Digital Visual Interface

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DVI_Connector.JPG
DVI Connector

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).

Contents

Overview

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.

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. 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, without packetization. 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 a second, separate link. 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. The DVI specification mandates a fixed single link cutoff point of 165MHz, 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 165MHz. 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.

Dual link can transmit about twice the amount of pixel information of single link, and thus is utilized for extremely high resolution displays, essentially being anything over 2.3 million pixels. Apple Computer's 30-inch Cinema HD Display was one of the first displays to use this connector, as its highest resolution was 2560 x 1600, which is well over 4 million pixels.

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

Connector types

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Dvi_types.png
DVI connector types (view of plug)

The DVI connector incorporates pins to pass through analog signals using the VGA standard. This feature was included in order to make DVI universal. The inclusion of the VGA pins allow either type of monitor (analog or digital) to be run from the same connector, however, not all DVI connectors implement them. Later, as high resolution displays became more common, the Dual link connector was introduced.

This means there are 4 types of DVI connectors:

  • DVI-D (digital only)
  • DVI-A (analog only)
  • DVI-I (integrated digital and analog)
  • DVI-DL (Dual link)

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 of the Apple Display Connector is electrically compatible with the VESA P&D/M1, but physically the shell of the connector is a different shape.

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.

Specifications

Digital

  • Minimum clock frequency: 21.76 MHz
  • Maximum clock frequency in single link mode: Capped at 165 MHz
  • Maximum clock frequency in dual link mode: Limited only by cable
  • Pixels per clock cycle: 1 (single link) or 2 (dual link)
  • Bits per pixel: 24
  • 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)
    • SXGA (1280 × 1024) @ 85 Hz with GTF blanking (159 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)
    • 2560 × 1600 pixels (on 30" LCD)

GTF == Generalized Timing Formula (a VESA standard)

Analog

  • RGB bandwidth: 400 MHz at -3dB

Connector

Pin numbers (looking at socket)
12345678 C1C2
910111213141516C5
1718192021222324C3C4
Pin assignments
Pin Name Function
1 TMDS Data 2- Digital red - (Link 1)
2 TMDS Data 2+ Digital red + (Link 1)
3 TMDS Data 2/4 shield
4 TMDS Data 4- Digital green - (Link 2)
5 TMDS Data 4+ Digital green + (Link 2)
6 DDC clock
7 DDC data
8 Analog Vertical Sync
9 TMDS Data 1- Digital green - (Link 1)
10 TMDS Data 1+ Digital green + (Link 1)
11 TMDS Data 1/3 shield
12 TMDS Data 3- Digital blue - (Link 2)
13 TMDS Data 3+ Digital blue + (Link 2)
14+5V Power for monitor when in standby
15 Ground Return for pin 14 and analog sync
16Hot Plug Detect
17 TMDS data 0- Digital blue - (Link 1) and digital sync
18 TMDS data 0+ Digital blue + (Link 1) and digital sync
19 TMDS data 0/5 shield
20 TMDS data 5- Digital red - (Link 2)
21 TMDS data 5+ Digital red + (Link 2)
22 TMDS clock shield
23 TMDS clock+ Digital clock + (Links 1 and 2)
24 TMDS clock- Digital clock - (Links 1 and 2)
C1 Analog Red
C2 Analog Green
C3 Analog Blue
C4 Analog Horizontal Sync
C5 Analog Ground Return for R, G and B signals

See also

External links

nl:DVI fr:Digital Video Interface

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