DVI and HDMI: Two New Types of Connectors Create a Cable Quandry

Two New Guys: DVI and HDMI:  by Alan Lofft, axiomaudio.com 

Recently, two new types of connectors—DVI and HDMI—began appearing in the latest generation of HDTV displays and DVD players.

DVI (Digital Visual Interface) is a large, multi-pin plug that carries digital HD video (it may carry analog video as well but it is not a DVI requirement).

HDMI (High Definition Multimedia Interface) uses a smaller plug that looks like an oversized USB plug. HDMI carries the same digital video as DVI but also carries a Dolby Digital audio data stream.

In addition to new HDTV displays, some new HD cable or HD satellite boxes and “up-converting” DVD players may have one or both of these digital video output connectors. The DVD player will up-convert a DVD's 480i or 480p output signals to 720p or 1080i to match the HDTV display's native resolution.

Both DVI and HDMI are high-speed digital connections that keep the video in the digital domain all the way from the HD cable box, HD satellite tuner or DVD player to the HDTV display, eliminating the extra digital-to-analog (D/A) and analog-to-digital (A/D) conversion that normally takes place with component video cables (remember, those are analog video connectors).

The theory sounds impressive, and there has been lots of excitement and hyperbole about these new digital HD connectors. The rationale goes like this: If component-video cables are the best analog video connectors you can use, then DVI or HDMI digital video connectors must be better because they maintain the video in the digital domain all the way to the HDTV display, right?

Not so fast.

The presumption is that if the conversion is done in the digital domain inside the DVD player, avoiding an extra conversion to analog (component video) and back again to digital when it reaches the HDTV display, then improved clarity and color should result. But here's where things get murky. What no-one is talking about is that even though the HD video is up- or down-converted and kept in the digital domain, when the digital video signal reaches the HD display in its native format, the digital TV still has to re-clock the digital video so that it exactly matches the TV's native resolution. And according to informed sources, re-clocking digital video may produce more picture degradation and video artifacts—not less—than a good analog component-video connection.

It's all in the chips
Two other factors come into play with DVI and HDMI that have the potential to cause video artifacts. The quality of internal digital video conversion chips may vary considerably from one product to another. Faroudja, Genesis, and Silicon Optix produce some of the best (and most expensive) digital video processing chips available, but there are lots of cheaper options from other chip-makers that a manufacturer can choose from to keep manufacturing costs down. Moreover, the quality of video A/D and D/A converter chips has improved tremendously, which means that even inexpensive DVD players these days are likely to have excellent video D/A processing through the component video outputs.

Finally, any device with DVI or HDMI outputs must carry a special digital anti-piracy code called HDCP (High-bandwidth Digital Content Protection) to thwart illegal copying. HDCP is mandated for both DVI and HDMI connections and uses an authentication protocol developed by computer chip-maker Intel. It involves the HD digital cable box or HD satellite receiver sending a kind of digital “hand-shake” to the HD receiving device (the TV display) to ensure that it's licensed to receive the HD content. These anti-copy codes are embedded in the digital video data stream and must be removed by the video processor in the HD display.

The result? The display's internal video processor has to do extra digital “work” with no benefits to picture quality, so possible degradation may take place. Using the analog component video outputs bypasses the anti-piracy codes, so the TV display doesn't have to deal with the extra processing.

Cable length vs. image quality
As to lengths of video cable, if the video output drivers of an A/V receiver are well-designed with lots of output, component-video can be carried over 50 feet without degradation. But with DVI or HDMI connectors, engineering and cable theory suggests that running lengths greater than 3 meters (about 10 feet) risks digital drop-outs and resulting pixelization. Additionally, the capacitance factor becomes very important because the bandwidth of the uncompressed digital video signals running in a DVI or HDMI cable is much higher than an HD component-video analog feed, and therefore more critical with longer cable runs. If the error-correction of the receiving device is robust, it may be fine. Otherwise, consider going analog component video.

Do your own tests
One appealing aspect of video, of course, is that you can see the results of picture degradation, so you can try an analog component-video connection to your HD display and compare the results with a DVI or HDMI digital video connection. Look for any visible artifacts—noise in dark areas, frequent drop-outs or freeze-ups of the image—artificial “peaking” or sharpening of the image, with white halos around the edges of objects, as well as the overall sharpness and clarity of the HD images. Use the cable link that produces the most satisfying and realistic movie-like image.