At Sun, 31 Jul 2011 23:39:45 -0400, Pavel Roskin wrote:
Hello!
I have discovered that the microphone on my ASUS EeePC 1005PE only works in stereo. If recording in mono, the result is inaudible. When I open the stereo recording in audacity, I see that the two channels are in counterphase. If I convert stereo to mono in audacity, I get a straight line, and it's inaudible when played.
The driver itself requires stereo recording:
$ arecord --rate 44100 -D hw:0 -f S16_LE --channels 1 >test.wav Recording WAVE 'stdin' : Signed 16 bit Little Endian, Rate 44100 Hz, Mono arecord: set_params:1065: Channels count non available
However, pulse audio doesn't enforce it. So it's possible to record a mono file through pulse audio, and it's inaudible. Some programs simply assume a mono microphone and thus get no sound.
I can work it around by setting the level on one of the channels to 100% and to 0% on the other in alsamixer. However, the GNOME volume manager removes that imbalance when adjusting the input level. I would prefer to have a robust solution. Perhaps one of the channels should be inverted. Or maybe the difference between the channels should be reported as mono input. I don't really care about stereo input.
I'm using Linux 3.0 on Fedora 15. I checked out and tested the master branch of sound-2.6.git, and it has the same problem.
It looks like the problem is known. I found this comment in alc882_quirks.c:
/* DMIC fix
- This laptop has a stereo digital microphone. The mics are only 1cm apart
- which makes the stereo useless. However, either the mic or the ALC889
- makes the signal become a difference/sum signal instead of standard
- stereo, which is annoying. So instead we flip this bit which makes the
- codec replicate the sum signal to both channels, turning it into a
- normal mono mic.
*/
My laptop has ALC269, bit it also used the DMIC quirk: SND_PCI_QUIRK(0x1043, 0x83ce, "ASUS P1005HA", ALC269_DMIC),
It might be that ALC269 has a same or similar COEF like ALC271 or ALC889. For ALC271, the following verbs do the mono-mixing of d-mic,
{0x20, AC_VERB_SET_COEF_INDEX, 0x0d}, {0x20, AC_VERB_SET_PROC_COEF, 0x4000},
while for ALC889, the following do:
{0x20, AC_VERB_SET_COEF_INDEX, 0x0b}, {0x20, AC_VERB_SET_PROC_COEF, 0x0003},
Try the above via hda-verb once whether it has any effect on your machine.
Takashi