Digital Video Interface (DVI) output is a common way of connecting an external video display to a computer or audio-visual system with a standalone graphics card containing a DVI output port. DVI produces an RGB (Red-Green-Blue)-based video signal in either a digital or backward-compatible VGA analog output.
There are distinct differences in utility based on port design and signal outputs based on two different pinout designs: DVI-D and DVI-I.
Output: Digital Only vs. Hybrid
DVI-D ports are solely for digital signal output to flat-screen displays, such as LED screens, or digital projection systems. In contrast, DVI-I ports have greater utility since they possess digital signal outputs and contain analog signal outputs that are intended for older, CRT-based (VGA) video displays as well.
Interfacing a DVI-I port to an analog display requires a DVI to VGA adapter.
Each DVI port type has a similar pinout, with exception of DVI-I ports having an additional set of pin connectors in the shape of a square, either bisected by a contactless bar socket or a contactless plus-shaped socket. DVD-D connectors, in contrast, have a lone contactless bar socket in the same area with no additional pinouts.
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DVI ports output digital signals in transition minimized differential signaling format (TMDS), a form of high-speed serial signal output. Each TMDS socket has a limited bit transmission rate and, by extension, can only support a certain number of pixels at a standard 60 Hz screen refresh rate.
When selecting a video card, another aspect to consider is the size of your intended display. DVI ports come in single- and dual-link configurations. The difference is mainly the number of output channels and the maximal pixel array dimensions supported.
Single TDMS links are arranged in groups of four; three links, each with a red (R), a green (G), and a blue (B) image signal, and a fourth transmitting a pixel clock that synchronizes the RGB signal outputs.
Single- and Dual-Link DVI Ports
Single-link DVI ports (with 4 TMDS links) typically support resolutions up to 1920 x 1200 pixels, whereas dual-link ports (with 8 TMDS links) can support up to 2560 x 1600-pixel formats. For DVI-I port analog output, resolutions up to 1920 x 1200 pixels are supported for VGA displays.
Distinguishing between single-link and dual-link DVI ports is a simple matter of geometry. Single-link ports possess two adjacent 3 x 3 pinouts, whereas dual-link DVI ports possess a single 3 x 8 pinout.
Display Data Channel 2
All video cards with DVI ports contain display data channel (DDC) pins which afford bidirectional communication between the output and display devices. Newer DVI-I video cards use DDC2, a communication format that reads the extended display identification data (EDID) signal and will automatically match the correct signal output.
DDC2 will only respond to one form of detected output, digital or analog. Be advised that attaching both an analog cable and a DVI cable to some video cards may cause an EDID signal conflict, selecting the default output for the board. To prevent conflicts, it is advisable to only connect one output cable at a time.
Video Cards with two ports
Some video cards have two output ports, one digital (DVI-D) and one for analog (VGA). However, this does not necessarily mean you can run two monitors from the card. Most cards switch the output between one port or the other and cannot support outputs to both at the same time.
If you do require simultaneous monitor output and wish to run them from a single video card, consult the product specifications or call the technical support line from the manufacturer before buying.
Cables and Signal Strength
DVI cables around 3 meters in length have effective cable shielding for resolutions up to 1920 x 1200 pixels. If you require higher pixel resolutions or greater distances, you should do one of the following:
- Use larger diameter DVI cables
- Install a DVI signal extender
The latter option can be an expensive solution but might be the only viable solution for extended transmission distances (beyond 15 meters). We recommend larger diameter cabling as a start since it is slightly less expensive and simpler to implement.
If you are building your own system: Always consider the power consumption of your video card with the overall system power requirements to ensure that your power supply produces enough wattage to run the entire system properly. More robust video cards may require you to replace your existing power supply if you are upgrading your system.
Which is better?
Performance for either DVI port system is comparable. Your choice of DVI-I or DVI-D is simply a matter of whether you will ever need to connect to a VGA monitor. If so, DVI-I is your choice, due to its flexibility.