Public bug reported:
When using an S/PDIF output connected to an external Dolby Digital
decoder (Logitech Z906), Ubuntu's GNOME Sound Settings selects the
S/PDIF device correctly but automatically changes the PipeWire audio
profile from IEC958 AC3 5.1 passthrough to IEC958 stereo.
This causes the external decoder to receive only stereo PCM instead of
Dolby Digital 5.1. The correct 5.1 profile is available and works when
selected manually through pavucontrol or pactl, but GNOME Sound Settings
immediately overrides it.
System Information
Ubuntu version: Ubuntu 24.04 LTS
Audio system: PipeWire with PulseAudio compatibility layer
Session manager: WirePlumber 0.4.x
Hardware:
ASUS PRIME B450M-A motherboard
AMD Family 17h/19h HD Audio Controller
Realtek ALC887-VD codec
S/PDIF output via motherboard S/PDIF bracket
Logitech Z906 speaker system (Dolby Digital decoder)
Hardware path
Realtek ALC887-VD
|
|
S/PDIF IEC958 output
|
|
Logitech Z906 Dolby Digital decoder
The hardware supports Dolby Digital 5.1 passthrough.
Current behavior:
The card exposes these profiles:
output:iec958-stereo
output:iec958-ac3-surround-51
The AC3 profile works correctly.
Running:
pactl set-card-profile alsa_card.pci-0000_08_00.6 output:iec958-ac3-surround-51
results in:
alsa_output.pci-0000_08_00.6.iec958-ac3-surround-51
with:
Sample Specification: s32le 6ch 48000Hz
The Logitech Z906 immediately detects and decodes 5.1 Dolby Digital
correctly.
However:
Open Ubuntu Settings → Sound
Select "Digital Output (S/PDIF)"
The system changes the profile back to:
output:iec958-stereo
Result:
Sample Specification: s32le 2ch 48000Hz
The Z906 receives stereo only.
Expected behavior:
When selecting an S/PDIF output device, Ubuntu should preserve the
user's selected profile.
If the user previously selected: Digital Surround 5.1 (IEC958/AC3) Output
then selecting that output device again should not silently change it to:
Digital Stereo (IEC958) Output
The profile should either:
Be restored automatically, or
Be exposed clearly in GNOME Sound Settings as a selectable option.
Why this matters
S/PDIF is different from normal analog output.
For analog output:
The PC performs channel separation.
A 5.1 sink outputs six analog channels.
For S/PDIF:
The PC may need to send a compressed Dolby Digital/DTS stream.
The external receiver performs decoding.
A user with an external decoder expects:
Stereo content → stereo
Dolby Digital 5.1 content → 5.1
but currently Ubuntu forces the device into a stereo PCM profile unless
advanced tools are used.
Workaround:
The only working method is manually selecting:
output:iec958-ac3-surround-51
using: pactl set-card-profile or pavucontrol.
This is not reasonable for a normal desktop user.
Additional observation:
GNOME Sound Settings appears to override PipeWire/WirePlumber's profile
state instead of simply changing the active sink.
The profile change can be observed with:
pactl list cards | grep "Active Profile"
Before selecting S/PDIF:
Active Profile: output:iec958-ac3-surround-51
After selecting S/PDIF in GNOME Settings:
Active Profile: output:iec958-stereo
Possible solutions:
Make GNOME Sound Settings remember the last selected profile per device.
Expose IEC958 AC3 passthrough profiles in the normal Sound settings UI.
Do not automatically downgrade an explicitly selected passthrough profile to
stereo.
Allow WirePlumber's restored profile preference to take priority over GNOME's
device selection logic.
Severity:
Medium.
The hardware works and the feature exists, but the default desktop UI
prevents normal users from using supported hardware capabilities without
command-line tools.
** Affects: ubuntu
Importance: Undecided
Status: New
** Tags: amd64 apport-bug noble wayland-session
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https://bugs.launchpad.net/bugs/2161617
Title:
Selecting S/PDIF Digital Output in GNOME Sound Settings automatically
switches IEC958 AC3 5.1 profile back to stereo
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