Evidence at a glance
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What was done
Reverse engineering of the Fosi Audio MD3 DAC revealed hardware architecture and enabled a custom ESP32-S3 firmware replacement. The project provides tools for factory restoration and control, though it remains limited by hardware design constraints.
The Fosi Audio MD3 is a USB-C DAC and headphone amplifier featuring an ESP32-S3R8 microcontroller and a Savitech SA9312L audio bridge. Reverse engineering efforts documented the hardware pin map, DAC initialization, and the Savitech link protocol via disassembly. The project provides a from-scratch replacement firmware built with ESP-IDF 5.5.1 and LVGL 9.2, which maintains parity with stock features while adding a 24-hour day-wheel clock and a USB control channel. Claude, using Claude Code, contributed the stock firmware disassembly, the self-reflashing path, and the USB control channel implementation. The hardware presents several limitations, including a shared USB port that prevents simultaneous audio and photo synchronization, and a non-functional radio on the tested unit. Users are advised to dump the original 32 MB flash before modification, as no factory firmware is included in the repository. Detailed findings and hardware limits are available at https://raw.githubusercontent.com/benevolentfutures/fosi-md3/HEAD/README.md.
Reproduce it
Dump the full 32 MB flash twice before any modifications. Use the provided Python host tools in the tools/ directory to manage content, deploy firmware, and perform factory restoration. Do not write to the bootloader or partition table, and avoid sending unknown HID output reports to the Savitech chip.
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