AuraPlug: Synchronized LE Audio!

I was driving across the US when I had the idea for AuraPlug - that was nearly half a year ago, and I was moving to New York while burning through the last of my funds after working with an audio startup. The Sonic Sprint challenge was the perfect opportunity to bring my idea to life, and in a full circle moment I designed it on another road trip this summer!

Imagine plugging a thumbdrive-sized dongle into multiple speakers and synchronizing them in real time over Bluetooth without installing any software - thanks to Auracast that's possible!

Out of the box, the NRF5340 Audio Dev Kits support BIS (broadcast isochronous stream) and CIS (connected isochronous stream) audio applications: the first which allows broadcasting to multiple LE Audio devices from a single Bluetooth source, and the second facilitating True Wireless Stereo! (Previous wireless stereo implementations like Airpods were kind of a "hack" on top of the Bluetooth classic stack).

Early Prototypes

The Audio Dev Kit has an onboard mono DAC (Cirrus CS47L63) since the kits are designed to be used in tandem for testing True Wireless Stereo (using one Dev Kit per earpiece). This is unfortunate since I wanted to design a stereo plug around a single NRF5340 IC! I found some leads suggesting the PCM5102, a stereo DAC from Texas Instruments, and I roped my friend Michael into the project to develop firmware while I designed a handheld receiver unit.

The EByte E83 was the smallest hand-solderable package I could source on Amazon (yes I hand soldered all 65 BGA pads), and I also found a ubiquitous PCM5102 breakout. Validating concepts with known working components before jumping into a fully SMD product is generally a good idea :)

Zephyr OS was a steep learning curve - with no public documentation on using the E83 except the bones of those who tried before us - we wrote our own board overlay file, contended with memory issues to fit a graphics library in our program, and Michael implemented some tricks to combat latency & audio artifacts in about a month! He had no previous embedded dev experience, and I had never even looked into Zephyr.

Yet - we built built a receiver that could run for hours on a 100mAh battery, and were able to toggle between multiple broadcast isochronous streams on the receiver! To test this we used another E83 as a USB Audio transmitter.

SoAuraPlug only needs to tune into one stream and that should be pretty easy right?

Designing AuraPlug

My first design was un-manufacturable. Well - costly. The NRF5340 comes in both QFN and WLCSP packages, and to my surprise I had more issue designing with the larger QFN package! The via-in-pads and trace tolerances were pushing PCBWay's manufacturing capabilities, and I was quoted at $140 per board - without assembly!

This was my first foray into 4 layer impedance matched PCB design - and also camping. I was on a national park road trip across America with a friend, and I spent hours reading datasheets & reference designs on the road, downloading Altium talks whenever we passed through range of cell service.

It really only took 2 days to redesign my board around the WLCSP, and somewhere while driving through Colorado I sent them off to be manufactured. No via-in-pads, larger tolerances, and more board space meant I was even able to add NFC and status LED's onboard! I imagined it might be useful to pair AuraPlug to a source transmitter by using NFC with a mobile app.

The design also includes two buttons for volume, an NPM1100 for battery management, and a MAX17048 for monitoring battery levels. I felt smug finding a board mounted TRS audio jack from Same Sky, and designed a simple case to reinforce it.

Designing a Transmitter

I like to travel light and have a proclivity for breaking hardware, so I looked for small NRF5340 dongles to replace the Audio DK's and E83's I was using to test audio broadcasting. To my surprise there were none!

I whipped up a simple design using the same schematic as the AuraPlug, in an outline slightly thicker than an edge-mounted USB-C plug.

In Transit

At the time of writing, both designs are still in production as I wait for NRF5340-CEAA and NPM1100's to ship across the world. I'm at a tractor hacking conference in Iowa, anxiously awaiting their arrival and continuing to revise my designs as I pour over Rick Hartley PCB design lectures on YouTube.

The concept is simple: low-latency, synchronized audio across multiple speakers without any software - eventually to be implemented for use cases like silent disco or guided museum tours - and we hope to iterate on the handheld console to improve TWS for applications like movie theater broadcasts in multiple languages!

LE Audio is redefining accessible audio tech for everyone, and its great to be an early part of the never ending RnD journey. Thank you to everyone at PCBWay, Hackster, and Nordic that made our project possible! Updates coming soon :)