Defining [DVI] Digital Video Interface
researched by: John Vellone
WHAT IS DVI ?
DVI means Digital Visual Interface.
DVI is the most recent video interface designed to optimize the quality of a PC’s image on a flat panel LCD. It is a replacement for the short lived Plug & Display standard and a step up for the DFP format for first generation fully digital flat panels. Slowly DVI is becoming more and more popular with video card manufacturers, and most video cards today include both VGA and DVI outputs.
Digital Visual Interface was developed by the Digital Display Working Group (DDWG).
THE DVI STANDARD:
The DVI [Digital Visual Interface] is a standard for high-speed, high-resolution digital displays. Below is a 24 Pin DVI Female connector seen at the Computer. The connector diagram and pin-out table show a DVI-I Digital and Analog [RGB]; 29 pins [modified D style] connector. A DVI-D Digital only connector with 24 pins [modified D style] looks about the same with out the 4 analog ‘C’ pins.
There are at least three common types of DVI connections:
DVI-D (Digital) – Subdivided in single and dual link
DVI-I (Integrated Digital/Analog)
DVI-D – Fully Digital
DVI-D is used for direct digital connections between the video card and digital LCD. Theoretically this would offer a higher quality image than would be possible with analog (VGA). Internally, the video image generated by your computer is in digital format, to output the image to a monitor the video cards used to convert this digital information into RGB analog signals and send them to a monitor on a “multi-coaxial” cable. All CRT monitors are quite happy to use this analog signal directly because inside they employ analog circuits. However to display the VGA image on an LCD, the LCD has to first convert the information back into digital format (by high speed sampling). Most LCD’s do a great job of this but the idea behind DVI is to eliminate this double conversion to analog and back.
The drawback to this is that since the clock rates of the signals inside the cable are extremely high (several hundred mega bits per second), this puts a high demand on the construction of the DVI cable (shielding, twisting, impedance match, etc) and places a relatively severe limitation on the cable length (15 ft max). This makes DVI suitable for desktop use but not so great for use in switching splitting and transmitting over long distances (though all this is still possible given enough $$$)
SINGLE-LINK vs DUAL-LINK !
The DVI-D and DVI-I formats (described below) are available in either Single or Dual link formats. Dual-Link cables are backward compatible with Single-Link cables and allow you to achieve very high resolutions if your video card and LCD support it. The general rule of thumb is that single link cables are good to 1920 x 1080, while dual link cables can achieve a resolution of 2048 x 1536. So, check your LCD specs, if it says single link, then there is no point in getting a dual link cable (not quite true, since you can use a dual link cable later if you upgrade your LCD). The price difference between a single link an dual link cable is usually only a few bucks.
DVI-A – Analog
DVI-A format is used to carry a DVI signal to an analog display, such as a CRT monitor or an HDTV with a traditional VGA (HD15) connector. The DVI connector has reserved pins for the old analog RBG signals, given the fact that your DVI video card outputs these analog signals (check with your card mfg) the DVI-A cable is used to carry these RGB (and sync) signals to your display.
DVI-I – Integrated Digital and Analog
Although we have not listed any DVI-I cables in the above table, HRT can provide this type of DVI cable upon request. DVI-I cables are used for transmitting either a digital-to-digital signal or an analog-to-analog signal, in fact it has a DVI connector on both ends and can be used in place of a DVI-D cable. It will not work for transmitting a digital-to-analog or vice versa. Some LCD’s DVI inputs are purely digital and some may be labeled ‘DVI-I’ which means that the LCD can accept both DVI-D and DVI-A source signals.
HOW DO I DETERMINE WHAT KIND OF DVI FORMAT MY LCD OR VIDEO CARD SUPPORT?
You have to check your documentation that comes with the equipment. Most video cards output DVI-I format which means that you can connect them to a purely DVI-D, or DVI-A, or even VGA LCD. The trend for LCD’s is that they are mostly DVI-D. That explains why DVI-D cables sell the most.
But to help you try to identify the type, first locate the flat horizontal ‘blade looking’ pin to one side of the connector. This can help tell you if you are dealing with digital or analog. A flat pin with four surrounding pins is either DVI-I or DVI-A, while a flat pin alone denotes DVI-D.
The rest of the pins tell you if you are dealing with single- or dual-link. A solid 27-pin set (rows of 8) usually means dual-link, while two separated 9-pin sets (rows of 6) identifies a single-link cable.
One more thing, if you have a flat pin with four surrounding pins, then a solid 27-pin set indicates a DVI-I, while separated 8-pin and 4-pin sets means DVI-A. GOOD LUCK !
DVI CONNECTOR PINOUT:
|Pin #||Signal Name||Pin #||Signal Name||Pin #||Signal Name|
|1||TMDS Data2-||9||TMDS Data1-||17||TMDS Data0-|
|2||TMDS Data2+||10||TMDS Data1+||18||TMDSData0+|
|3||TMDS Data2/4 Shield||11||TMDS Data1/3 Shield||19||TMDS Data0/5 Shield|
|4||TMDS Data4-||12||TMDS Data3-||20||TMDS Data5-|
|5||TMDS Data4+||13||TMDS Data3+||21||TMDS Data5+|
|6||DDC Clock [SCL]||14||+5 V Power||22||TMDS Clock Shield|
|7||DDC Data [SDA]||15||Ground (for +5 V)||23||TMDS Clock +|
|8||Analog vertical sync||16||Hot Plug Detect||24||TMDS Clock –|
|C4||Analog Horizontal Sync||—||—||—||—|
|C5||Analog GND Return: (analog R, G, B)||—||—||—||—|
The pinout table above provides the pin out for a DVI-I connector, supporting both digital and analog signals. Single Link DVI uses 12 of the available pins providing a maximum bandwidth of 165MHz. Single Link connectors do not use pins 4, 5, 12, 13, 20 and 21. Dual link DVI is implemented when all 24 pins are used providing 2 x 165MHz bandwidth. The analog signal are implemented by the ‘micro-cross’ pins; C1 – C5.
The DDC interface pins use the I2C interface bus, as DDC Data for SDA, and DDC Clock as SCL.
TMDS; Transition Minimized Differential Signaling
The DVI-I connector may be used with a VGA connector by way of an adapter, while the DVI-D [Digital only] is not compatible.
An Apple computer may have either a DVI connector as described above or a Apple display connector [ADC] which is an Apple proprietary interface connector.