Bug 100555

Created attachment 130669 [details]
g-c-c screenshot

I also have the same issue with the internal sound card

If pulseaudio can open the "iec958" alsa device, pulseaudio will think that the card has digital output. If no such output exists in reality, it's an alsa bug.

http://alsa-project.org/main/index.php/Bug_Tracking

Looking at [0], couldn't the 2 port be what they are calling there the 'Digital surround passthrough'?


[0] http://alsa.opensrc.org/DigitalOut

The G930 headset seems to have "Dolby 7.1 Surround Sound" as one of the features[1]. If that's supported on Linux at all, I suppose it might be implemented via an iec958 device using passthrough. If that's the case, it's probably not an alsa bug.

Do you get audio from the headset no matter which output you select?

If someone wants to work on merging the two ports (I probably won't have time for it), ideally alsa would somehow indicate that on this device the analog and digital outputs actually go to the same place. If that's not going to happen, I think it would be possible to write custom profile configuration for this device in pulseaudio and ask for its inclusion in upstream (there are several other special configurations for particular sound cards too). The configuration could define two output mappings, and one output port that is used by both mappings.

Some documentation about the profile configuration:
https://www.freedesktop.org/wiki/Software/PulseAudio/Backends/ALSA/Profiles/

As for the internal sound card, merging the ports doesn't make sense. If there are both analog and digital outputs on the sound card, they should have separate ports. If there's no digital output, having the iec958 device available is an alsa bug.

[1] https://www.amazon.com/Logitech-Wireless-Surround-Headphones-Microphone/dp/B003VANOFY

Yes the headset is supposed to support (software) Dolby under windows. There is a even an hardware switch to enable/disable it, but it doesn't seem to do anything under linux.

I've sound with the two outputs, the volume is a bit different between these tough (100% is a lot louder with the analogic output).

If I'm going to the pavucontrol with the digital profile/port I get extra (advanced) encoding like AC3, DTS,.... in addition to pcm

It would be interesting to hear if some of the encodings work (AC3 and EAC3 are related to Dolby stuff, so they are the prime candidates, although I think neither of them is what "Dolby Surround 7.1" uses). To test an encoding, you'll need to enable the encoding in pavucontrol, find a file with that encoding, and play the file with a media player that is able to use pulseaudio's passthrough feature.

Here's a bit of documentation: https://www.freedesktop.org/wiki/Software/PulseAudio/Documentation/User/Passthrough/

Just tried now

For some reasons, if I'm checking all the codecs (AC3, EAC3, DTS, MPEG, AAC) and then changing the volume, only AAC (and sometimes, AC3, but this doesn't seems to be alway the case) stay checked in the interface.

I then tried to play a video (audio in AC3, not sure the  using gst-launch playbin and the sound is completely rubbish (static,...) and inaudible. Works well if AC3 is unchecked.

When trying to read the AC3 test file hosted at http://www.lynnemusic.com/surround.html, gstreamer even gives me a non-supported error if the AC3 checkbox is checked in pavucontrol

Note that in the profile of the device I only have stereo.

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