DVI is a versatile digital format that is now widely used in PCs, DVDs, projectors and display devices. The three-letter acronym is short for Digital Visual Interface, and this is one powerful video signal.
DVI was designed by the Digital Display Working Group (DDWG) for carrying an uncompressed digital video signal to a digital display device such as a LCD display or projector.
The protocol delivers the signal in a digital bit stream, sending frame and line information as discrete red, green and blue color components. After adding vertical and horizontal sync information to the blue component, it delivers the package along three data channels in a format called “Transmission Minimized Differential Signaling” (TMDS).
DVI also supports Extended Display Identification Data. EDID is the link between a computer graphics card and the display device, and it provides a “handshake” between the devices.
Many DVI graphics cards won't output video unless there is a DVI display device attached. Most routers, switchers and projectors use EDID to send information back to the DVI source. This gives the graphics card the signal that it’s okay to start sending digital video through the DVI cable. That same EDID information also tells the source device, such as a PC, to output a digital video signal at a specific resolution (i.e., the native resolution of the display device).
There are three types of DVI cable connections: DVI-Digital, DVI-Analog and DVI-Integrated (Digital & Analog): DVI-D eliminates the digital to analog to digital conversion between digital video source (computer) and digital display device; DVI-A converts digital signal to analog for use with VGA display devices; and DVI-I transmits either a digital-to-digital signal or an analog-to-analog signal so it can be used with both types of display devices. Normally, DVI-I will detect and choose the analog or digital portion of the interface depending on what the connected display device tells it to do.
Note that the analog signal is VGA and can be split out with an adapter on a true DVI‑I cable. DVI-I sources communicate with the display to decide on the preferred output for the monitor. It will not send both signals.
DVI cables come in a variety of pin configurations as well. It is important to make sure you have the right cable when you are interconnecting DVI devices. If you are connecting two DVI-D devices, then use a DVI-D cable. If both devices being connected support a Digital DVI connection (DVI-I or DVI-D) and they are compatible in terms of resolution, refresh rates and sync, using a DVI-D cable will ensure that you are using a digital connection rather than an analog connection, without the need to change any settings. You can spot a DVI-D cable easily by its lack of pins surrounding the flat blade.
In addition to the pin configurations for DVI-A, DVI-D and DVI-I, a cable can be either single or dual link. Single link cables use one 165 MHz TMDS transmitter, while dual links use two. The dual link DVI pins effectively double the power of transmission and provide an increase of speed and signal quality. Most importantly, the dual link is used for high resolution displays, such as those beyond 1600×1200 or 1920×1080. The second link adds bandwidth to allow for the higher resolution sources.
As the signal resolution increases — say, HD at 1920×1080 — the bandwidth required to run it also increases and the distance you can send the signal decreases. DVI specs recommend cable lengths up to 16 feet (five meters). For longer distances, the use of a DVI booster is recommended to eliminate potential signal degradation that can manifest as artifacts, flickering or a blank screen.
The potential for signal degradation over a longer digital cable is more of a limitation than for analog cables. Why? When a digital signal is run through a cable, the edges of the bits (represented by sudden transitions in voltage) are rounded off and compromise the integrity of the signal.
This rounding process increases dramatically with distance. Meanwhile, poor control over the impedance results in signal reflections — portions of the signal bounce off of the display end of the line, travel back down the cable and return, causing interference with information that is sent later on in the same bit stream. At some point, the data becomes unrecoverable, and with no error correction available, there’s no way to restore the lost information. Once the data is lost, it is gone forever.
Understanding how DVI works is an important part of a visualist’s responsibility. Things to watch out for are:
1. Hot swapping with DVI cables is not allowed! If you need to unplug a DVI cable you should power down the computer, or you can use a DVI detective to hold the signal.
2. Don’t use DVI to VGA adapters between two digital devices or you will end up with an analog signal.
3. Use short cable lengths, no more than 16 feet. Use a DVI booster if you need longer cable runs.
4. If one connection is analog and the other connection is digital, you’ll have to use an electronic converter box.
DVI is one of the best acronyms among the many found in the video realm. It provides better signal quality and you need a single cable rather than a bundle of component cables, which makes for a faster and easier load-in and load-out. A little knowledge of the protocol will take you a long way down the digital video road. Happy travels.
