The study of bats requires the ability to record and generate sound waves in the ultrasonic ranges. The required bandwidth for such systems is up to 100kHz, which is much higher than the common acoustic systems designed for humans. Studying bat echolocation capabilities and generating real time acoustic responses requires a minimal latency acoustic system. The few available systems on the market aren`t designed for real time signal processing and manipulation. In order to meet those constrains, a full acoustic system was developed. The system includes a Smart microphone module embedded with a microprocessor, a Speaker-amplifier and a Switch box which is capable of signal redirection and real time processing.
The PlayBack setup consists of three main modules: Smart Microphones, a Switch and speaker amplifiers. It is designed to monitor audio signals from multiple microphone modules and direct a signal from a selected microphone to an array of speaker modules including DSP options for audio processing in real time. The setup is designed to work at an audio bandwidth of 1-100 kHz at up to 1MS/S.
The casings for the modules have been designed in Fusion 360 and 3d printed Microphone Switch Amplifier
The system’s topology is such that the microphone modules are connected to the switch module using a standard RJ25 cable. The 6 wires of the cable are used to pass the following between the Smart microphones and the Switch: power to the module, trigger signal, the audio signal and an analog “quality” signal. The Switch module consists of two MCU`s (Micro Controller Unit), one is used as the Switch interface, and routes the signals while the other is used as a DSP MCU to implement signal processing in real time to the acoustic signal. The speaker amplifiers are connected through an RJ45 cable which in turn is used to supply power from the switch to the amplifiers and transmit the audio signals and the enable signal to the amplifier. The Switch module is connected to a Matlab GUI application running on a PC using an Xbee adapter for wireless operation. In addition, a Saleae logic analyzer is connected to the Switch allowing to monitor the analog signals and the trigger signal.
The Smart microphone module is designed to meet the required acoustic bandwidth of bat echolocation calls. In addition, since the system requirements are minimal latency for signal processing and signal manipulation, each of the microphones are embedded with a dedicated microprocessor which is responsible for the initial signal processing of the individual acoustic source. Such an approach allows spreading the computational power required by the system and simplifies the decision-making requirements of the Switch by reducing the acoustic information to a more simplified representation of an acoustic source. i.e. digital triggers / signal quality indication. This approach also allows future expending of the system since each of the microphones acts as an independent entity responsible for its own acoustic system. Adding more microphone to a system will not influence the required signal processing for the main unit.
The Switch Module is used as the center piece of the system. It connects all the Smart microphone modules and Speaker amplifiers, has an analog routing circuitry to route one of the microphone signals to several speakers. In addition, it acts as a power delivery system, converting a single supply source to several voltage levels for the various components of the system. The switch contains several status indicators such as active speaker LEDs, external trigger. Connectors for all the analog signals running through the system and generally configured status LEDs for future configurations. The switch module contains two MCUs, Switch MCU and DSP MCU, the Switch MCU is used as the system interface and responsible for routing the acoustic signals and monitor the trigger events from the Smart Microphones, while the DSP MCU is used as a DSP and implements various signal processing algorithms.
Generating sounds in the ultrasonic frequencies (above 20kHz) requires a dedicated amplifier. Since commercial amplifiers are usually designed for the human hearing range and are commonly implemented through power efficient amplifiers (Class D). These speakers work as digital switches at high frequencies above the human hearing thresholds. The high frequency digital switching is converted to an analog signal using the natural low pass filtering characteristics of the speaker coils. The attempt to use such an amplifier at an ultrasonic frequency will result in high frequency distortions if any sound is generated at all since the common amplifiers are implemented with filters to cut the ultrasonic ranges from the input signals.
To control the software parameters of the Smart Microphone, a GUI was created in Matlab. The Microphone has a UART communication over USB with the PC through the MCU`s micro USB connection or through an Xbee adapter board connected to its mate at the PC - as a wireless configuration. The GUI is designed with several section, Connection settings section, Parameters setting and display section, Messages section and Microphone updates section.
