# A Wearable Microphone Array Helmet for Automotive Applications
## 2 Materials and Methods
### Electronics Design  
The system, whose scheme is depicted in Figure 6, is composed of a helmet and a digital-to-analog (D/A) box. The helmet houses microphones and the electronic board to collect the signals, while the D/A box is connected either to a personal computer (PC) or an external acquisition system. When a PC is employed, the entire system (after A/D conversion) is full-digital, while a double A/D and D/A conversion is introduced when an external recording system is employed. However, this solution guarantees a robust signal-to-noise (S/N) ratio even inside a car cockpit, which is always electrically noisy.  
Figure 5: Ideal 3D directivity of the first \(\mathbf{v}=\) 16 Spherical Harmonics (Ambisonics 3 rd order).  
The system was designed to use the A\({}^{2}\)B bus by Analog Devices [56], which can handle up to 32 channels on a single Unshielded Twisted Pair (UTP), up to 15 m distance. An A\({}^{2}\)B network consists of a main node and up to 10 subordinate nodes arranged in daisy-chain. Dedicated transceivers handle bus access and data flow, eliminating the need for extra devices that would increase the cost and complexity of the system. An A\({}^{2}\)B network provides a bandwidth of 50 Mbit/s, enabling the transmission of up to 32 channels at the standard sample rate of 48 kHz with 16-, 24-, or 32-bits resolutions. This digital bus is particularly suitable for transmitting audio signals in both microphone arrays [57] and loudspeaker arrays [58], since it guarantees a deterministic latency of just two samples at 48 kHz and the synchronization between all the channels [59].  
The electronic board installed in the helmet is provided with eight 4-channels, 16 bits, audio A/D converters, and type TLV320 ADC6140 by Burr-Brown, and it also integrates the A\({}^{2}\)B transceiver to acquire and deliver the data via A\({}^{2}\)B bus. The A/D converters (specification are reported in Table 2) are connected to the transceiver via two Time-Division Multiplexing (TDM) 4 digital lines (TDM0 and TDM1). The signals are transferred from the helmet to the D/A box unit over an Ethernet cable that also carries the 5 Vdc power supply for the board installed inside the helmet. The ethernet cable was chosen as the physical layer of the A\({}^{2}\)B bus, being cheap and common, with robust connectors that are easy to insert and remove. Note that a standard Ethernet cable includes four UTP cables, of which one is used for the A\({}^{2}\)B bus and one for the power supply.  
The D/A box receives the digital signals from the A\({}^{2}\)B bus and transmits them to a PC via Universal Serial Bus (USB) or to an external acquisition system, after the D/A conversion. This feature is of particular interest for Noise, Vibration, and Harshness (NVH) applications since standard automotive equipment, like the SCADA by Siemens, are usually required by car makers. In addition, when such systems are employed, additional sensors like accelerometers or microphones can be recorded synchronously with the helmet array, e.g., opening to the implementation of more advanced ANC algorithms. The D/A box is made of two boards: an A\({}^{2}\)B main board and an A\({}^{2}\)B subordinate board. The A\({}^{2}\)B main board generates the A\({}^{2}\)B network and provides the clock to the A\({}^{2}\)B subordinate board and the USB connection to deliver digital data to a PC. The A\({}^{2}\)B subordinate board features two 16-channels, 16 bits, D/A converters, type ADAU1966 by Analog Devices (specifications are resumed in Table 2), and it is provided with 32 BNC connectors to connect to external  
Figure 6: Block scheme of entire system: A/D board installed inside the helmet (**left**), D/A box (**right**).  
acquisition systems. In addition, the D/A box produces all the supply voltages for the system, starting from the 12 V on-board battery of the vehicle. To isolate the system from disturbances of the vehicle's electrical system, usually noise, all the regulators are designed to ensure full galvanic insulation.