When HDTV became a reality in the late 1990s, it was found that a new type of video interconnect was necessary to carry the signal from the digital tuner or set-top box to the HD TV or monitor. This was due to several factors. First, the HD signal has a much higher resolution than regular NTSC television (re-dubbed 480i by the Advanced Television Standards Committee) and, second, the colors must be further isolated from one another to avoid unwanted video artifacts. To address these needs, the industry turned to the component video (PbPrY) and VGA connector standards that had been used by the monitors of video professionals and home computer users.

Enter the Digital Video Interface (DVI) Cable
As computer makers migrated from analog CRT based monitors to digital LCD flat-screens, it became apparent that a digital interconnect between the video card and the monitor would yield a sharper, cleaner picture. As LCD and plasma video monitors took their place alongside such new technologies as DLP, the need for a pure digital solution between the video source and the screen was deemed a high priority.

Taking a page, once again, from the computer world, the digital television industry adopted DVI (Digital Visual Interface). DVI consists of a single cable, like a VGA cable but with a different connector. There are three types of DVI cables now in use. The first is called the DVI-D interface. These cables are used for direct digital connection between your source video such as a set-top box or a computer video card and your HD TV or monitor. Due to the nature of digital, and the lack of a requirement to convert the already digital signal to analog, and then turn it back to digital at the monitor, this interconnect provides for the highest quality signal transfer.

The second type of DVI cable is referred to as DVI-A. This cable transmits an analog video signal to the HD TV or monitor. Due to the double conversion mentioned above, this method is inferior to the DVI-D; however, it still has better bandwidth and can transmit a higher quality picture than either a VGA or component video cable.

The third type of DVI cable is called the DVI-I. This is an integrated cable capable of carrying either a digital-to-digital signal, or an analog-to-analog signal. This versatility makes this the ideal cable for today's digital television, where a number of mix-and-match digital and analog sources coexist in the same system. It has been adopted by a number of manufacturers.

Digital and analog DVI formats are not interchangeable. This means that a DVI-D cable will not work on a DVI-A interface. DVI-D connectors are compatible with only other DVI-D cables or DVI-I, so it is necessary to figure out which your equipment needs before purchasing.

One of the major drawbacks to DVI is the limited cable length. Since the digital signal must be exact in it's timing and waveform integrity, there is a limit to how long of a run a DVI cable can support. Cables, by nature are very reactive having a certain impedance per foot. This impedance is made up of both capacitive and inductive reactances, both of which affect digital signals. At the present, 16 feet or 5 meters is seen as the theoretical limit for DVI-D, although this is influenced mostly by the video signal's ability to drive the cable. So a set-top box or other component with the ability to source more current than another would be able to drive a longer cable than another component with less drive capability.

Single or Dual Link
To add to the complexity, there are actually two DVI standards. One is called Single Link and the other is Dual Link. Both of these protocols send information using a digital information format called TMDS (Transition Minimized Differential Signaling). The difference is that Single Link cables utilize one 165 Mhz transmitter, while the Dual Link system uses two. Double Link DVI standards essentially double the bandwidth of the cable's signal carrying capability. For instance, a Single Link DVI can support a digital resolution of 1920 X 1080 at 60 Hz (about 2.5 Gbps), while a double link can support 2048 X 1536. While it is clear that a Single Link system is adequate for today's High Definition and Progressive Scan DVD video signals, a change from today's 1080i HD standard to the proposed 1080p (progressive) would require the use of the Double Link system.

Indentifying the DVI Cable You Need
Knowing the correct DVI cable to use for your application is essential if you are going to obtain the correct cable the first time. Check both of the female DVI plugs on the source and the monitor to determine what signals each is compatible with.

• If one or both connections are DVI-D, use a DVI-D cable.
• If one or both connections are DVI-A, use a DVI-A cable.
• If one connection is DVI-I or DVI-A and the other is VGA,, use a DVI-I or DVI-A to VGA cable or a standard DVI-I or DVI-A cable and a VGA adaptor.
• If both connections are DVI-I, use any DVI cable; however, to take full advantage of the equipment use a DVI-I cable.
• If your monitor is DVI-A and your source is DVI-D, you will need a converter to convert the digital signal to an analog signal.

How to Identify Your Digital Connectors

A Television Only Standard
While DVI-I Single Link is the current standard for High Definition digital television, a new connector was introduced in late 2004 that offers great promise for consumer products. Called HDMI (High-Definition Multimedia Interface), it is the first industry supported, all-digital audio/video/control interface. HDMI supports all ATSC digital television formats; standard, enhanced, high-definition digital video, as well as multi-channel digital audio and device control, all on a single cable.

The advantages of HDMI to the consumer are readily apparent. The wide bandwidth of HDMI (5 Giga-bits/second) is twice what is needed for HDTV and thus gives the format room for future advancement and added capabilities. Maximum cable lengths have been increased from 5 meters to 15 meters. The interface is simple and easy to use. Since video, audio and device control are carried by one cable, connecting equipment is convenient, easy, and without fear of error. Controlling each piece of equipment with one remote control may soon be a reality as HDMI devices network control information. And imagine your TV being smart enough to tell the DVD player what aspect ratio to send it. This is what HDMI offers.

- HDMI (formerly DVI-CE) is a digital only connection using either a 19 pin (Type A) or 29 pin (Type B) connector. It can support video, audio, control information in aggregate up to 5 Gbps.

Compatibility
One of the goals of HDMI was to be totally backwards compatible with DVI-D. Adaptors and cables are available to go in either direction so that HDMI equipped digital TVs will display video received from existing DVI-D equipped products and DVI-D equipped TVs will display video from HDMI sources.

HDMI, Designed with Security in Mind
Since virtually the entire video industry (Panasonic, JVC, Hitachi, Mitsubishi, Sony, Philips, Thomson CSA (RCA) and Toshiba, to name a few) is behind the HDMI format including content producers such as Fox Universal, and Columbia, as well as, distribution operations such as DirecTV, Dish Network, and Cable Labs, HDMI is set to pave the way for the distribution of High Definition DVD as well as for recordable (for personal use) HD broadcasts. This is because HDMI, as well as more recent DVI connections, directly supports HDCP or High-Definition Content Protection, a new digital copyright protection scheme that has the advantage of being blessed by the content producing community.

Setting the Bit
HDCP is a very secure protocol that works by an exchange of digital passwords or "keys" between the program source and the monitor. The source device will query the monitor to make sure that it is a valid HDCP device before the video is enabled. Non-HDCP sources will work with any digital video display including an HDCP enabled one, but HDCP sources will only work with DVI or HDMI compatible monitors. However, If the protection bit is not set by the content provider in the data stream, the HDCP protection is not activated. If the programming is not copy protected, the HDCP enabled components will work just like normal DVI devices. Any DVI or HDMI equipped component purchased today is already HDCP enabled, but most content providers are not yet enabling the HDCP bit. This will likely change when HD DVDs and stand-alone digital High-Definition video recorders come on the market.