asound.conf的作用
asound.conf允许对声卡或者设备进行更高级的控制,提供访问alsa-lib中的pcm插件方法,允许你做更多的复杂的控制,比如可以把声卡组合成一个或者多声卡访问多个I/O。
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Default plugin
pcm.!default { type hw card 0 } ctl.!default { type hw card 0 } 关键字default定义在alsa-lib中,将会访问hw:0,0声卡。 aplay -D default test.wav (== aplay -Dhw:0,0 test.wav)
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Plugin
rate_convert pcm.rate_convert { type rate slave { pcm rate 48000 } } 调用这个接口,比如之前44.1k的采样频率将会转换为48khz。 更复杂的用法: type plug # Format adjusted PCM slave STR # Slave name (see pcm_slave) # or slave { # Slave definition pcm STR # Slave PCM name # or pcm { } # Slave PCM definition [format STR] # Slave format (default nearest) or "unchanged" [channels INT] # Slave channels (default nearest) or "unchanged" [rate INT] # Slave rate (default nearest) or "unchanged" } route_policy STR # route policy for automatic ttable generation # STR can be 'default', 'average', 'copy', 'duplicate' # average: result is average of input channels # copy: only first channels are copied to destination # duplicate: duplicate first set of channels # default: copy policy, except for mono capture - sum ttable { # Transfer table (bidimensional compound of # cchannels * schannels numbers) CCHANNEL { SCHANNEL REAL # route value (0.0 ... 1.0) } } pcm_slave.sl3 { pcm "hw:1,0" format S16_LE channels 1 rate 16000 } pcm.complex_convert { type plug slave sl3 }
dmix
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 } aplay -f cd -D default test.wav
dsnoop
pcm.mixin { type dsnoop ipc_key 5978293 # must be unique for all dmix plugins!!!! ipc_key_add_uid yes slave { pcm "hw:0,0" channels 2 period_size 1024 buffer_size 4096 rate 44100 periods 0 period_time 0 } bindings { 0 0 0 1 } }
jack plugin
pcm.jackplug { type plug slave { pcm "jack" } } pcm.jack { type jack playback_ports { 0 alsa_pcm:playback_1 1 alsa_pcm:playback_2 } capture_ports { 0 alsa_pcm:capture_1 1 alsa_pcm:capture_2 } } aplay -Djackplug somefile arecord -Djackplug somefile
Virtual multi channel devices
如果你想链接两个或者多个alsa设备,以便你有一个虚拟的多通道设备。 # create a virtual four-channel device with two sound devices: # This is in fact two interleaved stereo streams in # different memory locations, so JACK will complain that it # cannot get mmap-based access. see below. pcm.multi { type multi; slaves.a.pcm "hw:0,0"; slaves.a.channels 2; slaves.b.pcm "hw:1,0"; slaves.b.channels 2; bindings.0.slave a; bindings.0.channel 0; bindings.1.slave a; bindings.1.channel 1; bindings.2.slave b; bindings.2.channel 0; bindings.3.slave b; bindings.3.channel 1; } # JACK will be unhappy if there is no mixer to talk to, so we set # this to card 0. This could be any device but 0 is easy. ctl.multi { type hw; card 0; } # This creates a 4 channel interleaved pcm stream based on # the multi device. JACK will work with this one. pcm.ttable { type route; slave.pcm "multi"; slave.channels 4; ttable.0.0 1; ttable.1.1 1; ttable.2.2 1; ttable.3.3 1; } # see above. ctl.ttable { type hw; card 0; } arecord -f S16_LE -r 44100 -c 4 -D multi | aplay -f S16_LE -r 44100 -c 4 -D multi