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Modern routing systems

From traditional broadcast facilities and production operations to IPTV headends and network operations centers, the level of technical complexity and variety of technology and formats installed has markedly changed in the last two decades. Routing systems mirror this change.

Why are new systems so complex?

The increased complexity of routing systems is in part due to a general increase in physical size and in the number of formats that they support. For example, most broadcast facilities today support at least the 525-line format and one of the HD formats. Many facilities also support SDI and NTSC and have both analog and digital audio — and perhaps Dolby E or Dolby AC3. This material change first affects routing, which is, in most cases, the sole method of delivering signals to production and air.

Routing systems have evolved from single-format frames to mixed format systems that can accommodate analog and digital audio and video, as well as time code, machine control and HDTV signals in one monolithic structure. This is particularly useful to facilities undergoing continuous change during the DTV transition and need systems that will accommodate new formats without requiring an expensive, new routing system on top of existing installed hardware.

This can be done either by adding modules of a newly needed format to an existing frame, or by adding a frame with new cards to an existing control infrastructure. Several companies offer frames that can combine signals from mixed formats.

Making the right connection

Increasingly, it is important that routers handle formats that carry emerging signal types. DVB-ASI — a 270Mb/s NRZ signal not to be confused with 270Mb/s SMPTE 259M serial digital video — carries compressed MPEG, with either single or multiple program transport streams. Fortunately, most modern routing systems can carry ASI in any level intended for SMPTE 259M, despite the difference in coding.

With complex mixed signal types, the control system must understand the signals and only route connections that make logical sense. Connecting an HD output to an SD input or ASI to an encoder input is illogical. Moreover, crosspoint assignments that make such connections possible can waste available I/O ports.

Many facilities have segregated signals in routing blocks to make such choices efficient. A couple of decades ago, it was easy to do so because the smallest I/O block physically possible in a router was typically between 10 × 10 and 16 × 16. Today, however, the block sizes are often 32 × 32 or larger, making it harder to separate signals into logical blocks. Most routing is broken up today into logical or virtual blocks, with the control system understanding what signal types should be connected.

Control systems' increased role

Control systems have become more complex. They now support virtualization and allow complex pathfinding between matrix types. For example, if an SDI signal is routed to an NTSC port, the control system can be organized to recognize that a digital-to-analog conversion and encode are required to make the connection valid. The signal is routed through an appropriate interface and then back to an analog crosspoint before heading to the intended destination. This reduces the workload for operators and simplifies training, though at the expense of programming time to plan the control system properly.

For many years, control system designs were based on two basic models. In one, all control panels were linked on party line (coax or twisted pair) links, tying many control panels to the potential failure of a single piece of wire or connector. Although these busses were high in speed and deterministic, the architecture limited the number of control panels that could be online at the same time. Other systems used discrete lines to each panel, with even more limited extension of the total system size.

Today, control systems often work on TCP/IP networks, facilitating flexible cabling and even extension of the control system across a WAN connection. This is ideal for facilities in multiple locations or systems that must be distributed throughout a large building.

Many modern control systems permit status and monitoring using SNMP and Web services, moving control systems into the realm of common IT. It is, however, tricky to organize VLANs that will operate with deterministic control over a complex routing system.

Size matters

In the last five years, the size of routing systems has exploded, with several monolithic systems exceeding 1000 × 1000 and some with more than 2000 I/O ports. The market for such enormous systems is limited. But features essential to large, mission-critical routing systems have become prevalent. These features include redundant crosspoint cards and I/O modules, along with highly fault-tolerant control systems.

SMPTE recently completed work on the 3GHz SDI standard, named SMPTE 424M-2006, and is already shipping hardware. This new interface can carry 1080p30 HDTV, replacing the former dual 1.5GHz links needed with a single link interface.

Altera and Gennum have both developed and are now shipping chip sets supporting 424M. Although at one time 1080p was considered impractical for terrestrial broadcast, there is considerable interest developing in it partly because of the availability of consumer displays that support it. In addition, recent research has shown that 1080p may be practical in nearly the same compressed bit rate as 1080i.

The effect on audio

Audio has benefited greatly from the development of advanced routing solutions. Several manufacturers now offer systems that handle both analog and digital audio, and even provide A/D and D/A conversion internal to the router. This permits the design of a facility that flexibly interfaces to legacy and new hardware without restrictions.

AES routing is sensitive to interruptions in the bit stream, however, and requires a synchronous router to avoid annoying disturbances in the audio. An AES synchronous router buffers all of the inputs so that all outputs are always framed the same, facilitating seamless cuts in the audio.

Another important part of routing AES is through clean switches, or soft switches, in which the two audio signals are blended at the point of transition to avoid a step function in the sound. An example of this is in a cut from high to low volume, which would produce out-of-band signals.

Other audio signal types that create challenges include compressed Dolby E and Dolby AC3. For all practical purposes, it is not possible to switch between two AC3 signals because the audio access unit is not constrained to fall on video frame boundaries. However, Dolby E was designed to be video-friendly and is constrained to video frames, facilitating cuts along with picture either as standalone or embedded audio.

Multichannel audio can also be handled by putting three AES bit streams into three parallel paths through routing, carrying L, R, C, Ls, Rs and LFE. An additional AES pair can be added to carry Lt, Rt for some applications. Switching must be arranged to make simultaneous cuts on all channels of the 5.1 signal to ensure signal integrity.


Routing switchers may evolve over time to bridge the gap between conventional unidirectional video transport and common duplex IT networks. As traffic moves more as files and less as video signals, video hardware manufacturers will most likely design hybrid systems that facilitate interplay between existing IT and broadcast hardware.

John Luff is a broadcast technology consultant.

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