ReChord

Re-Chord — re-harmonize the FiiO Echo Mini. A free, from-source custom firmware (CWF) for the Echo Mini — a "Rockbox for the FiiO".

Objective (the whole point)

ReChord is a complete custom firmware, written from source — not a patch, not a byte-mod of the stock binary. The goal is to have all the firmware source code available and modifiable, covering both halves of the device:

• UI — custom menus, screens, navigation, theme (what you see).
• DSP / audio — custom effects, EQ, codecs, playback (what you hear).

Everything compiles from source and is flashed as a normal firmware update. No binary patching of the stock firmware. No stock app layer left over.

Architecture (the key insight)

The Echo Mini runs two firmwares side by side, communicating over a hardware mailbox (MailBox A2B/B2A):

Firmware	IMG offset	Role	SDK source
fw1 (AP)	0x0007B8 – 0x057820	UI — menus, navigation, fonts, I2C, file browser	RKnanoD SDK — App/UI build (main.c + SDK_160_128/UI/ 45 .c)
section_3 (BB)	0x081A14 – 0x9BAA0E	Audio — codecs, DSP/EQ, playback engine	RKnanoD SDK — BB build (Main2.c + codecs)

• Both firmwares are two builds of the same Rockchip RKnanoD SDK (confirmed by string match: fw1 = 223 RKnanoD_MP3 strings vs only 32 rk3399-table). The AP is the main.c + UI/ build; the BB is the Main2.c + codecs build.
• We compile both SDK builds from source and write our own app layer on top (our UI for the AP, our DSP/effects for the BB).
• The mask ROM (0x00000000–0x02FFFFFF) is the fixed hardware HAL (LCD, I2S, DAC, USB, keys) — it is not firmware we replace; it is the "BIOS" our code calls.

This is NOT a decompilation project, and NOT a patch project. The reverse-engineering phase (2026) produced the knowledge (addresses, SDK integration, hardware map) that makes the from-source build possible. See docs/RE-HISTORY.md.

Status (Aug 2026)

Layer	Status
Rockchip SDK (BB: kernel + audio + codecs)	✅ compiles (53 .c)
BB custom boot (firmware_entry → rechord_main on the SDK)	✅ builds, links, QEMU-verified
BB display (our menu → LCD)	🟡 in progress (framebuffer → LCD DMA is the open problem)
AP / UI from scratch (menus, navigation, fonts)	⬜ next — needs fw1 mapped
DSP effects (Effect.c / EffectProcess)	⬜ hook ready, effects to write
Flash (pack IMG → copy + reboot)	✅ pipeline verified (byte-exact header/resources)

• Ghidra project: FIIO-3.7.0-Decomp, language ARM:LE:32:v8-m (both fw1/AP and section_3/BB imported; section_3 loads at 0x03000000).
• tools/pack_img.py replaces section_3 (BB) and preserves bootloader + resources. Replacing fw1 (AP) is the missing piece for the UI.

Full details: docs/STATUS.md · docs/dispatch-map.md · docs/community.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/                   # our from-source layer (rechord_app, UI, stubs)
tools/                      # build, analyze, pack scripts
docs/                       # STATUS, dispatch-map, community, hardware, flashing
stock/                      # official firmware backups (not distributed)

Roadmap

• Rockchip SDK compiles (kernel + audio + codecs) — the BB base
• BB custom boot (firmware_entry → our own rechord_main)
• QEMU smoke test (boot + framebuffer menu + text rendering)
• AP identified: fw1 = RKnanoC SDK (rk3399-table-RKNanoC, 103 .c)
• BB display: get our framebuffer to the LCD (find the ROM DMA/transfer)
• AP (fw1) UI: compile the RKnanoC SDK (drivers + FileSys + NANO_OS) + our menus
• DSP effects: custom EQ / bass / reverb in EffectProcess
• Flash the full CWF (AP + BB) and verify on hardware

Safety / flashing

• Flashing uses the official upgrade method (copy IMG + reboot) — stock is always restorable by copying the original back.
• No signature verification in hardware (confirmed by the community — docs/community.md).
• tools/pack_img.py preserves bootloader + resources; it replaces the code section(s) cleanly.
• 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.