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Heya! This is a pretty interesting and niche problem, but I’ll try to break it down and offer some suggestions.

Understanding the Audio Subsystem

Since your car’s media unit is running Linux, the first step is to figure out how the audio system is configured. Here’s how you can approach it:

  1. Check for Audio Devices:

    • Run ls /dev/snd to see if there are any ALSA (Advanced Linux Sound Architecture) devices. If you see files like pcmC0D0c (capture) or pcmC0D0p (playback), that’s a good sign.
    • Alternatively, run aplay -l or arecord -l to list playback and capture devices, respectively.
  2. Check for PulseAudio or PipeWire:

    • Run pactl info to see if PulseAudio is running.
    • Run pw-cli list-objects to check for PipeWire (a newer audio server).
  3. Check Kernel Logs:

    • Run dmesg | grep -i audio to see if the kernel has logged any audio-related devices or errors.
  4. Check for ALSA Mixer:

    • Run alsamixer to see if you can interact with the audio mixer. This might give you insight into available channels.

Recording All Audio Channels

If you’re unsure which audio channels are active, you can try to capture all possible channels using ffmpeg or arecord. Here’s how:

  1. Using arecord:

    • Run arecord -l to list available capture devices.
    • Use arecord -D hw:0,0 -f cd -c 2 -t wav output.wav to record from the first device (adjust hw:0,0 based on your device list).
  2. Using ffmpeg:

    • Run ffmpeg -f alsa -i hw:0 -ac 2 output.wav to record from the first ALSA device.
    • If you suspect multiple channels, you can try increasing the channel count (e.g., -ac 6 for 5.1 surround sound).
  3. Capture All Channels:

    • If you want to capture all possible channels, you might need to experiment with the channel count (-ac) and device ID (hw:X,Y). For example:
      ffmpeg -f alsa -i hw:0 -ac 8 output.wav
    • This assumes your device supports up to 8 channels. Adjust as needed.

Finding the Right Output

Since you mentioned wanting to feed the audio to a USB card, you’ll need to identify the correct audio output. Here’s how:

  1. Check USB Audio Devices:

    • Plug in your USB audio card and run lsusb to see if it’s recognized.
    • Run aplay -l or arecord -l to see if the USB card appears as an audio device.
  2. Route Audio to USB:

    • Once you’ve identified the USB card, you can use ffmpeg or arecord to capture audio and pipe it to the USB device. For example:
      ffmpeg -f alsa -i hw:0 -f alsa hw:1
    • Here, hw:0 is the source (your car’s audio system) and hw:1 is the USB card.

Debugging Tips

  • Check Permissions: Ensure you have the necessary permissions to access audio devices. You might need to run commands with sudo.
  • Experiment with Channels: If you’re not getting useful audio, try different channel counts or devices.
  • Look for Documentation: If you can identify the media unit’s manufacturer or model, there might be documentation or forums with more specific info.

Long-Term Solution

If you’re planning to keep this setup long-term, consider writing a script to automate the audio capture and routing. You could also explore using pavucontrol (PulseAudio Volume Control) or qpwgraph (PipeWire Graph) to manage audio routing more easily.

Good luck, and let me know how it goes! 🚗🎶

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