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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). VESA has approved DisplayPort as the new license-free
successor to DVI.
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 will 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 did not 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.
However, when using digital displays (such as LCD and TFT) with analog signals
(such as VGA), there is an array of discrete 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 center of each pixel, and there are also problems with crosstalk.
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 is 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.
There is a length limitation of 15-foot (4.6 m) in DVI cables. 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.
Extensions
In Radeon HD, audio signals are carried through DVI when the video card detects
a connected HDMI display, which is connected via the HDMI adapter which is
optionally supplied by the manufacturer (it appears that the 2400 Pro models do
not come with the required adaptor).[1]
Connector
Enlarge picture
Color coded DVI connector with pin descriptions
The DVI connector usually contains pins to pass the DVI-native digital video
signals. In the case of dual-link systems, additional pins are provided for the
second set of data signals.
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 (digital & analog)
The connector also includes provision for a second data link for high resolution
displays, though many devices do not 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 longer than the same pin on a DVI-D
connector, so it is not 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 screen 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 is not clear whether such
systems will eventually be able to play protected content, as the link is not
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 (3.7 Gbit/s)
* Maximum clock frequency in dual link mode: Limited only by cable quality (more
than 7.4 Gbit/s)
* 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)
* 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, 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 dB
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