ReChord

Re-harmonize the FiiO Echo Mini.

A free, from-source custom firmware for the Echo Mini — not a patch, not a byte-mod of the stock binary.

dual-core · from source · GPL-3.0

What works · Architecture · Build · Flash · Docs

Warning

Work in progress. ReChord is experimental, but no longer silent: the from-source fw1 boots, the UI/library work, and audio plays from 16/44.1 up to 24-bit / 96 kHz — Core B decodes and the AP drives the CS43131 DAC. Track-to-track gapless works within one format (same rate / channels / depth); a format change stays a hard boundary by design, because the DAC has to re-lock (an album that mixes rates, e.g. 96k and 44.1k, will show a short pause there). Still open: the 192 kHz edge case (FLAC max_block 16384 needs a ~143 KB decode window) and the DSP chain. Driver/hardware changes must be verified on physical hardware before merge.

What works today

• Boots standalone — ROM → our reset → our Main; the stock AP layer is gone.
• UI — monochrome house look (black/white + one grey, inverted selection, 1 px rules, no debug clutter outside the System view); menus, keys, file browser over eMMC + microSD (FAT16/32/exFAT), themes, Now Playing with title/artist, position/duration, stream format, i/N and SHUF/REP1, plus list wrap-around and cursor memory.
• Storage & USB — eMMC read/write, microSD read, USB mass storage, self-updater. With a cable in, "off" parks the player (dark + silent + still charging) and a power press wakes it — the rail cut only works on battery.
• Audio — Core B decodes FLAC / MP3 / WAV; the AP owns the DAC/I2S bring-up and feeds the CS43131 from a shared PCM ring.
• Hi-res — int32 left-justified end-to-end, source bit depth reported to the DAC; 24-bit at 44.1/48/88.2/96 kHz verified on device.
• Library — recursive media index (v3, dual INDEX.A/INDEX.B slots with a CRC-checked commit) with Album / Artist / Genre / Year / Label / Album Artist / Composer and All Songs; names sort case-insensitively, paths up to 208 chars. A reindex is only needed when the layout changes.
• Playback lists — playing a song inside a list queues that list, shows i/N, auto-advances, with Rockbox-style shuffle (reorders the list) and repeat off/all/one, both persisted in rechord.cfg.
• Resume — the resume setting (off/folder/track) restores the last track at its position plus the browser folder/row from /.rechord/state, written in place and deferred. It opens paused (never autoplays).
• Diagnostics — a RAM log that persists itself (append + rotate at 32 KiB, written when there is news, at most every 30 s) so a session can be read back without plugging the USB cable.

Architecture

The Echo Mini is a dual Cortex-M3 (RKnanoD). ReChord ships a from-source fw1 (AP) plus its own Core B image, started by our own CRU sequence:

                shared memory (0x0102xxxx..0x0106xxxx)
  ┌───────────────┐   A→B file rings  ┌──────────────────┐
  │  fw1  (AP)    │ ───────────────▶ │  Core B (CAL_M3) │
  │  UI/services  │   (current +     │  FLAC/MP3/WAV    │
  │  DAC / I2S    │    pre-opened)   │  decoder         │
  └───────┬───────┘ ◀─────────────── └──────────────────┘
          │           PCM ring (64 KiB)
          ▼
     CS43131 DAC  →  headphones

Part	Role
fw1 (AP)	Boot, UI, services, filesystem, player state machine, DAC/I2S bring-up, I2S TX-IRQ feeder that drains the PCM ring
Core B	Decodes the track (our codecs) and produces the PCM ring; data/pool/stack live in its own memory region
Mask ROM	Fixed hardware HAL (LCD, I2S, DAC, USB, keys) — not replaced; it is the "BIOS" our code calls

At a track boundary the AP hands Core B the next file without stopping the stream: the next file is pre-opened into a second file ring ~3 s before the end (with its codec opened and its FLAC prescan geometry published), and the boundary is a NEXT command over the live ring. Details: docs/rewrite/core-b-phase3.md · docs/rewrite/audio.md · docs/rewrite/player.md.

This is not a decompilation project and not a patch project. The reverse-engineering phase produced the addresses, the SDK integration and the hardware map that make a from-source build possible — see docs/RE-HISTORY.md.

Status

Area	State
fw1 boots standalone	✅
UI, file browser, storage, USB, self-updater	✅
DAC renderer (I2S1 → CS43131)	✅ audible
Core B bring-up + audio (B decodes, AP feeds)	✅
Hi-res 24-bit @ 44.1/48/88.2/96 kHz	✅
Library index + category navigation	✅
Bluetooth headphones (external UART module, A2DP source)	✅ verified on device: pairing, streaming and headphone keys (docs/rewrite/bluetooth.md)
Playback lists, i/N, shuffle, repeat	✅
Gapless between same-format tracks (Core B, BRNOMT-73)	✅ verified on device
Format-change boundary	hard by design (DAC re-lock, ~200 ms)
96 kHz margins (ring cover, FIFO refill)	🟡 every time budget halves at 96k; occasional crackles under investigation
192 kHz (FLAC block 16384)	⬜ rate is in the table; needs a ~143 KB decode window (the workbuf is 80,896 B)
384 kHz	⬜ not in the rate table / untested
DSP effects in the playback path	🟢 5–10 band EQ on Core B with runtime profiles (docs/rewrite/eq.md)
RMF1 module loader	🟡 host-tested; later milestone

Build & test

# Host tests (player: codecs/playlist/gapless/ReplayGain · UI · services ·
# drivers · modules)
sh firmware/player/tests/run_tests.sh
sh firmware/ui/tests/run_tests.sh
sh firmware/services/tests/run_tests.sh
sh firmware/drivers/tests/run_tests.sh
sh firmware/modules/tests/run_tests.sh

# Clean-room product image (our AP + Core B + from-source codecs)
make               # = make standalone-ap5m -> build/ReChord_AP5M.IMG
make cfw-ap        # ELF only -> build/cfw/rechord_ap.elf

# Reproducibility acceptance: two clean worktrees, byte-identical image
make clean-clone-check

make needs no stock image, no community/ SDK and no proprietary .lib. The legacy release/all and cfw-img targets were removed (BRNOMT-82).

Flash

Copy build/ReChord_AP5M.IMG to the device root as HIFIEC90.IMG, then eject (or unplug — the power key also scans) → confirm on the device prompt → power-cycle. Full procedure: docs/FLASHING.md. Recovery: docs/DEBRICK-LINUX.md.

Repository layout

firmware/rockchip/            # Rockchip RKnanoD SDK (source + codec .lib)
  ├── bbsystem/               # kernel base (Main2, CRU, DMA, interrupt, ...)
  ├── system/                 # OS, fileseek, module_overlay, sysservice, debug
  ├── audio/                  # AudioControl, Effect (EQ/DSP), codecs
  └── include/                # ReChord integration headers (armcc compat, etc.)
firmware/                     # our from-source layer
  ├── startup/                # AP boot: ap_startup/ap_main/ap_ui + vectors + UI loop
  ├── app/                    # clean-room app/registry glue (host-tested wiring)
  ├── ui/                     # screens (menu, browser, player, settings, library)
  ├── services/               # settings, log, input, FAT/exFAT fs, media index,
  │                           #   store, fw_update
  ├── drivers/                # panel, eMMC, SD (DW-MMC), USB MSC/CDC, I2S, DAC
  ├── player/                 # engine: player.c, playlist.c, bfs.c, ipc_*, codecs/
  │   └── codecs/             #   wav + minimp3 + dr_flac vendored (CC0 / MIT-0)
  ├── bb/                     # Core B image: decoder + codec_io over the file rings
  ├── bb_stub/                # minimal section_3 stub image (bb-stub target)
  ├── audio/                  # DAC/I2S HAL: CS43131 + rate table + TX-IRQ feeder
  ├── dsp/                    # effect modules (EQ, bass, volume) — not linked yet
  ├── modules/                # RMF1 module loader/registry — later milestone
  ├── theme/                  # theme data + loader
  ├── libc/                   # freestanding libc (mem/str/stdio/math shims)
  └── (dev trees: hook, qemu, mb1, config, util)
tools/                        # build, analyze, pack scripts
docs/                         # HANDOVER (start here), STATUS, re/, rewrite/
stock/                        # official firmware backups (not distributed)

Roadmap

• Rockchip SDK compiles (kernel + audio + codecs)
• BB custom boot (firmware_entry → our own rechord_main)
• AP (fw1) boots standalone with our own from-source UI
• Storage: eMMC + microSD, FAT/exFAT browser, USB MSC, self-update
• DAC renderer: I2S1 → CS43131 — first audible sound
• Two-core audio: our Core B image started by our CRU sequence
• Core B decodes (phase 3c) — the AP keeps the DAC and file service
• Hi-res path: int32 end-to-end, source depth to the DAC
• Library: recursive index + category navigation + All Songs
• Playback lists: i/N, shuffle, repeat (+ persistence)
• Gapless between same-format tracks on Core B — verified on device; a format change stays a hard boundary (the DAC must re-lock)
• Resume: last track/position + browser folder/row in /.rechord/state (resume off/folder/track), restored paused
• 96 kHz margins — 96k halves the ring cover and the TX-FIFO refill margin; occasional crackles there are still being chased
• 192 kHz (bigger decode window) and 384 kHz
• EQ 5–10 bands with runtime profiles on Core B (docs/rewrite/eq.md)
• ReplayGain track/album/auto normalization (docs/rewrite/audio.md)
• Backlight brightness 1–10 (PWM on GPIO2.4)
• RMF1 modules (loader host-tested; later milestone)

Documentation

• docs/HANDOVER.md — session log and decisions (start here)
• docs/THEMING.md — theme package format, skin language and runtime variables
• docs/STATUS.md — build status and history
• docs/rewrite/ — architecture: audio, player, DAC, Core B
• docs/re/ — reverse-engineering notes and decompilation
• docs/HARDWARE.md · docs/TRM-notes.md — SoC map
• docs/FLASHING.md · docs/DEBRICK-LINUX.md — flash / recovery

Safety

• Flashing uses the official upgrade method; stock is always restorable by copying the original image back.
• Images are validated by our packer (scatter-table gate, CRC trailer) before they can be flashed; recovery is via maskrom.
• No signature verification exists in hardware (community-confirmed). Flashing modified firmware carries risk — use at your own risk.

License

GPL-3.0 — see LICENSE. Vendored decoders keep their own licenses (minimp3 CC0, dr_flac MIT-0).

Disclaimer

ReChord is an independent project, not affiliated with FiiO, SNOWSKY or Rockchip. It builds on the publicly available Rockchip RKnanoD SDK and independent reverse-engineering notes. Firmware images are not distributed; obtain official firmware from FiiO.

Acknowledgments

• recho.atzkey.workers.dev by atzkey — the pioneering Echo Mini CFW. ReChord is a completely independent project in implementation and codebase, but we gratefully acknowledge RECHO as prior inspiration for custom firmware on this hardware.
• echo-mini-opus by berettavexee — reference implementation and technical insights for fixed-point LibOpus porting and memory-constrained audio decoding on the Rockchip RKnano platform.
• EchoPod by BasilSkyWalk (GPL-3.0) — two pieces of ReChord are adapted from its hardware work on the same player: the 1–10 backlight brightness mapping / 10 kHz GPIO2.4 PWM (panel_io_rknanoc.c), and the Bluetooth implementation — the external module's UART1 wiring and ASCII protocol, the GPIO2.0/2.14 audio routing and the stock housekeeping, the connect/scan/retry timings, the play/pause-notice rules and the idle power-down behaviour. ReChord's implementation is its own (firmware/drivers/bt_hw.c, firmware/services/bt_proto.c, bt_link.c, bt_store.c, ui/screens/bluetooth_screen.c), but the hardware contract and field findings come from EchoPod's bluetooth-audio branch. Details: docs/rewrite/bluetooth.md.