Display Data Channel
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The Display Data Channel or DDC is a digital connection between a computer display and a graphics adapter that allows the display to communicate its specifications to the adapter. The standard was created by VESA.
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Mechanical
The DDC link is carried on three pins – data, clock and ground – in a 15-pin VGA connector, a DVI connector or an HDMI connector.
Electrical
The current version of DDC, called DDC2B, is based on the I²C bus defined long ago by Philips. This is a serial bus that allows multiple bus masters, although DDC2B allows only one master - the graphics adapter. The monitor (e.g. a CRT or LCD) contains a read-only memory (ROM) chip programmed by the manufacturer with information about the graphics modes that the monitor can display.
Logical
The data in the monitor's ROM is held in a standard format called EDID (for Extended Display Information Data). This format is defined by the Video Electronics Standards Association (VESA).
DDC/CI
DDC/CI (CI = command interface) is an extension to DDC specified by VESA in August 1998. It allows a computer with a suitably designed graphics adapter to adjust monitor parameters such as brightness and colour balance, or to initiate degaussing. DDC/CI monitors are sometimes supplied with an external colour sensor, to allow automatic calibration of the monitor's colour balance.
The latest version of the specification is "Display Data Channel Command Interface (DDC/CI) Standard- Version 1.1", October 2004 (VESA document VESA-2004-10).
References
- Extended Display Identification Data (EDID) Standard, Version 3, 1997, VESA
- 6. DDC - VESA - Plug & Play - Power Management (http://pksun1.publikompass.it:80/html.ng/site=hwupgrade&spacedesc=genrot&size=468x60&pp=topc)