Solving The Problem Of Bad Mobile Audio

Tim Carroll, president and founder of Linear Acoustic, still remembers the day some four years ago when he realized there was a serious problem with mobile DTV audio.

Seated at a Washington, D.C., restaurant partially underground, Carroll and his broadcaster guest passed around a mobile DTV receiver tuned to a local station.

“The video looked great,” Carroll recalls. “We could pass the receiver around. It wasn’t breaking up. It was a good, strong solid signal, which is tough anywhere, but in an underground restaurant in Washington, was kind of shocking.”

But the enthusiasm soon turned to concern for Carroll. He turned the volume on the device all the way up, but he couldn’t hear program audio over the din of the restaurant.

To hear, he had to hold the device to his ear, which meant he couldn’t watch the picture. To see the picture, he had to take it away from his ear, but then he couldn’t hear the audio.

“That’s when it became apparent that the way we produce content and transmit content for everything other than mobile is probably not appropriate for mobile,” Carroll says. “But the thought of having broadcasters do multiple versions of live streams seems a little bit daunting.”

Perhaps the most important step to rectify the issue has been an ATSC recommendation that broadcasters add 10 dB of gain to mobile DTV audio, says Carroll. “That often is enough to push things over background noise.”

Standard over-the-air television — not mobile DTV — has a dynamic range of 15 dB, Carroll says. At the high end of the range could be sounds like explosions or the crescendo of a musical performance. At the low end, would be quiet sounds like a snapping twig or the flap of a fluttering flag in a gentle breeze. In between the extremes of this range is dialog.

In a fixed viewing environment like a living room, it is assumed that the noise floor for ambient sound is 10 dB below where the dialog is, Carroll says. “However, if the noise floor is 5 dB below dialog, like it may be in a noisy mobile environment, having the dialog fall into the noise floor is unhelpful,” he explains.

Since Carroll’s epiphany at the restaurant, however, the choices the public has for watching TV on the go have multiplied — both in terms of where content originates and the number of devices that can be used.

In this new multiplatform world, there is no single place to go to fix the problem of loud ambient noise, such as is the case with adding 10 dB of gain to mobile audio at a TV station or transmitter site, Carroll says. Rather, the solutions for the noisy background problem with today’s generation of smartphones and tablets stem from the native processing power of the devices, which will help them recognize whether or not they are being used on the go, as well as in new audio codecs.

“One example is Dolby Digital Plus, which has a portable mode,” he says. “That mode builds gain into the codec, so it is something the user wouldn’t even have to worry about. If the playback device knows it’s mobile, that is the mode it would pick.”

Other codec-related functions that could help have been demonstrated at industry trade shows. For example, Fraunhoffer IIS (Institute for Integrated Circuits) in Erlangen, Germany, won the 2012 NAB Technology Innovation Award for its Dialog Enhancement technology, which allows consumers to personalize their listening experience by controlling the volume of just the dialog, bringing it up relative to the rest of the audio mix. Both Dolby and DTS also have addressed dialog intelligibility in their codecs.

While TV broadcasters, like other content distributors going over the top to reach smartphones and tablets, will benefit from these developments, work being done on a next-generation digital TV standard supporting everything from ultra-HD to small, mobile receivers is a little bit closer to home.

Mark Richer, president of the Advanced Television Systems Group, says the standards body learned a lot from its experience with ATSC 1.0, 2.0 and mobile DTV that is helping to inform the work of the ATSC sub-group responsible for audio. ”ATSC 3.0 builds on and improves those experiences,” he says. “Loudness and dynamic range will appropriately adapt to the device and the content.”

The audio sub-group, chaired by NBCUniversal director and principal audio engineer Jim Starzynski, is working from a list of requirements “specifically formulated to make certain” the standard will address the future needs of TV broadcasters and their audiences —regardless of whether they are watching in their quiet living rooms or on a noisy city bus, Richer says.

One of those requirements is smart dynamic range control (DRC), Richer says. “Adapting loudness and dynamic range to suit the capabilities and limitations of varied devices and multiple listening environments is a must for the audio technology that is chosen for [ATSC] 3.0,” he says.

The new standard also will make it possible for viewers to personalize their audio experience making it easier to hear in noisy environments. “ATSC 3.0 audio will provide users with the option of varying the loudness of a TV program’s dialog relative to other elements of the audio mix for the purposes of increasing intelligibility,” he says. “And receivers will always have the option of performing additional processing.”

Richer adds that the ability to personalize the audio experience with ATSC 3.0 won’t be limited to mobile reception, however. The standard is also being developed to enable a mix of “assistive audio services,” such as multiple language dialog and special commentary using an efficient bit stream, he says.

Carroll says that mobile receivers may one day not only be able to determine that it’s being used on the go, but also what the ambient level of noise is. 

With that level of information, the device might automatically know to make adjustments to compensate for loud ambient noise, he says. But even if automatic compensation is beyond the capabilities of the device, simply giving users access to these sorts of controls that make personalizing their listening experience possible is critical going forward, he contends.

“Having that sort of control, whether it is automatic or manual, is a top-of-the-list requirement for making it easy to listen to audio on mobile devices.”

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