DVI And HDMI

[Gregg Lengling]

Digital A/V Interfaces For A New Age

By Alen Koebel


 
HDMI™ Is Real And Almost Here

In the previous Issue 69, February 2003, of Widescreen Review, I reported on the technology of DVI™ and its successor HDMI™ ("DVI And HDMI: Digital A/V Interfaces For A New Age," Issue 69). That article was written just as the HDMI specification was being officially released and prior to the 2003 Consumer Electronics Show (CES). HDMI seemed then merely an idea waiting to happen. Now however, only a few days after the CES as I write this, the situation is very different. The CES essentially served as the venue for HDMI's public "unveiling," with a number of significant announcements and demonstrations proving that HDMI is not only real but also (almost) here. The show also featured a large number of new displays and source devices with DVI interfaces that include HDCP content-protection technology. Significant enough in their own right, all of these products should be backward compatible with HDMI. This update describes these and other related developments and their importance to the future of consumer A/V products.

An Overview Of DVI And HDMI

I described the fundamentals of both DVI and HDMI in some detail in the previous issue. However, in case you missed that issue or don't have it handy for reference, I will briefly describe the capabilities of both interfaces here.

Digital Visual Interface (DVI) is a standard for digital video signals that was originally designed to connect a PC to a flat-panel monitor. In its most basic and common form, DVI provides enough bandwidth to handle all of the HDTV formats defined by the American Television Standards Committee (ATSC) in uncompressed form as well as computer formats as high as 1600 x 1200 (UXGA) at 60 Hz (higher frame rates are supported at lesser resolutions). It can even handle 1080p/60 (1920 x 1080 at 60 Hz progressive). An extended but rarely used form of the interface, known as dual-link DVI, can handle pixel formats as high as 2048 x 1536 (QXGA) at 60 Hz.

When applied to consumer electronics (CE) products, DVI most often includes an encryption protocol known as High-bandwidth Digital Content Protection (HDCP). Endorsed by most of the major Hollywood studios as well as by the satellite and cable industries, HDCP was specifically designed to prevent DVI signals from being copied. A DVI receiver (most often a display device) must be HDCP-capable in order for it to receive (and display) protected content from an HDCP-capable source (e.g., a set-top box).

When used to connect a PC to a flat-panel monitor, DVI usually transports 24-bit RGB data. However, when it is used to connect CE devices together, in particular those that are high-definition-capable, it also allows the transmission of digital component (YCbCr) data. This "version" of DVI, which also includes HDCP, is commonly identified as "DVI-HDTV."

High Definition Multimedia Interface (HDMI) is a new digital interface based on DVI that was specifically created for use on consumer products. Even though it is very new —V1.0 of the HDMI specification was officially released only last December—a number of Hollywood studios, satellite TV providers, and CE manufacturers have already endorsed it. A DVI connection, as its full name suggests, carries only picture information (i.e., video and sync). HDMI adds to that the ability to carry multi-channel digital audio—in either compressed or uncompressed forms—and to distribute basic control data between A/V components. To make it even more practical as a consumer interface, it uses a smaller connector than DVI and adds enhanced support for high-definition digital component formats beyond those supported by DVI-HDTV. And, as with DVI-HDTV, HDMI includes HDCP technology for protection of content.

HDMI is backward compatible with DVI. That is, DVI and HDMI devices can be connected together using simple, passive adapters, with the only loss being the new features added by HDMI (such as the transport of audio and control data). This is important given the large number of consumer products currently available with DVI interfaces. Most of these should not become obsolete if and when HDMI replaces DVI in newer products.