HDMI/DVI-D over Cat5e
There is nothing especially new about transporting audio and video signals over a UTP cable. Companies like Gefen and Intelix have manufactured solutions using this technology to transmit analog S-Video and Component Video for nearly a decade. UTP stands for Unshielded Twisted Pair and is engineering “shorthand” for Cat5E (amongst others). Signals run over UTP use a Balun (Balanced to Un-Balanced transformer loading) to create a differential signal. Due to the unique nature of a transformer, the signal is induced on the secondary windings and completely isolates the conduction path from the equipment at either end. This delivers an important benefit – freedom from ground loops. A second benefit can be extended length – transformers can easily increase voltage at the expense of current. Since a signal is transmitted through the change in voltage, decreasing signal current is seldom critical. This property of a Baluns makes them especially good for bandwidth limited low-voltage AC signals. HDMI and DVI-D signals carried on UTP based infrastructure can often be run for 150 feet or more!
A digital video distribution system based on UTP technology has many advantages and attributes which make such a design a popular solution. First, Cat5e cable is smaller than coaxial cables, making the cable runs easier to execute. Because a Balun driven system is, ultimately, a passive circuit, HDCP compliance is seldom affected. The cable itself is often less expensive and in the context of a very large installation this can quickly add up to hundreds of dollars just for the wire. Cat5e is easy to terminate; nearly any technician can masters the intricacies of the punch-down block during their first weeks on the job. Cheaper, easier and it runs longer distances without fear of ground-loops. And the UTP is inherently resistant to RFI common-mode noise. This sounds like a great solution!
All is not rosy in the world of the Balun, however. Baluns are not the ideal solution for every application. The actual resolution and in-use performance of an installation is directly proportional to the quality of the transformer used to power the system. And good transformers can be expensive! HDMI-to-Cat5e and DVI-to-Cat5 systems typically sell for $500 or more.
Balun-based systems also suffer length related degradation. For instance, one primary manufacturer of HDMI Balun solutions specifies 1080i performance to 200 feet but cautions that 1080p performance is only attainable to 150 feet. A further caveat states that performance may be compromised in facilities with excessive RF interference.
While twisted pair cables are easy to run, there are a lot of environmental issues that must be observed to ensure proper operation. Twisted pair cabling carrying an audio-visual signal should not be run within one foot of fluorescent lights or more than three feet parallel to high voltage lines. Twisted pair cabling, such as Cat 5, carrying an audio-visual signal should not be passively split or routed through a hub or switcher. Finally, you cannot transmit data and digital A/V over the same length of Cat5e.
One final issue detracts from the Cat5e solution. Due to the extreme bandwidth requirements of HDMI and DVI digital video these transformer-driven systems require two runs of Cat5e, one of which must be a shielded Cat5e. If the project is taking place in a retrofit situation you cannot use a single run or an unshielded dual run. At least one cable – the cable used for the actual image information – must be shielded. The chances of finding shielded Cat5e run in a building are only slightly better than the odds of winning the Irish Sweepstakes! Still, Cat5e is small and easy to route. And it is ultimately the least expensive solution if you must install a run of more than 100’.
An additional benefit of a Balun-based installation exists in commercial buildings where Plenum CMP-rated cabling is demanded by the National Electrical Code. At this time there are no HDMI cables made that are CMP-rated. Plenum Cat5e and Cat6, however, are readily available.
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