Tech Focus: Immersive Audio — The Front End Makes Its Move
Object-based broadcast sound spurs evolution of multi-element microphones
Despite a decades-long march to the nearly complete digitalization of the audio-signal chain, its endpoints remain solidly analog: loudspeakers and microphones. And, although speakers have received considerable attention in the very recent progression to immersive audio, with studios and remote units installing overhead and other playback points according to carefully engineered positioning plots, the input side of the transducer equation is less fully formed.
The shift to 5.1 surround over the past decade has seen the progression of a toolkit’s worth of multi-element microphones intended to support that format, from such companies as Soundfield, Holophone and Sennheiser. That evolutionary process may already be under way when it comes to microphones used to support the four additional overhead channels specified in AC-4, the broadcast iteration of Dolby’s Atmos format.Sennheiser’s Ambeo VR four-channel microphone, introduced in 2016, is ready for the immersive era, according to David Missall, manager, customer development and applications engineering. Ambeo VR combines first-order ambisonics — which, in addition to the horizontal plane, also addresses sound sources above and below the listener, using conventional microphone polar patterns — and proprietary software to convert basic surround images to more-detailed B-format images that include the height information that sets immersive apart from surround.
“Once you’ve gotten the information converted, you can output any configuration you need, from binaural to 5.1 to 7.1 to 11.2 and so on,” says Missall. “The [ambisonics] concept has been around for decades, and the technology to achieve it has been available for years now. What’s been missing ’til now is an application for it.”
The Ambeo VR microphone has been undergoing testing in several broadcast-sports environments, including the NFL in the U.S. and FIFA in Brazil and the UK. This includes testing to see if the conversion to a B-format signal can be done in the broadcast-mix console as part of the broadcast or if it needs to go first to a DAW for rendering, which could impact latency.
“Then there’s the business case that needs to be made for it,” Missall adds. “Do we have an audience yet? Should [broadcasters] charge for it or make it available for free? These are questions we’re still asking.”
In any case, he believes that the imminent convergence of virtual reality and sports media could be the catalyst in making the multichannel microphone a linchpin of immersive-audio capture.
Michael Godfrey, president of Holophone, which developed the H2-PRO surround microphone, specifically designed for capturing discrete 5.1-, 6.1-, and 7.1-channel audio, believes that, in a sense, the surround-microphone sector has been waiting for this moment to move from niche to mainstream.
“Immersive is just the latest buzzword,” he says. “Atmos and MPEG-H are amazing, but they still need actual 3D microphones to do the capture initially, or they will be conveying nothing but manufactured spaces to a small captive audience. Immersive single-point-source microphones [are] an accurate, proven and convenient way to capture ambiences for any postproduction mixing and output scheme, both for now and for the future.”
The Next Big Thing
Immersiveness will be broadcast audio’s latest inflection point, after stereo in the 1990s and 5.1 surround a decade ago. Both Dolby AC-4 and MPEG-H are candidate standards for the ATSC 3.0 standards, and the North American Broadcasters Association (NABA) board approved Dolby AC-4 as its recommendation for the next-generation audio standard for NABA’s membership (the U.S., Canada, and Mexico), the sonic counterpart to the 4K or UHD that will be its video format. AC-4 can support up to 11 channels of audio, including two to four on a new axis: height. It also supports object-based audio, which represents sound as a set of individual assets together with metadata describing their relationships and associations.
Immersive audio’s momentum could spur more microphone development. For instance, Godfrey notes that developments are in the works, with several new patents applied for, including more-accurate 3D head-tracking.
According to Gabriel Antonini, national sales support/business development manager, DPA Microphones, that company’s d:mension Series 5100 mobile surround microphone, comprising five highly linear elements, has become a favorite of game audio, a media sector that embraced immersive sound early on. It could form the basis for any further research and product development in that area, along with the company’s extensive work on tetrahedral microphones dating back two decades.
The market for the next generation of multi-element microphones will be split, he predicts, between the cutting edge of broadcast sports — “Most likely the NFL” — and online gaming. “There are a lot of players at the [immersive-audio] table already, but we’re at the point now in between [determining] what they want in the way of an immersive-microphone product and what they’ll actually need in one,” he says, adding that he believes that “the time is right” for more products that support immersive audio, including specialty microphones. “Streaming to smart devices and the home environment is [a necessary first step]. There must be a need to warrant the effort. This is [based] on creating content that the end user will pay to experience.”
Audio systems already have a growing number of tools for managing the number of channels or objects in next-generation audio for broadcast, such as auto-panning systems for mixes, creating a pathway for microphones that accommodate that. What Antonini expects to propel that further, including microphones, is broadcast’s migration to a streaming environment, where the number of channels is limited only by bandwidth, not infrastructure. “It’s getting easier to deliver multiple channels of audio.”