SDL_OpenAudio

Name

SDL_OpenAudio -- Opens the audio device with the desired parameters.

Synopsis

#include "SDL.h"

int SDL_OpenAudio(SDL_AudioSpec *desired, SDL_AudioSpec *obtained);

Description

This function opens the audio device with the desired parameters, and returns 0 if successful, placing the actual hardware parameters in the structure pointed to by obtained. If obtained is NULL, the audio data passed to the callback function will be guaranteed to be in the requested format, and will be automatically converted to the hardware audio format if necessary. This function returns -1 if it failed to open the audio device, or couldn't set up the audio thread.

To open the audio device a desired SDL_AudioSpec must be created.

SDL_AudioSpec *desired;
.
.
desired = malloc(sizeof(SDL_AudioSpec));

You must then fill this structure with your desired audio specifications.

desired->freq
The desired audio frequency in samples-per-second.
desired->format

The desired audio format (see SDL_AudioSpec)

desired->channels
The desired channels (1 for mono, 2 for stereo, 4 for surround, 6 for surround with center and lfe).
desired->samples
The desired size of the audio buffer in samples. This number should be a power of two, and may be adjusted by the audio driver to a value more suitable for the hardware. Good values seem to range between 512 and 8192 inclusive, depending on the application and CPU speed. Smaller values yield faster response time, but can lead to underflow if the application is doing heavy processing and cannot fill the audio buffer in time. A stereo sample consists of both right and left channels in LR ordering. Note that the number of samples is directly related to time by the following formula: ms = (samples*1000)/freq
desired->callback

This should be set to a function that will be called when the audio device is ready for more data. It is passed a pointer to the audio buffer, and the length in bytes of the audio buffer. This function usually runs in a separate thread, and so you should protect data structures that it accesses by calling SDL_LockAudio and SDL_UnlockAudio in your code. The callback prototype is:

void callback(void *userdata, Uint8 *stream, int len);

userdata is the pointer stored in the userdata field of the SDL_AudioSpec. stream is a pointer to the audio buffer you want to fill with information and len is the length of the audio buffer in bytes.

desired->userdata

This pointer is passed as the first parameter to the callback function.

SDL_OpenAudio reads these fields from the desired SDL_AudioSpec structure passed to the function and attempts to find an audio configuration matching your desired. As mentioned above, if the obtained parameter is NULL then SDL with convert from your desired audio settings to the hardware settings as it plays.

If obtained is NULL then the desired SDL_AudioSpec is your working specification, otherwise the obtained SDL_AudioSpec becomes the working specification and the desired specification can be deleted. The data in the working specification is used when building SDL_AudioCVT's for converting loaded data to the hardware format.

SDL_OpenAudio calculates the size and silence fields for both the desired and obtained specifications. The size field stores the total size of the audio buffer in bytes, while the silence stores the value used to represent silence in the audio buffer

The audio device starts out playing silence when it's opened, and should be enabled for playing by calling SDL_PauseAudio(0) when you are ready for your audio callback function to be called. Since the audio driver may modify the requested size of the audio buffer, you should allocate any local mixing buffers after you open the audio device.

Examples

/* Prototype of our callback function */
void my_audio_callback(void *userdata, Uint8 *stream, int len);

/* Open the audio device */
SDL_AudioSpec *desired, *obtained;
SDL_AudioSpec *hardware_spec;

/* Allocate a desired SDL_AudioSpec */
desired = malloc(sizeof(SDL_AudioSpec));

/* Allocate space for the obtained SDL_AudioSpec */
obtained = malloc(sizeof(SDL_AudioSpec));

/* 22050Hz - FM Radio quality */
desired->freq=22050;

/* 16-bit signed audio */
desired->format=AUDIO_S16SYS;

/* Mono */
desired->channels=1;

/* Large audio buffer reduces risk of dropouts but increases response time */
desired->samples=8192;

/* Our callback function */
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\n", SDL_GetError());
  exit(-1);
}
/* desired spec is no longer needed */
free(desired);
hardware_spec=obtained;
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.
/* Prepare callback for playing */
.
.
.
/* Start playing */
SDL_PauseAudio(0);

See Also

SDL_CloseAudio, SDL_AudioSpec, SDL_LockAudio, SDL_UnlockAudio, SDL_PauseAudio

SDL_OpenAudio (last edited 2008-04-17 08:18:28 by localhost)