ALSA output fails despite seemingly valid config

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1

Describe the problem
I am attempting to enable hardware output on an existing Libretime install that has been functioning with stream-only output for a few years. Using ALSA, I am unable to successfully do so - after configuring output in config.yml, libretime-liquidsoap service restarts continuously with error connecting to ALSA.

To reproduce
Steps to reproduce the behavior.

  1. Enter relevant configuration keys in config.yml for ALSA, in my case:
    system:
      - # Whether the output is enabled.
        # > default is false
        enabled: true
        # System output kind.
        # > must be one of (alsa, ao, oss, portaudio, pulseaudio)
        # > default is alsa
        kind: alsa
        device: default:CARD=U192k
  1. Restart libretime services

Expected behavior
After restart, Liquidsoap plays through specified device

Relevant log output or error messages
Error logged in /var/log/libretime/liquidsoap.log:

2025/10/06 21:07:13 [alsa:1:3] Using ALSA 1.2.2.
2025/10/06 21:07:13 [threads:3] Thread "alsa:1" aborts with exception Alsa error: Device or resource busy!
2025/10/06 21:07:13 [main:3] Shutdown started!

LibreTime version
LibreTime version: 4.3.0

Installation method and OS / Environment
The method used to install and the operating system / environment your installation is running on.

Operating system: Ubuntu 20.04.6 LTS
Method: install script

Additional information:

I have confirmed that ALSA configuration is valid and libretime user has appropriate groups etc to access it. For example, playing aplay -D default:CARD=U192k test.wav as libretime results in expected output.

radio.liq as generated by Libretime does seem to have the correct ALSA configuration, it contains the following lines:

# alsa:1
%ifndef output.alsa
log("output.alsa is not defined!")
%endif
%ifdef output.alsa
output.alsa(
  id="alsa:1",
  device="default:CARD=U192k",
  s
)
%endif

If I make a manually run a minimal liquidsoap script using the same device config (again, as the libretime user), it plays the single MP3 as expected:

source = single("test.mp3")
s = mksafe(source)
output.alsa(
  id="alsa:1",
  device="default:CARD=U192k",
  s
)

I am at a bit of a loss here - it seems like somehow the config is not being used by Libretime as expected, but everything I can see seems valid. This seems similar to ALSA broken · Issue #3125 · libretime/libretime · GitHub

I’ve seen elsewhere that PulseAudio is preferred to ALSA. I would be open to using PulseAudio however I am running Libretime on a dedicated server. PulseAudio is installed but not running, and I have not been able to find any clear documentation on configuring PulseAudio (alone or in conjunction with Libretime) headless, while (given my investigation) ALSA seems to be functioning correctly outside of Libretime.

2

As an update - I did get output working with PulseAudio after much poking around to get it running in system-wide mode and set up as a systemd service.

If my schedule permits, I would be happy to write some documentation about that as it may be useful for other users in the future.

Would it be helpful if I added the details of this to issue #3125?

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