I am stuck. Here's what I am trying to do:
A USB DAC may or may not be connected to my system. When it is connected,
sound should come out the USB DAC. When it is not connected, sound should
come out the internal DAC.
The solution that I implemented uses udev to detect when the USB DAC is
connected/disconnected. udev runs a script when it detects a change. The
script writes ~/.asoundrc with the following:
pcm.!default {
type hw
card $n
}
ctl.!default {
type hw
card $n
}
where n is 0 for the internal DAC and 1 for the USB DAC. I know these
names because /proc/asound/cards contains:
0 [PCH ]: HDA-Intel - HDA Intel PCH
HDA Intel PCH at 0xfe700000 irq 48
1 [default ]: USB-Audio - USB Audio CODEC
Burr-Brown from TI USB Audio CODEC at
usb-0000:00:1d.0-1.4, full
(when both are connected, and just the first entry when the USB DAC is not
connected).
I know that udev is working because the .asoundrc file changes as expected
when I connect/disconnect the USB DAC. If I start with sound coming out
the USB DAC, the sound output switches from the USB DAC to the internal DAC
when I disconnect the USB DAC. However, when I reconnect the USB DAC,
sound does not switch back to the USB DAC even though the .asoundrc file
changes appropriately. At this point, repeatedly
disconnecting/reconnecting the USB DAC has no effect.
Here's surprising observation #1: If after reconnecting the USB DAC I play
a sound file manually (using gst-launch playbin2
uri=file:/path/to/file.ogg), sound *does* come out the USB DAC. In fact,
sound will track the connecting/disconnecting of the USB DAC exactly as it
should when I execute a play command from the command line.
So, what is different about my application from the command line? Well,
you might wonder whether I am restarting my application whenever .asoundrc
changes. And you might have wondered even more if you had known that my
application is a daemon, so normally it runs continuously in the background
while responding to commands from the client. However, I understand that I
need to restart my application for the changes to .asoundrc to take effect,
so the script activated by udev restarts the daemon. I know that the
daemon restarts because the pid changes. I know that the client is talking
to the daemon with the new pid because I have the client ask the daemon for
its pid and then print out the response. Nevertheless, sound from my
application does not come out the USB DAC. Here is surprising observation
#2: if at this point I manually restart the daemon from the command line
(with exactly the same command that the script executes), sound
*does*switch to the USB DAC.
So, what is different about restarting the daemon from the command line
versus restarting it from the script. Maybe udev does other critical
things after resetting the daemon (even though my rules file has a name
without a numeric prefix to be sure that it runs after the rules in
/lib/udev/rules.d). I tried restarting the daemon from a subshell of the
script that runs after waiting several seconds. Still no switch. Maybe
the script runs the restart from an environment that somehow affects alsa.
I tried running the restart command with env -i. Still no switch.
Running the restart command from the command line is the second time the
restart directive is issued (the first being the one from the script).
Maybe the script needs to make the request more than once. I tried
issuing the command two or three times from the script. Still no switch.
If any of these experiments had succeeded I would have been surprised
because the switch from USB DAC to internal DAC works. It's only the
switch from disconnected to connected that does not work, and only with my
application. From the command line, the switch always works.
I feel infinitesimally close to a working solution, but I can't think of
anything else to try. I hope that someone has a suggestion.
--
Jeffrey Barish
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