Encrypted Audio Comms with XIAO 52840 Sense

Encrypted Audio Comms with XIAO 52840 Sense

The beginnings of wireless audio comms

I've been playing around with the NRF52840 boards I have, and now that I have basic audio functionality working via PDM microphone and PWM audio output, the next thing to do is test out an encrypted wireless communication scenario, so that's what I've done.

The devices use the AutoAnalogAudio and nrf_to_nrf libraries created by me to communicate wirelessly. I've also enabled encryption at the nrf_to_nrf layer, and the related code is an example of implementation, with the device randomly generating a new key and re-keying every 30 seconds. This will prevent others from decrypting the payloads or performing replay attacks outside the 30-second windows. 

This means that users would need to start the devices at the same time (within a 30-second window), so they start-up using the same key. After 30-seconds, a new key is generated and configured, so both devices will then use the same new key. The receiving device ensures that the old key matches before accepting the new key, so unauthorized users can't simply send a new key or replay an old key. Any power down of either device will result in lost communication unless the other device is restarted within the same 30-second window.

To use the sketches (posted lower down), users would need a XIAO 52840 Sense board and both of the aforementioned libraries installed, then just run the code. No wires to connect, no power supply issues to worry about, no counterfeit nrf24 devices to worry about, just a straightforward device that works.

There are still a few issues with synchronization of the audio vs how the device handles PWM data. Essentially, you can have the PWM pin default to a LOW or HIGH state when the current PWM sequence ends, which happens during radio communication when packets are lost or slow to transmit. Compared to other devices I've worked with, which maintain the current state of the pin when PWM or DAC data ends, this device will revert the pin to either HIGH or LOW which causes a popping or clicking sound. 

I'm not sure how to work around this currently, but will continue to experiment to try and achieve better results. In the mean-time users can try out the prototype code by downloading the sketches here: https://github.com/TMRh20/Sketches/tree/master/XIAO-EncryptedAudio

Recording and Playback of Audio on the XIAO NRF52840 Sense - Auto Analog Audio

 Recording and Playback of Audio on the XIAO NRF52840 Sense

Auto Analog Audio Library

So I've been struggling with the I2S interface of the NRF52 devices, and have given up for the time-being trying to get it to work properly. In the meantime I've made some decent headway with the PWM interface and reproducing audio that way. This is similar to the TMRpcm library for AVR devices, which also uses Pulse-Width-Modulation to reproduce audio. 

So far the AutoAnalogAudio library is in a very basic but functional state with an included example to demonstrate how to record and playback audio on the XIAO 52840 Sense. It is designed to input audio from the PDM microphone directly and output using Pin5 of the XIAO board via PWM. The audio signal is 16-bit, 16kHz audio, so of reasonable quality, and cannot currently be modified. The code is still in its infancy.

This makes it easy to record and transmit audio over radio link, since with the nrf_to_nrf radio library, users can broadcast the audio to another device very easily. 

There are still a few problems with it, mainly some synchronization issues, which result in a clicking sound when audio is fed directly from the microphone into the PWM output (Amp & Speaker). I'm not quite sure how to resolve it currently, so will leave things as-is. Update: Adjusting the timers slightly to make the PWM a bit slower than PDM input results in a smooth output signal.

A new release will not be made for a little while, so to try it out, just install the AAAudio library directly from ZIP. See GitHub at  https://github.com/TMRh20/AutoAnalogAudio