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DVI and HDMI: The Short and the Long of It

By Steve Somers, Vice President of Engineering

Most DVI input cables for computer monitors are two meters long just like the typical analog VGA input cable. In our world of systems design, two meters is too short. For AV digital interfaces to become commonplace, the norm will not be these simple, desktop hookups. Systems designers and integrators require long run distance capability just as they possess with analog signals. HDMI promises to extend the digital interface connection distance while providing additional, integrated signal support. Haven't heard of HDMI or, perhaps, not sure how it fits with DVI? The thrust of this article is to bring into focus the primary advantages, or disadvantages depending on your point of view, of HDMI, the digital High Definition Multimedia Interface.

DVI, or Digital Visual Interface, is designed for point-to-point connection with the plug-and-play concept in mind. Basic DVI only supports the computer video graphics interface... no audio interface, copyright protection, or any other feature set. While the DVI chip set has evolved to support audio and security, the physical size of the interface connector does not lend itself to system installations and consumer convenience.

DVI to dvi?

Figure 1

HDMI encompasses the original DVI electrical interface topology known within the electronics industry as TMDS, or transition-minimized differential signaling. Consider HDMI as a superset which includes DVI along with digital audio support at many rates including surround sound, copyright protection, and consumer control—all packaged into a connector about half the physical size of the original DVI connector. Perhaps DVI should now be referred to as dvi? The HDMI consortium, www.hdmi.org, formed in 2002 and released the initial specification that same year. In 2004, version 1.1 of the specification released. Refer to Figure 1 for HDMI basic performance parameters. The notion is that the HDMI is the consumer electronics single interconnect solution which compacts all needed electrical interfaces into one small package consumers consider easy to use, for example, one cable and Hollywood content protection pundits will endorse. Certainly, a smaller connector is better from a systems installation and integration point of view. HDMI utilizes a new, smaller 19-pin plug for single link DVI, or 29-pin plug for dual link DVI. As actual implementation scenarios unfold, I expect that the new HDMI connectors will most likely stay at the source or destination as short I/O patch cables; and not have as much practical impact on the ability of systems integrators to install HDMI distribution. We'll look at that a little later.

Most devices will only need the single-link DVI type support. As we'll see later, the single link solution supports most computer graphics rates people use along with all the popular digital television formats, including HDTV. In addition, HDMI supports more than just a point-to-point connection. This will be described shortly.

Figure 2

HDMI physical connections require precision shielded twisted pair cable. A cable is comprised of four shielded twisted pairs – one for the source clock signal and three for digital data – along with five individual wires for power, sub-communication functions, plus a ground reference for those functions. Dual link HDMI connections require the aforementioned plus three more STPs for additional TMDS data channels. HDMI still requires the same encoding, speed, and data management protocol as DVI.

HDMI supports both computer graphics and digital television formats. For DTV support, the reader is encouraged to read EIA/CEA 861B, which describes the widened scope of HDMI to include support for digital television and control features along with the traditional expectations for computer graphics support. A single-link HDMI has sufficient data bandwidth for all versions of SDTV and HDTV. It will support up to 1080p, 60 Hz. For computer graphics rates beyond 1600x1200/60 Hz and 1920x1080p/60 Hz, a dual-link system is needed. Refer to Figure 2 for a listing of the video and audio support.

Promises in the Distance

Have you heard that HDMI promises longer cable length than DVI? Curiously, both the DVI specification and the HDMI specification contain the same electrical performance requirements between the source and the sink, or receiver. So, how is it that HDMI connectivity is promoted to work at up to 75 feet (23m) versus the nominal 16.5 feet (5m) for the basic DVI? The answer is that the industry is learning how to make better cables for this type interface technology. Better cable manufacturing methods for precision shielded twisted pair cable coupled with consumer electronics volumes will make longer, low-cost HDMI cables a reality.

Run distance becomes critical due to particular timing tolerances that must be maintained within the DVI/HDMI specifications. Aside from the transmitter and receiver requirements where consistency should be straightforward, the cable becomes the largest variable affecting performance in the transmission system. Twisted pair cables, which DVI/HDMI uses, are subject to two types of skew which, when excessive, create timing error and cause data dropouts. These parameters are: intra-pair skew, with time differential between the two wires making up the pair itself, and inter-pair skew, with time differential between separate twisted pair lines within the cable assembly.

Figure 3 on the following page shows the skew time budget for all the components in the system, including the cable. Note that the total allowable intra-pair skew is only 0.8 of a bit cell, or about 4.8 picoseconds. Inter-pair skew may not exceed 1.20 times that of a pixel, or about 7.2 nanoseconds. Another parameter of high importance is far-end crosstalk, or FEXT. With STP, this is normally very low, but significant care is required at the receiver termination point. At least 5 – 6 dB of margin between cable loss and FEXT must be maintained for reliable operation.

Figure 3

From these small allowances, it should come as no surprise that cable construction for DVI and HDMI is of paramount importance. Typical inter-pair skew within a category 5 style cable is easily greater than 7.2 nanoseconds; but, in that cable, skew is introduced to minimize crosstalk. De-skewing data within networks is the burden of the data receiver. But, what about run distances past 75 feet? Certainly, a better solution is needed than even what HDMI promises. Read on.

Arrest Fear, Not Pirates

HDCP is an acronym for High-Bandwidth Digital Content Protection, which is another subsystem of the HDMI. Released in 2000 and mapped directly into the DVI, HDCP provides data security for the interface and, hopefully, arrests the piracy concerns of digital content providers. HDCP protocol is implemented partly via a two-way communication link within the I2C control interface used by the DDC Data Display Channel and the high speed TMDS connections used to deliver image information. The DDC is the existing link in the DVI where the source graphics system communicates with the display device to determine its resolution capabilities.

Figure 4

HDCP is robust and rivals the complexity of TCP/IP SSL, or secure socket layer, used for internet security. An authorized HDMI device will contain forty 56-bit secret key values along with a special identifier called the key selection vector, or KSV for short. These values, or codes, are provided by Digital Content Protection LLC, the designated third party set up to provide trusted keys to HDMI-licensed product manufacturers. Each time an HDMI device is connected in a system, a three-part authentication routine automatically occurs. First, shared values, or codes, between devices are exchanged; second, the KSV of each receiver is reported to the source; third, frame-by-frame ciphers are sent to the receivers that enable data decoding.

Referring to Figure 4, HDCP supports interconnection of devices via a hierarchy of sources, sinks, or receivers, and repeaters. All devices in an HDCP system communicate through a protocol designed to allow digital content to travel only to those devices which the source determines have the authorization to receive such content. The hierarchy supports seven levels of repeaters and up to a total of 128 devices. Repeaters only pass HDCP protocols two ways between sources and receivers without acting on data. However, the source must authenticate a repeater to be sure it is authorized to handle secure content traffic.

Control, Yourself

Through EIA/CEA 861B, there is a whole new frontier of optional control capability embedded within HDMI for the consumer, as well as us AV systems types. The embedded Consumer Electronics Control, or CEC, system currently provides fourteen functions that orchestrate plug-and-play control among consumer products. The CEC function is handled via a separate wire connection within the HDMI and contains its own protocol and quality of service, or QoS.

For example, in an installation where all devices use the HDMI, its daisy-chain connection methodology may connect a DVD player through an AV tuner/receiver to a television set and, thereby, provide the consumer the ability to seamlessly turn on all downstream devices from the DVD player, signal them to switch to the appropriate input, and launch the movie by simply pressing the PLAY function on the DVD player. The concept of CEC is simple, but the level of effort to implement all the functions in the HDMI is significant; although CEC implementation is optional. This notion leads to the requirement for some form of interoperability testing.

Can You Say: Interoperability?

In order to promote consumer confidence and device compatibility, HDMI adopters are bound by the compliance agreement which requires successfully passing specified interoperability testing before a product may bear the HDMI logo.

The Authorized Testing Center, an independent organization, performs the initial product compliance tests for products within the designed product categories. Within the HDMI specification, a product falls into one of the following base categories: sink, source, repeater, or cable. Once tested by the ATC, all succeeding products of the same category may then be tested and self-certified by the manufacturer of the product. Compliance to interoperability within the HDMI specification is mandated.

Life Past 75 Feet

Yes, for you system designers, there is HDMI life past 75 feet. Be on the lookout for new HDMI/DVI interface products that essentially double or quadruple the run length. A new world of interface components has been developed to take advantage of cheap UTP network cable. Runs to about 150 feet are now possible for both single link and dual link DVI and HDMI connections. The typical single link solution requires two runs of UTP: one for the video data and the second for all support communication functions. Even with two cables, it's cheap at twice the price. Just add a third UTP cable for the second link as long as your interface device supports dual link functionality.

For the really distance-challenged, several relatively low cost fiber-based solutions allow run distances to about 300 feet. Some of these laser diode-based interfaces are kept in the low cost realm by fixed connection of the fiber to the interface plug assembly. These solutions are not very installer friendly for runs requiring conduit, but may work out in scenarios where the fiber cable may run unprotected. Slightly more expensive solutions utilize the typical ST or SC fiber cable connectors for simple, reliable connections and installation within raceways and conduits using the fiber cable of your choice.

New Acronym to Learn: DBW

If you haven't already noticed this in the Sunday newspaper ads, take a close look at the detailed pitch for new flat screen televisions and rear projection DLP/LCD sets. Most now have HDMI inputs. Potential customers probably don't know what HDMI stands for, but it sounds important. To the customer, some ads pitch digital simplicity, one cable connection, and ultimate picture quality. So, for our industry, here's a new acronym for you... DBW, the "Digital Band-Wagon".

Digital interfacing has come full circle from the days of CGA/EGA graphics but with a completely new foundation streamlined to handle a lot more information and features than just the video image. It's here to stay; so we, that is, you and I, must embrace it and find ways to make it work for us in AV systems design. It's time to get on the Digital Band-Wagon or be left behind.

For Additional Information

HDMI Organization – specifications download:

DVI Specification – download:

TMDS Information:

Video Electronic Standards Association:

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