Increasing signal rates and the need to transmit media over greater distances are driving the requirement for an optical fibre infrastructure in today's broadcast network. Contribution-quality HD (1080i/720p) content cannot easily be carried more than 100m on copper without regeneration. The adoption of 1080p (50fps/60fps) is likely to be even more challenging, depending on cable type. Solving this problem with a copper connection requires the use of repeaters, a costly alternative that results in accumulated jitter. A better option is fibre, and many broadcasters have already incorporated fibre into their facilities.
Polatis' Trinity optical video routing switch is an end-to-end optical solution to switching fibre; the signal is maintained as optical throughout the network infrastructure. (See Figure 1.)
It is important to distinguish all-optical routing switches, from those devices with optical-to-electrical (O/E) interfaces, which convert the signal at each intermediate point (fibre-ready). The switcher is capable of routing virtually any audio or video signal, whether analog or digital, providing a universal and future-proof extension to today's formats.
Keeping the signal in the optical domain avoids the multiple reclocking steps that cause signal jitter to accumulate as well as require retiming with the house reference. When routing high-bandwidth signals over intermediate and long distances, end-to-end optical solutions can be far less expensive than using repeaters on copper or multiple O/E converters.
Optical performance requirements
Effective routing of optical signals places a unique set of optical performance requirements on the device, including parameters such as low optical insertion loss, high return loss and low optical signal crosstalk; in other words, it requires maximizing the transparency of the router to the signal that passes through it.
At the heart of the switch is optical path mapping technology, which directs the optical signal from an input set of fibre ports to an output set of fibre ports. The technology uses solid-state materials, piezoelectric ceramics, to direct collimated light across free space to accomplish the switching function. Accuracy of pointing can be controlled to within fractions of a micron, resulting in typical optical power loss through the router of 0.7dB — little more than that of a fibre connector itself.
In contrast to other photonic switches, the switch does not require an existing broadcast signal in order to create or maintain a path connection, and it can switch dark fibre. It is also capable of passing bidirectional signals and carrying signals of one or more wavelengths, such as in dense wavelength division multiplexing systems.
To fulfill the role as a mainstream router in HD networks, the switch had to match many of the capabilities of electrical routing switches, while allowing maximum flexibility for new fibre infrastructures. The switch is a non-blocking (Layer 1) switch, with crosspoint sizes from 4 × 4 to 32 × 32 in a single-stage matrix.
An option on the router incorporates smart reconfiguration, which allows the user to swap sources and destinations, creating an asymmetric matrix on demand. For example, a 16 × 16 matrix can become a 4 × 28 or 18 × 14 matrix. This can be particularly useful when interfacing fibre trunks to multiple devices with optical I/O.
Optical interfaces are typically single-mode fibre of LC connector type and are most popular in high-density applications. However, the switch can also be configured to carry multimode fibre signals. The switch currently supports telecom standard protocols, such as TL1, and broadcast control protocols, enabling the routing switch to directly interface to the Grass Valley SMS-7000, Encore and Jupiter control systems.
The routing switcher has been a key element in broadcast infrastructures for many years. And the control concepts have remained relatively constant even though the signal formats themselves have evolved dramatically.
It is, therefore, attractive to integrate the optical layer into the router control system in exactly the same way, with optical path switching controlled from conventional router control panels or from interfaces from the router controller to automation and facility management systems using existing control protocols. The optical layer can be interfaced to the conventional SD/HD layers by using tie lines with embedded O/E conversion or by taking advantage of optical interfaces integrated into broadcast equipment such as the Grass Valley Kameleon and GeckoFlex products. The router control system uses these tie lines to automatically route optical sources to SD/HD destinations or vice versa, as required.
Conclusion
Regardless of where the greatest need first emerges, optical routing switches such as the Trinity have the flexibility to handle the breadth of today's formats and the extensibility to manage future formats and line rates. By ensuring such new technologies are also compatible with existing control strategies and traditional routing architectures, new approaches can be more easily adopted. Optical routing will fulfill a key role in the next generation of broadcast networks.
Aaron Bent is vice president of marketing and business development for Polatis, and John Liron is the manager of advanced development for Grass Valley.
