Previously in The Audio Voice we explored the development and ongoing capabilities of Hearables in the form of True Wireless Stereo (TWS) earphones. The appeal of TWS is undeniable - surely Hearables in the form of smart earphones will become as ubiquitous as smartphones. Sooner or later we won't even have to take our smartphones out of our pockets anymore, just listen to the voice in our ears and respond with a quiet utterance. On-board music libraries, continuous access to the Internet, total hands-free control by voice command, and a health tracking functionality will infuse TWS earphones into our lives more intimately than even the smartphone.
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True Wireless Stereo (TWS) Earbuds are a fast-expanding application area for NFMI technology.
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Hurdles to overcome include very dense mechanical packaging and battery size constraints, making it hard to include additional features while chasing the long battery operation that users crave. Some of the most intriguing use cases are literally all day long and well beyond the battery charge duration of Hearables today. Imagine your Hearable working hands-free while keeping your smartphone pocketed, filtering outside sounds to what you would like to hear, or even acting as hearing sound enhancement and hearing protection at concerts.
Game-changing technology that would move Hearables' performance closer to consumers' expectations such as much higher density batteries and next-generation Bluetooth antenna technology are not low-hanging fruit. But more positive news is the gains made with the integration of Near-Field Magnetic Induction (NFMI) into TWS earphones. NFMI has proven pivotal for TWS Hearables, enabling longer battery life, stable performance, reduced signal latency, and increased security over existing Bluetooth designs.
Today, the dominant technology implementing the connection with the smartphone is Bluetooth, but this approach alone is not a fit for TWS earphones. Sending a stereo audio stream to two distinct earbuds is not possible using standard Bluetooth A2DP profile: it is a point-to-point solution. As current Bluetooth profiles do not allow for sending stereo audio from a smartphone to two wireless earbuds at the same time, the solution attempted by first generation TWS was to send a stereo stream to one earbud and next forward one of the audio streams to the other earbud reusing the same Bluetooth device. This does not work reliably with head movement, a hand touching or passing your ear, etc. would lead to dropouts, constant re-pairing, along with increased power consumption and in the end too many product returns.
Bluetooth is simply not the right choice for communication between earphones as these microwaves are mostly soaked up by the brain between the earphones. Aside from performance, there is the open question of long-term health impact implications of Bluetooth cross-body and especially ear-to-ear communications for users. NFMI topology uses Bluetooth from the smartphone as the server to one earpiece, which then passes the alternative audio channel to the second earpiece using NFMI rather than RF Bluetooth.
When we think of electromagnetic waves, we're typically thinking of radio frequency (RF), and in the case of wireless Personal Area Networks (PAN), that has been Bluetooth. RF signals are all essentially the same. The signal is loaded with energy and sent out of the antenna to travel until the signal runs out of energy; that's why RF is called a far-field transmission. Far-field transmission is appropriate over long distances, but there are real problems when lots of devices use RF or Bluetooth simultaneously. When too many devices use the same radio frequency too close to each other, they interfere with each other. When you pack a signal with enough energy to travel way past the device with which you need to speak, you're wasting power. We can't stop RF signals from propagating into space, and out there with it goes the handshake and encryption protocols... that's why Bluetooth devices have been hacked over a mile away.
Bluetooth has its appealing attributes including a large established user base, relatively low power consumption, and mature support. Many designers and product managers have come to accept Bluetooth's vulnerabilities but there is significant room for a better user experience. At trade shows, we all have seen too many wireless products (Bluetooth, Wi-Fi, ZigBee, cordless phones, baby monitors, and even microwave ovens) all using the same frequency bands shut each other down. The commercial success of RF wireless products and the resultant crowding of the bands is now the cause of their shortcomings. Then, we also have Bluetooth's lack of security and excessive latency vs. quality trade-offs.
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Bluetooth True Wireless Stereo enhanced with Cross-Body NFMI Streaming.
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Our focus here is Near-Field Magnetic Induction (NFMI), which offers characteristics appropriate for use with short-range wireless audio and data connection between earbuds. NFMI works at a very low frequency (typically 10.6 MHz), resulting in a radio that is ultra-low power. In the January 2017 edition of audioXpress you can find a Near-Field Magnetic Induction overview by Dr. Michael Abrams of FreeLinc, debating those benefits and applications.
FreeLinc began by manufacturing and distributing NFMI-enabled radio accessories to law enforcement and public safety officers, where the need for secure and reliable short-range wireless connectivity was the most immediate. There are now more than 2,500 US Public Safety agencies using FreeLinc products, including the FBI and the Secret Service. NFMI has evolved through multiple product cycles in the highly demanding law enforcement market.
The first practical NFMI integrated chip solution for TWS applications was introduced by NXP with the NxH2280 in 2016. The NxH2281 introduced further improved audio performance at the start of 2017. Future NxH series enhancements will maintain performance and low power while shrinking both cost and package size.
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NFMI uses a tightly coupled, low power, non-propagating field between devices - sort of a lossy transformer.
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What Is Near-Field Magnetic Induction?
NFMI is a modulated low-frequency magnetic field - not RF but a loose coupling of low power, non-propagating magnetic field between devices for near-field communication. Essentially, the operation can be modeled as a weakly coupled transformer - AC current in primary induces AC voltage in secondary winding. The antenna design rules are basic and range can be accurately defined by the size and positioning of the antenna(s) up to a distance of 1 meter. NFMI is human body friendly - the magnetic field goes through (body) tissue with negligible absorption so there is no degradation of signal strength, and the specific absorption rate is 10,000 times less than for Bluetooth. For these reasons, NFMI has been used for more than a decade in medical devices such as hearing aids.
Most of the second wave of hearable TWS earphones have evolved to NFMI designs such as: The Headphone and The Dash by Bragi, Yevo1 from Yevo Labs, Alpha Skybuds, Earin M2, Click Mymanu, Jabra Sport Elite, andIQ Buds Nuheara. Apple's AirPods use their own proprietary solution and there are viable Bluetooth TWS examples that have implemented sophisticated directional and diversity antenna schemes.
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NXP Semiconductors introduced the second-generation fully integrated NxH2280 NFMI single-chip radio transceiver in 2016. In 2017, the NxH2281 introduced further improved audio performance.
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RF and NFMI and How They Differ
Near-field communication as a modulated AC current (such as an audio signal) in the primary induces AC voltage in secondary winding (antenna). For TWS, NFMI earphone applications, typically a 2 mm x 6 mm antenna coil is used in each earpiece. Longer distances up to a meter can be achieved but with a larger antenna and optimum orientation. Multiple antennas can be used as well for stable low power over highly defined range - like your inflight entertainment business class seat area. A flight with a few hundred Bluetooth systems in use would never work, while NFMI could be interference free.
Magnetic fields decay much faster than Bluetooth so NFMI actually creates a very tight coverage area around you. Steep degradation of magnetic induction signal strength as function of distance reduces interference level, therefore, it increases robustness and privacy, enabling separate use of the same carrier frequency for multiple headsets in a shared area.
Within this area multiple devices reliably connect, and outside they simply cannot be seen. Invisible to Bluetooth and Wi-Fi, NFMI does not cause interference in other devices or wireless networks. Once you've moved a distance away from the coverage area, the same frequency can be used to create another distinct coverage area. It's not physically possible to overcrowd the airwaves with NFMI like RF. And NFMI uses a fraction of the power used by Bluetooth to move the same amount of information. The power level is so low that NFMI communications are below the EMC background thresholds set by the Federal Communications Commission.
Aside from TWS earphones, NFMI has been successfully applied in hearing aids, pacemakers, and other applications where the robustness and integrity of the wireless link must be trusted. Future near-field applications range from super low latency bi-directional streaming for gamers, in-ear monitoring (IEM) for musicians, and interference-free inflight entertainment headphones without the mess of cables.
A future issue of audioXpress magazine will feature in-depth and exclusive coverage of NFMI.
