Visualizza wikitesto di Modificare ad un file WAV la frequenza di campionamento e il numero di canali usando una linea di pipeline con la funzione 'gst parse launch()'

La funzione esterna ''gst_parse_launch()'' della libreria ''GStreamer'' consente di costruire e gestire in modo semplice una pipeline ''GStreamer''. La riga della pipeline gestita dalla funzione esterna ''gst_parse_launch()'' è un insieme di elementi separati da punti esclamativi (!). Le proprietà possono essere aggiunte agli elementi, sotto forma di: ''proprietà = valore''.

Sarà necessario avere installata nel sistema e richiamare nell'applicazione Gambas la libreria condivisa: "''libgstreamer-1.0'' ".

Mostriamo un esempio pratico, nel quale ad un file WAV la frequenza di campionamento verrà ridotta a 8000 hertz ed il numero dei canali ad 1 (mono):
 Library "libgstreamer-1.0"
 
 Private Enum GST_STATE_VOID_PENDING = 0, GST_STATE_NULL, GST_STATE_READY, GST_STATE_PAUSED, GST_STATE_PLAYING
 Private Const GST_FORMAT_TIME As Integer = 3
 
 <FONT Color=gray>' ''gst_init (int *argc, char **argv[])''
 ' ''Initializes the GStreamer library, setting up internal path lists, registering built-in elements, and loading standard plugins.''</font>
 Private Extern gst_init(argc As Pointer, argv As Pointer)
 
 <FONT Color=gray>' ''GstElement * gst_parse_launch (const gchar *pipeline_description, GError **error)''
 ' ''Create a new pipeline based on command line syntax.''</font>
 Private Extern gst_parse_launch(description As String, GError As Pointer) As Pointer
 
 <FONT Color=gray>' ''GstStateChangeReturn gst_element_set_state(GstElement *element, GstState state)''
 ' ''Sets the state of the element.''</font>
 Private Extern gst_element_set_state(gstelement As Pointer, state As Integer) As Integer
 
 <FONT Color=gray>' ''gboolean gst_element_query_duration(GstElement *element, GstFormat format, gint64 *duration)''
 ' ''Queries an element for the total stream duration in nanoseconds.''</font>
 Private Extern gst_element_query_duration(gselement As Pointer, formatI As Integer, duration As Pointer) As Boolean
  
 <FONT Color=gray>' ''gboolean gst_element_query_position (GstElement *element, GstFormat format, gint64 *cur)''
 ' ''Queries an element for the stream position in nanoseconds.''</font>
 Private Extern gst_element_query_position(gstelement As Pointer, gstformat As Pointer, cur As Pointer) As Boolean
 
 <FONT Color=gray>' ''void gst_object_unref(gpointer object)''
 ' ''Decrements the reference count on object.''</font>
 Private Extern gst_object_unref(gobject As Pointer)
 
 
 Public Sub Main()
  
  Dim audio As Pointer
  Dim durata, pos As Long
      
  gst_init(0, 0)
   
  audio = gst_parse_launch("uridecodebin uri=file://<FONT Color=darkgreen>''/percorso/del/file/audio.wav''</font> ! audioresample ! audioconvert ! audio/x-raw, <FONT Color=#B22222>rate=8000</font>, <FONT Color=#B22222>channels=1</font> ! wavenc ! filesink location=<FONT Color=gray>''/percorso/del/nuovo/file.wav''</font>", 0)
     
 <FONT Color=gray>' ''Avviamo la conversione audio:''</font>
  gst_element_set_state(audio, GST_STATE_PLAYING)
  
  Repeat
    gst_element_query_duration(audio, GST_FORMAT_TIME, VarPtr(durata))
    Wait 0.01
  Until durata > -1
 
  Repeat
    gst_element_query_position(audio, GST_FORMAT_TIME, VarPtr(pos))
    Write "\r\e[0mDurata: " & Time(0, 0, 0, durata / 1000000) & "      Pos. \e[31m" & Time(0, 0, 0, pos / 1000000)
    Wait 0.001
  Until pos >= durata
 
  gst_object_unref(audio)
    
  Quit
    
 End



=Riferimenti=
* http://docs.gstreamer.com/display/GstSDK/gst-launch
* http://gstreamer.freedesktop.org/
* http://gstreamer.com/