Auto Analog Audio & NRF52840: Now working on non-Mbed cores

 Auto Analog Audio & NRF52840: Now working on non-Mbed cores

Playing around with the AAAudio library

So I finally decided to make the necessary purchase and get myself a relatively cheap oscilloscope, and with that play around more with the AAAudio library. Even with the added view into the output signal and what is going on using the scope, I still haven't been able to figure out what I'm doing wrong with the I2S interface, so for now the audio is not of the highest quality, which is relatively good for voice, but not so good for higher frequency audio signals.

I have, however, made progress on using the non-MBED core for both input and output of audio using the PDM (microphone) and PWM (amp/speaker) interfaces, which opens up more of the capabilities of this device! In the end, once I actually quit working with the I2S interface, it wasn't that difficult getting the non-MBED core to work. This means that AAAudio should work on other devices like the Feather 52840 Express etc, but it hasn't been fully tested yet. The PWM interface can handle ~10-bit audio at 16Khz, and lower at higher sample rates.

It also allows users to adjust between 20khz and 16khz PDM samples now as well!

Users can specify which pins to use for PWM by editing the library src/NRF52840/AutoAnalogAudio.cpp file and specifying the GPIO pin and port numbers, else it defaults to GPIO #5, port 0 for output.

As of this writing, users would need to download and install the library from ZIP directly from here because I will not do a new release for a while. (Still testing and potentially updating the new capabilities)

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 

Networking and communication with Nordic NRF52 devices, NRF24L01 & Arduino

Networking and communication with Nordic NRF52 devices, NRF24L01 & Arduino

Opening up new possibilities

So I recently created a new library (nrf_to_nrf) to communicate with NRF24L01 devices with the new NRF52x devices like the XIAO BLE SENSE 52840 sent to me by Seeed Studio. These devices open up some new possibilities as far as networking go and can be put to use in many different applications. 

For example, the XIAO Sense 52840 has an on-board Gyro & Accelerometer making it useful for things like simple quadcopters, drones etc, and if used in combination with a high power NRF24-based transmitter, long distance control can be achieved with this tiny device. It is also very much suitable for Audio transmission and reception, allowing full-duplex audio transmission and reception at 1Mbps (using the ACK-Payload functionality) of the radio devices. 

In contrast to the NRF24L01 which has 6 addressable pipes and can transmit up to 32-byte payloads, the NRF52x devices have 8 addressable pipes and can transmit up to 127-byte payloads. This opens up things quite a bit when it comes to networking these devices, allowing mixed NRF24L01 and NRF52 networks, and when NRF52x devices are solely used, expands the number of devices in RF24Network and RF24Mesh networks and the maximum transmission sizes. The NRF52x devices also have on-board encryption capabilities, enabled in nrf_to_nrf, so users can now create encrypted networks and communications scenarios.

I really like the NRF52x devices, specifically this little XIAO board, there are no pins to connect to the radio, and no issues with power supplies etc. I recently also updated the Auto Analog Audio library I created to support recording on these devices as well, making it possible to transmit audio over radio link using that library in combination with the nrf_to_nrf library. The code is still in its initial phases, without full functionality yet.

Modifications required to use the full capabilities of NRF52 only networks:

nrf_to_nrf layer: Set the maximum payload size. Up to 127 with CRC disabled or 123 with 16-bit CRC. Use radio.setPayloadSize(125); or radio.enableDynamicPayloads(123);

RF24Network: Open RF24Network.h and set MAX_FRAME_SIZE to 111, Open RF24Network_config.h and set NUM_PIPES to 8

RF24Mesh: Open RF24Mesh_config.h and set MESH_MAX_CHILDREN to 6

See https://github.com/TMRh20 and https://github.com/nRF24 for all related libraries and documentation.