ALSA Audio API手册.docx
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ALSAAudioAPI手册
ALSAAudioAPI手册
ALSAAudioAPI手册
ThisdocumentattemptstoprovideanintroductiontotheALSAAudioAPI.ItisnotacompletereferencemanualfortheAPI,anditdoesnotcovermanyspecificissuesthatmorecomplexsoftwarewillneedtoaddress.However,itdoestrytoprovideenoughbackgroundandinformationforareasonablyskilledprogrammerbutwhoisnewtoALSAtowriteasimpleprogramthatusestheAPI.
AllcodeinthedocumentislicensedundertheGNUPublicLicense.IfyouplantowritesoftwareusingALSAundersomeotherlicense,thenIsuggestyoufindsomeotherdocumentation.
Contents
UnderstandingAudioInterface
WhataTypicalAudioApplicationDoes
AMinimalPlaybackProgram
AMinimalCaptureProgram
AMinimalinterrupt-drivenProgram
AMinimalDuplexProgram
HowToDoIt
OpeningTheDevice
SettingTheParameters
ReceivingAndDeliveringData
WhyYouMightWantToForgetAboutAllOfThis
UnderstandingAudioInterfaces
Letusfirstreviewthebasicdesignofanaudiointerface.Asanapplicationdeveloper,youdon'tneedtoworryaboutthislevelofoperation-itsalltakencareofbythedevicedriver（whichisoneofthecomponentsthatALSAprovides）.Butyoudoneedtounderstandwhatisgoingataconceptuallevelifyouwanttowriteefficientandflexiblesoftware.
Anaudiointerfaceisadevicethatallowsacomputertoreceiveandtosendaudiodatafrom/totheoutsideworld.Insideofthecomputer,audiodataisrepresentedastreamofbits,justlikeanyotherkindofdata.However,theaudiointerfacemaysendandreceiveaudioaseitherananalogsignal（atime-varyingvoltage）orasadigitalsignal（somestreamofbits）.Ineithercase,thesetofbitsthatthecomputerusestorepresentaparticularsoundwillneedtobetransformedbeforeitisdeliveredtotheoutsideworld,andlikewise,theexternalsignalreceivedbytheinterfacewillneedtobetransformedbeforeitisusefultothecomputer.Thesetwotransformationsaretheraisond'etreoftheaudiointerface.
Withintheaudiointerfaceisanareareferredtoasthe"hardwarebuffer".Asanaudiosignalarrivesfromtheoutsideworld,theinterfaceconvertsitintoastreamofbitsusablebythecomputerandstoresitintheparthardwarebufferusedtosenddatatothecomputer.Whenithascollectedenoughdatainthehardwarebuffer,theinterfaceinterruptsthecomputertotellitthatithasdatareadyforit.Asimilarprocesshappensinreversefordatabeingsentfromthecomputertotheoutsideworld.Theinterfaceinterruptsthecomputertotellitthatthereisspaceinthehardwarebuffer,andthecomputerproceedstostoredatathere.Theinterfacelaterconvertsthesebitsintowhateverformisneededtodeliverittotheoutsideworld,anddeliversit.Itisveryimportanttounderstandthattheinterfaceusesthisbufferasa"circularbuffer".Whenitgetstotheendofthebuffer,itcontinuesbywrappingaroundtothestart.
Forthisprocesstoworkcorrectly,thereareanumberofvariablesthatneedtobeconfigured.Theyinclude:
whatformatshouldtheinterfaceusewhenconvertingbetweenthebitstreamusedbythecomputerandthesignalusedintheoutsideworld?
atwhatrateshouldsamplesbemovedbetweentheinterfaceandthecomputer?
howmuchdata（and/orspace）shouldtherebebeforethedeviceinterruptsthecomputer?
howbigshouldthehardwarebufferbe?
Thefirsttwoquestionsarefundamentalingoverningthequalityoftheaudiodata.Thesecondtwoquestionsaffectthe"latency"oftheaudiosignal.Thistermreferstothedelaybetween
dataarrivingattheaudiointerfacefromtheoutsideworld,anditbeingavailabletothecomputer（"inputlatency"）
databeingdeliveredbythecomputer,anditbeingdeliveredtotheoutsideworld（"outputlatency"）
Bothoftheseareveryimportantformanykindsofaudiosoftware,thoughsomeprogramsdonotneedbeconcernedwithsuchmatters.
Whatatypicalaudioapplicationdoes
Atypicalaudioapplicationhasthisroughstructure:
open_the_device（）;set_the_parameters_of_the_device（）;while（!
done）{/*oneorbothofthese*/receive_audio_data_from_the_device（）;deliver_audio_data_to_the_device（）;}closethedevice
AMinimalPlaybackProgram
Thisprogramopensanaudiointerfaceforplayback,configuresitforstereo,16bit,44.1kHz,interleavedconventionalread/writeaccess.Thenitsdeliversachunkofrandomdatatoit,andexits.ItrepresentsaboutthesimplestpossibleuseoftheALSAAudioAPI,andisn'tmeanttobearealprogram.#include<stdio.h>#include<stdlib.h>#include<alsa/asoundlib.h>main（intargc,char*argv[]）{inti;interr;shortbuf[128];snd_pcm_t*playback_handle;snd_pcm_hw_params_t*hw_params;if（（err=snd_pcm_open（&playback_handle,argv[1],SND_PCM_STREAM_PLAYBACK,0））<0）{fprintf（stderr,"cannotopenaudiodevice%s（%s）\n",argv[1],snd_strerror（err））;exit
（1）;}if（（err=snd_pcm_hw_params_malloc（&hw_params））<0）{fprintf（stderr,"cannotallocatehardwareparameterstructure（%s）\n",snd_strerror（err））;exit
（1）;}if（（err=snd_pcm_hw_params_any（playback_handle,hw_params））<0）{fprintf（stderr,"cannotinitializehardwareparameterstructure（%s）\n",snd_strerror（err））;exit
（1）;}if（（err=snd_pcm_hw_params_set_access（playback_handle,hw_params,SND_PCM_ACCESS_RW_INTERLEAVED））<0）{fprintf（stderr,"cannotsetaccesstype（%s）\n",snd_strerror（err））;exit
（1）;}if（（err=snd_pcm_hw_params_set_format（playback_handle,hw_params,SND_PCM_FORMAT_S16_LE））<0）{fprintf（stderr,"cannotsetsampleformat（%s）\n",snd_strerror（err））;exit
（1）;}if（（err=snd_pcm_hw_params_set_rate_near（playback_handle,hw_params,44100,0））<0）{fprintf（stderr,"cannotsetsamplerate（%s）\n",snd_strerror（err））;exit
（1）;}if（（err=snd_pcm_hw_params_set_channels（playback_handle,hw_params,2））<0）{fprintf（stderr,"cannotsetchannelcount（%s）\n",snd_strerror（err））;exit
（1）;}if（（err=snd_pcm_hw_params（playback_handle,hw_params））<0）{fprintf（stderr,"cannotsetparameters（%s）\n",snd_strerror（err））;exit
（1）;}snd_pcm_hw_params_free（hw_params）;if（（err=snd_pcm_prepare（playback_handle））<0）{fprintf（stderr,"cannotprepareaudiointerfaceforuse（%s）\n",snd_strerror（err））;exit
（1）;}for（i=0;i<10;++i）{if（（err=snd_pcm_writei（playback_handle,buf,128））!
=128）{fprintf（stderr,"writetoaudiointerfacefailed（%s）\n",snd_strerror（err））;exit
（1）;}}snd_pcm_close（playback_handle）;exit（0）;}
AMinimalCaptureProgram
Thisprogramopensanaudiointerfaceforcapture,configuresitforstereo,16bit,44.1kHz,interleavedconventionalread/writeaccess.Thenitsreadsachunkofrandomdatafromit,andexits.Itisn'tmeanttobearealprogram.#include<stdio.h>#include<stdlib.h>#include<alsa/asoundlib.h>main（intargc,char*argv[]）{inti;interr;shortbuf[128];snd_pcm_t*capture_handle;snd_pcm_hw_params_t*hw_params;if（（err=snd_pcm_open（&capture_handle,argv[1],SND_PCM_STREAM_CAPTURE,0））<0）{fprintf（stderr,"cannotopenaudiodevice%s（%s）\n",argv[1],snd_strerror（err））;exit
（1）;}if（（err=snd_pcm_hw_params_malloc（&hw_params））<0）{fprintf（stderr,"cannotallocatehardwareparameterstructure（%s）\n",snd_strerror（err））;exit
（1）;}if（（err=snd_pcm_hw_params_any（capture_handle,hw_params））<0）{fprintf（stderr,"cannotinitializehardwareparameterstructure（%s）\n",snd_strerror（err））;exit
（1）;}if（（err=snd_pcm_hw_params_set_access（capture_handle,hw_params,SND_PCM_ACCESS_RW_INTERLEAVED））<0）{fprintf（stderr,"cannotsetaccesstype（%s）\n",snd_strerror（err））;exit
（1）;}if（（err=snd_pcm_hw_params_set_format（capture_handle,hw_params,SND_PCM_FORMAT_S16_LE））<0）{fprintf（stderr,"cannotsetsampleformat（%s）\n",snd_strerror（err））;exit
（1）;}if（（err=snd_pcm_hw_params_set_rate_near（capture_handle,hw_params,44100,0））<0）{fprintf（stderr,"cannotsetsamplerate（%s）\n",snd_strerror（err））;exit
（1）;}if（（err=snd_pcm_hw_params_set_channels（capture_handle,hw_params,2））<0）{fprintf（stderr,"cannotsetchannelcount（%s）\n",snd_strerror（err））;exit
（1）;}if（（err=snd_pcm_hw_params（capture_handle,hw_params））<0）{fprintf（stderr,"cannotsetparameters（%s）\n",snd_strerror（err））;exit
（1）;}snd_pcm_hw_params_free（hw_params）;if（（err=snd_pcm_prepare（capture_handle））<0）{fprintf（stderr,"cannotprepareaudiointerfaceforuse（%s）\n",snd_strerror（err））;exit
（1）;}for（i=0;i<10;++i）{if（（err=snd_pcm_readi（capture_handle,buf,128））!
=128）{fprintf（stderr,"readfromaudiointerfacefailed（%s）\n",snd_strerror（err））;exit
（1）;}}snd_pcm_close（capture_handle）;exit（0）;}
AMinimalInterrupt-DrivenProgram
Thisprogramopensanaudiointerfaceforplayback,configuresitforstereo,16bit,44.1kHz,interleavedconventionalread/writeaccess.Itthenwaitstilltheinterfaceisreadyforplaybackdata,anddeliversrandomdatatoitatthattime.Thisdesignallowsyourprogramtobeeasilyportedtosystemsthatrelyonacallback-drivenmechanism,suchasJACK,LADSPA,CoreAudio,VSTandmanyothers.#include<stdio.h>#include<stdlib.h>#include<errno.h>#include<poll.h>#include<alsa/asoundlib.h>snd_pcm_t*playback_handle;shortbuf[4096];intplayback_callback（snd_pcm_sframes_tnframes）{interr;printf（"playbackcallbackcalledwith%uframes\n",nframes）;/*...fillbufwithdata...*/if（（err=snd_pcm_writei（playback_handle,buf,nframes））<0）{fprintf（stderr,"writefailed（%s）\n",snd_strerror（err））;}returnerr;}main（intargc,char*argv[]）{snd_pcm_hw_params_t*hw_params;snd_pcm_sw_params_t*sw_params;snd_pcm_sframes_tframes_to_deliver;intnfds;interr;structpollfd*pfds;if（（err=snd_pcm_open（&playback_handle,argv[1],SND_PCM_STREAM_PLAYBACK,0））<
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