2526d5b087
Bluetooth (RTL8723D USB combo chip): - GPIO pin 357 (active low) enables WiFi/BT hardware via x6200_gpio_set() - bluez5_utils 5.64 (downgraded from 5.79 — HID input plugin broken in 5.79) - rtl8723d_config.bin added to overlay (missing from linux-firmware package) - S45wifi-bt: GPIO enable + modprobe btusb/uhid/hidp at boot - S85bt-keyboard: auto-connect loop with scan+connect every 20s WiFi (RTL8723DU): - out-of-tree lwfinger/rtw88 driver (RTW88_8723DU not in kernel 6.1 mainline) - linux-firmware RTL_RTW88 for rtw88/rtw8723d_fw.bin - regulatory.db for cfg80211 - wpa_supplicant with multi-network config in /etc/wpa_supplicant.conf - S46wifi: wpa_supplicant + udhcpc at boot Key findings: - RTL8723D USB WiFi (0bda:d723) requires out-of-tree rtw88 on kernel 6.1 - BT and WiFi share same USB device, both need GPIO 357 = 0 to power on - bluez5 5.79 HID input plugin not linked into bluetoothd (build system bug)
256 lines
11 KiB
Markdown
256 lines
11 KiB
Markdown
# X6200 Audio Architecture
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A detailed technical account of how audio works on the Xiegu X6200,
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reverse-engineered during Mestre development (May 2026).
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## Overview
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The X6200 has two distinct audio sources and one audio output path.
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Understanding the distinction between them is essential for any software
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that wants to produce sound on this radio.
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```
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┌─────────────────────────────────────────────────────────┐
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│ X6200 Hardware │
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│ │
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│ ┌──────────┐ ┌──────────┐ │
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│ │ RF Front │ │ Allwinner│ │
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│ │ End │ │ R16 │ (SoC / Application CPU) │
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│ │ + FPGA │ │ │ │
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│ └────┬─────┘ └────┬─────┘ │
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│ │ RX audio │ DAC audio │
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│ │ (analog) │ (digital → analog via codec) │
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│ └──────┬────────┘ │
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│ │ │
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│ ┌────▼─────┐ │
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│ │ Audio │ ← voice_rec bit selects source │
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│ │ MUX │ (controlled via I2C to MCU) │
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│ └────┬─────┘ │
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│ │ │
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│ ┌────▼─────┐ │
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│ │ PA │ ← Power Amplifier │
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│ │ (Class D)│ enabled by rxvol_set > 0 │
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│ └────┬─────┘ │
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│ │ │
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│ ┌────▼─────┐ │
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│ │ Speaker │ │
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│ └──────────┘ │
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└─────────────────────────────────────────────────────────┘
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```
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## The Two Audio Sources
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### Source 1: RX-Radio Audio (default)
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The RF front-end and FPGA process the received RF signal and produce
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an analog audio output — the classic "radio receiver sound". This is
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the default audio source at power-on. It is completely independent of
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the SoC and ALSA; it bypasses the codec entirely and is fed directly
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into the audio MUX.
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This is why you hear receiver noise immediately when the X6200 boots,
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even before Linux has fully started.
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### Source 2: SoC DAC Audio
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The Allwinner R16 SoC contains an integrated audio codec
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(`sun8i-a33-audio`) which can produce digital audio, converted to
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analog via the DAC. This is the source used by software running on
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the SoC — piHPSDR, aplay, PulseAudio, etc.
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This source goes through the standard Linux ALSA stack:
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```
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Application (piHPSDR, aplay)
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→ PulseAudio (mixing, resampling)
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→ ALSA (hw:0,0 / pcmC0D0p)
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→ sun8i-a33-audio codec driver
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→ DAC (analog output)
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→ Audio MUX
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→ PA
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→ Speaker
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```
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## The Audio MUX — The Critical Control Point
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A hardware multiplexer inside the MCU/FPGA selects which source
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reaches the power amplifier and speaker. This MUX is controlled
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via I2C, **not** via ALSA or any Linux kernel interface.
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### I2C Protocol
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The X6200 MCU sits on I2C bus 0, address `0x72`. All hardware control
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uses the X6200Control library protocol: a 6-byte write transaction
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consisting of a 2-byte register address followed by a 4-byte value.
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The audio MUX is controlled by the `voice_rec` bit in the
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`sple_atue_trx` register (command index 12, address `0x0030`):
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| voice_rec bit | Audio source heard in speaker |
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|---------------|-------------------------------|
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| 0 (default) | RX-radio (RF receiver) |
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| 1 | SoC DAC (software audio) |
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**To switch to SoC DAC audio:**
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```bash
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i2ctransfer -y 0 w6@0x72 0x00 0x30 0x08 0x00 0x00 0x00
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```
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**To switch back to RX-radio audio:**
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```bash
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i2ctransfer -y 0 w6@0x72 0x00 0x30 0x00 0x00 0x00 0x00
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```
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## The Power Amplifier — The Other Critical Control Point
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The speaker PA (power amplifier) is not always on. It is enabled
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as a side effect of setting the speaker volume via `x6200_control_rxvol_set()`.
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When `rxvol = 0` (the default after `x6200_control_init()`), the PA
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is effectively off and nothing is heard regardless of the MUX position
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or ALSA configuration.
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This is why setting the correct mixer levels alone is not sufficient —
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the MCU must be told to activate the PA via the I2C control protocol.
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## The ALSA Codec — Clock Dependency
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The `sun8i-a33-audio` codec requires its master clock (`pll-audio`,
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22.5792 MHz) to be running for DAC output to work. The kernel's
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clock framework only enables this clock when a consumer holds an
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active reference — i.e., when an application has the PCM device open.
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**This is the most subtle requirement:** if no process holds
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`/dev/snd/pcmC0D0p` open, the audio clocks are disabled and the
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codec produces no output, even if the ALSA mixer is correctly
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configured and voice_rec is set to DAC mode.
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PulseAudio, running in system mode, holds the PCM device open
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continuously. This is what keeps the clocks running at all times
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and makes reliable audio possible.
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```
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PulseAudio running → pcmC0D0p open → pll-audio enabled → codec active
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```
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Without PulseAudio (or equivalent), every `aplay` invocation would
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need to wait for the codec to stabilise after the clock starts —
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and the X6200 MCU may reset the MUX during this window.
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## The ALSA Mixer — Required Configuration
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The codec's internal mixer must be configured correctly for audio
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to flow from the DAC to the analog output. The critical controls are:
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| numid | Name | Required value |
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|-------|-----------------------------------|----------------|
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| 2 | AIF1 DA0 Playback Volume | 160, 160 |
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| 7 | Headphone Playback Volume | 58 |
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| 8 | Headphone Playback Switch | on, on |
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| 20 | AIF1 Data Digital ADC Cap. Switch | on, on |
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| 26 | AIF1 DA0 Stereo Playback Route | 3, 3 (Mix Mono)|
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| 28 | AIF1 Slot 0 Digital DAC PB Switch | on, on |
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| 31 | DAC Playback Switch | off, off |
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Note that `DAC Playback Switch` (numid=31) must be **off** — this is
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counterintuitive but correct. The signal path goes through AIF1,
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not the direct DAC path.
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These values are saved in `/var/lib/alsa/asound.state` and restored
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at boot via `alsactl restore` (init script `S50alsa`).
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## Complete Initialisation Sequence
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The following sequence must be performed in this exact order at boot:
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### Step 1: ALSA mixer state (S50alsa)
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```bash
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alsactl restore
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```
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Restores the correct mixer levels from `/var/lib/alsa/asound.state`.
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### Step 2: PulseAudio (S50pulseaudio)
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```bash
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pulseaudio --system --daemonize --disallow-exit --exit-idle-time=-1
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```
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Starts PulseAudio in system mode. This opens `pcmC0D0p` and keeps
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the ALSA clocks running continuously.
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### Step 3: X6200 hardware init (S70mestre-audio)
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```python
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lib.x6200_control_init() # Establishes I2C state, sends all_cmd
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lib.x6200_control_spmode_set(False) # Speaker mode (not headphone)
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lib.x6200_control_rxvol_set(50) # Enables PA, sets volume (0-100)
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```
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`x6200_control_init()` sends a full command packet to the MCU via I2C,
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establishing all register values. **It sets rxvol=0** (PA off) as
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its default. `rxvol_set(50)` must follow immediately to enable the PA.
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### Step 4: Switch audio MUX to SoC DAC (S70mestre-audio)
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```bash
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i2ctransfer -y 0 w6@0x72 0x00 0x30 0x08 0x00 0x00 0x00
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```
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Sets the `voice_rec` bit, switching the speaker from RX-radio audio
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to SoC DAC audio. From this point, any audio played via PulseAudio
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or directly to `hw:0,0` (44100 Hz, stereo, S16_LE) will be heard
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in the speaker.
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## Mestre's Implementation
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Mestre's init scripts implement this sequence:
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```
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S50alsa → alsactl restore
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S50pulseaudio → pulseaudio --system
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S70mestre-audio → x6200_control_init + rxvol_set + voice_rec switch
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S96keepalive → I2C PMU keepalive (prevents automatic power-off)
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```
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The audio MUX switch command (`voice_rec=on`) is permanent until the
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next `x6200_control_init()` call — it does not need to be repeated.
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However, the PMU keepalive (separate from audio) must run continuously
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or the radio will power off after approximately 180 seconds.
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## Volume Control
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Speaker volume is controlled via `x6200_control_rxvol_set(vol)` where
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`vol` is 0-100. This sends an I2C command to the MCU which adjusts
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an analog gain stage. **It is not reflected in the ALSA mixer** and
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cannot be read or set via `amixer`.
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The ALSA mixer `Headphone Playback Volume` (numid=7) and
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`AIF1 DA0 Playback Volume` (numid=2) affect the digital gain
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in the codec but are set to fixed values at boot and not used
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for runtime volume control in Mestre.
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## Known Quirks
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**rxvol_set defaults to 0 after init.** Every call to
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`x6200_control_init()` resets the speaker volume to 0 (PA off).
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Always call `rxvol_set(N)` immediately after `init()`.
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**voice_rec is reset by x6200_control_init().** If `init()` is called
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again (e.g., after a crash), the MUX reverts to RX-radio mode and
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the full init sequence must be repeated.
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**PulseAudio must start before mestre-audio.** If PulseAudio is not
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running when the audio MUX is switched, the codec clocks may not be
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active and the switch may have no audible effect.
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**44100 Hz stereo S16_LE** is the correct format for direct hardware
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playback (`hw:0,0`). PulseAudio handles resampling from other rates.
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## References
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- [gdyuldin/X6200Control](https://github.com/gdyuldin/X6200Control) —
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I2C control library source, including the `sple_atue_trx` register
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layout and `voice_rec` bit definition.
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- [gdyuldin/x6200_gui](https://github.com/gdyuldin/x6200_gui) —
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Reference implementation of the full audio init sequence
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(`audio_play_en()`, `audio_init()`, `radio_init()`).
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- [AetherRadio/X6100Control](https://github.com/AetherRadio/X6100Control) —
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X6100 control library (protocol-compatible with X6200), used to
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understand the register map before X6200Control source was available.
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- Allwinner R16 / sun8i-a33 audio driver: `sound/soc/sunxi/` in the
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Linux kernel tree. The codec is `sun8i-codec` with an AIF1 I2S
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interface to the SoC's audio PLL.
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