DVI and HDMI: The Short and the Long of It

By Steve Somers, Vice President of Engineering

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?

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 (23 meters) versus the nominal 16.5 feet (5 meters) for basic DVI? The answer is that the industry is learning how to make better cables for this type of 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 shows the skew time budget for all the components in a system transporting 1080p TMDS video, 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.

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 longer distances? 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.

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 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 only a limited number of repeaters and total 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 CTA 861G, 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 Blu-ray 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 Blu-ray player, signal them to switch to the appropriate input, and launch the movie by simply pressing the PLAY function on the Blu-ray 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?

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 system designers, there is HDMI life past 75 feet (23 meters). There are plenty of HDMI/DVI extension products that essentially double or quadruple the run length. A new world of extension components has been developed to take advantage of cheap and easy-to-terminate CATx network cable. Runs up to about 330 feet (100 meters) are now possible for DVI and HDMI connections, even for 4K video. For situations where the full 230 feet (70 meters) or 330 feet (100 meters) of CATx cable extension distance is not required, or if installing new cabling is impractical, HDMI cable equalizer products can be installed at the end of a cable run to restore signal integrity.

For the really distance-challenged, several relatively low-cost fiber-based solutions allow run distances to about 300 feet (90 meters), and distance up to 18 miles (30 kilometers) are possible over singlemode fiber. 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 LC fiber cable connector for simple, reliable connections and installation within raceways and conduits using the fiber cable of your choice.

HDMI vs DVI, or maybe both?

DVI introduced the TMDS digital video transmission format, but the DVI standard itself has never been updated. Instead, HDMI extends the TDMS format and is an actively maintained standard that adds features with every new revision; driving AV technology forward to support higher resolutions, multichannel audio, and quality enhancements such as HDR. DVI doesn’t appear on many new displays anymore, but the physical DVI connector has the advantage of being mechanically secure, whereas the standard HDMI connector doesn’t have a locking mechanism. Those who need the enhanced features of HDMI but prefer the more solid DVI connection will be glad to find that many Extron products with DVI connections support HDMI features. Since the underlying digital transmission method is the same for DVI and HDMI, low cost adapters are available to convert between the two formats.

For Additional Information

HDMI Organization – specifications download: www.hdmi.org

DVI Specification – download: en.wikipedia.org/wiki/Digital_Visual_Interface

TMDS Information: www.siimage.com

Video Electronic Standards Association: www.vesa.org