ALSA Api: How to play two wave files simultaneously?

问题

What is the required API configuration/call for playing two independent wavefiles overlapped ? I tried to do so , I am getting resource busy error. Some pointers to solve the problem will be very helpful.

Following is the error message from snd_pcm_prepare() of the second wavefile

"Device or resource busy"

回答1:

ALSA does not provide a mixer. If you need to play multiple audio streams at the same time, you need to mix them together on your own.

The easiest way this can be accomplished is by decoding the WAV files to float samples, add them, and clip them when converting them back to integer samples.

Alternatively, you can try to open the default audio device (and not a hardware device like "hw:0") multiple times, once for each stream you wish to play, and hope that the dmix ALSA plugin is loaded and will provide the mixing functionality.

回答2:

You can configure ALSA's dmix plugin to allow multiple applications to share input/output devices.

An example configuration to do this is below:

pcm.dmixed {
    type dmix
    ipc_key 1024
    ipc_key_add_uid 0
    slave.pcm "hw:0,0"
}
pcm.dsnooped {
    type dsnoop
    ipc_key 1025
    slave.pcm "hw:0,0"
}

pcm.duplex {
    type asym
    playback.pcm "dmixed"
    capture.pcm "dsnooped"
}

# Instruct ALSA to use pcm.duplex as the default device
pcm.!default {
    type plug
    slave.pcm "duplex"
}
ctl.!default {
    type hw
    card 0
}

This does the following:

creates a new device using the dmix plugin, which allows multiple apps to share the output stream
creates another using dsnoop which does the same thing for the input stream
merges these into a new duplex device that will support input and output using the asym plugin
tell ALSA to use the new duplex device as the default device
tell ALSA to use hw:0 to control the default device (alsamixer and so on)

Stick this in either ~/.asoundrc or /etc/asound.conf and you should be good to go.

For more information see http://www.alsa-project.org/main/index.php/Asoundrc#Software_mixing.

回答3:

You can use this configuration also

   pcm.dmix_stream
   {
       type dmix
        ipc_key 321456 
        ipc_key_add_uid true
        slave.pcm "hw:0,0"
   }


   pcm.mix_stream
   {
      type plug
      slave.pcm dmix_stream
   }

Update it in ~/.asoundrc or /etc/asound.conf

You can use command

For wav file

aplay -D mix_stream "filename"

For raw or pcmfile

aplay -D mix_stream -c "channels" -r "rate" -f "format" "filename"

Enter the value for channels, rate, format and filename as per your audio file

回答4:

following is a very simplified multi-thread playback solution (assuming both files are the same sample format, same channel number and same frequency):

starting buffer based thread per each file decoding (have to make this code 2 times - for file1 and for file2):

import wave
import threading

periodsize = 160
f = wave.open(file1Wave, 'rb')
file1Alive = True
file1Thread = threading.Thread(target=_playFile1)
file1Thread.daemon = True
file1Thread.start()

file decoding thread itself (also has to be defined twice - for file1 and for file2) :

def _playFile1():
    # Read data from RIFF
    while file1Alive:
        if file1dataReady:
            time.sleep(.001)
        else:
            data1 = f.readframes(periodsize)
            if not data1:
                file1Alive = False
                f.close()
            else:
                file1dataReady == True

starting merging thread (aka funnel) to merge file decodings

import alsaaudio
import threading

sink = alsaaudio.PCM(alsaaudio.PCM_PLAYBACK, device="hw:CARD=default")
sinkformat = 2
funnelalive = True
funnelThread = threading.Thread(target=self._funnelLoop)
funnelThread.daemon = True
funnelThread.start()

merge and play (aka funnel) thread

def _funnelLoop():
    # Reading all Inputs
    while funnelalive:
        # if nothing to play - time to selfdestruct
        if not file1Alive and not file2Alive:
            funnelalive = False
            sink.close()
        else:
            if file1dataReady and file2dataReady: 
                # merging data from others but first
                datamerged = audioop.add(data2, data2, sinkformat)
                file1dataReady = False
                file2dataReady = False
                sink.write(datamerged)
                time.sleep(.001)

来源：https://stackoverflow.com/questions/14398573/alsa-api-how-to-play-two-wave-files-simultaneously

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Linux

audio

alsa