Raymond, I have attached verbose PA log of microphone
plugging/unplugging. It does not seem to contain valuable info though.
Checking volume levels with alsamixer lead to interesting results. It
turns out that I have common 'Capture' control for both source ports and
separate controls 'Internal Mic Boost' and 'Mic Boost'. Source volume is
a sum of 'Capture' and corresponding 'Boost'.
PA prefers increasing boost volume when it sets source volume, so boost
goes up to 100% even at 28% source volume.
So I ended up with one of the boost controls maximized when I was making
my records. By switching to 'wrong' port I have moved boost to wrong
control, leaving current port without boost and thus without noise.
To illustrate this behavior a dumped amixer control levels for source
volumes from 1% to 102% with following script
for v in {1..102}; do
echo "Setting volume=$v%"
pactl set-source-volume 1 $v%
amixer scontents | sed -n "/'Capture'\|'Internal Mic Boost'/,/^ Front
Right/p"
echo
done > volume_amixer.log
You can check log of PulseAudio (volume_pulse.log) while performing this
test as well.
--------------------------
Finally:
It seems wrong, that PulseAudio prefers to increase 'Boost' control over
'Capture' control. 'Boost' should not be increased from zero value if
increasing 'Capture' control is enough to get overall requested volume.
** Attachment added: "volume_amixer.log"
https://bugs.launchpad.net/ubuntu/+source/pulseaudio/+bug/1085402/+attachment/3449877/+files/volume_amixer.log
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https://bugs.launchpad.net/bugs/1085402
Title:
Wrong auto selection of microphone input
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