Eseguire un file WAV usando l'API di SndFile e di Alsa

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.

Pertanto, coniugheremo le librerie di SndFile e di Alsa, assegnando a SndFile il compito di leggere ed estrarre i dati del file audio, ed ad Alsa quello di eseguire tali dati audio.

Mostreremo di seguito un semplice codice per l'esecuzione di un file wav:

Library "libsndfile:1.0.25"

Public Struct SF_INFO
  frames As Long
  samplerate As Integer
  channels As Integer  
  formatI As Integer
  sections As Integer
  seekable As Integer
End Struct
 
Private Const BUFFER_LEN As Short = 1024
Private Const SFM_READ As Byte = 16
 
' SNDFILE * sf_open (const char *path, int mode, SF_INFO *sfinfo)
' Open the specified file for read, write or both.
Private Extern sf_open(path As String, mode As Integer, SFinf As SF_INFO) As Pointer

' sf_count_t  sf_read_short(SNDFILE *sndfile, float *ptr, sf_count_t items)
' Read the data chunk in terms of frames (in the native short format).
Private Extern sf_read_short(sndfile As Pointer, ptr As Pointer, items As Integer) As Integer

' int  sf_close  (SNDFILE *sndfile)
' Close the SNDFILE and clean up all memory allocations associated with this file.
Private Extern sf_close(sndfile As Pointer) As Integer
 

Library "libasound:2"

Private Const SND_PCM_STREAM_PLAYBACK As Byte = 0
Private Const SND_PCM_ACCESS_RW_INTERLEAVED As Byte = 3
Private Const SND_PCM_FORMAT_U8 As Byte = 1

' int snd_pcm_open(snd_pcm_t **pcm, const char *name, snd_pcm_stream_t stream, int mode)
' Opens a PCM.
Private Extern snd_pcm_open(pcmP As Pointer, nome As String, stream As Integer, mode As Integer) As Integer

' const char * snd_strerror (int errnum)
' Returns the message For an Error code.
Private Extern snd_strerror(errnum As Integer) As String

' int snd_pcm_set_params (snd_pcm_t *pcm, snd_pcm_format_t format, snd_pcm_access_t access, unsigned int channels, unsigned int rate, int soft_resample, unsigned int latency)
' Set the hardware And software parameters In aPrivate Const MPG123_ADD_FLAGS As Byte = 2 simple way.
Private Extern snd_pcm_set_params(pcmP As Pointer, formatB As Byte, accessB As Byte, channels As Integer, rate As Integer, soft_resample As Integer, latency As Integer) As Integer

' snd_pcm_sframes_t snd_pcm_writei(snd_pcm_t *pcm, const void *buffer, snd_pcm_uframes_t size)
' Write interleaved frames to a PCM.
Private Extern snd_pcm_writei(pcmP As Pointer, buffP As Pointer, uframes As Long) As Integer

' int snd_pcm_drain (snd_pcm_t *pcm)
' Stop a PCM preserving pending frames.
Private Extern snd_pcm_drain(pcmP As Pointer) As Integer

' int snd_pcm_close(snd_pcm_t **pcm)
' Close PCM handle.
Private Extern snd_pcm_close(pcmP As Pointer)
 

Public Sub Main()

 Dim infile, buffer, handle As Pointer
 Dim sfinfo As New SF_INFO
 Dim err, frequenza, conto, frames, canali, somma As Integer
 

   buffer = Alloc(BUFFER_LEN * 4)

' Apre il file wav:
   infile = sf_open("/percorso/del/file.wav", SFM_READ, sfinfo)
   
' Ricava la frequenza di campionamento ed il numero di canali del file wav:
   With sfinfo
     frequenza = .samplerate
     canali = .channels
   End With

''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' Qui inizia la parte relativa alle funzioni di Alsa:

   err = snd_pcm_open(VarPtr(handle), "default", SND_PCM_STREAM_PLAYBACK, 0)
    If err < 0 Then Error.Raise("Playback open error: " & snd_strerror(err))

   err = snd_pcm_set_params(handle, 2, SND_PCM_ACCESS_RW_INTERLEAVED, canali, frequenza, 1, 500000)
    If err < 0 Then Error.Raise("Playback open error: " & snd_strerror(err))


' Inizia il ciclo per la scrittura dei dati e per l'esecuzione del file wav:
   Do

      ' Legge i dati del file wav e li carica nel buffer:
     conto = sf_read_short(infile, buffer, BUFFER_LEN)

' I dati audio vengono inviati al sub-sistema "PCM" di Alsa, attraverso la lettura della variabile
' qui denominata "buffer". La quantità unitaria di dati scritti nell'handle del sub-sistema PCM deve essere uguale
' al valore del 3° argomento della precedente funzione "sf_read_short" diviso il numero di canali del file audio:
     frames = snd_pcm_writei(handle, buffer, BUFFER_LEN / sfinfo.channels)
     If (frames < 0) Then Error.Raise("Errore alla funzione 'snd_pcm_writei': " & snd_strerror(err))
   
     somma += conto
     Write "\r" & CStr(Date(0, 0, 0, 0, 0, 0, (somma / frequenza) * 1000))
    
   Loop While conto <> 0
  
   ' Impedisce che al termine l'esecuzione venga troncata inaspettatamente:
   snd_pcm_drain(handle)

 ' Va in chiusura:
   snd_pcm_close(handle)
   sf_close(infile)
   Free(buffer)
 
End

Riferimenti

• Il sito del API di SndFile.