Low-power snoring detection on a smartwatch

For a sleep apnea study, Withings wanted snoring detection on an existing smartwatch, the nRF52-based ScanWatch 2. My assigned scope was research and power estimation. I also brought the audio path up in firmware, on my own initiative, because I wanted something testable in a real watch.

Drivers. I wrote C drivers for an Asahi Kasei AK5707 low-power audio ADC: I2C for control, I2S for audio, driving an analog microphone. No driver existed, only a register map and closed-source test software. I then adapted the watch firmware to feed an existing Withings snoring-detection model.

Wake-on-sound. The AK5707 has an acoustic activity analyzer that can wake the MCU on sound. On its own it was not enough: snoring is a continuous sound, so during snoring it woke the MCU almost constantly. Adding duty cycling fixed that. The analyzer listens 30 s out of every 120 s. This works because snoring lasts longer than one listening window.

Through a sealed case. The microphone sits inside a watertight casing. I measured the casing's frequency response, with an enclosed and an open reference mic on the same playback. I chose the filter empirically, by running detection on filtered recordings: a 400 Hz first-order high-pass, plus an adaptive detection threshold. Three people wore the prototype overnight, and I compared its detections against a Withings sleep mat.