AV over twisted pair: (Almost) all about the Base-T

Several technology types exist to facilitate the transmission of audio-visual signals over twisted-pair cabling, but today HDBase-T is the darling.

From the January, 2015 Issue of Cabling Installation & Maintenance Magazine

Several technology types exist to facilitate the transmission of audio-visual signals over twisted-pair cabling, but today HDBase-T is the darling.

By Patrick McLaughlin

The concept of "IP convergence"-- disparate building systems migrating to Internet Protocol (IP) as a means of information transmission, and using IP to communicate with one another or a central command-and-control center--has begun to move from far-fetched to within reach. Some building systems are moving to IP, which is the foundational step in the process of convergence.

A characteristic of IP-based building systems that is of keen interest to cabling-industry professionals is that twisted-pair copper and/or fiber-optic cabling make up the common physical-layer infrastructure for IP networks. Building systems adopting, then converging over, IP networks holds the promise of business potential for cabling professionals.

Taken as a whole, audio-video (AV) is one system type that is a candidate for IP migration. In a web-delivered seminar we hosted in early December, some of the technical possibilities associated with twisted-pair as an AV-supporting medium were discussed. The seminar was titled "Capabilities of and Applications for Twisted-Pair Copper Cabling Systems," (www.cablinginstall.com/webcasts), and within the seminar George V. Fournier, Jr., senior sales engineer with FSR Inc. (www.fsrinc.com) delivered a presentation titled "Twisted-Pair Cabling as an Audio-Video Transmission Medium."

Standalone or networked

In his presentation, Fournier explained that the venues in which AV signals can be supported by twisted-pair are extensive, and include spaces like board rooms, conference rooms, auditoriums and convention centers. Early in the presentation Fournier explained how technologies like baluns can be used to facilitate AV-over-twisted-pair. Baluns, he said, are "traditionally used to extend composite video, S-video, component video, analog computer video, audio, control--RS-232 and IR--or combinations" of video, audio, and control. Cabling circuits that incorporate baluns for AV-over-twisted-pair really are not networked, but rather exist as individual, standalone cabling runs. "Baluns are basically a voltage transformer creating a balanced electrical signal," Fournier explained. "The balun converts the voltage from the input signal to one suitable to run long distances--50 feet to as many as 900 feet--over Category cabling, depending on the cabling, electronics, and environment involved."

Other technology that can be used in an AV-over-twisted-pair setup is what Fournier referred to as "Cat X." An example of this technology type is FSR's TwisterPro, a UTP transmitter/receiver system. On the company's website it explains the TwisterPro product group "allows video, stereo audio, RS-232 data and IR signal transmission over ordinary, low-cost Cat 5, 5e, and 6 cable. These … devices ensure reliable, error-free transmission that is immune to interference from electrical noise. Cable runs of up to 1,000 feet are within the capabilities of the system."

Heralding HDBase-T

But a significant amount of Fournier's presentation detailed the latest AV-over-twisted-pair technology developments brought about by HDBase-T. Founded in 2010, the HDBase-T Alliance (www.hdbaset.org) promulgated the HDBase-T Specification 1.0 and later revised it with HDBase-T Spec 2.0. HDBase-T was established and maintains its primary focus as a whole-home, residential-network-based technology. However, its characteristics may make it attractive to enterprise network operators who count AV systems among their responsibilities.

The alliance has trademarked the term 5Play to describe its main capability, which is support for 1) full HD digital video, 2) audio, 3) 100Base-T Ethernet, 4) Power over HDBase-T, and 5) bidirectional control signals. Via HDBase-T, a single run of Category 5e or better cabling supports all five applications to 100 meters. But despite its apparent close fit with the notion of "IP convergence," HDBase-T technically is not an Internet Protocol application. "HDBase-T does not travel on an Ethernet network," Fournier explained in the seminar. "It is based on a different protocol that standard Ethernet equipment can't work with. That being said, HDBase-T also supports 100-Mbit/sec Ethernet over the same cable; so if you plug a network feed into an HDBase-T transmitter, it will send 100-Mbit/sec Ethernet to the receiver, in effect making the receiver an unmanaged switch."

Not really IP

The HDBase-T Alliance further elaborates in a "Frequently Asked Questions" page on its website: "Although HDBase-T uses coding technology and uses the physical medium used by IP, it is not IP. HDBase-T uses T-packets, a different packetization protocol." The alliance later details another HDBase-T capability: Ethernet Fallback mode. Its support of this mode "means the HDBase-T device can be plugged into an Ethernet-only infrastructure," the alliance says. "The device will ‘realize' it, and will enable only the Ethernet capabilities of the connection. Since an HDBase-T port is identical to an Ethernet port, users can plug in and have fully functioning Ethernet--no frustrations. Also, the manufacturer does not need to design a device with a separate HDBase-T vs. Ethernet port; it can be the same port that can be used for either HDBase-T (including Ethernet) or Ethernet-only connectivity."

The HDBase-T 2.0 Specification, released in mid-2013, introduced HDBase-T over fiber. The alliance anticipated the following question: "Why introduce fiber support? Isn't transmission over Category cable one of the tenets of HDBase-T?" It provided the following answer: "HDBase-T was developed with the belief that it should allow for the transmission of high-definition video over the most ubiquitous cable around, i.e. LAN cable, and be able to deal with commonplace noise and interference without compromising the quality, and without introducing latency.

"Having said that, there are some industries and segments, such as medical and military, who are extremely strict regarding interference, and demand fiber as the transmission medium. In order to cater to those segments, the HDBase-T Alliance developed fiber support in Spec 2.0. Fiber allows transmissions over longer distances than category cable."

The alliance also addresses important distinctions between Power over HDBase-T (POH) and Power over Ethernet (PoE). Its FAQ page includes the question, "Are PoH and PoE interchangeable?" along with the answer, "Yes and no." The answer continues, "They are two different standards. If you happen to connect PoH equipment into a PoE line, the maximum common power will be passed between them, i.e. according to PoE. PoE Plus transmits up to 30W, while PoH transmits 100W. If the device needs more than 30W in this case, the power sourcing equipment will shut off the power, and the powered device will not work … As of today, HDBase-T's 100W power transmission can power most TV sets up to 60 inches. Most TV manufacturers strive to comply with the Energy Star specifications. These are constantly evolving, and reducing the maximum power allowed per TV set."

As mentioned by the HDBase-T Alliance, its 2.0 specification includes fiber cabling in consideration of industries like medical and military, and those industries' extreme concerns about signal interference. That recognition by the alliance suggests that it views HDBase-T as a technology not just for home-entertainment systems, but also for multimedia systems in corporate, institutional and other environments. Although by strict definition it may not be "IP convergence," the transmission of AV over twisted-pair or fiber-optic cabling, via HDBase-T, may be a technical and business opportunity for many.

Patrick McLaughlin is our chief editor.

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