SDL_AudioSpec(3) SDL API Reference SDL_AudioSpec(3)NAME
SDL_AudioSpec - Audio Specification Structure
STRUCTURE DEFINITION
typedef struct{
int freq;
Uint16 format;
Uint8 channels;
Uint8 silence;
Uint16 samples;
Uint32 size;
void (*callback)(void *userdata, Uint8 *stream, int len);
void *userdata;
} SDL_AudioSpec;
STRUCTURE DATA
freq Audio frequency in samples per second
format Audio data format
channels Number of channels: 1 mono, 2 stereo
silence Audio buffer silence value (calculated)
samples Audio buffer size in samples
size Audio buffer size in bytes (calculated)
callback(..) Callback function for filling the audio buffer
userdata Pointer the user data which is passed to the callback function
DESCRIPTION
The SDL_AudioSpec structure is used to describe the format of some audio data. This structure is used by SDL_OpenAudio and SDL_LoadWAV.
While all fields are used by SDL_OpenAudio only freq, format, samples and channels are used by SDL_LoadWAV. We will detail these common
members here.
freq The number of samples sent to the sound device every second. Common values are 11025, 22050 and 44100. The higher the
better.
format Specifies the size and type of each sample element
AUDIO_U8
AUDIO_S8
AUDIO_U16 or AUDIO_U16LSB
AUDIO_S16 or AUDIO_S16LSB
AUDIO_U16MSB
AUDIO_S16MSB
AUDIO_U16SYS
AUDIO_S16SYS
channels The number of seperate sound channels. 1 is mono (single channel), 2 is stereo (dual channel).
samples When used with SDL_OpenAudio this refers to the size of the audio buffer in samples. A sample a chunk of audio data of
the size specified in format mulitplied by the number of channels. When the SDL_AudioSpec is used with SDL_LoadWAV sam-
ples is set to 4096.
SEE ALSO
SDL_OpenAudio, SDL_LoadWAV
SDL Tue 11 Sep 2001, 22:58 SDL_AudioSpec(3)
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SDL_OpenAudio(3) SDL API Reference SDL_OpenAudio(3)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=(SDL_AudioSpec *)malloc(sizeof(SDL_AudioSpec));
You must then fill this structure with your desired audio specifications.
desired->freq
desired->format
desired->samples
desired->callback
void callback(void *userdata, Uint8 *stream, int len);
userdata is the pointer stored in 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
SDL_OpenAudio reads these fields from the desired SDL_AudioSpec structure pass 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 desirec 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=(SDL_AudioSpec *)malloc(sizeof(SDL_AudioSpec));
/* Allocate space for the obtained SDL_AudioSpec */
obtained=(SDL_AudioSpec *)malloc(sizeof(SDL_AudioSpec));
/* 22050Hz - FM Radio quality */
desired->freq=22050;
/* 16-bit signed audio */
desired->format=AUDIO_S16LSB;
/* Mono */
desired->channels=0;
/* 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
", SDL_GetError());
exit(-1);
}
/* desired spec is no longer needed */
free(desired);
hardware_spec=obtained;
.
.
/* Prepare callback for playing */
.
.
.
/* Start playing */
SDL_PauseAudio(0);
SEE ALSO
SDL_AudioSpec, SDL_LockAudio, SDL_UnlockAudio, SDL_PauseAudio
SDL Tue 11 Sep 2001, 22:58 SDL_OpenAudio(3)