IntroductiontoSoundProgrammingwithALSA.docx

IntroductiontoSoundProgrammingwithALSA.docx

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IntroductiontoSoundProgrammingwithALSA

IntroductiontoSoundProgrammingwithALSA

ALSAstandsfortheAdvancedLinuxSoundArchitecture.Itconsistsofasetofkerneldrivers,anapplicationprogramminginterface（API）libraryandutilityprogramsforsupportingsoundunderLinux.Inthisarticle,IpresentabriefoverviewoftheALSAProjectanditssoftwarecomponents.ThefocusisonprogrammingthePCMinterfacesofALSA,includingprogrammingexampleswithwhichyoucanexperiment.

YoumaywanttoexploreALSAsimplybecauseitisnew,butitisnottheonlysoundAPIavailable.ALSAisagoodchoiceifyouareperforminglow-levelaudiofunctionsformaximumcontrolandperformanceorwanttomakeuseofspecialfeaturesnotsupportedbyothersoundAPIs.Ifyoualreadyhavewrittenanaudioapplication,youmaywanttoaddnativesupportfortheALSAsounddrivers.Ifyourprimaryinterestisn'taudioandyousimplywanttoplaysoundfiles,usingoneofthehigher-levelsoundtoolkits,suchasSDL,OpenALorthoseprovidedindesktopenvironments,maybeabetterchoice.ByusingALSAyouarerestrictedtousingsystemsrunningaLinuxkernelwithALSAsupport.

HistoryofALSA

TheALSAProjectwasstartedbecausethesounddriversintheLinuxkernel（OSS/Freedrivers）werenotbeingmaintainedactivelyandwerelaggingbehindthecapabilitiesofnewsoundtechnology.JaroslavKysela,whopreviouslyhadwrittenasoundcarddriver,startedtheproject.Overtime,moredevelopersjoined,supportformanysoundcardswasaddedandthestructureoftheAPIwasrefined.

Duringdevelopmentofthe2.5seriesofLinuxkernel,ALSAwasmergedintotheofficialkernelsource.Withthereleaseofthe2.6kernel,ALSAwillbepartofthestableLinuxkernelandshouldbeinwideuse.

DigitalAudioBasics

Sound,consistingofwavesofvaryingairpressure,isconvertedtoitselectricalformbyatransducer,suchasamicrophone.Ananalog-to-digitalconverter（ADC）convertstheanalogvoltagesintodiscretevalues,calledsamples,atregularintervalsintime,knownasthesamplingrate.Bysendingthesamplestoadigital-to-analogconverterandanoutputtransducer,suchasaloudspeaker,theoriginalsoundcanbereproduced.

Thesizeofthesamples,expressedinbits,isonefactorthatdetermineshowaccuratelythesoundisrepresentedindigitalform.Theothermajorfactoraffectingsoundqualityisthesamplingrate.TheNyquistTheoremstatesthatthehighestfrequencythatcanberepresentedaccuratelyisatmostone-halfthesamplingrate.

ALSABasics

ALSAconsistsofaseriesofkerneldevicedriversformanydifferentsoundcards,anditalsoprovidesanAPIlibrary,libasound.ApplicationdevelopersareencouragedtoprogramusingthelibraryAPIandnotthekernelinterface.Thelibraryprovidesahigher-levelandmoredeveloper-friendlyprogramminginterfacealongwithalogicalnamingofdevicessothatdevelopersdonotneedtobeawareoflow-leveldetailssuchasdevicefiles.

Incontrast,OSS/Freedriversareprogrammedatthekernelsystemcalllevelandrequirethedevelopertospecifydevicefilenamesandperformmanyfunctionsusingioctlcalls.Forbackwardcompatibility,ALSAprovideskernelmodulesthatemulatetheOSS/Freesounddrivers,somostexistingsoundapplicationscontinuetorununchanged.Anemulationwrapperlibrary,libaoss,isavailabletoemulatetheOSS/FreeAPIwithoutkernelmodules.

ALSAhasacapabilitycalledpluginsthatallowsextensiontonewdevices,includingvirtualdevicesimplementedentirelyinsoftware.ALSAprovidesanumberofcommand-lineutilities,includingamixer,soundfileplayerandtoolsforcontrollingspecialfeaturesofspecificsoundcards.

ALSAArchitecture

TheALSAAPIcanbebrokendownintothemajorinterfacesitsupports:

Controlinterface:

ageneral-purposefacilityformanagingregistersofsoundcardsandqueryingtheavailabledevices.

PCMinterface:

theinterfaceformanagingdigitalaudiocaptureandplayback.Therestofthisarticlefocusesonthisinterface,asitistheonemostcommonlyusedfordigitalaudioapplications.

RawMIDIinterface:

supportsMIDI（MusicalInstrumentDigitalInterface）,astandardforelectronicmusicalinstruments.ThisAPIprovidesaccesstoaMIDIbusonasoundcard.TherawinterfaceworksdirectlywiththeMIDIevents,andtheprogrammerisresponsibleformanagingtheprotocolandtiming.

Timerinterface:

providesaccesstotiminghardwareonsoundcardsusedforsynchronizingsoundevents.

Sequencerinterface:

ahigher-levelinterfaceforMIDIprogrammingandsoundsynthesisthantherawMIDIinterface.IthandlesmuchoftheMIDIprotocolandtiming.

Mixerinterface:

controlsthedevicesonsoundcardsthatroutesignalsandcontrolvolumelevels.Itisbuiltontopofthecontrolinterface.

DeviceNaming

ThelibraryAPIworkswithlogicaldevicenamesratherthandevicefiles.Thedevicenamescanberealhardwaredevicesorplugins.Hardwaredevicesusetheformathw:

i,j,whereiisthecardnumberandjisthedeviceonthatcard.Thefirstsounddeviceishw:

0,0.Thealiasdefaultreferstothefirstsounddeviceandisusedinalloftheexamplesinthisarticle.Pluginsuseotheruniquenames;plughw:

forexample,isapluginthatprovidesaccesstothehardwaredevicebutprovidesfeatures,suchassamplingrateconversion,insoftwareforhardwarethatdoesnotdirectlysupportit.Thedmixanddsharepluginsallowyoutodownmixseveralstreamsandsplitasinglestreamdynamicallyamongdifferentapplications.

SoundBuffersandDataTransfer

Asoundcardhasahardwarebufferthatstoresrecordedsamples.Whenthebufferissufficientlyfull,itgeneratesaninterrupt.Thekernelsounddriverthenusesdirectmemoryaccess（DMA）totransfersamplestoanapplicationbufferinmemory.Similarly,forplayback,anotherapplicationbufferistransferredfrommemorytothesoundcard'shardwarebufferusingDMA.

Thesehardwarebuffersareringbuffers,meaningthedatawrapsbacktothestartwhentheendofthebufferisreached.Apointerismaintainedtokeeptrackofthecurrentpositionsinboththehardwarebufferandtheapplicationbuffer.Outsideofthekernel,onlytheapplicationbufferisofinterest,sofromhereonwediscussonlytheapplicationbuffer.

ThesizeofthebuffercanbeprogrammedbyALSAlibrarycalls.Thebuffercanbequitelarge,andtransferringitinoneoperationcouldresultinunacceptabledelays,calledlatency.Tosolvethis,ALSAsplitsthebufferupintoaseriesofperiods（calledfragmentsinOSS/Free）andtransfersthedatainunitsofaperiod.

Aperiodstoresframes,eachofwhichcontainsthesamplescapturedatonepointintime.Forastereodevice,theframewouldcontainsamplesfortwochannels.Figure1illustratesthebreakdownofabufferintoperiods,framesandsampleswithsomehypotheticalvalues.Here,leftandrightchannelinformationisstoredalternatelywithinaframe;thisiscalledinterleavedmode.Anon-interleavedmode,whereallthesampledataforonechannelisstoredfollowedbythedataforthenextchannel,alsoissupported.

Figure1.TheApplicationBuffer

OverandUnderRun

Whenasounddeviceisactive,dataistransferredcontinuouslybetweenthehardwareandapplicationbuffers.Inthecaseofdatacapture（recording）,iftheapplicationdoesnotreadthedatainthebufferrapidlyenough,thecircularbufferisoverwrittenwithnewdata.Theresultingdatalossisknownasoverrun.Duringplayback,iftheapplicationdoesnotpassdataintothebufferquicklyenough,itbecomesstarvedfordata,resultinginanerrorcalledunderrun.TheALSAdocumentationsometimesreferstobothoftheseconditionsusingthetermXRUN.ProperlydesignedapplicationscanminimizeXRUNandrecoverifitoccurs.

ATypicalSoundApplication

ProgramsthatusethePCMinterfacegenerallyfollowthispseudo-code:

openinterfaceforcaptureorplayback

sethardwareparameters

（accessmode,dataformat,channels,rate,etc.）

whilethereisdatatobeprocessed:

readPCMdata（capture）

orwritePCMdata（playback）

closeinterface

Welookatsomeworkingcodeinthefollowingsections.IrecommendyoucompileandruntheseonyourLinuxsystem,lookattheoutputandtrysomeofthesuggestedmodifications.Thefulllistingsfortheexampleprogramsthataccompanythisarticleareavailablefordownloadfrom

Listing1.DisplaySomePCMTypesandFormats

#include

intmain（）{

intval;

printf（"ALSAlibraryversion:

%s\n",

SND_LIB_VERSION_STR）;

printf（"\nPCMstreamtypes:

\n"）;

for（val=0;val<=SND_PCM_STREAM_LAST;val++）

printf（"%s\n",

snd_pcm_stream_name（（snd_pcm_stream_t）val））;

printf（"\nPCMaccesstypes:

\n"）;

for（val=0;val<=SND_PCM_ACCESS_LAST;val++）

printf（"%s\n",

snd_pcm_access_name（（snd_pcm_access_t）val））;

printf（"\nPCMformats:

\n"）;

for（val=0;val<=SND_PCM_FORMAT_LAST;val++）

if（snd_pcm_format_name（（snd_pcm_format_t）val）

!

=NULL）

printf（"%s（%s）\n",

snd_pcm_format_name（（snd_pcm_format_t）val）,

snd_pcm_format_description（

（snd_pcm_format_t）val））;

printf（"\nPCMsubformats:

\n"）;

for（val=0;val<=SND_PCM_SUBFORMAT_LAST;

val++）

printf（"%s（%s）\n",

snd_pcm_subformat_name（（

snd_pcm_subformat_t）val）,

snd_pcm_subformat_description（（

snd_pcm_subformat_t）val））;

printf（"\nPCMstates:

\n"）;

for（val=0;val<=SND_PCM_STATE_LAST;val++）

printf（"%s\n",

snd_pcm_state_name（（snd_pcm_state_t）val））;

return0;

}

Listing1displayssomeofthePCMdatatypesandparametersusedbyALSA.ThefirstrequirementistoincludetheheaderfilethatbringsinthedefinitionsforalloftheALSAlibraryfunctions.Oneofthedefinit