The Audio Tracker is a simple audio recording device for wildlife monitoring applications. It is able to record 700h of continuous audio data at 11kHz samplerate using a single 3.7V 1000mAh LiPo battery. Up to 1500h can be recorded on a 32GB SD card and 2 batteries.
The clever part is the analog font-end involving a low-power analog preamplifier and a Sigma-Delta modulator that directly sends PDM data to the MCU. Advantages of using Sigma-Delta modulation is double: Low-power and noise shaping removing the need for anti-aliasing filters.
A low-power STM32L476 has been chosen for this project because the peripheral availability:
SDMMC for interfacing SD Card
DFSDM for interfacing the Sigma-Delta modulator
The MCU is sleeping most of the time, while DFSDM works with DMA to transfer incoming audio sample to memory buffer. About every 600ms, the DMA HT/TC interrupt is fired and the MCU is woke up. Samples are then compressed using ADPCM algoritm and stored onto the SD Card. The whole process is most often 6ms long, but the memory buffer must accomodate casual very long writing times (close to 1s) due to SD internal housekeeping.
The microphone is a Knowles® MEMS device, with analog output. It is waterproof.