Effective routing of video and its associated audio is one of the most critical functions in modern broadcasting operations, and routing technology has become considerably more complex in the past 10 years. When specifying a routing switcher, systems engineers face numerous challenges related to the simultaneous use of multiple video and audio formats on a variety of physical interconnections. The adoption of HD video formats and increasingly dense audio formats has increased the overall complexity of these multiformat systems, which are also expected to accommodate audio and video processing. Internal router processing creates system and operational efficiencies by simplifying control, and a flexible input/output arrangement allows easy reconfiguration between uses.
Today’s complex routing environment
A decade ago, when broadcast signals were primarily in SD with associated discrete AES stereo audio signals, routing was relatively simple. All it took was a separate router for each signal type, two control levels and an occasional need to synchronize the signals.
The emergence of HD broadcasting has opened the doors to massive improvements in picture quality as well as the home audio experience through surround-sound technology, but at the same time has created a requirement to manage and route many more audio channels. Now, each video signal typically requires a stereo track, a surround track and sometimes different language or regional audio tracks. In fact, it’s not out of the question to expect an 1152 x 1152 video router to support a 36,864 x 36,864 embedded audio matrix.
Embedding the audio tracks in the video signal simplifies signal transport, but the two signal types still need to be separated for production applications. The resulting physical connections can include AES on coaxial cable or balanced-pair cable with multiple channels of audio in multiplexed audio digital interface (MADI) streams (typical for audio consoles).
With the emergence of Dolby E as the de facto compressed audio format in professional surround-sound applications, many video feeds contain embedded Dolby E. This reduces the number of physical connections but introduces additional complexities since Dolby E is data and not audio, and therefore can’t be routed or mixed without additional signal processing. Also, no part of a Dolby E packet can be changed without loss of at least a video frame’s worth of audio. Dolby E must never be sample-rate converted or subject to sample slips.
On a basic level, a router passes all signals from input to output transparently, but it’s not that simple in modern production environments. Most large routers come equipped with hybrid capabilities that enable routing of video and audio within the same chassis and accept a mixture of embedded and discrete audio signals. However, with smaller routers, architectural and input/output flexibility surmount the ability to route multiple signal formats.
Overlaying these complexities is the commercial pressure to reduce the costs of installation, operation and maintenance. Fiber cable is lighter, less lossy and is now available at a lower cost than copper coaxial cable, but fiber transmitters and receivers at each end remain more costly than their coax counterparts, at least for now. These are the factors that keep systems engineers up at night as they plan and purchase routing equipment to deal with the migration from SD to HD and then 3-D and beyond.