HDMI/DVI-D over Copper
HDMI LLC doesn’t specify any length
limitations to an HDMI interconnect. Neither does the Digital Display Working
Group, the founders of the DVI-D standards. As with all cables, signal
attenuation becomes too high at a certain length. Instead, HDMI specifies a
minimum performance standard. Any cable meeting that specification is
compliant. Different construction quality and materials will enable cables of
different lengths to be manufactured. In
practice the maximum reliable length for a DVI-D or HDMI connection running a
1080 X 1920 signal (1080p) is about 75 feet.
The use of active devices may
extend that run to 100 feet or slightly more.
Performance at these lengths is directly proportional to the quality of
the driving and receiving electronics in the source and load components.
Copper HDMI and DVI-D cables are complex and expensive pieces of wire. An HDMI interconnect has 19 conductors set in a specific geometric arrangement to hold the characteristic impedance stable. Setting cable impedance on a twisted pair is a very difficult undertaking as the twists-per-inch and the diameter of the conductor and dielectric work together to form this important electrical characteristic. Changing the shape of the cable through tight bends, or changing the twists-per-inch through inadvertently stretching the cable under tension can easily and adversely affect signal propagation. In short, HDMI and DVI cables are delicate!
Because of this complex construction, HDMI interconnects can be seen as an expensive solution when compared to a Cat5e-based Balun solution. For instance, a quality 10 meter HDMI interconnect might have a retail point of $150.00 or more, compared with $12.00 for shielded Cat5e. If that is the only number one were to consider then there is no competition. However, the HDMI interconnect is complete and ready to install. The Cat5e-based Balun solution requires the transformers and short jumper cables to connect to the source and load components. A 10 meter Cat5e solution really costs $12.00 (bulk Cat5e) + $500.00 (Baluns) + $50.00 (2each 1meter HDMI interconnects) = $562.00. Now the $150.00 cost of the manufactured HDMI interconnects look like a bargain!
If a copper based solution has a price advantage, it also has a performance advantage. While the frequency response of any conductor will drop in direct proportion to the square of the length of the cable, the high frequency attenuation in an HDMI environment doesn’t impinge on its ability to deliver a full 1080p image out to the maximum practical length. For example, the RapidRun Digital system can support a 480p HDMI signal at 65 feet. It will also pass 720p, 1080i and 1080p at that same length.
HDMI copper has its drawbacks, too. HDMI interconnect technology is delicate and must be installed with care. Wire runs should observe best installation practices and avoid sharp turns that will exceed the minimum bend radius of the cable. Wire ties and cable stringers must not compress the diameter of the cable or otherwise disturb the geometric relationship of the various conductors. Finally, this is a larger cable to pull. And HDMI connector will not fit into a conduit smaller than 1.25-inches. A DVI connector requires at least 1.75-inches. And these diameters don’t allow for tight bends, so no 90 degree elbows allowed! The RapidRun Digital system from Impact Acoustics allows a run to be placed in a .75-inch with up to a 90 degree manufactured sweep. It is currently the most compact solution available.
Copper HDMI and DVI solutions can also be difficult if compliance with NEC standards is imperative for the installation. Most manufacturers don’t make a CL2 rated HDMI or DVI interconnect (all Impact Acoustics SonicWave product is CL2 rated for in-wall use). At this time no manufacturer is making a CMP rated HDMI interconnect. This means that, even though these are low voltage solutions, they must be housed in conduit if they are installed in a plenum air-space in a commercial installation. Further and unlike the Cat5e solution, HDMI and DVI-D interconnects are not field-terminable. You cannot cut to fit! This requires careful planning before the components are placed and the cable is pulled. Two-feet too long can be a challenge, and two-feet too short can be a nightmare!
Finally, copper-based solutions don’t do anything to address ground loops. This isn’t a huge problem in most quality digital video installations, but it can be an issue if ceiling-mounted projectors or wall-mounted plasma displays are powered from a different A/C mains circuit. Copper-based solutions ensure compliance with HDCP, though.
An important characteristic of longer length HDMI solutions is found in the wire gauge. The longer the cable, the thicker the copper must be to ensure performance. The two variables of copper-based HDMI solutions are the quality of the source and load electronics and the stability of the copper assembly in terms of impedance and resistance. These can be calculated and compensated, but forethought is required.
