The QUAD 3 integrated amplifier lacks direct input selection via remote control, requiring users to cycle through sources sequentially. To automate input switching, the author performed reverse engineering on the device's firmware.
The firmware is a raw application image for the ARM Cortex-M3 architecture, likely running on an STM32F100 microcontroller. Analysis revealed that the internal code already supports direct input selection via a function that handles source changes, including mute, routing, and display updates.
The author obtained the official firmware binary, version 1.14j, from the manufacturer. The analysis confirmed the absence of RSA or ECDSA digital signatures, allowing for potential modifications. The system uses a NEC-like IR protocol for remote control commands.
By identifying unused IR commands and mapping them to the internal source-switching function, the author enabled direct input selection. This approach avoids hardware modifications, relying instead on the existing firmware logic to manage the audio path and state transitions.
