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XBee Walkie Talkie




Hardware components:
Goldilocks Analogue
Still as prototype currently, but functionality can be recreated with MCP4822 DAC, Microphone Amplifier, and Headphone Amplifier, together with Arduino Uno.
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MAX9744
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MAX9814
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MCP4921 DAC
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A000066 iso both
Arduino UNO & Genuino UNO
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Arduino Wireless Shield (Xbee)
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Story

XBee Walkie Talkie

I’m building an advanced Arduino clone based on the AVR ATmega1284p MCU with some special features including a 12 bit DAC MCP4822, headphone amplifier, 2x SPI Memory (SRAM, EEPROM), and a SD Card. There are many real world applications for analogue outputs, but because the Arduino platform doesn’t have integrated DAC capability there are very few published applications for analogue signals. A Walkie Talkie is one example of using digital and analogue together to make a simple but very useful project.

Goldilocks Analogue – Prototype 3Goldilocks Analogue XBee Walkie Talkie

The actual Walkie Talkie functionality is really only a few lines of code, but it is built on a foundation of analogue input (sampling), analogue output on the SPI bus to the MCP4822 DAC, sample timing routines, and the XBee digital radio platform. Let’s start from the top and then dig down through the layers.

XBee Radio

I am using XBee Pro S2B radios, configured to communicate point to point. For the XBee Pro there needs to be one radio configured as the Coordinator, and the other as a Router. There are configuration guides on the Internet.

I have configured the radios to wait the maximum inter-character time before sending a packet, which implies that the packets will be set only when full (84 bytes). This maximises the radio throughput. Raw throughput is 250 kbit/s, but the actual user data rate is limited to about 32 kbit/s. This has an impact on the sampling rate and therefore quality of speech that can be transmitted.

Using 8 bit samples, I have found that about 3 kHz sampling generates about as much data as can be transmitted without compression. I’m leaving compression for another project.

For more detail: XBee Walkie Talkie

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