Manpages - SDL_ConvertAudio.3

SDL_ConvertAudio - Convert audio data to a desired audio format.

#include “SDL.h”

int SDL_ConvertAudio*(*SDL_AudioCVT *cvt);

SDL_ConvertAudio takes one parameter, cvt, which was previously initilized. Initilizing a SDL_AudioCVT is a two step process. First of all, the structure must be passed to SDL_BuildAudioCVT along with source and destination format parameters. Secondly, the cvt->*buf* and cvt->*len* fields must be setup. cvt->*buf* should point to the audio data and cvt->*len* should be set to the length of the audio data in bytes. Remember, the length of the buffer pointed to by buf show be len***len_mult bytes in length.

Once the SDL_AudioCVT*structure is initilized then we can pass it to *SDL_ConvertAudio, which will convert the audio data pointer to by cvt->*buf*. If SDL_ConvertAudio returned 0 then the conversion was completed successfully, otherwise -1 is returned.

If the conversion completed successfully then the converted audio data can be read from cvt->*buf*. The amount of valid, converted, audio data in the buffer is equal to cvt->*len***cvt*->*len_ratio*.

  /* Converting some WAV data to hardware format */
  void my_audio_callback(void *userdata, Uint8 *stream, int len);

  SDL_AudioSpec *desired, *obtained;
  SDL_AudioSpec wav_spec;
  SDL_AudioCVT  wav_cvt;
  Uint32 wav_len;
  Uint8 *wav_buf;
  int ret;

  /* Allocated audio specs */
  desired=(SDL_AudioSpec *)malloc(sizeof(SDL_AudioSpec));
  obtained=(SDL_AudioSpec *)malloc(sizeof(SDL_AudioSpec));

  /* Set desired format */
  desired->freq=22050;
  desired->format=AUDIO_S16LSB;
  desired->samples=8192;
  desired->callback=my_audio_callback;
  desired->userdata=NULL;

  /* Open the audio device */
  if ( SDL_OpenAudio(desired, obtained) < 0 ){
    fprintf(stderr, "Couldn't open audio: %s
  ", SDL_GetError());
    exit(-1);
  }
          
  free(desired);

  /* Load the test.wav */
  if( SDL_LoadWAV("test.wav", &wav_spec, &wav_buf, &wav_len) == NULL ){
    fprintf(stderr, "Could not open test.wav: %s
  ", SDL_GetError());
    SDL_CloseAudio();
    free(obtained);
    exit(-1);
  }
                                              
  /* Build AudioCVT */
  ret = SDL_BuildAudioCVT(&wav_cvt,
                          wav_spec.format, wav_spec.channels, wav_spec.freq,
                          obtained->format, obtained->channels, obtained->freq);

  /* Check that the convert was built */
  if(ret==-1){
    fprintf(stderr, "Couldn't build converter!
  ");
    SDL_CloseAudio();
    free(obtained);
    SDL_FreeWAV(wav_buf);
  }

  /* Setup for conversion */
  wav_cvt.buf=(Uint8 *)malloc(wav_len*wav_cvt.len_mult);
  wav_cvt.len=wav_len;
  memcpy(wav_cvt.buf, wav_buf, wav_len);

  /* We can delete to original WAV data now */
  SDL_FreeWAV(wav_buf);

  /* And now we're ready to convert */
  SDL_ConvertAudio(&wav_cvt);

  /* do whatever */
  .
  .
  .
  .

SDL_BuildAudioCVT, SDL_AudioCVT