Unwired for Sound

BACK in the mid 1990s, Ericsson engineers Sven Mattisson and Jaap Haartsen looked at the tangle of wires linking devices on their desks and wondered whether there was a better way to transfer digital information. After years of hard work their solution replaced traditional RS-232 cables with 2.4-GHz wireless connectivity, and with it our world started to change.

Mattisson and Haartsen’s employers coaxed other companies, including Nokia, IBM and Toshiba, to join the initiative, which soon became an open standard. By 1998, these companies had formed the Bluetooth Special Interest Group (SIG), an organization tasked with developing and promoting the wireless technology.

The first consumer Bluetooth product—a hands-free mobile headset—hit the shelves in 1999. Soon after a host of commercial Bluetooth equipped devices were released, including a mobile phone (the Sony Ericsson T36), mice, keyboards, printers and hands-free car kits. By 2003, product shipments had grown to one million per week.

But it was that first product, relaying voice from handset to headset, that pointed the way to the technology’s leading success. This was of little surprise to insiders because Ericsson had originally encouraged the standard’s development due to its potential for connecting peripheral devices—primarily headsets—to mobiles. And among the first ‘profiles’ (software that customizes the Bluetooth protocol stack to optimize it for a particular application) adopted was one for wireless voice and headsets.

But growth into higher-end audio applications stalled because Bluetooth tech’s 1 Mbps maximum raw data throughput (which diminishes to around 721 kbps in practice because of the intentional transmission delays built into the protocol during operation) proved insufficient to support streaming audio. Things changed as Bluetooth tech rapidly made its way into virtually every smartphone on the planet and music consumption underwent a parallel revolution with the invention and development of the MPEG-1 or MPEG-2 Audio Layer 3 (‘MP3’) compression format.

In response to consumer demand, the Bluetooth SIG introduced the Advanced Audio Distribution Profile (A2DP) which was accompanied by a codec that supported stereo audio streaming. Soon after, several commercial codecs brought further sound enhancements.

Today, Bluetooth audio is big business: “With close to one billion Bluetooth audio products shipped last year, wireless audio is the largest Bluetooth market,” noted Mark Powell, CEO of the Bluetooth SIG, in a statement. Nonetheless, wired headphones and speakers are still the choice of many because Bluetooth streamed music still lacks CD-like quality, and the technology lacks flexibility such as the capability to stream to more than one device simultaneously.

But now, the announcement by the Bluetooth SIG of ‘LE Audio’ promises to address these weaknesses. By switching from a Bluetooth Classic radio to a Bluetooth LE version, introducing a new optimized codec and adding new features to the Bluetooth protocol stack, the SIG is promising “the next generation of Bluetooth audio”. Is the hype justified?

Music everywhere
In 1987, The Fraunhofer Institute in Germany began researching high quality, low bitrate audio coding. The aim was to come up with a method of compressing audio files such that they could be easily stored and moved between the bandwidth and memory constrained computers of the era. In early 1988, The Moving Picture Experts Group (MPEG) was established and later that year the group called for an industry wide audio-coding standard. Soon after, coding developed by Dieter Seitzer, who’d helped Fraunhofer with its research, was integrated into the MP3 standard, along with software from co-contributors.

MP3 spawned a huge new business whereby tracks ‘ripped’ from purchased CDs could easily be shared across the Internet. Initially, music companies claimed this amounted to piracy, but the industry was legitimized by commercial offerings such as Apple’s iTunes.

Early MP3 players were equipped with wired headphones, but even then, consumers craved the convenience of wireless. Later, as consumers migrated their MP3 libraries over to Bluetooth equipped smartphones, the Bluetooth SIG recognized the opportunity and pushed through A2DP. The profile defines how to unidirectionally transfer an audio stream in two-channel stereo, either to or from the Bluetooth host. This profile in turn relies on the Generic Audio/Video Distribution Profile (GAVDP).

However, even with MP3 compression and A2DP, streaming audio across a Bluetooth link is unreliable. Additional compression of the data by a codec is required so that it can be accommodated by the link’s restricted throughput. The codec then decompresses the data on the other side of the link to retrieve the audio information stored in the original MP3 file. To that end, the SIG included mandatory support within A2DP for a low-complexity subband codec (SBC) designed to ensure reasonable audio quality at low-to-medium bit rates while using modest processing power. However, while A2DP can theoretically support up to Bluetooth Classic’s maximum operational 721 kbps, SBC is limited to a nominal maximum of 345 kbps. And in practice, bit rates using the codec are generally around 256 kbps, limiting audio quality.

A2DP does include optional support for codecs including Advanced Audio Coding (AAC), High-Efficiency AAC (HEAAC) and Adaptive Transform Acoustic Coding (ATRAC). The profile can also be extended to support manufacturer-defined codecs, such as the Qualcomm-owned aptX and aptX-HD which is available to wireless headphone makers under license and offers bit rates of 354 kbps and 576 kbps respectively. A recently introduced codec from Sony, LDAC, offers 990 kbps but requires an (optional and relatively scarce) Bluetooth Enhanced Data Rate (EDR) radio at both ends of the link. The audio quality of wireless headphones using higher bit rate commercial codecs is improved compared with SBC but, according to audiophiles, still lacks the dynamic range, depth, timbre and lifelike quality of wired headphones.

Hear the quality
Despite these concerns, Bluetooth audio headphone sales are thriving. Analyst statista noted that 2017 shipments of the products reached 140 million units, compared to around 67 million shipments of the next biggest Bluetooth sector, wearables. Wireless speakers do well too; however, the growth of Bluetooth audio in other applications has been curbed because of technical limitations.

Chief among these is that the technology can only stream audio to a single device. That’s not a problem for wireless headphones or speakers where each side is connected by a wire but is a significant challenge for products such as ‘earbuds’. Current earbud products work by wirelessly connecting the audio source (for example, a smartphone) to one of the earbuds with the target earbud then employing clever software to wirelessly relay information to its partner. The system works, but it requires significant processing power to keep everything in sync, adding cost and compromising battery life.

The Bluetooth SIG promises LE Audio will resolve these technical challenges—while giving audio quality a further boost—through the introduction of ‘LE Isochronous communication’. In part, the quality boost will come from a codec to replace SBC. In addition to enhancing the audio experience, the Low Complexity Communication Codec (LC3) is specifically designed to save battery power.

According to Manfred Lutzky, Head of Audio for Communications at Fraunhofer IIS, LC3 offers great promise. “Extensive listening tests have shown that LC3 will provide improvements in audio quality over SBC included with [Bluetooth] Classic Audio, even at a 50 per cent lower bit rate,” he said in a statement. Combined with the Bluetooth LE radio’s inherent lower power consumption compared with Bluetooth Classic, the lower throughput afforded by LE Audio will have a significant impact on battery life. While no official figures are yet available, engineers estimate that batteries powering LE Audio should last at least 40 per cent longer than in an identical Bluetooth Classic audio application. That provides an opportunity to extend playback time or shrink earbud size by using smaller batteries while keeping playback time the same.

LE Isochronous communications (which makes use of the LE Isochronous Channels recently introduced as part of Bluetooth 5.2 works by transferring ‘timebounded’ data between devices. With this technology multiple devices (such as a pair of earbuds or stereo wireless speakers) can receive unique audio streaming channels (for example, separate left and right channels from a stereo source) that are precisely synchronized, perfectly preserving the precise musical timing. The Bluetooth SIG has dubbed the technology ‘Multi-Stream Audio’.

“Developers will be able to use the Multi-Stream Audio feature to improve the performance of products like truly wireless earbuds,” noted Nick Hunn, CTO of WiFore Consulting and Chair of the Bluetooth SIG Hearing Aid Working Group, in a statement. “For example, they can provide a better stereo imaging experience … and make switching between multiple audio source devices smoother.”

Multi-Stream Audio significantly increases the number of applications for LE Audio. The Bluetooth SIG cites the example of ‘Broadcast Audio’, a technique which enables an audio source to broadcast one or more audio streams to an unlimited number of receivers. Such a technique encourages consumers to share music from a smartphone with family and friends or allows enterprises to share audio information to large groups in public places.

“People will be able to select the audio being broadcast by silent TVs in public venues, and places like theaters and lecture halls will be able to share audio to assist visitors with hearing loss as well as provide audio in multiple languages,” said Peter Liu of Bose Corporation and member of the Bluetooth SIG Board of Directors, in a statement.

The LE Audio Evaluation Platform extends battery life by around 40 percent compared to today’s off-the-shelf Bluetooth Classic audio solutions

Next steps
Commercial LE Audio solutions are not available today. Instead, what the Bluetooth SIG announced early this year was its intention to release the full specification later in 2020.

“While the introduction of Bluetooth 5.2 brought the LE Isochronous Channels needed for LE Audio, work remains to be done on other important elements of the technology before we’ll see commercial chips,” explains Kjetil Holstad, Bluetooth chipmaker Nordic Semiconductor’s Director of Product Management.

But that doesn’t mean developers can’t start thinking about new Bluetooth audio products. For example, Nordic Semiconductor has developed an evaluation platform that allows developers to start assessing the fledgling technology for next generation wireless speakers, over-the-ear headphones, and ‘true’ wireless earbuds.

“Developers keen to understand LE Audio’s benefits over conventional Bluetooth audio can experiment with Nordic’s LE Audio Evaluation Platform to gain knowledge that will accelerate the design of prototypes when the specification is finally adopted,” says Holstad.

While chipmakers like Nordic are moving forward, commercial products like LE Audio headphones and wireless speakers are still several years from hitting the stores. But the message that the future of wireless audio will be based on Bluetooth LE comes over loud and clear. Crystal clear.