Versione delle 08:13, 22 set 2016

E' possibile ottenere un file audio formato WAV da un file Midi mediante una linea di pipeline gestita dalla funzione gst parse launch() del API di GStreamer.

Mostriamo un esempio pratico:

Private midi As Pointer


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
Private stato As Integer = GST_STATE_PLAYING

' gst_init (int *argc, char **argv[])
' Initializes the GStreamer library, setting up internal path lists, registering built-in elements, and loading standard plugins.
Private Extern gst_init(argc As Pointer, argv As Pointer)

' GstElement * gst_parse_launch (const gchar *pipeline_description, GError **error)
' Create a new pipeline based on command line syntax.
Private Extern gst_parse_launch(description As String, GError As Pointer) As Pointer

' GstStateChangeReturn gst_element_set_state(GstElement *element, GstState state)
' Sets the state of the element.
Private Extern gst_element_set_state(gstelement As Pointer, state As Integer) As Integer

' gboolean gst_element_query_duration(GstElement *element, GstFormat format, gint64 *duration)
' Queries an element for the total stream duration in nanoseconds.
Private Extern gst_element_query_duration(gselement As Pointer, formatI As Integer, duration As Pointer) As Boolean
 
' gboolean gst_element_query_position (GstElement *element, GstFormat format, gint64 *cur)
' Queries an element for the stream position in nanoseconds.
Private Extern gst_element_query_position(gstelement As Pointer, gstformat As Pointer, cur As Pointer) As Boolean

' void gst_object_unref(gpointer object)
' Decrements the reference count on object.
Private Extern gst_object_unref(gobject As Pointer)


Public Sub Main()
 
 Dim posizione, durata As Long
 Dim dur, pos As Date
     
  gst_init(0, 0)
  
  midi = gst_parse_launch("filesrc location=/percorso/del/file.mid ! decodebin ! audioconvert ! audioresample ! volume volume=7.0 ! wavenc ! filesink location =/tmp/file.wav", 0)
       
' Avviamo la riproduzione del file Midi per la creazione del file audio WAV:
  gst_element_set_state(midi, GST_STATE_PLAYING)
  
  While stato = GST_STATE_PLAYING
    gst_element_query_duration(midi, GST_FORMAT_TIME, VarPtr(durata))
    gst_element_query_position(midi, GST_FORMAT_TIME, VarPtr(posizione))
    dur = Date(0, 0, 0, 0, 0, 0, durata / 1000000)
    pos = Date(0, 0, 0, 0, 0, 0, posizione / 1000000)
    If (posizione > 0) And (pos >= dur) Then Break
    Write #File.Out, "\rDurata: " & Date(0, 0, 0, 0, 0, 0, durata / 1000000) & "      Pos. " & Date(0, 0, 0, 0, 0, 0, posizione / 1000000)
    Wait 0.01
  Wend
  
  gst_object_unref(midi)
  Print "\nCreazione del file audio WAV terminata."
  Quit
      
End

Riferimenti

• http://docs.gstreamer.com/display/GstSDK/gst-launch
• http://gstreamer.freedesktop.org/
• http://gstreamer.com/