Versione delle 12:06, 9 lug 2017

La libreria di VorbisFile consente di estrarre informazioni di carattere generale da un file audio OggVorbis e di convertirlo in un file audio WAV.

Per poter fruire in Gambas delle risorse del API di Ogg Vorbis si dovrà utilizzare la libreria: "libvorbisfile.so.3.3.4"

Di seguito mostreremo un semplice codice per convertire un file OggVorbis in file WAV. Poiché l'uso della libreria esterna Libvorbisfile prevede il richiamo della Struttura esterna molto complessa: OggVorbis_File, al fine di poter gestire detta Struttura esterna in modo assolutamente sicuro, riserveremo un'area di memoria di dimensione pari a quella, preliminarmente calcolata, occupata dalla Struttura esterna.

Il codice Gambas potrà essere il seguente:

 Library "libvorbisfile:3.3.4"

Public Struct vorbis_comment
  user_comments As Pointer
  comment_lengths As Pointer
  comments As Integer
  vendor As Pointer
End Struct

Public Struct ogg_sync_state
  data As Pointer
  storage As Integer
  fill As Integer
  returned As Integer
  unisynced As Integer
  headerbytes As Integer
  bodybytes As Integer
End Struct

Public Struct ogg_stream_state
  body_data As Pointer
  body_storage As Long
  body_fill As Long
  body_returned As Long
  lacing_vals As Pointer
  granule_vals As Pointer
  lacing_storage As Long
  lacing_fill As Long
  lacing_packet As Long
  lacing_returned As Long
  header[282] As Byte
  header_fill As Integer
  e_o_s As Integer
  b_o_s As Integer
  serialno As Long
  pageno As Long
  packetno As Long
  granulepos As Long
End Struct

Public Struct vorbis_dsp_state
  analysisp As Integer
  vi As Pointer
  pcm As Pointer
  pcmret As Pointer
  pcm_storage As Integer
  pcm_current As Integer
  pcm_returned As Integer
  preextrapolate As Integer
  eofflag As Integer
  lW As Long
  W As Long
  nW As Long
  centerW As Long
  granulepos As Long
  sequence As Long
  glue_bits As Long
  time_bits As Long
  floor_bits As Long
  res_bits As Long
End Struct

Public Struct oggpack_buffer
  endbyte As Long
  endbit As Integer
  buffer As Pointer
  ptr As Pointer
  storage As Long
End Struct

Public Struct vorbis_block
  pcm As Pointer
  opb As Struct Oggpack_buffer
  lW As Long
  W As Long
  nW As Long
  pcmend As Integer
  mode As Integer
  eofflag As Integer
  granulepos As Long
  sequence As Long
  vd As Pointer
  localstore As Pointer
  localtop As Long
  localalloc As Long
  totaluse As Long
  reap As Pointer
  glue_bits As Long
  time_bits As Long
  floor_bits As Long
  res_bits As Long
  internal As Pointer
End Struct

Public Struct ov_callbacks
  fread As Pointer
  _ov_header_fseek_wrap As Pointer
  void As Pointer
  ftell As Pointer
End Struct

Public Struct OggVorbis_File
  datasource As Pointer
  seekable As Integer
  offset As Long
  end_ As Long
  oy As Struct Ogg_sync_state
  links As Integer
  offsets As Pointer
  dataoffsets As Pointer
  serialnos As Pointer
  pcmlengths As Pointer
  vi As Pointer
  vc As Pointer
  pcm_offset As Long
  ready_state As Integer
  current_serialno As Long
  current_link As Integer
  bittrack As Float
  samptrack As Float
  os As Struct Ogg_stream_state
  vd As Struct Vorbis_dsp_state
  vb As Struct Vorbis_block
  callbacks As Struct Ov_callbacks
End Struct

Public Struct vorbis_info
  version As Integer
  channels As Integer
  rate As Long
  bitrate_upper As Long
  bitrate_nominal As Long
  bitrate_lower As Long
  bitrate_window As Long
End Struct
 
' int ov_fopen(const char *path,OggVorbis_File *vf)
' Opens and initialize an OggVorbis_File structure.
Private Extern ov_fopen(path As String, vf As OggVorbis_File) As Integer
 
' ogg_int64_t ov_pcm_total(OggVorbis_File *vf,int i)
' Returns the total pcm samples of the physical bitstream or a specified logical bitstream.
Private Extern ov_pcm_total(vf As OggVorbis_File, i As Integer) As Long

' vorbis_info *ov_info(OggVorbis_File *vf,int link)
' Returns the vorbis_info struct for the specified bitstream.
Private Extern ov_info(vf As OggVorbis_File, linkI As Integer) As Vorbis_info
  
' double ov_time_total(OggVorbis_File *vf,int i)
' Returns the total time in seconds of the physical bitstream or a specified logical bitstream.
Private Extern ov_time_total(vf As OggVorbis_File, i As Integer) As Float

' vorbis_comment *ov_comment(OggVorbis_File *vf,int link)
' Returns a pointer to the vorbis_comment struct for the specified bitstream.
Private Extern ov_comment(vf As OggVorbis_File, lnk As Integer) As Pointer

' long ov_read(OggVorbis_File *vf, char *buffer, int length, int bigendianp, int word, int sgned, int *bitstream)
' Decode a Vorbis file within a loop.
Private Extern ov_read(OVf As Pointer, buffer As Pointer, length As Integer, bigendianp As Integer, word As Integer, sgned As Integer, bitstream As Pointer) As Long
  

Public Sub Main()
 
 Dim vf As New OggVorbis_File
 Dim vi As Vorbis_info
 Dim commenti As Vorbis_comment
 Dim fin As Pointer
 Dim eo, current_section, err, i As Integer
 Dim rit As Long
 Dim datiPCMgrezzi, fileOGG As String
 Dim pcmout As New Byte[](4096)
 Dim f As File
 Dim b As Byte
  
  fileOGG = "/percorso/del/file.ogg"
  datiPCMgrezzi = "/tmp/datiPCMgrezzi"
   
  f = Open datiPCMgrezzi For Create
    
  err = ov_fopen(fileOGG, vf)
  If err < 0 Then Error.Raise("Il file caricato potrebbe non essere del formato Ogg !")
  
' Estrae alcune informazioni generali del file audio Ogg caricato:
  vi = ov_info(vf, -1)
 
  Print "File audio:                              "; fileOGG
  With vi
    Print "Frequenza di campionamento:              "; .rate; " Hertz"
    Print "Canali di uscita:                        "; .channels
    Print "Bitrate:                                 "; .bitrate_nominal; " bps"
    Print "Durata del file ogg:                     "; Date(0, 0, 0, 0, 0, 0, ov_time_total(vf, -1) * 1000)
    Print "Lunghezza dei campioni decodificati:     "; ov_pcm_total(vf, -1); " byte"
  End With
  Print "Commenti presenti nel file ogg:"
  commenti = ov_comment(vf, -1)
  With commenti
    For b = 0 To .comments - 1
      i = 8 * b
      Print "   "; String@(Pointer@(.user_comments + i))
    Next
    Print "   "; String@(.vendor)
  End With
 

' Ciclo per la decodifica dei dati Ogg e per la scrittura del file contenente i dati grezzi PCM:
  While eo = 0
    rit = ov_read(vf, pcmout.Data, pcmout.Count, 0, 2, 1, VarPtr(current_section))
    If rit = 0 Then
      eo = 1
    Else
' Scrive il file contenente i dati grezzi PCM:
      pcmout.Write(f, 0, rit)
    Endif
  Wend
 
' Va a creare il file WAV finale:
  crea_file_wav(datiPCMgrezzi)


' Va in chiusura:
  Free(vf)
  f.Close
  fclose(fin)

End


Private Procedure crea_file_wav(datiGrezzi As String)
 
 Dim fl, fwav As File
 Dim i, i2 As Integer
 Dim dati As New Byte[](Stat(datiGrezzi).Size)
 Dim bh As Byte[] = [&52, &49, &46, &46, &00, &00, &00, &00, &57, &41, &56, &45, &66, &6D, &74, &20, &10, &00, &00, &00, &01, &00, &02, &00,
                    &44, &AC, &00, &00, &10, &B1, &02, &00, &04, &00, &10, &00, &64, &61, &74, &61, &00, &00, &00, &00]    ' blocco d'intestazione del file wav futuro: 2 canali, 16 bit, hz 44100
                    
  fl = Open datiGrezzi For Read
  dati.Read(fl)
     
  i = Lof(fl)
  fl.Close
  i2 = i + 36
  
' Imposta il valore dimensionale di 4 byte a partire dal 5° byte del futuro file wav:
  bh[4] = i2 And &FF
  bh[5] = Shr(i2 And &FF00&, 8)
  bh[6] = Shr(i2 And &FF0000&, 16)
  bh[7] = Shr(i2 And &FF000000&, 24)
  
' Imposta il valore dimensionale di 4 byte a partire dal 41° byte del futuro file wav.
' Esso è relativo alla quantità effettiva dei dati audio grezzi.
  bh[40] = i And &FF
  bh[41] = Shr(i And &FF00&, 8)
  bh[42] = Shr(i And &FF0000&, 16)
  bh[43] = Shr(i And &FF000000&, 24)
     
  bh.Insert(dati, 44)

  fwav = Open "/percorso/del/file.wav" For Create

' Inizia la scrittura del file wav:
  bh.Write(fwav, 0, bh.Count)
  
  Print "\n\nDurata del file WAV: "; Date(0, 0, 0, 0, 0, 0, ((i * 8) / (44100 * 16 * 2)) * 1000)
   
  dati.Clear
  bh.Clear
  fl.Close
  fwav.Close
 
End

Riferimenti

• http://www.xiph.org/vorbis/doc/
• http://www.vorbis.com/setup_linux/