Dear all ALSA Users,
Recently my team and I are making an attempt to record audio signals from an
open source sound card called 8 Sounds USB
(http://sourceforge.net/p/eightsoundsusb/wiki/Main_Page/) and were facing great
difficulty in the most fundamental of issues, being unable to get any signals
recorded from the sound card/mic array system for months until we found out
that the issue could be solved by enabling audio capture on the sound card in
Alsamixer as shown on the page: (http://alsa.opensrc.org/Record_from_mic) .
With the capture enabled, we were able to actually record down signals from all
the microphones in the array with the Audacity application, and play them back
without a glitch. However, we are really puzzled as to why enabling capture in
Alsamixer solved the problem, so this comes to our first question of:
1. How does enabling capture in alsamixer actually allow for a device to be
able to start recording signals when previously, without capture enabled,
although we could detect the device on our system (but that too is quite fuzzy,
with certain commands, we were able to detect the device, while with other
means we did not detect the device on Ubuntu) in audacity at least, we could
not get any recorded signals (we can initiate the recording but get zero
inputs, no spikes whatsoever) ?
what changes did alsamixer actually make to ALSA (its drivers etc) that
would have miraculously enabled the signals to be recorded?
Also, we made an observation that might be interesting and we are not sure if
it has anything to do with the alsamixer enabling capture issue. Basically on
the Ubuntu laptop that we have, we identified our sound card to be numbered as
device 1. So when we do a listing of the /proc directory tree for this sound
card, we get:
augustine@augustine-ubuntu:~$ tree /proc/asound/card1
/proc/asound/card1
├── id
├── pcm0c
│ ├── info
│ ├── sub0
│ │ ├── hw_params
│ │ ├── info
│ │ ├── status
│ │ └── sw_params
│ └── xrun_debug
├── pcm0p
│ ├── info
│ ├── sub0
│ │ ├── hw_params
│ │ ├── info
│ │ ├── status
│ │ └── sw_params
│ └── xrun_debug
├── stream0
├── usbbus
├── usbid
└── usbmixer
4 directories, 17 files
And when we do a cat of the following directory when we are no doing any
recording with Audacity, we get:
augustine@augustine-ubuntu:~$ cat /proc/asound/card1/pcm0c/sub0/status
state: RUNNING
owner_pid : 4614
trigger_time: 26673.762479721
tstamp : 28544.782938570
delay : 926
avail : 926
avail_max : 926
-----
hw_ptr : 82477944
appl_ptr : 82477018
And this is the case all the time we are doing a recording, regardless of
whether we have enabled or have not enabled Alsamixer capture for our sound
card device numbered 1 on our machine. The only difference it seems, when we
enable capture, is that we actually get a audio recording, as opposed to not
getting anything when we don’t.
Lastly, to complete the observation, we found that when we stop the recording
in Audacity and do the same cat command, we get:
augustine@augustine-ubuntu:~$ cat /proc/asound/card1/pcm0c/sub0/status
closed
So our second question would be:
2. What exactly does the /proc/asound/card1/pcm0c/sub0/status file tell us, and
if we see a “RUNNING” state regardless of whether we have enabled alsamixer
capture or not (as implied by what we observed), then does this suggest that
alsamixer is independent from the /proc/asound/card1 directory?
We really appreciate all your help and thank you all in advance! We have always
been struggling to wrap our heads around ALSA and issues arising from getting
our mic array + sound card system to work with our Ubuntu 14.04 machine, and
any help here will be appreciated! Please feel free to chip in with comments
even if they do not answer our questions too.
PS: I’m not too sure if my mail should go to the developers mailing list too.
If it should, please let me know and I’ll post it up there too.
Best regards,
Augustine Koh
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