For other meanings of DVI, please see DVI (disambiguation).
The digital visual interface or digital video interface (DVI) is a video connector designed to maximize the visual quality of digital display devices such as flat panel LCD computer monitors and digital projectors. It was developed by an industry consortium, the Digital Display Working Group (DDWG).
Existing standards, such as VGA, are analog and designed for CRT based devices. As the source transmits each horizontal line of the image, it varies its output voltage to represent the desired brightness. In a CRT device, this is used to vary the intensity of the scanning beam as it moves across the screen. However, in digital displays, instead of a scanning beam there is an array of pixels and a single brightness value must be chosen for each. The decoder does this by sampling the voltage of the input signal at regular intervals. When the source is also a digital device (such as a computer), this can lead to distortion if the samples are not taken at the centre of each pixel, and in general the crosstalk between adjacent pixels is high.
DVI takes a different approach. The desired brightness of the pixels is transmitted as a list of binary numbers. When the display is driven at its native resolution, all it has to do is read each number and apply that brightness to the appropriate pixel. In this way, each pixel in the output buffer of the source device corresponds directly to one pixel in the display device, whereas with an analog signal the appearance of each pixel may be affected by its adjacent pixels as well as by electrical noise and other forms of analog distortion.
The DVI connector incorporates pins to pass through analog signals using the VGA standard. 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 only)
- DVI-A (analog only)
- DVI-I (integrated digital and analog)
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.
One oversight in DVI is that USB signals were not incorporated into the connector. This has been addressed in the VESA M1-DA connector used by InFocus on their projector systems, and in the now-defunct Apple Display Connector used by Apple Computer. The VESA M1 connector is essentially the VESA Plug & Display (P&D) connector, which was itself originally named the Enhanced Video Connector (EVC). The pinout Apple Display Connector is electrically compatible with the VESA P&D/M1, but physically the shell of the connector is a different shape.
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 data format used by DVI is based on the PanelLink™ serial format devised by the semiconductor manufacturer Silicon Image Inc. This uses Transition Minimized Differential Signaling. A single DVI link consists of four twisted pairs of wire (red, green, blue, and clock) to transmit 24 bits per pixel. The timing of the signal almost exactly matches that of an analog video signal; The picture is transmitted line by line with blanking intervals between each line and each frame, and no packetiziation. No compression is used and DVI has no provision for only transmitting changed parts of the image. This means the whole frame must be constantly retransmitted. The DVI connector has provision for two separate links. When the bandwidth of a single link is exceeded, the second link may be enabled with alternate pixels being transmitted on each link. The second link may also be used when more than 24 bits per pixel is required, in which case it carries the least significant bits.
Some new DVD players , TV sets (including HDTV sets) and video projectors have DVI/HDCP connectors; these are physically the same as DVI connectors but transmit an encrypted signal using the HDCP protocol for copyright protection. Computers with DVI video connectors can theoretically use HDTV sets as display.
- Minimum clock frequency: 21.75 MHz
- Maximum clock frequency: 165 MHz
- Pixels per clock cycle: 1 (single link) or 2 (dual link)
- Bits per pixel: 24
- Maximum display resolutions (single link):
- HDTV (1920 × 1080) @ 60 Hz (5% LCD blanking)
- UXGA (1600 × 1200) @ 60 Hz (GTF blanking)
- SXGA (1280 × 1024) @ 75 Hz (GTF blanking)
- SXGA (1280 × 1024) @ 85 Hz (GTF blanking)
- Maximum display resolutions (dual link):
- QXGA (2048 × 1536) @ 75 Hz (GTF blanking)
- HDTV (1920 × 1080) @ 85 Hz (GTF blanking)
- RGB bandwidth: 400 MHz at -3dB
Note: GTF == Generalized Timing Formula (a VESA standard) Source: DVI Specification 1.0 from DDWG