ARLA/CLUSTER: Walkie Talkie experimental em 434 MHz com o sistema de
voz digital Codec2
João Costa > CT1FBF
Quarta-Feira, 23 de Julho de 2014 - 13:33:29 WEST
434 MHz Codec2 Walkie Talkie
Mike McCauley VK4AMM reports he has developed a 434 MHz Walkie Talkie
using the amateur radio Codec2 digital voice mode
On the Codec2 reflector he posted:
Just a quick report on a recent project I built: A low bandwidth/duty
cycle Codec2 packetised Walkie Talkie
It implements a Codec2 http://www.rowetel.com/blog/codec2.htmlencoded
walkie-talkie on an STM32F4 Discovery board
Each station has:
STM32F4 Discovery board (using onboard MEMS microphone and Audio out
chip to headphone, and user button as PTT) and ST Firmware Codec2 in
Optional HopeRF RFM22 radio module (connected to SPI1) at 434MHz
RadioHead http://www.airspayce.com/mikem/arduino/RadioHead drivers and
Arduino compatibility code
The free pdm_fir FIR filter from volkov.oleg
The MEMS Microphone is sampled at 1024 kHz and FIR filtered to 4 kHz
bandwidth (this all happens in an interrupt handler).
When the PTT button is pressed, microphone samples (160x16bits at a
time = 20 ms of voice) are compressed with Codec2 MODE_3200, producing
8 bytes of compressed voice. These 8 bytes are then sent by
RadioHead::RH_RF22 driver (sent using the broadcast address).
When the PTT is not pressed, it listens for 8 byte messages received
from RadioHead::RH_RF22. Each one is decompressed with Codec2 and the
resulting 160x16 bit = 20 ms samples are duplicated to stereo and sent
to the output DAC by DMA, and can be heard through headphones.
The RF22 modem is configured for GFSK_Rb125Fd125 (125 kHz bandwidth,
125 kHz deviation GFSK), which gives good quality sound and plenty of
extra air space for interleaved transmitters (possibly privately
Over the air, by analog radio, it just sounds like 50 Hz buzz.
Lower data rates also work. Min workable RF22 modulation is prob about
9.6 kbps (based on RadioHead addressing/preamble/header/crc overheads
You can also use RadioHead's RH_Serial driver for transmitting fair
quality voice over 9600 baud serial.
Should also (courtesy of RadioHead) be able to support the very
interesting RFM95 LoRa family of radios, with long range and spread
Or, using the RadioHead RH_RFM69 driver, you could also have AES
encryption of the radio data for extra security.
Or, switch between all types of RadioHead driver.
Latest version and build instructions etc can be found at
This is NOT a finished product. It is intended to be a
proof-of-concept for further experimentation by other amateurs.
Mike McCauley VK4AMM
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Read the David Rowe VK5DGR blog at
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