Digital Visual Interface Totally Explained

Everything about Digital Visual Interface totally explained

The Digital Visual Interface (DVI) is a video interface standard designed to maximize the visual quality of digital display devices such as flat panel LCD computer displays and digital projectors. It was developed by an industry consortium, the Digital Display Working Group (DDWG). It is designed for carrying uncompressed digital video data to a display. It is partially compatible with the High-Definition Multimedia Interface (HDMI) standard in digital mode (DVI-D).

Overview

The DVI interface uses a digital protocol in which the desired illumination of pixels is transmitted as binary data. When the display is driven at its native resolution, it'll 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.
   Previous standards such as the analog VGA were designed for CRT-based devices and thus didn't use discrete time display addressing. As the analog 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.
   DVI cable connectors are designed in such a way as not to allow the user to connect the cable in an incorrect position or orientation. DVI connectors are available in five models, differing in the way they handle analog or digital transfers. In the digital transfer one or two channels are present. Video and monitor cards which are exclusively digital can't be connected to analog, but can be connected to equipment that handles both analog and digital signals. The DVI standard also supports the Display Data Channel (DDC) and the Extended Display Identification Data (EDID) which allows computers to communicate with different monitor extensions.
   "DVI-I" stands for "DVI-Integrated" and supports both digital and analog transfers, so it works with both digital and analog monitors. "DVI-D" stands for "DVI-Digital" and supports digital transfers only.

Technical discussion

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 (TMDS). 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 without packetization. No compression is used and there's no support for only transmitting changed parts of the image. This means that the whole frame is constantly re-transmitted. The specification (see below for link) does, however, include a paragraph on "Conversion to Selective Refresh" (under 1.2.2), suggesting this feature for future devices.
   With a single DVI link, the largest resolution possible at 60 Hz is 2.75 megapixels (including blanking interval). For practical purposes, this allows a maximum screen resolution at 60 Hz of 1915 x 1436 pixels (standard 1.33 ratio), 1854 x 1483 pixels (1.25 ratio) or 2098 x 1311 (widescreen 1.6 ratio). The DVI connector therefore has provision for a second link, containing another set of red, green, and blue twisted pairs. When more bandwidth is required than is possible with a single link, the second link is enabled, and alternate pixels may be transmitted on each, allowing resolutions up to 4 megapixels at 60 Hz. The DVI specification mandates a fixed single link maximum pixel clock frequency of 165 MHz, where all display modes that require less than this must use single link mode, and all those that require more must switch to dual link mode. When both links are in use, the pixel rate on each may exceed 165 MHz. The second link can also be used when more than 24 bits per pixel is required, in which case it carries the least significant bits. The data pairs carry binary data at ten times the pixel clock reference frequency, maximum 1.65 Gbit/s x 3 data pairs for a single DVI link.
   Like modern analog VGA connectors, the DVI connector includes pins for the display data channel. DDC2 (a newer version of DDC) allows the graphics adapter to read the monitor's extended display identification data (EDID). If a display supports both analog and digital signals in one input, each input can host a distinct EDID. If both receivers are active, analog EDID is used.
   The maximum length of DVI cables isn't included in the specification since it's dependent on bandwidth requirements (the resolution of the image being transmitted). In general, cable lengths from 1-15 feet (4.5m) will work for displays at resolutions of 1920x1200. Cable lengths up to 50 feet (15m) can be used with displays at resolutions up to 1280x1024. For longer distances, to eliminate the video degradation, the use of a DVI booster is recommended. DVI boosters may or may not use an external power supply.

Connector

As well as digital signals, the DVI connector includes pins providing the same analog signals found on a VGA connector, allowing a VGA monitor to be connected with a simple plug adapter. This feature was included in order to make DVI universal, as it allows either type of monitor (analog or digital) to be operated from the same connector.
The DVI connector on a device is therefore given one of three names, depending on which signals it implements:

DVI-D (digital only)
DVI-A (analog only)
DVI-I (integrated, digital & analog)

The connector also includes provision for a second data link for high resolution displays, though many devices don't implement this. In those that do, the connector is sometimes referred to as DVI-DL (dual link).
   The long flat pin on a DVI-I connector is wider than the same pin on a DVI-D connector, so it isn't possible to connect a male DVI-I to a female DVI-D by removing the 4 analog pins. It is possible, however, to connect a male DVI-D cable to a female DVI-I connector. Many flat panel LCD monitors have only the DVI-D connection so that a DVI-D male to DVI-D male cable will suffice when connecting the monitor to a computer's DVI-I female connector.
   DVI is the only widespread video standard that includes analog and digital transmission options in the same connector. Competing standards are exclusively digital: these include a system using low-voltage differential signaling (LVDS), known by its proprietary names FPD (for Flat-Panel Display) Link and FLATLINK; and its successors, the LVDS Display Interface (LDI) and OpenLDI.
   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 copy protection. Computers with DVI video connectors can use many DVI-equipped HDTV sets as a display; however, due to Digital Rights Management, it isn't clear whether such systems will eventually be able to play protected content, as the link isn't encrypted. USB signals are not incorporated into the connector, but were earlier incorporated into the VESA Plug and Display connector used by InFocus on their projector systems, and in the Apple Display Connector, which was used by Apple Computer until 2005.

Specifications

Digital

Minimum clock frequency: 21.76 MHz

Maximum clock frequency in single link mode: Capped at 165 MHz (up to 3.96 Gbit/s)

Maximum clock frequency in dual link mode: Limited only by cable quality (up to 7.92 Gbit/s)

Pixels per clock cycle: 1 (single link) or 2 (dual link)

Bits per pixel: 24 (single and dual link) or 48 (dual link only)

Example display modes (single link):

HDTV (1920 × 1080) @ 60 Hz with 5% LCD blanking (131 MHz)
UXGA (1600 × 1200) @ 60 Hz with GTF blanking (161 MHz)
WUXGA (1920 × 1200) @ 60 Hz (154 MHz)
SXGA (1280 × 1024) @ 85 Hz with GTF blanking (159 MHz)
WXGA+ (1440 x 900) @ 60 Hz (107 MHz)
WQUXGA (3840 × 2400) @ 17 Hz (164 MHz)

Example display modes (dual link):

QXGA (2048 × 1536) @ 75 Hz with GTF blanking (2×170 MHz)
HDTV (1920 × 1080) @ 85 Hz with GTF blanking (2×126 MHz)
WQXGA (2560 × 1600) @ 60 Hz with GTF blanking (2x174 MHz) (30" Apple, Dell, HP, NEC, Quinux, and Samsung LCDs)
WQUXGA (3840 × 2400) @ 33 Hz with GTF blanking (2x159 MHz)

GTF (General Timing Formula) is a VESA standard which can easily be calculated with the Linux gtf utility.

Analog

RGB bandwidth: 400 MHz at -3 dBFurther Information

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