), White Y (bit 1-0).
  27–34: high significant bits for Red X, Red Y, Green X, Green Y, Blue X, Blue Y, White X, White Y.
  To decode actual value, rearrange bits as follows:
  High significant bits 7-0 for (channel), low significant bits for (channel).
    Actual value is between 0.000 and 0.999, but encoded value is between 000h and 3FFh.
35: ESTABLISHED TIMING I
  bit 7-0: 720×400@70 Hz, 720×400@88 Hz, 640×480@60 Hz, 640×480@67 Hz,
           640×480@72 Hz, 640×480@75 Hz, 800×600@56 Hz, 800×600@60 Hz
36: ESTABLISHED TIMING II
  bit 7-0: 800×600@72 Hz, 800×600@75 Hz, 832×624@75 Hz, 1024×768@87 Hz (Interlaced),
           1024×768@60 Hz, 1024×768@70 Hz, 1024×768@75 Hz, 1280×1024@75 Hz
37: Manufacturer's Reserved Timing
38–53: Standard Timing Identification.  2 bytes for each record.
  First byte
    Horizontal resolution.  Multiply by 8, then add 248 for actual value.
  Second byte
    bit 7-6: Aspect ratio.  Actual vertical resolution depends on horizontal resolution.
      00=16:10, 01=4:3, 10=5:4, 11=16:9
    bit 5-0: Vertical frequency.  Adds 60 to get actual value.

54–71: Descriptor Block 1
  54–55: Pixel Clock (in 10 kHz) or 0
  If Pixel Clock is non null:
    56: Horizontal Active (in pixels)
    57: Horizontal Blanking (in pixels)
    58: Horizontal Active high (4 upper bits)
        Horizontal Blanking high (4 lower bits)
    59: Vertical Active (in pixels)
    60: Vertical Blanking (in vertical pixels/lines)
    61: high significant bits for Vertical Active (4 upper bits)
        high significant bits for Vertical Blanking (4 lower bits)
    62: Horizontal Sync Offset (in pixels)
    63: Horizontal Sync Pulse Width (in pixels)
    64: Vertical Sync Offset (in lines) (4 upper bits)
        Vertical Sync Pulse Width (in lines) (4 lower bits)
    65: high significant bits for Horizontal Sync Offset (bit 7-6)
        high significant bits for Horizontal Sync Pulse Width (bit 5-4)
        high significant bits for Vertical Sync Offset (bit 3-2)
        high significant bits for Vertical Sync Pulse Width (bit 1-0)
    66: Horizontal Image Size (in mm)
    67: Vertical Image Size (in mm)
    68: high significant bits for Horizontal Image Size (4 upper bits)
        high significant bits for Vertical Image Size (4 lower bits)
    69: Horizontal Border
    70: Vertical Border
    71: Interlaced or not (bit 7)
        ? (bit 6)
        Stereo or not (bit 5)
        Separate Sync or not (bit 4-3)
        Horizontal Sync positive or not (bit 2)
        Vertical Sync positive or not (bit 1)
        ? (bit 0)
  If Pixel Clock is null:
    56: 0
    57: Block type
      FFh=Monitor Serial Number, FEh=ASCII string, FDh=Monitor Range Limits, FCh=Monitor name, 
      FBh=Colour Point Data, FAh, Standard Timing Data, F9h=Currently undefined, F8h=defined by manufacturer
    58: Unknown
    59–71: Descriptor block contents.
      If block type is FFh, FEh, or FCh, the entire area is a text string.
      If block type is FDh:
        59–63:
          Min Vertical frequency, Max Vertical frequency, 
          Min Horizontal frequency (in kHz), Max Horizontal frequency (in kHz), pixel clock (in MHz (multiply by 10 for actual value))
        64–65: Secondary GTF toggle
          If encoded value is 000A, bytes 59-63 are used.  If encoded value is 0200, bytes 67–71 are used.
        66: Start horizontal frequency (in kHz).  Multiply by 2 for actual value.
        67: C. Divide by 2 for actual value.
        68-69: M (little endian).
        70: K
        71: J. Divide by 2 for actual value.
      If block type is FBh:
        59: W Index 0.  If set to 0, bytes 60-63 are not used.  If set to 1, 61–63 are assigned to white point index #1
        64: W Index 1.  If set to 0, bytes 65-68 are not used.  If set to 2, 65–68 are assigned to white point index #2
        White point index structure:
          First byte
            bit 3-2: low significant bits for White X (bit 3-2), White Y (bit 1-0)
          Second to third byte: high significant bits for White X, White Y.
          Fourth byte: Gamma.  Divide by 100, then add 1 for actual value.
          To decode White X and White Y, see bytes 25-34.
      If block type is FAh:
        59–70: Standard Timing Identification.  2 bytes for each record.
          For structure details, see bytes 38-53.

72–89: Descriptor Block 2
90–107: Descriptor Block 3
108–125: Descriptor Block 4
126: Extension EDID Block(s).  In EDID 1.1, it is ignored, and should be set to 0.
127: Checksum.

For example, here is a summary of the data reported by an Envision EN-775e monitor:

  Monitor Name                 EPI EnVision EN-775e
  Monitor ID                   EPID775
  Model                        EN-775e
  Manufacture Date             Week 26 / 2002
  Serial Number                1226764172
  Max. Visible Display Size    32 cm × 24 cm (15.7 in)
  Picture Aspect Ratio         4:3
  Horizontal Frequency         30–72 kHz
  Vertical Frequency           50–160 Hz
  Maximum Resolution           1280×1024
  Gamma                        2.20
  DPMS Mode Support            Active-Off

Supported Video Modes:
  640×480                    140 Hz
  800×600                    110 Hz
  1024×768                   85 Hz
  1152×864                   75 Hz
  1280×1024                  65 Hz

Monitor Manufacturer:
  Company Name                 Envision, Inc.