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| − | '''PortAudio''' è una libreria ''open-source'' multipiattaforma per l'ingresso e l'uscita audio in tempo reale. La libreria fornisce funzioni che permettono al software di acquisire e di inviare flussi audio in tempo reale dalle interfacce audio hardware del computer.
| + | #redirect[[Eseguire_un_file_audio_mediante_le_funzioni_esterne_del_API_di_PortAudio_e_SndFile]] |
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| − | Poiché ''PortAudio'' non è progettato per leggere i dati audio utili da un file audio, bisognerà ''appoggiarsi'', per questo, ad un'altra risorsa come, ad esempio, le funzioni della libreria ''SndFile''.
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| − | '''SndFile''' è una libreria, scritta in C, per leggere e scrivere file che contengono suono campionato (come il formato WAV ed il formato AIFF) attraverso una libreria interfaccia standard.
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| − | Pertanto, bisognerà coniugare entrambe le librerie, assegnando a ''SndFile'' il compito di leggere ed estrarre i dati del file audio, ed a ''PortAudio'' quello di eseguire tali dati audio.
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| − | Mostreremo di seguito un semplice codice per l'esecuzione di un file wav 44100 hertz, 2 canali, 16-bit:
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| − | Public Struct SF_INFO
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| − | frames As Long
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| − | samplerate As Integer
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| − | channels As Integer
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| − | formatI As Integer
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| − | sections As Integer
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| − | seekable As Integer
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| − | End Struct
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| − | Public Struct PaStreamParameters
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| − | device As Integer
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| − | channelCount As Integer
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| − | sampleFormat As Long
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| − | suggestedLatency As Integer
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| − | hostApiSpecificStreamInfo As Pointer
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| − | End Struct
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| − | Public Struct PaDeviceInfo
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| − | structVersion As Integer
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| − | name As String
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| − | hostApi As Integer
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| − | maxInputChannels As Integer
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| − | maxOutputChannels As Integer
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| − | defaultLowInputLatency As Integer
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| − | defaultLowOutputLatency As Integer
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| − | defaultHighInputLatency As Integer
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| − | defaultHighOutputLatency As Integer
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| − | defaultSampleRate As Integer
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| − | End Struct
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| − | Private Const paInt16 As Byte = 8
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| − | Private Const paClipOff As Byte = 1
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| − | Private Const FRAMES_PER_BUFFER As Short = 1024
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| − | Private Const NUM_CHANNELS As Byte = 2
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| − | Private Const BUFFER_LEN As Short = 8192
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| − | Private Const MAX_CHANNELS As Byte = 2
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| − | Private Const SFM_READ As Byte = 16
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| − | Private Const paNoError As Byte = 0
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| − | Private PaStream As Pointer
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| − | Private infile As Pointer
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| − | Library "libsndfile:1.0.25"
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| − | <FONT color=gray>' ''SNDFILE * sf_open (const char *path, int mode, SF_INFO *sfinfo) --> Apre un file per la lettura''</font>
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| − | Private Extern sf_open(path As String, mode As Integer, SFinf As SF_INFO) As Pointer
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| − | <FONT color=gray>' ''sf_count_t sf_read_short(SNDFILE *sndfile, float *ptr, sf_count_t items) --> Legge i dati del file wav''</font>
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| − | Private Extern sf_read_short(sndfile As Pointer, ptr As Pointer, items As Integer) As Integer
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| − | <FONT color=gray>' ''int sf_close (SNDFILE *sndfile) --> Chiude il file precedentemente aperto''</font>
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| − | Private Extern sf_close(sndfile As Pointer) As Integer
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| − | Library "libportaudio:2.0.0"
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| − | <FONT color=gray>' ''PaError Pa_Initialize( void )''
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| − | ' ''Library initialization function - call this before using PortAudio.''
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| − | ' ''This function initializes internal data structures and prepares underlying host APIs for use.''</font>
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| − | Private Extern Pa_Initialize() As Integer
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| − | <FONT color=gray>' ''PaDeviceIndex Pa_GetDefaultOutputDevice( void ) --> Retrieve the index of the default output device.''</font>
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| − | Private Extern Pa_GetDefaultOutputDevice() As Integer
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| − | <FONT color=gray>' ''PaDeviceInfo* Pa_GetDeviceInfo( PaDeviceIndex device )''
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| − | ' ''Retrieve a pointer to a PaDeviceInfo structure containing information about the specified device.''</font>
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| − | Private Extern Pa_GetDeviceInfo(device As Integer) As PaDeviceInfo
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| − | <FONT color=gray>' ''PaError Pa_OpenStream( PaStream** stream, const PaStreamParameters *inputParameters,''
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| − | ' ''const PaStreamParameters *outputParameters, double sampleRate, unsigned long framesPerBuffer,''
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| − | ' ''PaStreamFlags streamFlags, PaStreamCallback *streamCallback, void *userData )''
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| − | ' ''Opens a stream for either input, output or both.''</font>
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| − | <FONT size=1>Private Extern Pa_OpenStream(Pstream As Pointer, inputParameters As PaStreamParameters, outputParameters As PaStreamParameters, FsampleRate As Float, framesPerBuffer As Long, streamFlags As Byte, streamCallback As Pointer, userData As Pointer) As Integer</font>
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| − | <FONT color=gray>' ''PaError Pa_StartStream( PaStream *stream ) --> Commences audio processing.''</font>
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| − | Private Extern Pa_StartStream(Pstream As Pointer) As Integer
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| − | <FONT color=gray>' ''PaError Pa_WriteStream(PaStream * stream, const void * buffer, unsigned long frames)''</font>
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| − | Private Extern Pa_WriteStream(Pstream As Pointer, buffer As Pointer, frames As Integer) As Integer
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| − | <FONT color=gray>' ''PaError Pa_CloseStream (PaStream *stream) --> Closes an audio stream.''</font>
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| − | Private Extern Pa_CloseStream(Pstream As Pointer) As Integer
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| − | <FONT color=gray>' ''PaError Pa_IsStreamStopped (PaStream * stream) --> Returns one (1) when the stream is stopped, zero (0) when the stream is running.''</font>
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| − | Private Extern Pa_IsStreamStopped(Pstream As Pointer) As Integer
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| − | <FONT color=gray>' ''PaError Pa_Terminate (void) --> Library termination function - call this when finished using PortAudio.''
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| − | ' ''This function deallocates all resources allocated by PortAudio since it was initialized by a call to Pa_Initialize().''</font>
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| − | Private Extern Pa_Terminate() As Integer
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| − | <FONT color=gray>' ''PaError Pa_StopStream (PaStream * stream) --> Terminates audio processing. It waits until all pending audio buffers have been played before it returns.''</font>
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| − | Private Extern Pa_StopStream(Pstream As Pointer) As Integer
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| − | '''Public''' Sub Button1_Click()
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| − | Dim data As Pointer
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| − | Dim sfinfo As New SF_INFO
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| − | Dim pasp As New PaStreamParameters
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| − | Dim padi As New PaDeviceInfo
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| − | Dim err, frequenza, conto As Integer
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| − | <FONT color=gray>' ''Se l'applicativo sta già eseguendo un file wav, esce dalla routine:''</font>
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| − | If Pa_IsStreamStopped(PaStream) = 0 Then Return
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| − | <FONT color=darkorange><B>data</b></font> = Alloc(BUFFER_LEN * 4)
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| − | <FONT color=gray>' ''Inizializza la libreria di "PortAudio":''</font>
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| − | err = Pa_Initialize()
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| − | If err <> paNoError Then Error.Raise("Impossibile inizializzare la libreria 'PortAudio' !")
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| − | <FONT color=gray>' ''Apre il file wav:''</font>
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| − | infile = sf_open("''/percorso/del/file.wav''", SFM_READ, sfinfo)
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| − | <FONT color=gray>' ''Ricava la frequenza di campionamento del file wav:''</font>
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| − | frequenza = sfinfo.samplerate
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| − | With pasp
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| − | .device = Pa_GetDefaultOutputDevice()
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| − | .channelCount = NUM_CHANNELS
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| − | .sampleFormat = paInt16
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| − | padi = Pa_GetDeviceInfo(.device)
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| − | .suggestedLatency = padi.defaultLowOutputLatency
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| − | .hostApiSpecificStreamInfo = Null
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| − | End With
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| − | err = Pa_OpenStream(VarPtr(PaStream), Null, pasp, frequenza, FRAMES_PER_BUFFER, paClipOff, Null, Null)
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| − | If err <> paNoError Then Error.Raise("Impossibile aprire il flusso di dati !")
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| − | If IsNull(PaStream) = False Then
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| − | err = Pa_StartStream(PaStream)
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| − | If err <> paNoError Then Error.Raise("Impossibile avviare il flusso di dati !")
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| − | <FONT color=gray>' ''Inizia il ciclo per la scrittura dei dati e per l'esecuzione del file wav:''</font>
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| − | While True
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| − | <FONT color=gray>' Legge i dati del file wav e li carica nel buffer "data". La quantità unitaria di dati caricati nella variabile "data"''
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| − | ' ''deve essere uguale al doppio del 3° argomento della successiva funzione "Pa_WriteStream":''</font>
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| − | conto = sf_read_short(infile, <FONT color=darkorange><B>data</b></font>, BUFFER_LEN * 2)
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| − | <FONT color=gray>' ''Scrive i dati, presenti nel buffer "data", nella variabile "PaStream":''</font>
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| − | err = Pa_WriteStream(PaStream, <FONT color=darkorange><B>data</b></font>, BUFFER_LEN)
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| − | If err <> paNoError Then Error.Raise("Impossibile scrivere i dati per l'esecuzione !")
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| − | If conto = 0 Then Exit
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| − | <FONT color=gray>' ''Attende un centesimo di secondo per consentire di agire su eventuali oggetti presenti sul Form:''</font>
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| − | Wait 0.01
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| − | Wend
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| − | Endif
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| − | <FONT color=gray>' ''Va in chiusura:''</font>
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| − | err = Pa_CloseStream(PaStream)
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| − | If err <> paNoError Then Error.Raise("Impossibile chiudere il flusso di dati !")
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| − | err = Pa_Terminate()
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| − | If err <> paNoError Then Error.Raise("Impossibile chiudere la libreria 'libportaudio' !")
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| − | sf_close(infile)
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| − | Free(data)
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| − | '''End'''
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| − | '''Public''' Sub Button2_Click()
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| − | Dim err As Integer
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| − | <FONT color=gray>' ''Arresta l'esecuzione del file wav e va in chiusura:''</font>
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| − | err = Pa_StopStream(PaStream)
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| − | If err <> paNoError Then Error.Raise("Impossibile arrestare l'esecuzione dei dati audio !")
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| − | err = Pa_Terminate()
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| − | If err <> paNoError Then Error.Raise("Impossibile chiudere la libreria 'libportaudio' !")
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| − | sf_close(infile)
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| − | '''End'''
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| − | =Riferimenti=
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| − | * [http://www.portaudio.com/ Il sito di PortAudio]
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| − | * [http://portaudio.com/docs/v19-doxydocs/index.html Il sito del API di PortAudio].
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| − | * [http://ispac.ing.uniroma1.it/scarpiniti/files/LabEMM/Less9.pdf Una introduzione a PortAudio]
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| − | * [http://www.mega-nerd.com/libsndfile/api.html Il sito del API di SndFile].
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