DVI comes in two ?classes?: single and dual link. Just to make things interesting, it is also important to note that DVI comes in three different ?formats?: DVI-D (digital only), DVI-I (digital and analog), and the so-called DVI-A (analog only).

An Introduction to DVI and Digital Connectivity

| DVIGear Technology Connection

An Introduction to DVI and Digital Connectivity
from DVIGear Technology Connection

DVI comes in two "classes": single and dual link. Just to make things interesting, it is also important to note that DVI comes in three different "formats": DVI-D (digital only), DVI-I (digital and analog), and the so-called DVI-A (analog only).


These days new connector types are showing up with alarming frequency on all types of consumer electronics equipment. Whether you look on notebooks or desktops, DVD players, cable boxes or satellite receivers, or even on TV's, flat panel displays or projectors, one is likely to encounter a bewildering array of connectivity designed, it seems, just to confuse us.

In the evolving world of high-tech displays, new connector types also abound, but most are of great significance and value to consumers. This is especially true with the latest wave of digital video connections found on everything from today's newest flat panel monitors to HDTV sets.

The technology behind these connections allows for a direct digital interface between your source and your display, which provides stellar pixel-for-pixel accuracy, and improved image quality that is simply impossible to achieve with conventional analog technology. If one peers more deeply, you'd see that inside your computer the video information is processed digitally and sent to the display. In the old days, CRT monitors would process the signal in the analog domain; however, with today's flat panel display, this processing is done digitally. The process of converting the digital signal in the PC from digital to analog and then back to digital again is incredibly inefficient and results in noise, sampling errors and other artifacts that can visibly degrade the picture quality. In an all-digital system, the signal stays digital from the PC to the display without the need to be converted to the analog domain. Eliminating this circuitous conversion process results in noise-free images with perfect color and pixel-perfect accuracy.

In addition to more accurate transmissions, digital connectivity also allows for higher bandwidth transmissions, which equates to greater on-screen resolution. As higher resolution sources and displays are created, bandwidth transmission through traditional analog cables becomes more problematic. In most applications, the cable becomes the limiting factor as its internal capacitance acts like a low pass filter, degrading the signal bandwidth (especially high frequencies) as cable lengths are increased.

At the forefront of PC-based digital displays is a connectivity standard known as DVI (Digital Visual Interface). DVI comes in two "classes": single and dual link. Just to make things interesting, it is also important to note that DVI comes in three different "formats": DVI-D (digital only), DVI-I (digital and analog), and the so-called DVI-A (analog only). The reason for these permutations was that originally the DVI-I format was designed to be a backward-compatible format that could provide support for older analog equipment such as CRT monitors. Whereas the DVI-D and DVI-A formats have only one (digital or analog) signal present, the DVI-I format has both an analog and a digital signal available concurrently. Because of this flexibility and improved performance DVI has been well received as a new connectivity standard.

Single link DVI supports a bandwidth of up to 165 MHz, whereas dual link can support a bandwidth of twice that (up to 330 MHz). To approximate your bandwidth requirements, you can simply multiply your horizontal resolution by your vertical resolution by your refresh rate. For example, if your PC is running at XGA resolution (1024x768) with a 60 Hz refresh rate (see your monitor settings on the Control Panel), then 1024 x 768 x 60 = 47,185,920 or about 47 MHz. Since this is less than 165 MHz, we can correctly deduce that a standard XGA display signal will easily transmit over a single link DVI connection. In fact, a single link DVI connection can support standard HDTV at 720p and 1080i, as well as full 1080p HDTV resolution (1920 x 1080 x 60Hz = 124 MHz)! Dual link DVI connections can support QXGA (2048x1536) and beyond without any problems. Furthermore, the newest DVI processor chips will extend this range to even greater bandwidth at single (225 MHz) and dual (450 MHz) link resolutions. For most consumer applications today, single link DVI is more than sufficient, but that could change in the future…

With greater bandwidth capabilities, cable length limitations will become even more critical. Currently, the standard specification for DVI is up to 5 meters (16.4 ft.). However, high quality copper cables and also fiber optic cables are available that can transport the DVI signal to much greater distances. Fiber cables can easily transmit a DVI signal 100 meters (328 ft.) and beyond! While copper DVI cables are usually limited in length, electronic repeaters can be used to electrically re-generate the signal, thus allowing it to be sent further distances. A repeater accomplishes this task by receiving the incoming signal and then reconstructing a pristine copy that is optimized for re-transmission down long cable runs. Using this approach, repeaters have been used to send DVI signals hundreds of feet when properly placed in the signal chain. The main drawback of using fiber cables or even repeaters is the cost associated with these products.

In the past, there have been only a few manufacturers that could supply DVI cables with lengths over 5 meters. However, more recently, some manufacturers (e.g., DVIGear) have introduced high performance copper cables that can work well up to 15 meters (49 ft.). Currently it is possible to send a high resolution image of 1600x1200 @ 60 Hz across a 15 meter DVI cable with little to no signal degradation.

While DVI is a popular digital display connector, it is certainly not the only type in use today. There are many connector types in circulation, including: DFP, M1, P&D, LVDS, OpenLDI, and ADC to name a few.

One of the older connector types that is in circulation, but isn't appearing on new devices is the DFP type, also known as the MDR20 standard. The DFP connector's demise is largely attributable to its lack of versatility based on two factors: it only offers single link bandwidth and it is digital-only.

One standard that increasingly more popular is the M1 (a.k.a EVC or P&D) standard. This connector allows for the same capabilities as DVI-I, that is to say, digital single or dual link, plus analog transmissions. But in addition, the M1 connector also supports USB (control) and FireWire. These added capabilities are attractive to manufacturers of compact business projectors such as InFocus, Dell and HP as these connectors offer great flexibility and take little space on an input panel.

ADC or the Apple Display Connector is yet another format which is a proprietary design created by Apple Computers. It combines all of the capabilities of the M1 standard - digital single or dual link or analog transmissions, USB (control), and FireWire. However, ADC also includes the ability to power the display through the connector. This allows displays with the ADC connector to have one total connection which allows for a sleek and simple looking display setup.

On the technical side, a common thread of all these connectivity standards is that they are all based on Panel Link Technology from a company called Silicon Image. This commonality allows can provide an easier means of signal adaptation from one format to another. When converting display signals to other non-Panel Link based standards, it can become more costly, and there are more chances for signal degradation.

One non-Panel Link based technology is the OpenLDI, LVDS, or MDR-36, which proprietary to SGI (Silicon Graphics). This connector appears on SGI's 1600SW monitor, a very popular digital monitor. However, for this monitor to be connected to a VGA, DVI, or any other digital source, the signal must pass through a SGI Multilink Adapter, which is no longer being manufactured. Because OpenLDI is not based on Panel Link technology, it is no longer being used by SGI and will eventually completely die out.

Coming back again to DVI connectivity, there are other more subtle aspects that should be mentioned. One aspect that is getting a lot of attention in the world of consumer electronics is HDCP (High-bandwidth Digital Content Protection). Created by chip giant Intel, HDCP is an encryption technology designed to protect digitally transmitted picture content from illegal copying and distribution. Backed by content producers (studios) who strongly desire to protect their investments (e.g. movies, sporting events, TV shows, etc.), HDCP's underlying charter is to protect and maintain control of copyrighted materials. It does so through the use of an encryption process that renders the signal unusable unless a special HDCP decoder is used to decrypt the signal. This decoder relies on the use of special cipher keys which must be implanted in all HDCP devices (transmitters and receivers) so that they can "talk" to each other in a secure fashion; however, a difficulty arises when a non-HDCP compliant device is added into the signal chain. For information on HDCP, see DVIGear's article entitled: "HDCP - For Better or for Worse."

Another noteworthy point of DVI technology is a tiny piece of information contained within the DVI signal known has EDID (Extended Display Identification Data). EDID is small block of data that contains fundamental information about a display (monitor, projector, plasma, LCD, etc.) and its capabilities. The EDID data can include: vendor information, maximum image size, color characteristics, factory pre-set timings, frequency range limits, and character strings for the monitor name and serial number. Some computers require the receipt of this EDID information in order to work properly. In addition, the EDID information allows a computer source to optimize its output for a particular display. In some applications where the sources needs to see the EDID data and something in the application is prohibiting this information from being sent, there are devices that exist that can store the information and trick the computer into thinking that the display is set up a certain way, allowing the application to work properly. These EDID devices can solve many mysterious problems that otherwise can baffle even the most seasoned experts. For more information, please call DVIGear application support.

In closing, let's take a look at a new a rapidly-growing form of digital connectivity called HDMI (High Definition Multimedia Interface). Yet another digital connectivity acronym, you'll surely be hearing more about HDMI in the future. A direct outgrowth of DVI, the HDMI connector is deceptively small and looks more like a jumbo USB type connector. Nonetheless HDMI offers a formidable array of signal connections, including all forms of digital video (HDTV), PCM digital audio (7.1) and bi-directional communication for remote control functions. HDMI is fully HDCP compliant and is 100% backward compatible with DVI. HDMI is truly revolutionary in that it allows for interconnection of sources, receivers and displays, all using a single cable type for digital audio, video and control. Today more and more consumer electronics devices are appearing with HDMI connectivity. HDMI cabling solutions are somewhat more robust than DVI and the connector's smaller form factor has been well received by installers as it is easier to route through internal structures. For more information on HDMI, please see DVIGear's article entitled: "Smaller Connector, Bigger Options".

If you are interested in learning more about digital connectivity or have a special application request, please contact DVIGear for more information.


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