Design considerations for Bluetooth headsets [multiple images]

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In order to quickly design a Bluetooth headset product that can bring a pleasant experience to end users, it is necessary to consider multiple factors such as Bluetooth chips, Bluetooth protocol stacks and headset configuration software, software and hardware development kits, reference designs, interoperability testing, and local technical support. This article discusses and analyzes these design considerations.

Bluetooth headsets are in great demand due to their ease of use, especially in countries where the use of mobile phones while driving is strictly prohibited by law. Since Bluetooth headsets are truly independent of the phone, mobile phone users can enjoy the latest mobile phone features such as built-in cameras and PDA functions from multiple different manufacturers without having to change headsets every time.

The huge market demand has driven the diversification of the end-user market. The headset market is now divided into low-end, mid-end and high-end, which allows Bluetooth headset providers to choose their target market so that they can provide more features that differentiate their products from competing products, or choose to enter the low-end, low-cost and high-volume market.

Figure 1: Block diagram of a mono Bluetooth headset.

Low-end, easy-to-use, low-cost mono headsets are still very popular and can be bundled with new mobile phones. Mid-range headsets are more attractive to experienced Bluetooth users who usually want more features such as noise cancellation, LCD screen, call vibration and voice recognition. The headset itself is more like a mini phone for this application. Sound quality and voice clarity are very important to branded headset manufacturers. To improve sound quality, mid-range headset chips now have on-chip DSPs to run echo cancellation and noise suppression software such as Clear Voice Capture (cVc).

The headset market is further fragmented as manufacturers introduce headsets designed specifically for women. Designed to fit women with long hair and sunglasses, these headsets are more like jewelry, worn around the neck or like a brooch. These products may have small earpieces that are carefully inserted into the ear and can be removed after each call, which is better than traditional designs.

These new headset products are driving more device-level integration and require additional features such as DSPs for improved sound quality through echo cancellation and noise suppression, on-chip battery charging circuits, and switch-mode power supplies.

Design Challenges

Today's Bluetooth headset designers face many challenges, including not only the size and weight of the final product, but also other issues such as power consumption, sound quality and interoperability, in addition to other pressures such as time to market, overall cost and final "Bluetooth Certification Body (BQB)" testing. Even in addition to all these factors that need to be considered, the headset itself must not only be powerful, but also must be practical, easy to use and beautiful in appearance to attract a wide range of users. Therefore, when designing a Bluetooth headset, it is necessary to consider multiple factors such as Bluetooth chips, Bluetooth protocol stacks and headset configuration software, software and hardware development kits, reference designs, interoperability testing and local technical support.

Bluetooth chip

The Bluetooth chip itself is just one of many components in the headset. When considering which chip is best for the headset design, there are several factors that need to be carefully considered, namely cost, size, feature integration and power consumption.

Figure 1 is a block diagram of the design principle of a typical mono headset. Ideally, we can integrate many functions into the chip to reduce the material cost, design size and weight of the design. If all the modules in Figure 1 are included in the chip, the cost may be very high because it will require multiple circuit blocks in one package. For example, stacking technology can integrate analog, digital circuits and multiple voltages in the same package. Although such chips are available on the market, they are too expensive for high-volume, low-cost designs. A more practical solution is to integrate the RF radio, memory, interface (including voice codec), microcontroller, battery charging circuit, clock generator, DAC and switch mode power supply module (SMPSU) on one chip, leaving only the unbalanced transformer, filter, speaker, microphone, battery, user button and LED outside the chip.

Figure 2: Bluetooth headset software architecture.

In addition, we should also consider the package type. The package should be as small as possible and lead-free to meet global environmental requirements. It should also be easy to produce to keep manufacturing costs to a minimum. For example, a package such as TFBGA with an outer size of 8×8mm and a pin pitch of 1.0mm can achieve a good balance between manufacturability and size. In addition, the battery is the main factor affecting the overall size of the headset. The size of a typical lithium-ion battery designed for headsets is about 5×12×45mm, which is much larger than the 8×8mm chip package.

Bluetooth Software

The underlying hardware and firmware of Bluetooth are basically determined by the Bluetooth specification, and they are very similar in target terminal applications such as mobile phones, computer keyboards and mice. The main difference between different suppliers lies in the level and scope of their interoperability testing. Although some device-level features for headsets such as battery charging circuits and SMPSUs have been described above, many headset-specific features are written and defined in software.

There are two types of headset chips: Flash-based chips, which are suitable for small-volume production or pilot production, with the headset software stored in flash; and ROM-based chips, where the headset software is stored in on-chip ROM and user-configurable key codes are stored in off-chip EEPROM. Therefore, in ROM-based chips, the headset features are loaded from the EEPROM into the Bluetooth device at power-up, while flash-based chips have the advantage of being able to upgrade the software to accommodate new phones. But today, with extensive interoperability testing, it is no longer necessary to limit yourself to flash-based chips in mass production. Most designs for mass applications have now moved to ROM-based to reduce costs and reduce risk by allowing the use of EEPROM to store code.

The Bluetooth protocol stack and headset profile are stored in read-only memory (ROM) and included in the headset software. A module-specific chip reference parameter called the "Permanent Storage Key (PS Key)" is used to set the information, and the PS Key is stored in an external small rewritable memory (EEPROM).

Figure 2 shows the headset-specific software system architecture running on the headset-specific Bluetooth ROM chip.

In addition to radio calibration, the Bluetooth standard requires that each device be assigned a unique identification code called a Bluetooth address, which is set using a module-specific PS Key.

The module-specific PS Key is used to configure the dedicated parameters of each module, which are mainly aimed at the radio performance of the module. Bluetooth will provide recommended test plans and test software, and provide support for the establishment of a Bluetooth headset production test system.

The Human Machine Interface (HMI) configuration key is set once during the development process (when defining the headset user interface) and is subsequently programmed into each headset.

Software and hardware development system

As chips on the market today become more complex, chip manufacturers will provide high-quality development systems to minimize the time to market for new products. A typical hardware development system will include a well-designed, fully functional application design development board with headset software preloaded into the Bluetooth device, as well as the appropriate software, application guides, data sheets, and user setup guides required to change the PS Key value. For most Bluetooth headset designs, it is best to have a ready-made headset solution that allows for sufficient user configuration in the PS Key to meet end-user requirements. This approach is ideal for quickly launching a headset product that has been fully interoperability tested and has relatively low risk.

But in order to change the deeply embedded functions, a software development kit (SDK) must be used, such as the CSR BlueLab SDK, which enables the highest level of headset customization.

Figure 3: Block diagram of the overall Bluetooth headset solution.

Reference Design

The Bluetooth antenna and RF design is still the most complex part of the entire Bluetooth headset design and requires approval before it can be carried out. A good reference design is very valuable because it defines the actual PCB layout and wiring that can be used to ensure that any on-board components will not cause interference or EMI problems. Bluetooth device manufacturers should be able to provide everything needed for reference design and its production, including BOM with all known material prices, design schematics, and film files for PCB layout and wiring to ensure that the final design can be successful at the first time.

Interoperability

Interoperability is the primary consideration when designing any Bluetooth product. CSR's laboratory in Cambridge specializes in interoperability testing to ensure that designs based on CSR's Bluetooth chips can interoperate with all existing and new Bluetooth products. Extensive testing makes CSR products unique and helps produce reliable and easy-to-use Bluetooth headset products. Extensive interoperability testing can also ensure a very pleasant and immediate end-user experience, and this first-class end-user experience can reduce the end-user technical support that requires resources.

Conclusion

The design of Bluetooth headsets is not only related to the chip, but also to the overall solution provided by wireless chip manufacturers (such as CSR). It requires careful consideration of the entire end-to-end design process from board layout and wiring, software, selection of suitable chips to reference design, product development and production testing (including interoperability testing). In addition, high-quality local technical support is also very important to ensure the success of terminal products and customer satisfaction.

By: Karen Parnell
Product Marketing Manager
Email: karen.parnell@csr.com
CSR