Extended display identification data(EDID)

1. EDID background

    we have product, need to connect difference monitor over VGA display port. In order to guarantee to set factory default  resolution of that monitor, we need to get video timing information for monitor by DDC interface(usually I2C bus). The information also call  Extended display identification data (EDID), (EDID) is a data structure provided by a digital display to describe its capabilities to a video source (e.g.graphics card, Set-top box). It is what enables a modern personal computer to know what kinds of monitors are connected to it. EDID is defined by a standard published by the Video Electronics Standards Association (VESA). The EDID includes manufacturer name and serial number, product type, phosphoror filter type, timings supported by the display, display size, luminance data and (for digital displays only) pixel mapping data.

2.EDID 1.3 data format

   EDID has 128 bytes, detail information please refer http://en.wikipedia.org/wiki/Extended_display_identification_data. We only focus on those video timing relate information.

   Usually EDID raw data like below(Sample monitor DELL 1907FP)

0x00 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0x00 0x10 0xAC 0x14 0x40 0x32 0x36 0x33 0x42

0x0A 0x10 0x01 0x03 0x0E 0x26 0x1E 0x78 0xEE 0x68 0x75 0xA2 0x5A 0x49 0x9F 0x23

0x13 0x50 0x54 0xA5 0x4B 0x00 0x71 0x4F 0x81 0x80 0x01 0x01 0x01 0x01 0x01 0x01

0x01 0x01 0x01 0x01 0x01 0x01 0x30 0x2A 0x00 0x98 0x51 0x00 0x2A 0x40 0x30 0x70

0x13 0x00 0x78 0x2D 0x11 0x00 0x00 0x1E 0x00 0x00 0x00 0xFF 0x00 0x43 0x39 0x37

0x35 0x39 0x36 0x33 0x37 0x42 0x33 0x36 0x32 0x0A 0x00 0x00 0x00 0xFC 0x00 0x44

0x45 0x4C 0x4C 0x20 0x31 0x39 0x30 0x37 0x46 0x50 0x0A 0x20 0x00 0x00 0x00 0xFD

0x00 0x38 0x4C 0x1E 0x51 0x0E 0x00 0x0A 0x20 0x20 0x20 0x20 0x20 0x20 0x00 0x7A

0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00

0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00

0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00

0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00

0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00

0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00

0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00

0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00

  We care about ”Descriptor Blocks“, There are four "Descritor Blocks" from byte 54(Byte 0 is first byte), 18 bytes per block,we call them DTD. DTD is in first Descriptor Block in normal way, but it can be in any of the four. We read first two bytes of the block, we can judge it is DTD if they are nonzero, some monitor manufacturer may have more than one DTD, that need us to choose which one is most suitable.

Let’s decode DTD

0x30 0x2A 0x00 0x98 0x51 0x00 0x2A 0x40 0x30 0x70 0x13 0x00 0x78 0x2D 0x11 0x00 0x00 0x1E

byte0,1 is pixle clock, unit 10Khz,MSB in byte1, LSB in byte 0,

0x2A30=10800, that is 10800x10Khz=108Mhz.

byte2 and byte4 upper 4 bites is Horizontal active,

0x500 = 1280 pixel

byte5 and byte7 upper 4 bits is Vertical active

0x400 = 1024 pixel

byte3 and byte4 lower 4 bits is Horizontal banking

0x198= 408 pixel

byte6 and byte7 lower 4 bits is Vertical banking

0x2a=42 pixel

byte8 and byte11(bit7-6) is Horizontal sync offset

0x30 = 48

byte9 and byte11(bit5-4) is Horizontal sync pulse width

0x70 = 112

byte10 upper 4 bits and byte11(bit3-2) is Vertical sync offset

0x1 = 1

byte10 lower 4 bits and byte11(bit1-0) is Vertical sync pulse width

0x3 = 3

byte17  Interlaced or not (bit 7)
           Stereo or not (bit 6-5) ("00" means not)
           Separate Sync or not (bit 4-3)
           Vertical Sync positive or not (bit 2)
           Horizontal Sync positive or not (bit 1)
           Stereo Mode (bit 0) (unused if 6-5 are 00)

0x1E means HSync and VSync are positive.

And we need get refresh rate from EDID Standard timing identification(byte38-53)

0x81 0x80 0x01 0x01 0x01 0x01

First byte: Horizontal resolution. Multiply by 8, then add 248 for actual value

0x81 * 8 +248 = 1280

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 (00=1:1 prior to v1.3)

10b means 5:4

bit 5-0

Vertical frequency. Add 60 to get actual value.

0 + 60 = 60Hz

Now we got timing parameters

General timing

Screen refresh rate 60Hz

Pixel frequency 108Mhz

Horizontal timing (line)

Visible area= 1280 pixels

HSync width = 112 pixels

Front porch = 48 pixels

Back porch = 408-112-48 = 248 pixels

Vertical timing(line)

Visible area = 1024 pixels

VSync width = 3 pixels

Front porch = 1

Back porch = 42 - 1- 3 = 38 pixels

We can get below information by decoding full EDID  structure

========================================

Display Information (EDID)

========================================

  EDID Version 1.3

  Manufacturer: DEL

  Model: 4014

  Serial#: 1110652466

  Year: 2006 Week 10

  Serial Number: C9759637B362

  Monitor Name: DELL 1907FP

  Display Characteristics:

     Monitor Operating Limits: From EDID

          H: 30-81KHz V: 56-76Hz DCLK: 140MHz

     Analog Display Input: Input Voltage - 0.700V/0.300V

     Sync: Separate Composite Sync on Green

     Max H-size in cm: 38

     Max V-size in cm: 30

     Gamma: 2.20

     DPMS: Active yes, Suspend yes, Standby yes

     RGB Color Display

     Chroma

        RedX:     0.634 RedY:     0.354

        GreenX:   0.287 GreenY:   0.621

        BlueX:    0.138 BlueY:    0.077

        WhiteX:   0.313 WhiteY:   0.329

     Default color format is primary

     First DETAILED Timing is preferred

  Detailed Timings

     108 MHz 1280 1328 1440 1688 1024 1025 1028 1066 +HSync +VSync

  Supported VESA Modes

     720x400@70Hz

     640x480@60Hz

     640x480@75Hz

     800x600@60Hz

     800x600@75Hz

     1024x768@60Hz

     1024x768@75Hz

     1280x1024@75Hz

     Manufacturer's mask: 0

  Standard Timings

     1152x864@75Hz

     1280x1024@60Hz

========================================

3. How to setup other resolution timing that not in EDID?

   In Linux, we can use “cvt” command to calculate timing setting for one resolutionn.

$ cvt 1280 1024 60

# 1280 is horizontal size , 1024 is vertical size, 60 is refresh rate.

#  we will get mode line like below

108 MHz 1280 1328 1440 1688 1024 1025 1028 1066 +HSync +VSync

I have checked some latest monitor in market. Most of them are not VESA timing, but suitable for themselves. Except one "KTC" monitor, I have tried to use its EDID timing to setup, however, the display shift right about 100 pixels. It is very unprofessional!
( Also we can have quick refer another interesting web site http://tinyvga.com/vga-timing. )