How to avoid stereo duplication / improve audio quality by replicating CoreAudio filtering

[JayFoxRox]

This commit talks about stereo duplication: https://github.com/davidjo/snd_hda_macbookpro/commit/063110b132b69ea8e6223d3c6870df47137ccacb and this is also mentioned in the README changes in https://github.com/davidjo/snd_hda_macbookpro/commit/6640c79c061b7596b01901cf3f581af94cb9a557

I wonder if this (current-state) is enough for good audio, or if it's possible to get full access to the speaker array to do something like https://github.com/chadmed/asahi-audio (the author also wants to find a more generic plug-and-play solution: https://gitlab.freedesktop.org/pipewire/pipewire/-/issues/2210).

//Edit:

Here's the resources I found already:

• Lengthy discussion about alsa and pulse configs, but not channel-specific tuning / generic DSP: https://github.com/davidjo/snd_hda_macbookpro/issues/23
• Talks about volume being incorrect in old version and more generic presets for EasyEffects: https://github.com/davidjo/snd_hda_macbookpro/issues/13

[davidjo]

The auto duplication only occurs if you set the audio to the stereo output (for ubuntu in Audio Settings). You should also see a 4 channel surround output - that requires a 4 channel stream if you set audio output to that. All you have to do is create the 4 channel stream. (Note that unlike the Apple way this is a left/right tweeter left/right woofer order). (Apples is left tweeter/left woofer/right tweeter/right woofer). If you look thro the issues here (possibly closed issues) various attempts at generating 4 channel streams from pulse or alsa have been tried ie actual user configuration scripts have been attempted - you shouldnt need to change the actual kernel driver. (Unfortunately Ubuntu 22.04 is not using pipewire - next non-LTS version will I believe).

[slynobody]

I got working 4-channel sound on an Imac qualitywise near macos with these changes:

• /usr/share/alsa-card-profile/mixer/profile-sets/default.conf change 'channel-map' under [Mapping analog-surround-40] front-left,front-right,rear-left,rear-right > front-left,rear-left,rear-right,front-right
• /usr/share/pipewire/pipewire-pulse.conf changes to stream.properties = {

  node.latency = 1024/48000

  node.autoconnect = true resample.quality = 4

  channelmix.normalize = true

  channelmix.mix-lfe = true channelmix.upmix = true channelmix.upmix-method = psd # none, simple

  channelmix.lfe-cutoff = 150

  channelmix.fc-cutoff = 12000

  channelmix.rear-delay = 12.0 channelmix.stereo-widen = 0.2

  channelmix.hilbert-taps = 0

  dither.noise = 0

  } then restart according services or reboot and change audio to 'surround 4.0'

it even works very nice with wine if you enable '4-channel-audio' in wine-settings (if the 4.0-device is visible / used)

what does not work yet is

• kernel 5.19-support (only through changes in scripts)
• kernel 6-support
• working headphone (very, very low quality; crash when headphone inserted at login-time

[davidjo]

Nice work. Headphones need to switch back to 2 channel - the headphone audio path is separate from the speaker path so any downmixing needs doing to be done in pipewire.