Triax or hybrid cable for HD digital TV signals?

HD Camera Editor’s Note: This is part one of a three part series.

There are basically two ways to transmit an HD signal from the camera head to the base station: triax or SMPTE311 hybrid fiber cable. A triax cable carries multiple signals over a solid or stranded conductor surrounded by an inner shield and an outer shield. It was first used publicly at the Dorado Open Golf Tournament in Miami in 1975, and was the de facto camera standard by the time ABC covered the 1976 Winter Olympic Games in Innsbruck, Austria.

The use of triax allowed a small cable with a maximum diameter of 14 mm to run multiple signals. It replaced huge and complicated cables, such as TV-81, which were very difficult to manage and impossible to repair. Triax soon became an international standard for professional video camera applications. Europe’s triax cables have three different outer diameters—8, 11 or 14 mm—and are mainly in red. The rest of the world uses 6, 9 and 13 mm cables.

The triax cable–connector combination, capable of withstanding extreme abuse but easy to repair or replace in the field, dominated the professional camera market until the introduction of high-definition (HD) digital signals. But HD required much greater bandwidth, and in 1998 SMPTE approved Standard 311M (copper-fiber hybrid cable) and 304M (details of the connector mating interface and optical & electrical performance). In 1999, the European Broadcasting Union published Technical Recommendation R100-1999, “Connectors for camera cables with fibre Optical transmission.” In 2007, the SMPTE 304M was reviewed and updated to better reflect the state of the technology.

With the rise of HD video applications, such as 1080i and 720p, the bandwidth demands on camera cables increased considerably. These applications run on a bit stream of 1.5 Gb/s and a resultant bandwidth of approximately 750 MHz. The path from the camera to the base station is one of the few places where HD video has not been compressed: using compressed digital signals introduces several seconds of delay because of the encoding process. The latest HD format, 1080p, mainly used in sports events, needs a bit stream of 3 Gb/s.

Belden offers hybrid fiber cables, consisting of two single-mode fiber optic cables and four copper components for power and data, with a special connector (Lemo 3K.93C, Canare FC-Series or Fischer 1053). Neutrik’s® OpticalCon, Rosenberger OSI  HD614 and ADC’s ProAX™ are substitutes, but have different designs.

Despite the initial belief that hybrid cable was the HD substitute for triax, it became apparent that SMPTE 311M was actually very different from triax. The optical fibers can be broken more easily than copper components, and field repairs are virtually impossible.  As a result, many installations required extra 311M cables, just in case the primary cables failed. These difficulties have led a number of designers and installers to begin reconsidering triax as a high-definition cable.

By the way, all camera manufacturers offer cameras that operate on either hybrid fiber or triax using different adapters. The end user should decide what fits best for them!

Belden is well prepared, because we have both cable types in our product portfolio, including terminated hybrid fiber cables. Using proper termination of the cable eliminates potential water ingress and/or damage to fiber within the connector. On customer request, we can produce the hybrid fiber cable for large-sized TV cameras in studios with an additional sheath (O.D. 12 mm instead of 9.2 mm) so that it does not get jammed between the camera pedestal dolly and the floor in the studios.

HD signals can travel further on fiber, but triax covers significant distances (up to 2 km over standard triax—more than enough for most arena applications). Where longer runs are needed, a single strand of single-mode fiber is the solution. I will come back with more details on this solution in my next blog.

If you need more details about our solutions let me know and Email me at werner.eich@belden.com

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