Zylia BeagleBone MicArray Cape

Description


The MicArray Cape provides two electret microphones along with A/D converter, low noise preamplifiers and FPGA to allow recording of surrounding audio. Each cape, along with its BeagleBone host, becomes an audio sensor. Any number of sensors can create a net where every device is synchronized with each other.

The developed cape is capable of audio signal recording with a resolution of 16 bits and sampling rate of 48 kHz for both microphones. It is sufficient for most professional and amateur recording purposes. Built-in EM172 microphones are characterized with 80dB signal to noise ratio. For more demanding users there is a single 3.5mm Jack connector that allows to attach an external microphone pair. A hardware switch is used to select between built-in and external microphone for each channel independently.

What makes the MicArray Cape different from a typical audio cape is an ability of wireless synchronization with a master reference clock signal (currently under development). 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 other similar tasks. 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. The synchronization module is optional, the cape can record without it.

In order for the captured audio data to be useful, precise position of all microphones need to be known. The MicArray 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.

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.

Software Support

 * BeagleBone MicArray Cape is supported by Debian Jessie (source files, git-source and instructions how to install)

Getting Started
Required setup:
 * A BeagleBone MicArray Cape
 * A BeagleBone Black with supported software
 * A WiFi adapter
 * A 5V DC power supply

Follow the instructions below to start using your MicArray Cape:
 * 1) Mount the BeagleBone to the connectors on the bottom side of the MicArray Cape.
 * 2) Make sure the SD card is inserted in the slot at the bottom of the BeagleBone Black and has image that supports the MicArray Cape.
 * 3) Plug in the WiFi adapter (we highly recommend using USB hub).
 * 4) Plug in a 5V DC power supply to the DC connector on the BeagleBone.
 * 5) Log in to the BeagleBone Black via ssh or using connected keyboard and a display.
 * 6) Download required software which will be available here (installation of additional software may be required, e. g. GStreamer in version ≥ 1.4.0) and follow further instructions that are in the README file.

Specifications
Following are some specifications of the BeagleBone MicArray Cape:

Signal Usage
The BeagleBone MicArray Cape uses following signals:
 * GND: P9_1
 * VDD_3V3: P9_3, P9_4
 * SYS_5V: P9_7, P9_8
 * SYS_RESET: P9_10
 * SPI1: P9_28, P9_29, P9_30, P9_31

Connected but currently not used:
 * UART4: P9_11, P9_13, P8_33, P8_35
 * UART5: P8_31, P8_32, P8_37, P8_38
 * P9_19, P9_20, P9_25

Distributors

 * Zylia

Product Images
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