# Design and Implementation of a Multi-Function Hydrophone for Underwater Acoustic Application
## 1 Introduction
The first digital hydrophone system was standalone and was used to record the raw underwater acoustic data. The second digital hydrophone system was a digital passive acoustic transducer array whose main capability was to provide directional sound source information for hydroacoustic surveys. In another work [13], the raw underwater acoustic data stream was recorded by an underwater acoustic MODE, and a secure digital (SD) card with 64 GB capacity was used to store raw signals. Unfortunately, such application costs high power consumption, and the storage is limited. An underwater acoustic recorder was designed using a single board computer [14], the maximum sampling frequency was 48 kilo-symbols per second (kSPS), and the power consumption was about 1.5 watts. The current design of digital hydrophones has become more mature. From the perspective of new products launched in recent years, many devices were developed by different famous companies. For example, the smart digital hydrophone named icListen was developed by Ocean Sonics [15]. IcListen can be used as an acoustic recorder, it has ultra-low noise and a wide bandwidth range from 10 Hz to 200 kHz. There are many series of icListens for different applications, for more details, please refer to the website [15]. A snap recorder for storing the underwater acoustic data stream was developed by Logger-head Instruments company [16], the maximum sampling frequency was 96 kHz, and the maximum working depth was 7000 m when a titanium case was used. There are also some other commercial smart hydrophones. For example, the remote underwater digital acoustic recorder was designed by Cetacean Research Technology [17], the SoundTrap recorder was designed by Ocean Instruments [18], and the DORI underwater acoustic recorder was designed by Abyssens [19]. Many different underwater acoustic devices have been designed in recent years. However, such devices typically have a single function, or their parameters are not configurable, or they have high power consumption, do not have a compact size, etc. In addition, the use of commercial underwater recorders is not the best and feasible choice, since their high cost and lack of flexibility. Multi-function and parameters-configurable underwater acoustic device design is a new trend and requirement, since it is suitable for autonomous underwater vehicles (AUVs), unmanned underwater vehicles (UUVs), etc. The key technical difficulty is how to implement such multiple functions under the limited resources of a micro-controller unit (MCU). To address the problems mentioned above, a compact, multi-function, parameters-configurable hydrophone is designed and implemented in this paper. In particular, the multiple functions include local data preservation, real-time raw data upload, and direct sequence spread spectrum (DSSS) receiver. The software and the hardware of the proposed multi-function hydrophone are introduced in detail. Specifically, the principles and the flowcharts of different functions are described. The pre-amplifier, the analog-to-digital converter (ADC), and MCU are also depicted. To address the limited resources of MCU, memory multiplex is used, and some integrated algorithms from the digital signal processing (DSP) library are used to improve efficiency. Finally, sea trials verified the proposed multi-function hydrophone. The trial results demonstrate that the proposed multi-function hydrophone has good potential to explore the ocean. The proposed multi-function hydrophone can be used for underwater acoustic communication, underwater ambient noise collection and analysis, marine pre-warning, etc. The remaining paper is organized as follows. Section 2 describes the system overview including the hardware and the software. In Section 3, the experimental results are described and analyzed in detail. Finally, some conclusions are drawn in Section 4.