Versione delle 12:06, 18 ago 2018

Libao è una libreria multi-piattaforma che permette ai programmi di inviare dati audio PCM ai dispositivi audio nativi su una vasta gamma di piattaforme.

E' necessario avere installata nel proprio sistema e richiamare in Gambas la libreria condivisa: "libao:4.1.1"

Per generare ed eseguire un'onda sinusoidale con le risorse del API di Libao, sono possibili almeno due modalità.

1^a modalità

La prima modalità prevede l'uso nell'applicativo Gambas della sola principale libreria dinamica condivisa: "libao.so.4.1.1"

Library "libao:4.1.1"

Public Struct ao_sample_format
  bits As Integer             ' bits per sample
  rate As Integer             ' samples per second (in a single channel)
  channels As Integer         ' number of audio channels
  byte_format As Integer      ' Byte ordering in sample, see constants below
  matrix As Pointer           ' input channel location/ordering
End Struct

Private Const AO_FMT_LITTLE As Integer = 1

' void ao_initialize(void)
' Library setup.
Private Extern ao_initialize()

' int ao_default_driver_id(void)
' Driver information.
Private Extern ao_default_driver_id() As Integer

' ao_device * ao_open_live(int driver_id, ao_sample_format *format, ao_option *option)
' Driver information.
Private Extern ao_open_live(driver_id As Integer, fmt As Ao_sample_format, option As Pointer) As Pointer

' int ao_play(ao_device *device, char *output_samples, uint_32 num_bytes)
' Write samples to the device. Channels are interleaved. 1 indicates success.
Private Extern ao_play(device As Pointer, output_samples As Pointer, num_bytes As Integer) As Integer

' int ao_close(ao_device *device)
' Closes audio device.
Private Extern ao_close(device As Pointer) As Integer
  
' void ao_shutdown(void)
' Library teardown.
Private Extern ao_shutdown()

  
Public Sub Main()

 Dim device As Pointer
 Dim ao_sf As New Ao_sample_format
 Dim default_driver, buf_size, sample, i, err As Integer
 Dim freq As Single = 440.0
 Dim buffer As Byte[]
   
' Inizializza la libreria 'libao':
  ao_initialize()
   
' Imposta il driver audio come predefinito:
  default_driver = ao_default_driver_id()

' Imposta le caratteristiche audio in uscita:
  With ao_sf
    .bits = 16
    .channels = 2
    .rate = 44100
    .byte_format = AO_FMT_LITTLE
  End With

' Apre il driver:
  device = ao_open_live(default_driver, ao_sf, Null)
  If IsNull(device) Then Error.Raise("Errore nell'apertura del dispositivo audio !")

' Elabora il suono:
  buf_size = ao_sf.bits / 8 * ao_sf.channels * ao_sf.rate

  buffer = New Byte[buf_size]
   
  For i = 0 To ao_sf.rate - 1

' Genera l'onda sinusoidale:
    sample = CInt(0.75 * 32768.0 * Sin(2 * Pi * freq * CSingle(i / ao_sf.rate)))

' Imposta il medesimo suono nei canali destro e sinistro:
    buffer[4 * i] = sample And &FF
    buffer[4 * i + 2] = sample And &FF
    buffer[4 * i + 1] = Shr(sample, 8) And &FF
    buffer[4 * i + 3] = Shr(sample, 8) And &FF
   
  Next
    
  err = ao_play(device, buffer.Data, buf_size)
  If err < 1 Then Error.Raise("Errore nell'esecuzione dei dati audio !")
   
 
' Va in chiusura:
  ao_close(device)
  ao_shutdown()

End

2^a modalità

La seconda modalità prevede l'uso sia della libreria "libao.so.4.1.1" che della libreria "libalsa.so".

Library "libao:4.1.1"

Public Struct ao_sample_format
  bits As Integer             ' bits per sample
  rate As Integer             ' samples per second (in a single channel)
  channels As Integer         ' number of audio channels
  byte_format As Integer      ' Byte ordering in sample, see constants below
  matrix As Pointer           ' input channel location/ordering
End Struct
 
Private Const AO_FMT_LITTLE As Integer = 1

' void ao_initialize(void)
' Library setup.
Private Extern ao_initialize()

' int ao_default_driver_id(void)
' Get information about a particular driver.
Private Extern ao_default_driver_id() As Integer

' ao_device * ao_open_live(int driver_id, ao_sample_format *format, ao_option *option)
' Open a live playback audio device for output.
Private Extern ao_open_live(driver_id As Integer, fmt As Ao_sample_format, option As Pointer) As Pointer
 
' void ao_shutdown(void)
' Unloads all of the plugins and deallocates any internal data structures the library has created.
Private Extern ao_shutdown()


Library "/usr/lib/x86_64-linux-gnu/ao/plugins-4/libalsa"

' int ao_plugin_test()
' Determine if parameters are requires for this particular plugin.
Private Extern ao_plugin_test() As Integer

' int ao_plugin_device_init(ao_device *device)
' Initialize internal data structures.
Private Extern ao_plugin_device_init(ao_dev As Pointer) As Integer

' int ao_plugin_open(ao_device *device, ao_sample_format *format)
' Prepare the audio device for playback.
Private Extern ao_plugin_open(ao_dev As Pointer, fmt As Ao_sample_format) As Integer

' int ao_plugin_play(ao_device *device, const char *output_samples, uint_32 num_bytes)
' Play num_bytes of audio data.
Private Extern ao_plugin_play(ao_dev As Pointer, output_samples As Pointer, num_bytes As Integer) As Integer

' int ao_plugin_close(ao_device *device)
' Close the audio device.
Private Extern ao_plugin_close(ao_dev As Pointer) As Integer

' void ao_plugin_device_clear(ao_device *device)
' Free the internal data structures.
Private Extern ao_plugin_device_clear(ao_dev As Pointer)


Public Sub Main()

Dim device As Pointer
Dim ao_sf As New Ao_sample_format
Dim default_driver, buf_size, sample, i, err As Integer
Dim freq As Single = 440.0
Dim buffer As Byte[]
  
' Inizializza la libreria 'libao':
 ao_initialize()
  
' Imposta il driver audio come predefinito:
 default_driver = ao_default_driver_id()

' Imposta le caratteristiche audio in uscita:
 With ao_sf
   .bits = 16
   .channels = 2
   .rate = 44100
   .byte_format = AO_FMT_LITTLE
 End With

' Apre il driver:
 device = ao_open_live(default_driver, ao_sf, Null)
 If IsNull(device) Then Error.Raise("Errore nell'apertura del dispositivo audio !")

 err = ao_plugin_test()
 If err = 0 Then Error.Raise("Il driver audio ha bisogno che vengano impostate alcune opzioni !")
  
 ao_plugin_device_init(device)
 
 ao_plugin_open(device, ao_sf)
  
' Elabora il suono:
 buf_size = ao_sf.bits / 8 * ao_sf.channels * ao_sf.rate

 buffer = New Byte[buf_size]
  
 For i = 0 To ao_sf.rate - 1

' Genera l'onda sinusoidale:
   sample = CInt(0.75 * 32768.0 * Sin(2 * Pi * freq * CSingle(i / ao_sf.rate)))

' Imposta il medesimo suono nei canali destro e sinistro:
   buffer[4 * i] = sample And &FF
   buffer[4 * i + 2] = sample And &FF
   buffer[4 * i + 1] = Shr(sample, 8) And &FF
   buffer[4 * i + 3] = Shr(sample, 8) And &FF
  
 Next
   
 err = ao_plugin_play(device, buffer.Data, buf_size)
 If err < 1 Then Error.Raise("Errore nell'esecuzione dei dati audio !")
  

' Va in chiusura:
 ao_plugin_close(device)
 ao_plugin_device_clear(device)
 ao_shutdown()
 
End

Riferimenti

• http://www.xiph.org/ao/doc
• http://manual.freeshell.org/libao-1.0.0