In the broadcast video world, 75Ω coax is used virtually every-where to carry signals between routers, tape decks, cameras, distribution amps, etc. While this coax provides good performance for SDTV signals, the 1485Mb/s bandwidth of HDTV signals limits coax runs to approximately 200ft to 500ft. This distance can be insufficient for many on-location venues where HDTV is being used, including sports arenas, concert halls and stadiums. In addition, these runs often aren't long enough for permanent installations in TV studios or video production houses. And, besides the distance concerns, there are many situations where existing coax cable tray space is at a premium and adding more coax is expensive.
Fiber-optic cables can easily overcome this distance limitation because their attenuation at high frequencies is significantly lower than coax. Fiber also overcomes the concerns of limited space and weight in crowded existing coax cable trays. Unfortunately, most of the video hardware in use today — even equipment designed specifically for HDTV — is still equipped with only BNC connectors for coax cable. Most of it has no built-in provision for fiber-optic cable.
The solution to the distance limitation of coax and the lack of native fiber connectivity is coax-to-fiber and fiber-to-coax media converters. While the marketplace does offer several devices to provide these functions, historically they have suffered from two types of problems.
The first problem is size and packaging. Many converters are line cards that must be installed in equipment racks. This precludes their use in the field, or in temporary applications where installation in a rack is not possible. Even “throw-down” box converters can be too large and bulky for many applications.
Smaller, “in-line” converters often exhibit the second problem: insufficient video performance. Some of these units cannot meet the specifications for the SMPTE optical and electrical protocols for HDTV. The result is compromised or intermittent video quality, insufficient optical link distance or, in the worst cases, the inability to even establish a link over the fiber-optic cable run.
Stratos converters meet SMPTE protocols 259M, 292M, 305M and 310M and will operate correctly and reliably from 19.4Mb/s to 1485Mb/s. They also support DVB-ASI operation. These converters are approximately 4in long by 0.75in high and 0.75in wide, and are equipped with a 75Ω BNC connector on one end and an ST simplex fiber-optic connector on the other end. Using nine-micron, single-mode fiber-optic cable, they provide a typical link distance of 15mi at 1485Mb/s.
The devices fully meet or exceed all SMPTE specs at distances of 3mi at 1.485Gb/s and 6miat 270Mb/s. The converters use custom optics, mechanical engineering and controlled impedance circuit design technologies for high-end telecom and datacom optical transceivers that operate at speeds of 2.5Gb/s or greater. The units are cost-competitive with other, larger converters while providing high reliability and high-quality video performance.
The small size of the units allows their use virtually anywhere — directly on cameras and equipment, in the middle of a cable run, or in rack-mount panels. They are contained in rugged, die-cast metal housings overmolded with a compliant, nonconductive elastomer. This packaging can withstand rugged handling and provides high reliability in the field.
Simplex transmitters and simplex receivers are available. The transmitter (in a red-colored housing) features built-in line equalization for signal integrity with various lengths of attached coax. The receiver (in a blue-colored housing) features signal re-clocking for all SMPTE 259M and 292M rates and 75Ω line-driven electrical outputs to support significant coax cable runs. Re-clocking provides random jitter control and allows broadcasters to use the receiver remotely in the field.
The different housing colors of the transmitter and the receiver reduce the chance of installation errors caused by personnel with minimal technical background or by knowledgeable personnel in a fast-paced, on-location event. A multi-color LED performance indicator, visible on the outside of the unit, provides a clear indication of go/marginal/no-go link status. This helps ensure reliable in-field monitoring, whether or not the user has any significant technical knowledge.
These devices include a digital-diagnostics interface (DDI) for high-reliability applications such as live video feeds or uplinks. DDI-monitored parameters include device temperature, device power-supply voltage, received optical power (receiver only), laser bias current (transmitter only) and transmitted optical power (transmitter only). DDI also offers alarm flags to warn when specific operating conditions exceed the device's parameters. An LED on each unit indicates these data, which can be accessed through the four-pin connector. The connector also provides DC power to the device, which can operate over a wide power supply voltage range of 4.5VDC to 16VDC.
Other devices and equipment that support these video media converters include universal 110V/220V wall-plug power adapters, a single unit in-line DDI interface box and a 1RU shelf, which holds 18 converters for rack-mount installations.
These video media converters facilitate the transport, processing and display of HDTV signals over significant distances, without loss of signal quality. They allow an existing infrastructure of coax-based video equipment to interface easily with fiber-optic cable. Because the units are small and self-contained, they eliminate the need for installation in equipment racks, providing flexibility in deployment.
Mark Benton is director of product development at Stratos Lightwave.
