DVI and HDMI Video Connections
By: Thomas Steves, Ram Electronics Industries, Inc.
During the next few months, many manufactures will release new consumer electronic products with DVI (Digital Visual Interface) and HDMI (High Definition Multimedia Interface) connectors. DVD players, cable/satellite set-top boxes and High Definition televisions and projectors are the prime targets for these connections that provide superior digital picture quality and eliminate the need for unnecessary analog to digital conversions. DVI and HDMI were adopted and approved for HDTV connections because of HDCP (High-Bandwidth Digital Content Protection) compliance, which provides copyright protection. The HDMI connector also includes high quality digital sound capability as well as control signals incorporated into the interface. The digital signals carried by these cables are different than current analog signals and cause unique problems when transmitting the signal.
How do DVI/HDMI systems work (or not) in basic terms? A DVI or HDMI system's ability to transfer the video signal from source to display depends on:
1. A transmitter's silicon and board layout ability to transmit the signal properly.
2. The cable's ability to carry the signal with the lowest possible jitter, skew, crosstalk and attenuation.
3. A receiver's capability to recover the incoming data signal and verify the HDCP signal.
The three components work as one system and their success is interdependent. Source device and receiver device incompatibilities (and inadequacies) are certainly sources for problems. Improvements in HDMI and DVI silicon and advancements in board layouts and circuit designs will improve the source and receivers abilities to provide and recover signals.
Cables are generally not a source of problems - until cable lengths factor into the system. Short cables for RPTV's, Plasma and LCD displays are not usually a problem when using well-made DVI and HDMI cables. The longer lengths required for Projectors and similar applications are a common source of problems. Outputs from many of today's Set Top Boxes and DVD players that have DVI connectors are considered to be limited to 5 meters (approximately 15 feet) with standard DVI cable design. In the typical Home theater setup the equipment or signal source may be located greater than the recommended 5 meters DVI distances. Projectors are often 14 to 20 feet or more from the signal sources requiring cable lengths of 20 to 30 feet or more. This is a serious problem for cable manufacturers. Small gauge twisted pair TMDS lines have too much accumulated attenuation (among other parameters) to deliver the signal in a condition that the receiver can recover properly. There are now a number of companies offering copper based solutions at more reasonable prices than fiber or other exotic cable solutions. HDMI is designed to perform better over longer lengths, but actual performance constraints are similar to DVI. Currently, booster amplifiers or fiber optic extenders are required for long DVI cable applications, adding hundreds of dollars in additional cost.
There are many quality factors involving cables besides gauge or insulation properties of the cable, and not all of them are related to amplitude degradation, jitter degradation, propagation delay or skew of the pristine cable. Crushing or bending of these cables can have a severe impact on their performance. Design of the cables can have a large impact on how well cables can maintain their performance when bent, twisted or compressed. A nitrogen gas injected foamed PE insulated cable is going to have less loss at very high frequencies, which is why it was originally developed for CATV applications. Unfortunately, what it will also have is less consistent impedance and greater return loss due to reflections than a non-foamed or hard cell foam insulated cable. These characteristics become worse yet when the cable is bent or compressed. As you can see, trying to improve one characteristic of a cable can certainly involve tradeoffs in other characteristics.
The challenge for manufacturers of long length DVI and HDMI cables is to continue to find ways to optimize important performance characteristics of their cables while not adversely affecting other performance characteristics and/or the durability of the cable. This is not always easy, and certainly not fool proof. Cables, of all aAV components are quite possibly the most vulnerable to "Murphy's Law". Many problems that were easy to overcome with an analog audio, video or digital audio cable can be problematic with a DVI/HDMI interface cable. Someone always wants to use an unnecessary adapter or two, connect several cables together for a longer length, or cut and solder to run them through a conduit. This can easily change the digital video cable performance characteristics causing a number of picture quality problems. These are relatively high bandwidth cables, which really are affected by skin effect, skew, impedance variations and other factors that are quite insignificant in other cables.
One interesting aspect of HDMI is that many companies are bragging about the new capabilities of HDMI to perform well at extreme cable lengths. Many of those are the silicon vendors, while most of the big name high quality bulk cable manufacturers seem to ignore this whole market segment entirely. This is certainly a bonanza for vendors of fiber optic and other solutions to the long transmission dilemma, which are good solutions, but not always a cost competitive solution for consumers.
The result is that the market for lower cost copper (or other metal) DVI/HDMI solutions has become a marketing dominated area much like other "high end" AV market segments without consideration for overall long-lasting quality of the cables. This will probably not change soon.
Factors as seemingly unimportant as surge protection of connector outputs, connector layouts, connectors themselves and termination on source and receiver components can have a large effect on performance of the system. It is impossible for a cable vendor to know how a cable will be used and with what components.
Why don't we see complete specifications published for DVI/HDMI Cables?
A) Nobody else does. Suppliers don't want to make it any easier for other manufacturers to try to duplicate their performance.
B) It is very hard for a laymen to determine what factors actually influence the performance of a cable.
C) There are many factors to consider, and some factors have more or less impact depending on the silicon, and implementation of source and receivers.
In a typical HDTV application a quality single link cable will be sufficient 99% of the time. Most computer graphic cards outputs are still only single link and are capable of the higher resolutions associated with dual link only on their analog outputs. Some new high performance monitors and video cards are now being seen with dual TMDS links and require dual link DVI cables. The HDMI standard is backward compatible and you can use a HDMI to DVI cable to connect to a monitor if only DVI is present at the equipment. However, DVI does not pass digital audio signals as HDMI and a separate audio connection is needed for the audio. Use either coaxial or toslink (fiberoptic) connections for the audio portion if you are using digital audio.
-Use DVI/HDMI cable connection when the two connecting devices have them for the best picture quality.
-Minimize the length of the cable and use the shortest possible run.
-A single cable of proper length connected directly to the equipment is the optimum configuration.
-If multiple sources are connected to a single display use a quality switch that is HDCP compliant.
-Purchase your cables and switches from sources that have quality product and guarantees that allow you time to try them in the application.
-Test the cables with the sources and display prior to installing them behind a wall or in conduit, plus handle the cables properly during installation.
Things are changing. The major silicon vendors are doing a great job of supplying better chips with better information as to best practices for implementation. Things are getting better, and someday this assembly will be another commodity with predictable limitations graspable from general specifications. As for now, the best course for consumers is to try a cable in your system, and use what works.