Audio Routers Grow in Importance in Production Trucks

The goal of signal-routing systems is to simplify the process of getting signal A to point B, but achieving that goal keeps becoming more complicated.

Even as the use of multipoint routers aboard remote-broadcast trucks has increased in recent years, aimed at eliminating the rat's nests of patch-bay cables, there remain multiple approaches to the issue, including the use of separate audio and video routers, as well as units that embed audio with the video signals, and the move by some manufacturers to offer IP transport for audio signals.

Kerry Wheeles, director of product marketing at Harris Broadcast, sees the choice in truck-based routing as headed toward the convergent model, with audio embedded within the video signal. However, he adds, the ability to process and route the audio regardless of the format (for example, embedded or not) is still required.

In response, Harris Broadcast's Platinum line of routing switchers offers a multipath architecture that allows any mixture of discrete and/or embedded audio signals to be de-embedded, processed, routed, and re-embedded as required, making it a perfect fit for production environments. To transport audio, voice, and data via studio-to-transmitter or interfacility links, Harris Broadcast's Intraplex product range supports audio over IP. Wheeles expects that, eventually, all signals will migrate to an IP environment but says that's not going to happen everywhere overnight.

A convergence of baseband and IP is going to be the final [environment], he says, but, just as with the transitions from analog to digital and SD to HD, it's not going to happen overnight. There are hundreds of devices out there, and we'll be looking at working in a hybrid distributed environment for a while to come.

Tom Harmon, president/CEO of router manufacturer Utah Scientific, says the hybrid environment the industry finds itself in is transitory, on the way to what he predicts will be a full IP environment once sufficient bandwidth is ubiquitous and affordable. In the meantime, however, manufacturers are developing various ways to make audio as accessible as possible even as it remains embedded with video for transport efficiency. Whereas Harris redesigned its routers' internal architecture, Utah Scientific uses dedicated I/O cards to demux audio into its own TDM 2304 2304 matrix within the router.

The cards are also used in conjunction with a very new product, introduced at NAB Show 2013, that converts AES audio signals to MADI, and vice versa, for routing to digital audio consoles on trucks. In fact, while the evolution of routers to a single integrated AV signal is taking years to happen, Harmon is amazed by how rapidly MADI has become the norm for audio-signal transport aboard remote-broadcast trucks, and he says he's now seeing the format migrate into the broadcast plant.

If you'd have told me even 18 months ago that MADI would be being used all over broadcast, I'd have said it's just for the trucks, he says of the protocol, also called AES-10, which was issued in 1991 and used mainly in recording and post studios until broadcast-truck designers rediscovered it and have been widely implementing it.

But as the broadcasters are replacing their [plant] infrastructure with digital [consoles], he continues, they're looking at how MADI is being used in the trucks and saying what MADI is doing in the trucks is cool and we think it'll work in the studio, too. We can move 64 channels of audio on MADI fiber, too. It's not as noticeable as it was in the trucks, [which are] either digital or they're not; the studios are still in the process of transition [from analog].

Tim Walker, product manager for routers at Miranda Technologies, agrees that the digital audio console on remote trucks and in broadcast studios has become both an extension of the router and a control point for digital audio tapped from the router through MADI conversion. (In fact, the success of that combination has been used as a marketing route in the U.S. market for several manufacturers, including Stagetec and Lawo, which make digital consoles with integrated audio routers.)

He's finding that the ability to easily de-embed up to 16 channels per video stream and route up to 64 mono audio channels of sound through a MADI-connected router is amping up demand for even larger throughput, with clients inquiring as to the feasibility of 128 and even 256 channels over a single cable as well as supporting 32 channels of embedded audio in a single SDI stream.

The growing need for more audio, he says, is being driven by increasing numbers of multichannel audio productions and multiple-language support.

According to Walker, Miranda's existing router topology can handle larger streams by leveraging the SMPTE 424M standard, which allows bitrates of 2.970 Gbps over a single-link coax cable. These bitrates are sufficient for 1080p video at 50 or 60 frames per second. The SMPTE 425M Level B mapping is a multiplex scheme where two links of 1.5-Gbps HD-SDI video are transported over a 3G-SDI stream, with each link carrying 16 channels of embedded audio.

Mark Nelsen, market segment manager for production, Miranda, says that, typically, only eight to 10 mono channels of each 16-channel stream are being used for such applications as 5.1 surround and alternate-language audio but that could be stretched by new applications, such as descriptive AV services for visually impaired viewers.

The challenges for digital audio in a more complex AV router environment will focus on such issues as phase coherency and audio-to-video latency, all byproducts of extracting embedded audio from a video stream, routing it to processing, then re-embedding it, for which Miranda's low-latency design has recently been granted a patent. Says Nelsen,

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