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The Bluetooth® Special Interest Group (SIG) identified audio as a key market for Bluetooth wireless technology virtually from its adoption. Soon after Bluetooth 1.0 was ratified in 1999, the first Bluetooth consumer product, a hands-free mobile headset, hit the shelves. In addition to earning a “Best in Show Technology Award” at (the now defunct) COMDEX, an influential computer trade event, the headset provided an early pointer to Bluetooth’s commercial success.
Much of this success was due to the design engineers developing innovations to overcome the limitations of Bluetooth’s one megabit per second nominal raw data throughput (around 721kbps in practice) to ensure it was adequate to cope with voice transmission. One of these ploys was to limit sampling of the original analog voice signal to reduce the amount of data that needed to be wirelessly transmitted.
The benchmark for high-quality digital sound is the CD. The technology samples the original analog signal at 44.1kHz with 16-bit precision (1.41Mbps), a sampling rate sufficient to capture all the sounds humans are capable of hearing across the 20kHz bandwidth of the ear. But for voice-only systems, design engineers can take advantage of the ear’s particular sensitivity to sound with a frequency between 800Hz and 4kHz, a bandwidth sufficient to encompass 80 percent of the information carried by the sound waves and more than adequate to understand speech. The reduction in sampled bandwidth allows for a proportional reduction in sampling to rate to 8kHz. Even when retaining 16-bit precision, this sampling rate reduces the data throughput requirement to 128kbps.
But clever analog-to-digital sampling is not the only trick in the design engineer’s playbook. And they need others because, while 128kbps is within Bluetooth’s capability in ideal circumstances, it only takes a little interference from a nearby 2.4GHz source such as Wi-Fi® for the wireless link to struggle. So, to boost reliability, design engineers also employ codecs. An algorithm compresses the digital data to reduce the workload on the wireless link while a companion algorithm decompresses the data at the other end to retrieve the original information.
While wirelessly transmitting monoaural voice data is one thing, streaming stereo audio is quite another. The introduction of the MPEG-1 or MPEG-2 Audio Layer 3 (‘MP3’) format—which came about to ease storage and file sharing requirements in the early days of the internet—helped because source files were already compressed to some extent before the codec went about its work to further reduce the throughput requirement.
The Bluetooth SIG also came to the aid of audio engineers by introducing additional firmware to customize the Bluetooth protocol for audio applications. Named the Advanced Audio Distribution Profile (A2DP), the firmware facilitates audio streaming between two Bluetooth devices. But while A2DP and MP3 helped overcome Bluetooth’s throughput limitations, audio quality was still nothing to boast about. The Bluetooth’s SIG’s next move was to introduce a low-complexity sub-band codec (SBC) to boost audio quality at low-to-medium bit rates while using modest processing power.
The group also left the door open for others by including optional support in A2DP for proprietary codecs such as Advanced Audio Coding (AAC), High-Efficiency AAC (HE-AAC) and Adaptive Transform Acoustic Coding (ATRAC), and manufacturer-defined codecs such as the Qualcomm’s aptX and aptX-HD and Sony’s LDAC. Today, almost all of today’s commercial wireless headphones use Bluetooth and a manufacturer-defined codec. However, despite all the development, even the best of these is hardly an audiophile’s nirvana.
Lack of audio quality doesn’t seem to be harming headphones sales, though. According to analyst Statista, 2017 shipments of wireless headphones reached 140 million units. Many of these products are of the over-the-ear type, and for good reason. Bluetooth is only able to stream audio to a single device, such as the left-hand speaker of wireless headphones. Electronics on that side of the product process the incoming stream into left and right channels, forwarding the right channel, by a wire, to the other speaker. The challenge comes when there is no wire; for example, with a pair of in-ear headphones, or “earbuds.” Engineers overcome this by processing the incoming Bluetooth audio stream in one earbud and then wirelessly retransmitting the second channel to the other earbud. It’s a satisfactory solution but is processor intensive and puts a strain on batteries.
The Bluetooth SIG recently announced its intention address both audio quality and power consumption by swapping out “classic” Bluetooth technology for Bluetooth Low Energy. Bluetooth LE offers the same one megabit per second raw data throughput of its bigger brother but with much lower power consumption.
Key to the Bluetooth SIG’s claims that LE Audio will enhance wireless stereo streaming is the introduction of something it calls “LE Isochronous Communication.” This technology overcomes the drawbacks of conventional Bluetooth Audio by making it possible for multiple devices, such as a pair of earbuds, to each receive their own unique audio streaming channel. The key to success is that the channels are “time bounded”; so, for example, the left and right channels retain the original precise musical timing upon replay.
LE Audio also brings a new codec to bear on the challenge of streaming stereo audio. Called the “Low Complexity Communication Codec” (LC3), the Bluetooth SIG claims the software is specifically designed to save battery power while also improving audio quality. The group also says LC3 “will provide improvements in audio quality over the SBC codec … even at a 50 percent lower bit rate.” .Design engineers estimate that batteries powering LE Audio should last at least 40 percent 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.
Steven Keeping gained a BEng (Hons.) degree at Brighton University, U.K., before working in the electronics divisions of Eurotherm and BOC for seven years. He then joined Electronic Production magazine and subsequently spent 13 years in senior editorial and publishing roles on electronics manufacturing, test, and design titles including What’s New in Electronics and Australian Electronics Engineering for Trinity Mirror, CMP and RBI in the U.K. and Australia. In 2006, Steven became a freelance journalist specializing in electronics. He is based in Sydney.