HOWTO: fix mic settings on Acer Aspire 3810T notebook in Linux

Once upon a time... You try a Linux Skype version (e. g., 4.3.0.37-1 for Debian) on Acer Aspire 3810T notebook and no sound is heard from the internal microphone.

The microphone is not muted in AlsaMixer (alsamixer); in PulseAudio Volume Control (pavucontrol) mic looks OK too.

Let's record some mic sound using Audacity audio editor... Here is a screenshot:

As you can see, the right channel ("R") is simply inverted left channel ("L"); let's call that "R=-L".

Let's look at the system environment details...

$ uname -a
Linux Notebook1 4.5.0-1-amd64 #1 SMP Debian 4.5.1-1 (2016-04-14) x86_64 GNU/Linux

$ cat /proc/asound/version
Advanced Linux Sound Architecture Driver Version k4.5.0-1-amd64.

$ dmesg | grep Acer
[    0.000000] DMI: Acer Aspire 3810T/Aspire 3810T, BIOS V1.28 08/10/2010
...

$ dmesg | grep snd_hda_codec_realtek
[    3.796475] snd_hda_codec_realtek hdaudioC0D0: autoconfig for ALC269: line_outs=1 (0x14/0x0/0x0/0x0/0x0) type:speaker
[    3.796496] snd_hda_codec_realtek hdaudioC0D0:    speaker_outs=0 (0x0/0x0/0x0/0x0/0x0)
[    3.796510] snd_hda_codec_realtek hdaudioC0D0:    hp_outs=1 (0x15/0x0/0x0/0x0/0x0)
[    3.796522] snd_hda_codec_realtek hdaudioC0D0:    mono: mono_out=0x0
[    3.796531] snd_hda_codec_realtek hdaudioC0D0:    dig-out=0x1e/0x0
[    3.796540] snd_hda_codec_realtek hdaudioC0D0:    inputs:
[    3.796550] snd_hda_codec_realtek hdaudioC0D0:      Mic=0x18
[    3.796559] snd_hda_codec_realtek hdaudioC0D0:      Internal Mic=0x12

$ dmesg | grep HDA
[    3.854039] input: HDA Digital PCBeep as /devices/pci0000:00/0000:00:1b.0/sound/card0/input13
[    3.855277] input: HDA Intel Mic as /devices/pci0000:00/0000:00:1b.0/sound/card0/input14
[    3.855365] input: HDA Intel Headphone as /devices/pci0000:00/0000:00:1b.0/sound/card0/input15
[    3.855451] input: HDA Intel HDMI/DP,pcm=3 as /devices/pci0000:00/0000:00:1b.0/sound/card0/input16

$ cat /proc/asound/pcm
00-00: ALC269 Analog : ALC269 Analog : playback 1 : capture 1
00-01: ALC269 Digital : ALC269 Digital : playback 1
00-03: HDMI 0 : HDMI 0 : playback 1

$ cat /sys/module/snd_hda_intel/parameters/model
(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null)

Nothing interesting, except the Realtek ALC269 audio codec.

Thus, it's time to look at the microphone itself.

Terminal markings (DATA, VDD, CLK, GND) suggest it's a digital microphone (not analog one), and it looks like a mono (1-channel) one.

Fortunately, there is a way to fix the problem: you should create a modprobe configuration file with appropriate settings for Intel HDA driver kernel module.

If you're interested, you can get some info about some kernel modules:

$ /sbin/modinfo snd-hda-intel
...
$ /sbin/modinfo snd-hda-codec-realtek
...

But let's get back to the configuration.
Create, e. g., a /etc/modprobe.d/sound.conf file and put an appropriate configuration string (i. e., options snd-hda-intel model=MODEL, where MODEL is a specific model string) into it:

$ sudoedit /etc/modprobe.d/sound.conf
...

Let's try some model names that look usable.

1. "laptop-amic".
   Some minor changes in dmesg output:

   $ dmesg | grep snd_hda_codec_realtek
   ...
   [    3.803435] snd_hda_codec_realtek hdaudioC0D0:    inputs:
   [    3.812954] snd_hda_codec_realtek hdaudioC0D0:      Mic=0x18
   [    3.817411] snd_hda_codec_realtek hdaudioC0D0:      Internal Mic=0x19
   [    3.817414] snd_hda_codec_realtek hdaudioC0D0:      Internal Mic=0x12
   

   And major effects on mic sound output:
   
   Yes, funny enough, it's a stereo low-level noise, and there's no intelligible sound (!); and that's not strange: "amic" stands for "analog microphone", so it's just a useless option for digital ones.
2. "laptop-dmic".
   
   
   R=-L (no significant changes).
3. "alc269-dmic".
   
   
   R=L.
4. "alc271-dmic".
   
   
   R=-L (no significant changes).
5. "inv-dmic".
   
   
   Some audio signal in the left channel, and no signal (zero level) in the right one (R=0).

Conclusion: "inv-dmic" represents the real configuration (if you want pseudo-stereo, you can also try "alc269-dmic"); so here is my suggested config:

$ cat /etc/modprobe.d/sound.conf
options snd-hda-intel model=inv-dmic

$ cat /sys/module/snd_hda_intel/parameters/model
inv-dmic,(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null),(null)

External links:

1. 1. 1. https://www.kernel.org/doc/Documentation/sound/alsa/HD-Audio-Models.txt
      2. https://www.kernel.org/doc/Documentation/sound/alsa/HD-Audio.txt
      3. https://www.kernel.org/doc/Documentation/sound/alsa/alsa-parameters.txt
   2. https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/sound/pci/hda/patch_realtek.c (kernel/git/torvalds/linux.git - Linux kernel source tree)
2. ALSA - Debian Wiki
3. HdaIntelSoundHowto - Community Help Wiki (Ubuntu)