The Linkplay A31 is a WiFi audio streaming module (System on Module, SoM) developed to be used in wireless speakers and wireless audio systems. The module seems to be sold under different names (Rakoit, Wiimu).
Because the module itself does not have a DAC and amplifier, it is typically connected to another board in many consumer products. For example, in the DYON Area L WiFi speaker it is connected to an C.AP8064.05 amplifier board (PCB back, PCB front with serial cable attached to the A31, MVsilicon AP8064 MCU datasheet).
The aim of this project is to replace the vendor firmware by an open source solution. With the following instructions you can install an OpenWrt system with network streaming capabilities for Apple Airplay devices.
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The Linkplay A31 V04 module (PCB front, datasheet) is based on a MediaTek MT7688AN MCU (32-bit MIPS 24KEc) with 64MB RAM and 16MB W25Q128 SPI NOR flash. The module offers pins for I2S, I2C, ethernet, USB, UART and 5 GPIOs.
The module has an undocumented serial port used as Linux console ttyS1 (RX/TX pins next to the WiFi antenna, full PCB). The serial port works with 5V (not 3V3) and 57600 8N1.
The module uses a patched version of U-Boot 1.1.3 (Ralink UBoot Version 4.3.0.0), a (MediaTek SDK) patched Linux kernel 2.6.36 and a weird combination of proprietary and OSS/GNU operating system components.
Other users thankfully have comprehensivly analysed the system and vendor application layer: Crymeiriver started reverse engineering some years ago, Jan21493 added a ton of more information and AndersFluur wrote an inofficial API documentation.
mt7628an_linkplay_a31.dts is a Device Tree Source file which describes the hardware components of the Linkplay A31 WiFi module.
The DTS configures the A31 as an I2S slave (see A31 manual section 2.3) using a generic S/PDIF transmitter. For I2S master configuration, you'll need to add a real codec, see DTS files of other boards if needed.
Original firmware has a 'firmware' partition with size 0x730000 at 0x250000, a jffs2 'user' partitition with size 0x80000 at 0x980000 and a jffs2 'user2' partition with size 0x600000 at 0xa00000. The DTS drops both user partitions in favour of a bigger 'firmware' partition (writeable, with OverlayFS).
The WiFi module GPIOs 1-5 (PCB pins 23, 22, 27, 26, 19) are wired to MT7688AN pins 14-18, one can access them from Linux via GPIOs 494 (GPIO2), 495 (GPIO1), 496 (GPIO4), 497 (GPIO3) and 498 (GPIO5).
Make sure to read the general OpenWrt docs here and here and at least install the necessary build packages.
Then proceed like this:
git clone https://github.com/hn/linkplay-a31.git
git clone https://git.openwrt.org/openwrt/openwrt.git
cd openwrt
git checkout v23.05.2
./scripts/feeds update -a
./scripts/feeds install -a
../linkplay-a31/openwrt-linkplay-a31/prepare-openwrt-a31.sh
# edit 'files/etc/config/wireless' and set your WiFi credentials
make -j$(nproc) defconfig download clean world
Output images are found in ./bin/targets/ramips/mt76x8.
The script includes a patch which works around the MT7688 from crashing the entire system when I2S audio playback is stopped manually.
The OpenWrt image can easily be tested without writing anything to flash (no permanent modifications). You can use self-compiled packages or packages provided by this project.
Install Kermit (apt-get install ckermit),
connect the serial console of the A31
and execute ../linkplay-a31/linkplay-a31-serial-upload-kermit
(change port /dev/ttyUSB0 if necessary).
Reset (e.g. powercycle) the A31.
The Kermit script will interrupt the A31 boot process,
upload the openwrt-ramips-mt76x8-linkplay_a31-initramfs-kernel.bin image
via loadb and boot the system from RAM (bootm).
Be patient, the serial upload will take roughly 20-25 minutes to finish.
You might want to edit /etc/config/wireless within the
test system (followed by wifi reconf) to set your WiFi credentials.
The image contains the package shairport-sync-mini and
therefore you can use an Apple device to stream music to it.
Alternativly, you can use aplay for wav-files or mpg123
for music streams (non-ssl only). Do not download large files
to the system, space is extremely limited.
With fw_printenv one can list the NVRAM contents of the vendor firmware
(config fw_env.config).
There exist various documents on how to backup and flash OpenWrt devices.
It might even be possible to upload the OpenWrt sysupgrade.bin via the
vendor's firmware update web interface, but this has not been tested
at all.
✋ Flash memory has a limited number of write cycles. Avoid unnecessary system upgrades or writing large files (always use tmpfs in /tmp if possible).
It is recommended to boot the system via serial and flash the image from within the running system with
mtd -r write /tmp/openwrt-ramips-mt76x8-linkplay_a31-squashfs-sysupgrade.bin firmware
The Linkplay A31 does not have a DAC and amplifier, so it
is necessary to control volume and mute status via
serial commands (e.g. AXX+VOL+007, sent via ttyS0 57600 8N1
to the amp board).
Audio is not always that straightforward with Linux, so this project uses a possibly confusing (some might call it clever) ALSA setup to control the amplifier status:
- The I2S S/PDIF output device does not have a mixer, so client programs (e.g. shairport-sync) would not be able to control the volume.
- Therefore a Linux dummy sound device (with dummy mixer) is activated (
kmod-sound-dummy). - With a specially crafted asound.conf, default audio output is routed to the I2S device (the amp board accepts audio format S16, rate 44,100 Hz) and default audio control is routed to the dummy device (mixer). Client programs happily change the volume of the dummy mixer, but the real audio data is passed unchanged (volume 100%) to the amplifier board (assuming that the client program itself does no software mixing, it is recommended to disable any client volume/mixer processing).
- A Linkplay Emulator Daemon is installed to monitor both devices and send serial control commands to the amplifier board when sound is played or the volume changes.
The terms 'emulator' and 'daemon' are a bit exaggerated, the code only emulates three serial commands and does not daemonize itself, but hey, it works :)
- The DYON Area L WiFi speaker plays a particularly annoying
welcome message ("Geniesse WiFi Musik" in german) when you
switch to the Airplay source (
AXX+PLM+001). This message is not stored on the A31 module but on the amp board.