True wireless headsets (TWS headsets) continue to become more attractive due to an increase in battery life, enhanced features, attractive design and better price points. With headset manufacturers focusing on miniaturization and design improvements and quickly adopting features to enhance user experience they are able to attract the most demanding consumers in a strong and competitive market.
Looking at these new in-ear systems, at first glance they look like rather simple devices. On the contrary, TWS systems require a lot of electronics to be smart and user friendly, as illustrated in the high-level system overview in Figure 1.
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Figure 1: System Overview (Source: ams AG)
When thinking about TWS headsets in daily use, there are several ways for enhancing user experience and hassle-free user interface integration. A key issue that TWS system designers face is playtime duration, as battery space can be particularly limited in TWS headphones. Typical battery sizes of 25-80mA/h can be reached, which results in a playtime of 2-4 hours. Once the battery is empty, the earbud needs a fresh charge before it is ready for its next use.
Currently, the most advanced TWS headphones are shipped with a charger cradle for charging the batteries, rather than having a wire connected to each of the earpieces. The cradle includes a bigger battery and acts as handy compartment – as it is easy to lose the tiny earphones. This allows the user to charge the earbuds on-the-go without being dependent on a power outlet. The goal of this cradle/earbud configuration is to guarantee fully-loaded batteries at all times. This avoids the frustration of realizing at the beginning of a workout that your earbuds are not ready to use because the batteries are empty! Another aspect to enhance user experience is automatic start-up and pairing of the earbuds. The user doesn’t want to wait for the devices to pair or start when the earbuds have been inserted in the ear. It should be a seamless process without pressing any buttons to begin the pairing.
To make a standard TWS headset smart and user friendly, a key requirement is data exchange between the charger cradle and earbuds.
If the cradle senses the battery status of the earbuds, it can automatically start re-charging the earbuds. This continuous re-charging process is necessary due to the quiescent current consumption caused by the always-on Microcontroller Unit (MCU), as shown in Figure 2. Conversely, if the earbud senses an empty charging cradle, it can automatically inform the user via a Bluetooth notification to charge the cradle’s battery.
In terms of automatic start up and pairing, a smart connection would also be beneficial. If the cradle informs the earpiece that the compartment lid has been opened, the earbuds wake from their sleep mode and prepare the BT pairing process without the need to press a button on the earbuds to enable them.
As well as the enhanced user experience, a link between cradle and earbud can enable better industrial design, software updates, personalization of earbuds (name, EQ data) and transfer of music data to the earbuds, to name just a few application examples of a feature-rich and differentiating product in the market.
To get a clearer picture of the technical implementation, let’s dig deeper and have a look at the system in more detail, as shown in Figure 2.
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Figure 2: Detailed block diagram (Source: ams AG)
On the cradle side, the most important thing is, of course, the Li-Ion Battery and accompanying charger to enable the charging of its battery with the help of a standard 5V supply connected to a USB outlet. Power management blocks – like LDOs and DCDC converters – distribute the required supply voltages to the MCU and other devices placed in the cradle. A dedicated 5V supply is mandatory to provide power to the earbuds for charging their batteries. The always-on MCU acts as the central control unit of the cradle and is usually connected to several other sensors (lid detection, earbud detection), as well as to the charger to receive cradle-battery-status updates.
After a trigger event – such as opening the lid, inserting an earphone, or a request sent by the earphone – it exchanges the required information or sends commands/ data to the earbuds.
On the earbud side, the topology is basically very similar, but of course the Bluetooth SOC is additionally required. The MCU in the earbud directly communicates with the MCU on the cradle side, exchanging information back and forth.
Sensor-wise, there may be additional devices such as proximity sensors for ear-insertion detection, accelerator sensors, hear rate sensors for fitness devices, temperature sensors, and touch sensors.
As shown in Figure 2, multiple pins are needed to implement the smart functionality of the charger cradle and the earpieces. This fact comes with several disadvantages: to reach a high customer-acceptance level, TWS solutions can’t be dramatically larger than their wired competitors. So, the placement of additional poles on the earbud always leads to a compromise between space and features. In addition, the design and appearance are also affected negatively if several poles need to be placed on the earpiece. Certainly, one option is implementing a BLE (Bluetooth Low Energy Connection) link, however this would significantly affect the bill of material cost and increase software implementation effort.
A more elegant compromise is to enhance the features of the standard and mandatory two poles which are used for charging (GND and 5V) the earbuds. If the functionality of the two-wire connection is extended to allow for charging and simultaneous communication to the earpiece in parallel, all the smart and user-friendly features could be realized without drawbacks in physical space or design expectations. The resulting user experience can be further improved with a dedicated app, which benefits from the load of information the earbud is now able to provide to any smart device. A few examples are shown in the list below.
Battery status left
Battery status right
Cradle battery status
Name of left and right earbud or if it matches
Check for SW updates for charger cradle
Notification info about empty cradle battery (especially with altered battery)
To incorporate the functions listed above, a few modifications to Figure 2 are necessary that however make the system slightly more complicated. We need to find a way to utilize the 5 Volt power supply signal line to be able to transfer power as well as data over a single wire.
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Figure 3: Proposal for data communication over 5V power line (Source: ams AG)
A possible and simplified timing diagram is shown in Figure 3, which shows the 5V power signal line with data modulated directly to the signal line. The host side provides the 5V supply to be transferred to the client to charge a battery and the client could potentially modulate data which can be transferred to the host side. In ideal case semi-duplex communication can also be implemented where client and master share the single power line to alternatively modulate data, so as to be able to exchange data between the cradle and earbud.
For a proper implementation of such a single-wire-communication principle, various new system blocks are necessary to replace the two-serial-communication interface signal lines shown in Figure 2. On the host side, the easiest way to implement such a modulation principle is with a coil to reject high-frequency modulation content and a modulation resistor to modulate a voltage drop to the 5V power supply signal line. In addition to the coil, there is also a data modulator which could be implemented by a simple current sink.
When designing such a system, it’s important to find a good trade-off between modulation current and modulation voltage level to ensure the system is not sensitive to external electromagnetic interference. On the other hand, the modulation current used also influences the overall power consumption of the communication system. Another tricky but important parameter beside the absolute modulation current is its slew rate. Steep current ramps may cause electro-magnetic emissions which can result in reception problems with mobile phones, Bluetooth, or FM radios. There are regulations that have to be fulfilled, otherwise a final product might not receive a license to be sold in some markets. Furthermore, the modulator is also the line reader which is intended to read the modulated data from the client device – indicated in blue in Figure 4 – whereas the green data represents data generated by the host to be send to the client.
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Figure 4: Example half duplex modulation protocol (Source: ams AG)
In this proposal, each frame is divided into 64 slots transferring 30 bits of data from the host to the client and vice versa . Each frame starts with a synchronization pulse that is generated by the host and which is necessary for a client to synchronize its clocks – as the host and the client don’t share the same clock, and so the client needs to extract its clock from the data stream and synchronization pulse. On the other end of the frame, the client terminates each frame with a synchronization pulse for the host to indicate both devices are in sync. Needless to say, this example requires some pre-synchronization sequencing which can be part of a possible host-detection circuit. This block on the client is necessary to ensure that data modulation only happens if both host and client are connected. For this purpose, a possible solution is that the host starts emitting pulses to explore if a client is connected to the power terminal. Once the Startup Sync Detector detects the synchronization pulses, it can wake up the MCU inside the earbud to start responding to the sync pulses, and to indicate presence of a valid client and so begin synchronizing with each other. Line reader and data modulator fulfil the same purpose as the host side – to read and transmit data to and from host.
The coil LCLIENT and RMODC are used to block high-frequency content and modulate data to the supply line. Furthermore, the resistors help to receive better signal integrity, but this is more relevant if longer signal lines are available in the system. For short signal traces there is no impedance matching of the transmission line and PCB necessary. A further important consideration when looking more closely at the transmission line is the DC resistance. To reduce charging time of a TWS earbud, it is important to keep DC resistance low to avoid big voltage drops which might lead to reduced charger current, due to low input voltage at the charger input. Small form factor coils in particular often provide a high resistance, which is contra-productive to our goal of keeping the resistance at a minimum level, and so maximize the charging current while minimizing the charging time for end users.
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Figure 5: Two-wire power and communication block diagram (Source: ams AG)
Certainly, the TWS system evolution is at the very beginning with all its features and form factors. However, it will quickly reach its peak due to a strong and rapid growing competitive market and demanding customers who are constantly trying to push the limits of physical design. Miniaturization combined with extended battery life is key to ensuring TWS systems can find their way in everyone’s life – perhaps without even being noticed. These key requirements lead to a general problem of the proposed implementation structure, shown in Figure 5 . The system integration of the required blocks for the described communication interface (line reader, clock extraction unit, data modulator, startup sync detector) is of course not an easy task. Considering the existing size constraints inside small earbuds, it is most unlikely to be possible to use discrete components in the final form factor. In addition, the system inherits some complexity, therefore development requires good understanding and experience in analog and digital design. For many headphone companies, the effort is big enough to resign and stick with the drawbacks of adding additional poles, or simply not add any intelligence to their system.
An off-the-shelf power:communication (POW:COM) solution such as ams AG’s can significantly lower the hurdles of enabling a smart TWS system. This solution consists of the AS3442 (host device, inside the cradle) and the AS3447 (client device, inside the earbud) and adds even more functionality without adding too much more effort and development time. The interface to the AS3442/47 is a standard I2C interface to make integration effort low, whereas the communication between the two devices is a tailor-made communication interface which meets the technical requirements mentioned earlier. The interface provides a net data transfer rate of 1kBit/s. This data transfer rate includes all the necessary overhead as well as error handling to transfer error free data like battery status, serial numbers or user names. Data can be exchanged back-and-forth between cradle and earbud with simple I2C commands. Inside the AS3447, a dedicated memory space (“mailbox exchange register”) can be used to update e.g. the battery voltage level register. If the earbud MCU updates the value in the register, the cradle automatically gets an interrupt and is able to read out the value. That way, the cradle MCU always knows the battery voltage of the earbud and can decide if re-charging is necessary.
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Figure 6: POW:COM integration example True Wireless Earbud (Source: ams AG)
Of course, if the cradle battery becomes empty and needs to notify the earbud, the procedure is the same but in the other direction. In addition to the data exchange feature, the device offers several GPIOs which can be used to wake up or control external devices like the MCU, the Bluetooth SoC, external battery charger, sensors, or LEDs. A possible integration example of the POW:COM system is shown in Figure 6. It is quite clear to see that the complexity of the system shrinks tremendously if using the POW:COM system instead of multiple discrete functional blocks as shown previously. The integration of AS3442 and AS3447 into a TWS allows system designers to easily enable smart TWS systems while fulfilling the trend to miniaturization and extending battery life.
Horst Gether is Senior Product Manager and has joined ams AG in 2003 as application engineer for audio amplifiers and digital portable media players. In 2010 Horst became product manager for active noise cancelling products responsible for the technical product definition. He holds a master’s degree in automation engineering and is a member of the AAP Expert Listeners Panel.
Martin Denda is Staff Application Engineer and has joined ams AG in 2014 as application engineer for the business line audio & pressure sensors. His focus is on developing active noise cancellation headphones and customer support. Martin holds a master's degree in audio-engineering from the technical university of Graz.