Visualizza wikitesto di Ridurre a 8-bit la risoluzione di campionamento di un file audio WAV con le funzioni esterne del API di Sox

La libreria '''Sox''' contiene risorse per poter gestire ampiamente i file audio: riproduzione e registrazione, conversione di vari formati audio in altri formati, nonché applicazione di vari effetti. ''Sox'' consente di eseguire numerosissimi formati di file audio.

Per poter diminuire a 8-bit la risoluzione di campionamento di un file WAV, campionato ad esempio a 16-bit, bisognerà impostare con il valore della Costante ''SOX_ENCODING_UNSIGNED'' il membro "''->encoding.encoding''" della Struttura ''sox_format_t'', relativa ai nuovi dati del nuovo file wav.

E' necessario avere installata nel proprio sistema e richiamare in Gambas la libreria condivisa: "''libsox.so.3.0.0'' ".

Mostriamo un esempio, nel quale un file di formato WAV a 16-bit, 44100 hertz, viene diminuita a 8-bit la risoluzione di campionamento e a 8000 hertz la frequenza di campionamento:
 Library "libsox:3.0.0
 
 Public Struct sox_signalinfo_t
   rate As Float
   channels As Integer
   precision As Integer
   length As Long
   mult As Pointer
 End Struct
 
 Public Struct sox_encodinginfo_t
   encoding As Integer
   bits_per_sample As Integer
   compression As Float
   reverse_bytes As Integer
   reverse_nibbles As Integer
   reverse_bits As Integer
   opposite_endian As Integer
 End Struct
 
 Public Struct sox_effects_globals_t
   plot As Integer
   global_info As Pointer
 End Struct
 
 Public Struct sox_effects_chain_t
   effects As Pointer
   table_size As Integer
   length As Integer
   ibufc As Pointer
   obufc As Pointer
   global_info As Struct Sox_effects_globals_t
   in_enc As Pointer
   out_enc As Pointer
 End Struct
 
 Public Struct sox_effect_handler_t
   name As Pointer
   usage As Pointer
   flags As Integer
   sox_effect_handler_getopts As Pointer
   sox_effect_handler_start As Pointer
   sox_effect_handler_flow As Pointer
   sox_effect_handler_drain As Pointer
   sox_effect_handler_stop As Pointer
   sox_effect_handler_kill As Pointer
   priv_size As Long
 End Struct
  
 Public Struct sox_effect_t
   global_info As Pointer
   in_signal As Struct Sox_signalinfo_t
   out_signal As Struct Sox_signalinfo_t
   in_encoding As Pointer
   out_encoding As Pointer
   handler As Struct Sox_effect_handler_t
   obuf As Pointer
   obeg As Long
   oend As Long
   imin As Long
   clips As Long
   flows As Long
   flow As Long
   priv As Pointer
 End Struct
  
 Private Enum SOX_EOF = -1, SOX_SUCCESS, SOX_EHDR = 2000, SOX_EFMT, SOX_ENOMEM, SOX_EPERM, SOX_ENOTSUP, SOX_EINVAL
 Private Const SOX_ENCODING_UNSIGNED As Integer = 2
 Private Const SOX_UNSPEC As Integer = 0
 Private Const SOX_EFF_LENGTH As Integer = 8
 
 <FONT Color=gray>' ''int sox_format_init(void)''
 ' ''Client API: Initialize effects library. SOX_SUCCESS if successful.''</font>
 Private Extern sox_init() As Integer
 
 <FONT Color=gray>' ''sox_format_t * sox_open_read(char const *path, sox_signalinfo_t const *signal, sox_encodinginfo_t const *encoding, char const *filetype)''
 ' ''Opens a decoding session for a file. returns the handle for the new session, or null on failure.''</font>
 Private Extern sox_open_read(path As String, signal As Sox_signalinfo_t, encoding As sox_encodinginfo_t, filetype As String) As Pointer
  
 <FONT Color=gray>' ''sox_format_t * sox_open_write(char const *path, sox_signalinfo_t const *signal, sox_encodinginfo_t const *encoding, char const *filetype, sox_oob_t const *oob, sox_bool(*overwrite_permitted)(const char *filename)''
 ' ''Opens a decoding session for a file. returns the handle for the new session, or null on failure.''</font>
 Private Extern sox_open_write(path As String, signal As Sox_signalinfo_t, encoding As Sox_encodinginfo_t, filetype As String, oob As Pointer, filename As String) As Pointer
 
 <FONT Color=gray>' ''sox_effects_chain_t * sox_create_effects_chain(sox_encodinginfo_t const * in_enc, sox_encodinginfo_t const * out_enc)''
 ' ''Initializes an effects chain.''</font>
 Private Extern sox_create_effects_chain(in_enc As Sox_encodinginfo_t, out_enc As Sox_encodinginfo_t) As Sox_effects_chain_t
 
 <FONT Color=gray>' ''sox_effect_t * sox_create_effect(sox_effect_handler_t const * eh)''
 ' ''Creates an effect using the given handler.''</font>
 Private Extern sox_create_effect(eh As Sox_effect_handler_t) As Sox_effect_t
 
 <FONT Color=gray>' ''sox_effect_handler_t const * sox_find_effect(char const * name)''
 ' ''Finds the effect handler with the given name.''</font>
 Private Extern sox_find_effect(name As String) As Pointer
 
 <FONT Color=gray>' ''int sox_effect_options(sox_effect_t *effp, int argc, char * const argv[])''
 ' ''Applies the command-line options to the effect.''</font>
 Private Extern sox_effect_options(effp As Sox_effect_t, argc As Integer, argv As Pointer[]) As Integer
 
 <FONT Color=gray>' ''int sox_add_effect(sox_effects_chain_t * chain, sox_effect_t * effp, sox_signalinfo_t * in, sox_signalinfo_t const * out)''
 ' ''Adds an effect to the effects chain, returns SOX_SUCCESS if successful. Returns SOX_SUCCESS if successful.''</font>
 Private Extern sox_add_effect(chain As Sox_effects_chain_t, effp As Sox_effect_t, inS As Sox_signalinfo_t, ouS As Sox_signalinfo_t) As Integer
 
 <FONT Color=gray>' ''int sox_flow_effects(sox_effects_chain_t * chain, sox_flow_effects_callback callback, void * client_data)''
 ' ''Runs the effects chain, returns SOX_SUCCESS if successful. Returns SOX_SUCCESS if successful.''</font>
 Private Extern sox_flow_effects(chain As Sox_effects_chain_t, callback As Pointer, client_data As Pointer) As Integer
 
 <FONT Color=gray>' ''void sox_delete_effects_chain(sox_effects_chain_t *ecp)''
 ' ''Closes an effects chain.''</font>
 Private Extern sox_delete_effects_chain(chain As Sox_effects_chain_t)
 
 <FONT Color=gray>' ''int sox_close(sox_format_t * ft)''
 ' ''Closes an encoding or decoding session.''</font>
 Private Extern sox_close(ft As Pointer) As Integer
 
 <FONT Color=gray>' ''int sox_quit(void)''
 ' ''Close effects library and unload format handler plugins. Returns SOX_SUCCESS if successful.''</font>
 Private Extern sox_quit()
 
 
 '''Public''' Sub Main()
 
  Dim fileAUDIO As String
  Dim err As Integer
  Dim sfIn, sfOut As Pointer
  Dim sigIn, sigOut As New Sox_signalinfo_t
  Dim encoIn, encoOut As New Sox_encodinginfo_t
  Dim chain As Sox_effects_chain_t
  Dim eff As New Sox_effect_t
  Dim argsP As New Pointer[10]
  Dim st As Stream
  Dim s As Short
 
  fileAUDIO = "<FONT Color=gray>''/percorso/del/file/audio.wav''</font>"
 
 <FONT Color=gray>' ''Inizializza la libreria "libsox":''</font>
  err = sox_init()
  If err <> SOX_SUCCESS Then Error.Raise("Impossibile inizializzare la libreria 'libsox' !")
 
  sfIn = sox_open_read(fileAUDIO, Null, Null, Null)
  If sfIn == 0 Then Error.Raise("Impossibile aprire il file audio !")
 
  sigIn = sfIn + SizeOf(gb.Pointer)
  With sigIn
    Print "File audio \e[31msorgente\e[0m:        ", String@(Pointer@(sfIn))
    Print "Frequenza di campionamento: ", .rate; " hertz"
    Print "Canali:                     ", .channels
    Print "Risoluzione:                ", .precision; " bit"
    Print "Quantità dati audio:        ", .length * .channels; " byte"
    Print "Durata:                     ", Str(Time(0, 0, 0, (((.length * .channels) * 8) / (.rate * .channels * .precision)) * 1000))
  End With
 
 <FONT Color=gray>' ''Sostituisce la necessaria funzione "memcpy()" di C, con le seguenti 5 righe:''</font>
  sfOut = Alloc(SizeOf(gb.Byte), 728)
  st = Memory sfOut For Write
  For s = 0 To 727
    Write #st, Byte@(sfIn + CInt(s)) As Byte
  Next
  st.Close
 
  encoOut = sfOut + (SizeOf(gb.Pointer) * 5)
  With encoOut
    <FONT Color=#B22222>.encoding = SOX_ENCODING_UNSIGNED</font>
    <FONT Color=#B22222>.bits_per_sample = 8</font>
  End With
 
  sigOut = sfOut + SizeOf(gb.Pointer)
  With sigOut
    <FONT Color=#B22222>.rate = 8000</font>
    .precision = 16
    .length = SOX_UNSPEC
  End With
 
  Free(sfOut)
 
 <FONT Color=gray>' ''Imposta il "percorso" del nuovo file da creare:''</font>
  sfOut = sox_open_write("<FONT Color=gray>''/percorso/del/nuovo/file/audio.wav''</font>", sigOut, encoOut, Null, 0, Null)
  If sfOut == 0 Then Error.Raise("Impossibile aprire il nuovo file audio per la scrittura !")
 
  encoIn = sfIn + (SizeOf(gb.Pointer) * 5)
 
  chain = sox_create_effects_chain(encoIn, encoOut)
  
  eff = sox_create_effect(sox_find_effect("input"))
  argsP[0] = sfIn
  err = sox_effect_options(eff, 1, argsP)
  If err <> SOX_SUCCESS Then Error.Raise("Errore nella funzione 'sox_effect_options' !")
  err = sox_add_effect(chain, eff, sigIn, sigIn)
  If err <> SOX_SUCCESS Then Error.Raise("Errore nella funzione 'sox_add_effect' !")
  
  If sigIn.rate <> sigOut.rate Then
    eff = sox_create_effect(sox_find_effect("rate"))
    err = sox_effect_options(eff, 0, Null)
    If err <> SOX_SUCCESS Then Error.Raise("Errore nella funzione 'sox_effect_options' !")  
    eff.handler.flags = eff.handler.flags Or SOX_EFF_LENGTH
    err = sox_add_effect(chain, eff, sigIn, sigOut)
    If err <> SOX_SUCCESS Then Error.Raise("Errore nella funzione 'sox_add_effect' !")
  Endif
 
  If sigIn.channels <> sigOut.channels Then
    eff = sox_create_effect(sox_find_effect("channels"))
    sox_effect_options(eff, 0, Null)
    sox_add_effect(chain, eff, sigIn, sigOut)
  Endif
  
  eff = sox_create_effect(sox_find_effect("output"))
  argsP[0] = sfOut
  sox_effect_options(eff, 1, argsP)
  sox_add_effect(chain, eff, sigIn, sigOut)
 
 <FONT Color=gray>' ''Esegue i dati audio:''</font>
  sox_flow_effects(chain, Null, Null)
  
 <FONT Color=gray>' ''Va in chiusura:''</font>
  Free(argsP.Data)
  sox_delete_effects_chain(chain)
  sox_close(sfOut)
  sox_close(sfIn)
  sox_quit()
 
 '''End'''



=Riferimenti=
* http://sox.sourceforge.net/libsox.html
* http://fossies.org/dox/sox-14.4.1/index.html
* http://sox.sourcearchive.com/documentation/14.3.2-3/index.html