Playing the sound in the real world - Multimedia

When we think of video games, we often think of the music and sounds that accompany them. Providing those special effects used to be very difficult. Keep reading to learn how SDL makes this important task very easy.

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By: A.P.Rajshekhar

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March 27, 2007

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Up to now I have shown you the code snippets. Now it's time for a full fledged application. So here goes.

First the includes:

#include "SDL.h"
#include "SDL_audio.h"

Then comes the main part and opening the audio:

int main()
{
extern void mixaudio(void *unused, Uint8 *stream, int
len);
SDL_AudioSpec fmt;

/* Set 16-bit stereo audio at 22Khz */
fmt.freq = 22050;
fmt.format = AUDIO_S16;
fmt.channels = 2;
fmt.samples = 512; /* A good value for games */
fmt.callback = mixaudio;
fmt.userdata = NULL;

/* Open the audio device and start playing sound! */
if ( SDL_OpenAudio(&fmt, NULL) < 0 ) {
fprintf(stderr, "Unable to open audio: %sn",
SDL_GetError());
exit(1);
}

//can call other functions like mixing and playing
functions
:
PlaySound("start.wav");
:
SDL_CloseAudio();//closes the audio and setting the fmt
to null
}

The next part is playing the wav file. For this we need a structure that keeps track of the current sound data, position and length. The following is the structure:

#define NUM_SOUNDS 2
struct sample {
Uint8 *data;
Uint32 dpos;
Uint32 dlen;
} sounds[NUM_SOUNDS];

The next part is playing the file. The function goes likes this:

void PlaySound(char *file)
{
int index;
SDL_AudioSpec wave;
Uint8 *data;
Uint32 dlen;
SDL_AudioCVT cvt;

/* Look for an empty (or finished) sound slot */
for ( index=0; index<NUM_SOUNDS; ++index ) {
    if ( sounds[index].dpos == sounds[index].dlen ) {
      break;
    }
}
if ( index == NUM_SOUNDS )
    return;

/* Load the sound file and convert it to 16-bit stereo
at 22kHz */
if ( SDL_LoadWAV(file, &wave, &data, &dlen) == NULL ) {
    fprintf(stderr, "Couldn't load %s: %sn", file,
SDL_GetError());
    return;
}
SDL_BuildAudioCVT(&cvt, wave.format, wave.channels,
wave.freq, AUDIO_S16, 2, 22050);
cvt.buf = malloc(dlen*cvt.len_mult);
memcpy(cvt.buf, data, dlen);
cvt.len = dlen;
SDL_ConvertAudio(&cvt);
SDL_FreeWAV(data);

/* Put the sound data in the slot (it starts playing
immediately) */
if ( sounds[index].data ) {
free(sounds[index].data);
}
SDL_LockAudio();
sounds[index].data = cvt.buf;
sounds[index].dlen = cvt.len_cvt;
sounds[index].dpos = 0;
SDL_UnlockAudio();
}

Finally we have to define the callback function for the SDL_AudioSpec which is:

void mixaudio(void *unused, Uint8 *stream, int len)
{
int i;
Uint32 amount;
for ( i=0; i<NUM_SOUNDS; ++i ) {
    amount = (sounds[i].dlen-sounds[i].dpos);
    if ( amount > len ) {
      amount = len;
    }
    SDL_MixAudio(stream, &sounds[i].data[sounds[i].dpos],
amount, SDL_MIX_MAXVOLUME);
    sounds[i].dpos += amount;
}
}

That brings us to the end of this article. I have left several aspects unexplained. The reason is that just explaining them as stand alone functions wouldn't do any good. They have to be understood in the context of rendering and scenes. SDL_MixAudio for mixing and the APIs for timer, threading, networking and CD-ROM access are among such functions. In the next article in this ongoing series I will be moving towards rendering using OpenGL with SDL as the base framework. In the rendering and animations, the real utility of the above mentioned APIs will be revealed. Till next time...