Initial Testing, Notes & Progress on new PCB for Wireless Audio Prototyping

 Initial Testing, Notes & Progress on new PCB for Wireless Audio Prototyping

 Testing out the new circuit boards and getting the built-in microphone working

 If you weren't following along I just received some new custom built circuit boards from PCBWay.com and have been testing things out extensively. Everything works! I even got the custom-added I2S microphone working with the nRF52840.

The boards support functionality like voice transmission over the built-in radio, and can function as a radio, intercom or simple music player etc. 

                                            A full test of the new circuit boards

Getting the Microphone working with the AutoAnalogAudio library: 

The nRF52840 Feather Express/Sense does have an I2S interface, but it only supports up to a 24-bit window, whereas the SPH0645LM4H expects 32 SCK pulses per audio frame (18 bits of data + 14 bits of padding) and a 64 x BCK signal, where the nRF52840 typically does a 48 x BCK signal.

After a bit of searching, I found that the appropriate signal could be created by the PWM peripheral of the 52840, and the I2S interface would just need to be configured with special settings to grab the 18-bits of data properly.

The main drawback with using this mode is the I2S interface can only operate in either Input or Output mode, they can't be run at the same time unless both I2S Input & Output devices use the same settings. I had to make some modifications to the AutoAnalogAudio library, released in v1.54.0, in order to allow switching between I2S modes.

Currently, this mode is only supported for input devices. 

                                                                The Feather Express 52840 

Configuration for the Microphone: 

To use the new special settings, users need to set the aaAudio.manualI2S variable to True before calling the aaAudio.begin(2,0); function.

With the Feather Express, the LRCK or WS pin needs to be configured from the default of 29, to something like 28, which is A3 on the Feather Express 52840

Results: 

 I've put together a full Radio Transceiver example for this PCB, found at https://github.com/TMRh20/FeatherAudio/tree/main/Examples/I2S_TransmitterAndReceiver

In order to use the example, upload to two Feather 52840 Express or Sense boards, and plug them into the custom PCB, or manually attach external I2S microphone & amplifier.  

The example features 3 buttons, the PTT button is used to transmit, then there are two for Volume Up/Down. If users press and hold Volume Down, then Press Volume Up, the device will go into a continuous transmission mode, not requiring the PTT button to be pressed.

The device also toggles the onboard red LED when data is received.

 Audio is relatively high quality at 16-bits and 32kHz, so it is pretty clear and useful for many different applications.

 

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)