Applied Technology:  Coaxial Cable Improvements for 4K Transport

As higher resolutions become the “norm” in television production and broadcasting, improvements in coaxial cables and associated connectors to enhance performance in the 12G operating range provides a more practical solution for 4K transport.

High-definition television (typically 720 or 1080 lines of resolution) has been part of the consumer viewing experience for nearly two decades, and has for the most part been satisfactory for transmitting news and entertainment programming. However, improvements in both camera sensor and video display technology have spurred a movement to even higher resolutions (2,000+ lines), and this ultra-high definition (UHD) video seems destined to become the “norm” in television production and broadcasting. Moving 4K UHD signals within the broadcasting facility presents several challenges though, as the required 12G data rate greatly limits transmission distances achievable via conventional coaxial cable. While fiber optic technology could provide a solution for some broadcasters, the requirement for electrical-to-optical and optical-to-electrical signal conversions can make this an expensive proposition in situations where a large number of signals have to be transported over fairly short distances, such as within an individual broadcasting facility. The use of such active devices also adds other potential points of failure, thus decreasing system reliability.

Making the 12G Connection

Canare Corporation of America, an established manufacturer of cables for the broadcast industry, has addressed this problem by developing a new generation of coaxial cable and improved connector technology, making it possible to satisfactorily transmit UHD 12G signals up to 100 meters or more without increasing the cable diameter or necessitating special considerations in handling during installation. The new L-5.5CUHD cable has an outside diameter of only 7.7 millimeters, the same as the company’s L-5CFB cable used to transport 3G HDTV signals. The company is aggressively pursuing development of additional cables optimized for 12G applications, including an 11.1 mm diameter cable with the ability to transport 12G signals up to 140 meters. An associated family of improved BNC connectors and a new 12G-capable patchbay have also been developed to take full advantage of such high-performance cables. These connectors address the need to maintain a 75-ohm characteristic impedance even after repeated use, and are available for both printed circuit boards and cable end applications. 

Coaxial Cable Transport Limitations

Coaxial cable was developed over 100 years ago and has been used successfully for carrying video signals for more than 80 years. All transmission lines must obey the laws of physics, and coax is no exception. High frequency response is limited by capacity between the shield and center conductor, and care must be taken to ensure that all components used to terminate/couple cables maintain a uniform characteristic impedance. Capacity effects and even slight impedance mismatches become even more critical when transmitting high data rate digital television signals due to the “smearing” of square wave transitions and reflections due to even slight changes in characteristic impedance of the circuit. The diameter of the dielectric material separating the inner conductor and shield can be increased to minimize shunt capacitance between conductors, but this necessarily increases the physical diameter of the cable, making it more difficult to handle and install due to increased weight and greater care needed to avoid exceeding the cable’s maximum bending radius.

Cable/Connector Improvements

Much research in dielectric materials and other cable components have been undertaken in recent years to produce coaxial cables for handling the progressively higher frequencies demanded by the television industry without making them so large as to be unwieldy. Similar work has gone on to improve the broadcast industry-standard BNC coaxial cable connectors to provide a closer match to the characteristic impedance of the cables with which they are used. Even a slight impedance mismatch can have very deleterious effects at high data rates. These impedance “bumps” cause signal reflections, producing distortion in the high frequency square wave transitions and limiting transmission distances that might otherwise be achieved by the improved cable designs.

Through the use of improved designs, such connector impedance discontinuities can be kept to a minimum. Performance in this area is quantified by “return loss,” which expresses in Decibels the ratio of signal being transmitted to that reflected back to the transmission source due to mismatches. Through careful mechanical and electrical design, as well as precision machining and a high level of quality control in production, return losses of up to 15 dB at 12G are achievable in BNC cable connectors, and as much as 10 dB in printed circuit board mounted BNC receptacles used for UHD signals.

4K UHD Transport

With the accelerating industry-wide movement to 4K UHD image capture and distribution, content providers and broadcasters are being faced with the need to have a reliable method of transporting extremely high data rate signals within their facilities. At the same time, business economics must be considered. While fiber optic connectivity may seem like a logical choice due to its proven ability to handle high data rate transport over long distances, this technology may not be the best choice for UHD signal runs within most broadcast or production facilities with moderate distances separating studios, edit rooms and equipment areas. Careful consideration should be given to the use of high-quality advanced technology coaxial cable in planning for migration to 4K technology, either in a phased approach in an existing plant or in the construction of a new operation. 

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