Digital visual interface

The digital visual interface (DVI) is a form of video connector made to maximize the display quality of flat panel LCD monitors and high-end video graphics cards. It was developed by an industry consortium, the Digital Display Working Group (DDWG).

Existing standards, such as VGA, are analog, as are the monitors they are plugged in to. However, LCD monitors and plasma screens internally use a digital signal. Using VGA cabling results in the computer signal being converted from the internal digital format to analog on the VGA cable, then back to digital in the monitor for display. Obviously a better solution, provided by DVI, would be to simply supply the original digital signal to the monitor directly.

DVI also includes the ability to drive analog signals using the VGA standard as well. This feature was included in order to make DVI universal, in that it supports both types of monitors with a single connector. However the designers then confused matters considerably by defining three different DVI's with or without digital and analog portions.

There are three types of DVI connections:

DVI-D (Digital)
DVI-A (Analog)
DVI-I (Integrated Digital/Analog)

Another shortcoming of DVI is that it lacks USB pass-throughs. This is somewhat odd considering that the standard was originally developed by Intel, also the developers of USB. This has been addressed in the non-standard M1-DA connector used by InFocus on their projector systems.

The data format used by DVI is based on the PanelLink™ serial format devised by the semiconductor manufacturer Silicon Image Inc. This uses transition-minimized digital signalling (TMDS), which is a method of transmitting high-speed serial data whilst emitting the least possible electromagnetic interference (EMI). An advantage of TMDS over parallel formats is that the video data can be sent down a smaller number of wires, thereby reducing the price of the connectors and cables. A basic DVI-D link consists of four twisted pairs of wire (red, green, blue and clock) to transmit 24 bits per pixel, although for higher display resolutions a dual link (eight pairs, using reserved pins in the standard connector) can be used.

Like modern analog VGA connectors, the DVI connector includes pins for the display data channel, version 2 (DDC2) that allows the graphics adaptor to read the monitor's extended display identification data (EDID).

The DVI specification can be downloaded free of charge from the website of the Digital Display Working Group at " class="external">http://www.ddwg.org/.

DVI is the only widespread standard that includes analogue and digital transmission options in the same connector. Competing standards are exclusively digital: these include a system using low-voltage differential signalling (LVDS), known by its proprietary names FPD (for Flat-Panel Display) Link and FLATLINK; and its successors, the LVDS Display Interface (LDI) and OpenLDI.