The purpose of developed 3D AudioSense Cape is to record surrounding audio synchronously along with other capes. Each cape, along with its BeagleBone host, becomes an audio sensor. Multiple sensors forms a network which effectively creates a wireless distributed microphone array. Recorded audio stream is transmitted over WiFi connection while the synchronization is provided by a separate low-latency radio channel.
A single 3D AudioSense Cape consists of following components:
What the 3D AudioSense Cape allows to do
What makes the 3D AudioSense Cape different from a typical audio cape is an ability of wireless synchronization with a master reference clock signal. The synchronization allows multiple capes to capture the sound field synchronously with no more than a single sample phase deviation. This unique feature makes recorded sound possible to be processed by algorithms which allow sound source separation, localization and so on. The synchronization signal along with recording control information is transmitted wirelessly from a single master device over a dedicated ISM band low-latency radio channel. A dedicated master device is responsible for an appropriate clock signal generation for the whole sensor network. Control commands are also sent by it. This allows very high flexibility of microphone array arrangement.
In order for the captured audio data to be useful, precise position of all microphones need to be known. 3D AudioSense Cape is equipped with a buzzer that allows it to send audio pulses. These pulses allows other capes to localize it by measuring sound propagation time. Precise localization is possible thanks to the wireless synchronization of all capes.
Detailed hardware description
The microphone preamplifier was specially designed for operation in a presence of electrical noise generated by digital circuitry. Both analog and digital components share the same power source which turned out to be a significant noise generator for the weak microphone audio signal. Both low and high frequency noise filters were applied to the power of the analog part of the cape. A special care was taken to the PCB layout in order to separate analog and digital parts and signal tracks from each other. The result is high quality audio signal free from any unwanted digital signal interference.
Analog signals from microphone preamplifiers are connected to line inputs of the audio converter chip. The analog input of the A/D converter provides analog programmable gain amplifiers (PGA) which can be used to adjust signal levels just before digitalization.
The same audio chip contains D/A converter with built-in headphone amplifier. Output of the amplifier is connected directly to a small speaker. The speaker is a 2kHz resonant buzzer which task is to transmit short audio pulses of that frequency. The connection of the buzzer to the D/A converter allows to precisely control shape of transmitted waveform.
The radio module operates in ISM frequency band, therefore no radio licensing is required. A special care was taken to the radio module selection. Due to necessity of reference clock transmission, the radio module cannot perform any channel coding and data encapsulation in packets. This ensures low-latency signal propagation from the master transmitter device to each cape. The drawback of such solution is higher susceptibility to interference. Fortunately appropriate FPGA modules togeather with analog PLL device are there to eliminate any impairments to synchronization clock and data transmission.
The FPGA chip is what binds everything together. The use of an FPGA is necessary due to presence of external wireless synchronization. FPGAs are an excellent choice when it comes to time critical signal processing. The key task of the FPGA device is decoding of incoming synchronization signal by separating audio clock from control data stream. Timestamps, received through the wireless channel, are then embedded into audio stream that comes from the A/D converter. The resulting data stream is then sent to the host BeagleBone board via SPI interface.
3D AudioSense is a research project which focuses on the capturing of spatial audio scene using a distributed wireless sensor network.