ReChord

Re-Chord — re-harmonize the FiiO Echo Mini. A free, compile-from-source firmware for the Echo Mini digital audio player (Rockchip RKnanoC, ARM Cortex-M3), built to be modified — starting with the DSP / EQ.

What is ReChord?

ReChord is a custom firmware project for the FiiO Echo Mini. It builds a free, modifiable firmware from source, so you can add DSP effects (bass boost, reverb, custom EQ), change the UI, and flash it to your device.

The firmware is ~90% Rockchip RKnanoD SDK (the chip vendor's own code, legitimately available) + ~10% FiiO-specific app layer (UI, menus, services). ReChord compiles the SDK from source and rebuilds the FiiO layer as clean, modifiable C — a "Rockbox for the Echo Mini".

This is NOT a decompilation project anymore. The decompilation phase (2026) produced the knowledge and SDK integration that power ReChord. See docs/RE-HISTORY.md for the reverse-engineering notes that remain useful.

Project history

ReChord started as a reverse-engineering project: decompiling the stock FiiO Echo Mini firmware (all 23 versions, v1.2.5 → v3.8.0) with Ghidra, correlating each version's changes with its official changelog, and naming 852 functions (30.7%) to understand how the device works.

That phase revealed the key insight that changed the project's identity: the Echo Mini firmware is ~90% Rockchip RKnanoD SDK — the chip vendor's own source code — plus a thin ~10% FiiO app layer (UI, menus, services). Reconstructing the firmware from Ghidra's decompiled output would have been impractical (unreadable param_1/undefined4 C). Instead, ReChord:

1. Compiles the real Rockchip SDK from source (53 files: kernel, audio, codecs) — the foundation.
2. Rebuilds the FiiO app layer as clean, modifiable C, using the decompilation as the specification.
3. Focuses on DSP: the goal is to add custom effects (bass boost, reverb, EQ presets) to Effect.c and flash a modified firmware.

The reverse-engineering work is archived in docs/RE-HISTORY.md and docs/re/ — it remains the reference for the hardware map and the FiiO layer.

Status (Aug 2026)

ALL 53 Rockchip SDK source files compile with arm-none-eabi-gcc:

Layer	Files	Status
Kernel (OS, drivers, power)	29	✅ compiles
Audio (incl. DSP Effect.c)	8	✅ compiles
Codec wrappers (AAC, FLAC, MP3, OGG, DSD...)	16	✅ compiles
Codec .lib binaries	22	📦 ready to link

The DSP-effects mod target is ready: Effect.c's EffectProcess() is the per-frame audio hook. Modify it → recompile → flash.

Remaining to a flashable custom firmware:

1. Linker script (place codecs + kernel at segment addresses)
2. Link → flat binary
3. FiiO app/UI layer (from RE knowledge)
4. Flash via tools/pack_img.py

Full details: docs/STATUS.md

Quick build check

# any SDK .c compiles (see docs/STATUS.md for full include list)
arm-none-eabi-gcc -mcpu=cortex-m3 -mthumb -fsyntax-only \
  -include firmware/rockchip/include/armcc_compat.h \
  -Ifirmware/rockchip/include -Ifirmware/rockchip \
  -Ifirmware/rockchip/audio/Include -Ifirmware/rockchip/audio/AudioControl \
  -Ifirmware/rockchip/audio/Common -Ifirmware/rockchip/audio/RkEQ/Effect \
  -Ifirmware/rockchip/audio/RecordControl -Ifirmware/rockchip/audio/ID3 \
  -Ifirmware/rockchip/audio/Wav/WAV_LIB -Ifirmware/rockchip/audio/SSRC/resampler \
  -Ifirmware/rockchip/system/os -Ifirmware/rockchip/system/fileseek \
  -Ifirmware/rockchip/system/module_overlay -Ifirmware/rockchip/system/sysservice \
  -Ifirmware/rockchip/bbsystem \
  firmware/rockchip/bbsystem/Main2.c

Repository layout

firmware/rockchip/          # Rockchip RKnanoD SDK (source + codec .lib)
  ├── bbsystem/             # kernel base (Main2, CRU, DMA, interrupt, ...)
  ├── system/               # OS, fileseek, module_overlay, sysservice
  ├── audio/                # AudioControl, Effect (EQ/DSP), codecs
  └── include/              # ReChord integration headers (armcc compat, etc.)
firmware/                   # (SDK + FiiO layer + decompiled reference)
tools/                      # build, analyze, pack scripts
docs/                       # STATUS, RE-HISTORY, memory map, flashing
stock/                      # official firmware backups (not distributed)

Roadmap

• SDK kernel compiles (29 files)
• SDK audio compiles (8 files, incl. DSP Effect.c)
• SDK codecs compile (16 wrappers + 22 .lib ready)
• Linker script + link → section_3 binary
• QEMU smoke test (Cortex-M3)
• FiiO app/UI layer (from RE knowledge)
• First custom DSP effect
• Flash + verify on hardware

Safety / flashing

• Flashing uses the official upgrade method (copy IMG + reboot) — the stock firmware is always restorable by copying it back.
• tools/pack_img.py preserves the bootloader and resources; only section_3 (code) is replaced.
• See docs/FLASHING.md for the safe procedure.

Disclaimer

ReChord is an independent project. It is not affiliated with FiiO, SNOWSKY, or Rockchip. It uses the publicly-available Rockchip RKnanoD SDK and independent reverse-engineering notes. Firmware images are not distributed; obtain official firmware from FiiO. Flashing modified firmware carries risk — use at your own risk.