Convertire la frequenza di campionamento di un file WAV con le funzioni esterne del API di SDL2
Da Gambas-it.org - Wikipedia.
La libreria SDL 2 è un API multi-piattaforma contenente funzioni per la gestione multimediale dell'audio e del video.
Sarà necessario avere installate nel sistema e richiamare in Gambas la libreria condivisa: "libSDL2-2.0.so.0.3000.0 ".
Mostriamo di seguito un esempio di codice per convertire la frequenza di campionamento di un file WAV da 44100 hertz a 22050 hertz:
Library "libSDL2-2.0:0.3000.0"
Public Struct SDL_AudioSpec
freq As Integer
format As Short
channels As Byte
silence As Byte
samples As Short
padding As Short
size As Integer
callback As Pointer
userdata As Pointer
End Struct
Public Struct SDL_AudioCVT
needed As Integer
src_format As Short
dst_format As Short
rate_incr As Float
buf As Pointer
len As Integer
len_cvt As Integer
len_mult As Integer
len_ratio As Float
filters[10] As Pointer
filter_index As Integer
End Struct
Private Const SDL_INIT_AUDIO As Integer = 16
Private Const SDL_AUDIO_MASK_BITSIZE As Integer = 127
' int SDL_Init(Uint32 flags)
' Initialize the SDL library.
Private Extern SDL_Init(flags As Integer) As Integer
' SDL_AudioSpec* SDL_LoadWAV_RW(SDL_RWops * src, int freesrc, SDL_AudioSpec* spec, Uint8** audio_buf, Uint32* audio_len)
' Loads a WAVE from the data source.
Private Extern SDL_LoadWAV_RW(SDL_RWops As Pointer, freesrc As Integer, SDL_spec As SDL_AudioSpec, audio_buf As Pointer, audio_len As Pointer) As SDL_AudioSpec
' int SDL_BuildAudioCVT(SDL_AudioCVT * cvt, SDL_AudioFormat src_format, Uint8 src_channels, int src_rate, SDL_AudioFormat dst_format, Uint8 dst_channels, int dst_rate)
' Initialize an SDL_AudioCVT structure for conversion.
Private Extern SDL_BuildAudioCVT(SDL_cvt As SDL_AudioCVT, src_format As Integer, src_channels As Byte, src_rate As Integer, dst_format As Integer, dst_channels As Byte, dst_rate As Integer) As Integer
' int SDL_ConvertAudio(SDL_AudioCVT* cvt)
' Converts audio data to a desired audio format.
Private Extern SDL_ConvertAudio(SDL_cvt As SDL_AudioCVT) As Integer
' void SDL_Quit(void)
' Clean up all initialized subsystems.
Private Extern SDL_Quit()
' SDL_RWops * SDL_RWFromFile(const char *file, const char *mode)
' Create a new SDL_RWops structure for reading from and/or writing to a named file.
Private Extern SDL_RWFromFile(filewav As String, mode As String) As Pointer In "libSDL2_mixer-2.0"
Public Sub Main()
Dim wav, convertito As String
Dim data, pbb As Pointer
Dim lun, cvtfreq, bitsize, blockalign, avgbytes As Integer
Dim spec As New SDL_AudioSpec
Dim cvt As New SDL_AudioCVT
Dim fl As File
Dim bb As Byte[]
Dim st As Stream
wav = "/percorso/del/file.wav"
convertito = "/percorso/del/file.wav/convertito"
' Imposta la nuova frequenza di campionamento, nella quale il file wav sarà convertito:
cvtfreq = 22050
' Inizializza la libreria SDL2:
If SDL_Init(SDL_INIT_AUDIO) < 0 Then Error.Raise("Impossibile inizializzare la libreria SDL2 !")
If IsNull(SDL_LoadWAV_RW(SDL_RWFromFile(wav, "rb"), 1, spec, VarPtr(data), VarPtr(lun))) Then
Error.Raise("Impossibile caricare il file audio: " & wav & " !")
Endif
If SDL_BuildAudioCVT(cvt, spec.format, spec.channels, spec.freq, spec.format, spec.channels, cvtfreq) = -1 Then
Error.Raise("Impossibile realizzare il CVT !")
Endif
cvt.len = lun
cvt.buf = Alloc(SizeOf(gb.Short), cvt.len)
' Inizia la procedura per trasferire con sicurezza i dati dall'area di memoria, puntata dal Puntatore "data", all'area di memoria puntata dal Puntatore "cvt.buf".
' Per velocizzare tale procedura si utilizzerà il Metodo "Write" di un vettore di tipo "Byte[]".
bb = New Byte[cvt.len]
' Innanzitutto vengono usati i "Memory Stream" per leggere i dati contenuti dal puntatore "data":
st = Memory data For Read
' I dati letti sono caricati nel vettore "bb" di tipo "Byte[]":
bb.Read(st, 0, bb.Count)
st.Close
' I dati ora presenti nel vettore "bb" vengono, quindi, scritti nell'area di memoria puntata dal Puntatore "cvt.buf":
st = Memory cvt.buf For Write
bb.Write(st, 0, bb.Count)
st.Close
If SDL_ConvertAudio(cvt) = -1 Then Error.Raise("Conversione fallita !")
bitsize = spec.format And SDL_AUDIO_MASK_BITSIZE
blockalign = (bitsize / 8) * spec.channels
avgbytes = cvtfreq * blockalign
' Scrive l'header del file WAV in uscita:
fl = Open convertito For Create
Write #fl, &46464952 As Integer ' RIFF
Write #fl, (cvt.len_cvt + 36) As Integer
Write #fl, &45564157 As Integer ' WAVE
Write #fl, &20746D66 As Integer ' fmt
Write #fl, 16 As Integer ' chunk size
Write #fl, 1 As Short ' uncompressed
Write #fl, spec.channels As Short ' channels
Write #fl, cvtfreq As Integer ' sample rate
Write #fl, avgbytes As Integer ' average bytes per second
Write #fl, blockalign As Short ' block align
Write #fl, bitsize As Short ' significant bits per sample
Write #fl, &61746164 As Integer ' data
Write #fl, cvt.len_cvt As Integer ' size
bb = New Byte[cvt.len_cvt]
' Vengono usati i "Memory Stream" per far leggere dal vettore "bb" i dati (ormai convertiti) contenuti dal puntatore "cvt.buf":
st = Memory cvt.buf For Read
bb.Read(st, 0, bb.Count)
' Scrive i valori contenuti dal vettore "bb" nel file precedentemente aperto:
bb.Write(fl, 0, bb.Count)
' Va in chiusura:
st.Close
Free(cvt.buf)
fl.Close
SDL_Quit()
End
