Introduction:

The Digital Visual Interface - also referred to as DVI, has been appearing on home theater gear for quite a number of years.

It was originally developed by DDWG (Digital Display Working Group) in 1999. It brought about numerous different types of DVI cables and connectors to support the various signals that can be carried over a DVI interconnect.

However, it did not take long before this whole new mess of interconnects complicated itself even further.

Soon, we saw the advent of a new digital interface that is closely related to DVI - the High Definition Multimedia Interface - or simply HDMI, and with it a whole new range of HDMI interconnects - HDMI to DVI cables, HDMI splitters, adaptors and HDMI connectors.

DVD players, digital cable and satellite set-top boxes, High Definition plasmas and LCD TVs, and home theater projectors, all represent prime targets for these digital connections.

The new technology behind these connectors allows for direct digital connection between a digital source and your display leaving you with pixel-for-pixel accurate images on your display.

It is true that the DVI standard was soon superseded by the HDMI - a standard that is fully compatible with digital DVI, but that also carries digital audio on the same interconnect. Yet DVI is still a very popular interface; in fact, it is estimated that by end 2006, there will be some 50 million DVI enabled devices in use and this figure is expected to reach the 100 million by 2008! This means that the possibility to come across some DVI enabled device is there and will still remain there for the years to come.

The Digital Visual Interface Basics:

The DVI connector can accommodate up to two TMDS (transition minimized differential signaling) links, the VESA Display Data Channel (DDC) and the Extended Display Identification Data (EDID) services, and even the red, green, blue and horizontal and vertical sync lines of an RGBHV analog video signal for backward compatibility with analog VESA video.

The use of TMDS means that DVI is fully compatible with both P&D and DFP (Digital Flat Panel) standards as well as HDMI.

Each link in the DVI standard is made up of 3 data channels for RGB information and has a maximum bandwidth of 165 Mhz; this equates to 165 million pixels per second. Color support is up to 24-bit depth, 8 bits per color. Each incoming 8-bit color data word is encoded by the DVI standard into a 10-bit transition minimized DC-balanced character, meaning that each pixel is represented by a total of 30-bits. This means that effective data throughput over a single link DVI cable is 4.95Gbps while in the case of dual link DVI, max. data throughput is 9.9Gbps.

The only real major limitation of the DVI standard is the supported maximum cable length; this can be a serious hurdle to overcome in a dedicated home theater setup when connecting a DVI source with a ceiling-mounted video projector.

The use of twisted-pair cable instead of coaxial cable to carry high bit-rate digital video without error correction can lead to severe problems with signal degradation over distance.  At long cable lengths, cable quality becomes a critical factor that will affect maximum bit-rate and therefore signal resolution. The specified standard limit for maximum DVI cable length is 5 meters. However, the actual maximum cable length depends on cable quality and signal bit-rate; some manufactures claim that their high quality cables can even run up to 15 meters (50 feet) at maximum bit-rate without loss of signal.

 

The 165 Mhz maximum bandwidth per link on a DVI cable means that full resolution 60Hz - 1920 x 1080 HDTV is well within the available bandwidth on a DVI single link interface. The use of dual link DVI effectively doubles the power of transmission and provides an increase of speed and signal quality. At the same time, this necessitates the use of additional twisted pairs in a DVI cable, and therefore additional pins on the DVI connector to handle the second link.

Worth taking note here that as matters stand today, there are very few devices on the market which actually use the dual-link interface. Going for a dual link DVI cable won't hurt anything, but in most cases, it won't help either, unless it is implemented in both the source equipment and the display device.

Exploring the different DVI Connectors

The DVI connector is a 37.0 mm wide plug. The DVI specifications refer to two different types of DVI cable connectors - depending on the type of signal you need to carry:

• DVI-Digital (DVI-D) supports digital displays only.

• DVI-Integrated (DVI-I) supports both digital and analog displays.

In addition, analog only DVI is often referred to as DVI-A.

The DVI standard specifies a DVI connector based on a 24 pin layout - however only 12 are implemented in a single link DVI interconnect. Further more, the implementation of analog support in a DVI cable leads to an extra four pins plus a ground plane on the DVI-I connector to accommodate the red, green, blue and horizontal sync (R,G,B and H) lines of an RGBHV analog video signal (the vertical sync lies on another pin). The table below depicts the five main DVI connector configurations in common use today.

How to determine the type of DVI Cable to use?

It is this clear that the DVI connector can take various forms. Further more, the fact that it makes use of the same TMDS used in P&D, DFP, and HDMI standards, as well as its backward compatibility with analog VESA video, has led to the development of numerous different types of DVI cables to cater not only for DVI to DVI connections, but also for DVI to analog flat panel monitors and CRTs, DVI to DFP displays, and also DVI to HDMI connections.

Hence, great care should be taken when selecting your DVI cables and connectors. This is critical if you want to get the right product the first time - after all, quality DVI cables are expensive so mistakes can turn out to be costly.

The first step is to carefully examine the connection points on your source and display device to identify the type of female DVI plugs installed on your gear and therefore, the type of signals they are compatible with.

Consult also your manuals before investing in a DVI cable. This is necessary as most often, manufacturers will make the female plugs with all available pins to prevent pins being broken off of mismatched cables. This means that most female DVI plugs installed on home theater equipment will look like dual link DVI-I, but this does not necessarily mean that all pins are actually implemented. Therefore, be sure to look for some label identification, or check the product documentation to make sure you know what type it is.

When selecting your DVI cable, it is important to take note of the following points:

• If you have DVI-D, you can use either a DVI-D or DVI-I cable - provided the female plug on your gear can take the extra pins on the DVI-I male connector.

• If you have DVI-A, you can use either a DVI-A or DVI-I cable - again, this applies as long as the female plug on your gear can take the extra pins on the DVI-I male connector on the DVI cable.

• If you have plugs that are DVI-I, they will accept any type of DVI cable.

• If you have mismatched plugs, such as DVI-D and DVI-I or DVI-A and DVI-I, you may use either a DVI-I cable or the cable that matches the other plug. For example, you may use a DVI-D cable on a DVI-I to DVI-D connection, but you cannot use a DVI-A cable.

Note: You may not mismatch a DVI-D and a DVI-A connection.
 

How to identify a DVI Cable

A most important difference when selecting your DVI connector is analog support - look at the receptacles on your devices to determine if they can take the four pins around the flat blade on the DVI connector.

• A flat blade alone denotes a DVI-D connection.

• If there are two holes above and two holes below the flat blade, then it is either a DVI-I or a DVI-A. In other words, you would then need to determine whether both the digital and the analog modes are enabled on your DVI port -- check your user's manual.

The pin-sets will vary depending on whether the DVI cable is single or dual-link. A 24-pin connector using 3 rows of 8-pins each stands for a dual-link cable, while three rows of 6-pins each represent a single-link cable. However, the exact pins implemented will depend on whether it is a DVI-I single link or a DVI-A; for the exact pin-out details, check the DVI Connection Chart illustrated above.

NOTE: To distinguish from DVI-I and DVI-A, check the pin-sets. A solid 24-pin set is for a DVI-l; a separated 8-pin and 4-pin set is for DVI-A.


Finally, please remember that it is not possible to go from a digital signal to an analog signal.

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