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



function SDL_OpenAudio( desired, obtained : PSDL_AudioSpec ) : Integer;


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 TSDL_AudioSpec must be created.

desired : PSDL_AudioSpec;
desired := PSDL_AudioSpec( malloc( SizeOf( SDL_AudioSpec ) ) );
You must then fill this structure with your desired audio specifications.


The desired audio frequency in samples-per-second.

desired. format

The desired audio format (see TSDL_AudioSpec)

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


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 userdata field of the TSDL_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.


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

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

If obtained is nil then the desired TSDL_AudioSpec is your working specification, otherwise the obtained TSDL_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.


// Prototype of our callback function 
procedure my_audio_callback( userdata : Pointer; stream : PUint8; len : integer );
// Open the audio device
desired, obtained : PSDL_AudioSpec;
hardware_spec : PSDL_AudioSpec;
// Allocate a desired SDL_AudioSpec
desired := PSDL_AudioSpec( malloc( SizeOf( SDL_AudioSpec ) ) );

// Allocate space for the obtained SDL_AudioSpec
obtained := PSDL_AudioSpec( malloc( SizeOf( SDL_AudioSpec ) ) );

// 22050Hz - FM Radio quality
desired.freq := 22050;

// 16-bit signed audio
desired.format := AUDIO_S16LSB;

// Large audio buffer reduces risk of dropouts but increases response time
desired.samples := 8192;

// Our callback function
desired.callback := my_audio_callback;

desired.userdata := nil;

// Open the audio device
if ( SDL_OpenAudio(desired, obtained) < 0 ) then
MessageBox( 0, PChar( Format( 'Couldn''t Open Audio : %s', [SDL_GetError] ) ), 'Error', MB_OK or MB_ICONHAND );

// desired spec is no longer needed
hardware_spec := obtained;
// Prepare callback for playing
// Start playing
SDL_PauseAudio( 0 );

See Also

TSDL_AudioSpec , SDL_LockAudio , SDL_UnlockAudio , SDL_PauseAudio