I have two different streams of PCM samples. I开发者_如何学JAVAs it possible to somehow mix them and play with ALSA on the same output device?
I read it may be possible using the mixer but I'm finding it difficult to understand given the lack of documentation. Could anyone please provide some more information of any kind about how this may be implemented (if it actually is)?
Thanks!
For that, check if you are having .asoundrc file our system. If its not there, then create one and you can place it in your /home folder. Update it with these settings:
pcm.!default {
type plug
slave.pcm "dmixer"
}
pcm.dmixer {
type dmix
ipc_key 1024
slave {
pcm "hw:1,0"
period_time 0
period_size 1024
buffer_size 4096
rate 44100
}
bindings {
0 0
1 1
}
}
ctl.dmixer {
type hw
card 0
}
Use $aplay -l
to get a list of the devices on your system. The hw:X,Y comes from this mapping of your hardware, where X is the card number, while Y is the device number.
Now, open the terminal, and play your first PCM stream with:
$aplay -f cd pcm_sound1.wav
And in other tab, play your second PCM stream:
$aplay -f cd -D default pcm_sound2.wav
It will mix your two PCM data streams.
Cheers,
I'm not sure if this deserves a separate answer. I used HSC's answer and the examples at "ALSA Wiki - The Dmix Howto". The Dmix documentation is indeed a bit frustrating.
Here is an example ~/.asoundrc
that works for me. The "bindings" section is unnecessary, but the HOWTO says it helps "multichannel chips work faster" (not sure if I care?). If I bind "0 1 1 0" then it switches the right/left channels. If I bind "0 1 1 1" then I get garbage in my right ear... Apparently ipc_key
is a required parameter; I just chose a random integer.
# example 1
pcm.hw3mix {
type dmix
ipc_key 1939 # must be unique
slave {
pcm "hw:3"
period_time 0
period_size 1024 # must be power of 2
buffer_size 4096
rate 44100
}
bindings {
0 0
1 1
}
}
pcm.!default hw3mix
The last line makes my new hw3mix
the default device.
I simplified the above to:
# example 2
pcm.hw3mix {
type dmix
ipc_key 1939 # must be unique
slave { pcm "hw:3" }
}
pcm.!default hw3mix
and it seems to work just fine.
As suggested on the ALSA Wiki link above, I can further simplify it to:
# example 3
pcm.dsp0 {
type plug
slave.pcm "dmix"
}
pcm.!default dsp0
and this can be further simplified to:
# example 4
pcm.!default {
type plug
slave.pcm "dmix"
}
or even:
# example 5
pcm.!default plug:dmix
However, the last three examples don't allow me to specify parameters for the Dmix plugin. I wanted to create a device which mixes multiple streams to "hw:3". I think this has to be specified as a parameter to Dmix since it is not the default device, so I went with my second example above. In my actual ~/.asoundrc
, there is also a block for "hw:0,0" called "hw0mix", so by changing the comments on the following lines, I can switch the output of (newly created) streams between my headphones and speakers:
#pcm.!default hw0mix
pcm.!default hw3mix
NB: The above refers to devices like "hw:0,0" and "hw:3" ... it was a bit hard to find out how to ensure stable device numbers for my various USB sound devices, I found the instructions here under "Ordering multiple cards of the same type". The configuration consists of one line in a file in /etc/modprobe.d/
. That's how I get numbers like 0 and 3 to refer to the same devices across reboots.
精彩评论