Very simple synthetizer with SDL_audio

audio.c

#include <SDL/SDL.h>
#include <stdio.h>
#include <math.h>

#define NUM_NOTES 37

float notes[NUM_NOTES] = {0};
float freqs[NUM_NOTES] = {0};

int volume = 32;

void audio_callback(void *udata, Uint8 *stream, int len)
{
    static size_t i = 0;
    size_t j = i;
    float freq = 200 + 0.001*i;
    for(; i < j + len; i++)
    {
        int n;
        stream[i-j] = 0;
        for(n = 0; n < NUM_NOTES; n++)
        {
            stream[i-j] += notes[n]*sin(2.0*M_PI*i * freqs[n] / 44100.0);
        }
    }
}

void interactive_mode(void);

int main(int argc, char **argv)
{
    SDL_AudioSpec audiospec;

    SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER);

    audiospec.freq = 44100;
    audiospec.format = AUDIO_S16SYS;
    audiospec.channels = 2;
    audiospec.samples = 1024;
    audiospec.callback = audio_callback;
    audiospec.userdata = NULL;
    if(SDL_OpenAudio(&audiospec, NULL) < 0)
    {
        fprintf(stderr, "Impossible d'ouvrir l'audio\n");
        return 1;
    }

    {
        int n;
        float f = powf(2, -1.0/12.0);
        float p = 1046.5f;
        for(n = NUM_NOTES - 1; n >= 0; n--)
        {
            freqs[n] = p;
            p *= f;
        }
    }

    SDL_PauseAudio(0);
    SDL_SetVideoMode(200, 100, 32, 0);

    interactive_mode();

    SDL_CloseAudio();
    SDL_Quit();
    return 0;
}

void interactive_mode(void)
{
    int quit = 0;
    SDL_Event event;
    while(!quit)
    {
        while(SDL_PollEvent(&event))
        {
            if(event.type == SDL_KEYDOWN)
            {
                switch(event.key.keysym.sym)
                {
                case SDLK_ESCAPE:
                    quit = 1;
                    break;
                case SDLK_a:
                    notes[0] = volume;
                    break;
                case SDLK_w:
                    notes[1] = volume;
                    break;
                case SDLK_s:
                    notes[2] = volume;
                    break;
                case SDLK_e:
                    notes[3] = volume;
                    break;
                case SDLK_d:
                    notes[4] = volume;
                    break;
                case SDLK_f:
                    notes[5] = volume;
                    break;
                case SDLK_t:
                    notes[6] = volume;
                    break;
                case SDLK_g:
                    notes[7] = volume;
                    break;
                case SDLK_y:
                    notes[8] = volume;
                    break;
                case SDLK_h:
                    notes[9] = volume;
                    break;
                case SDLK_u:
                    notes[10] = volume;
                    break;
                case SDLK_j:
                    notes[11] = volume;
                    break;
                case SDLK_k:
                    notes[12] = volume;
                    break;
                default:
                    break;
                }
            }
            else if(event.type == SDL_QUIT)
                quit = 1;
        }

        if(!quit)
        {
            int i = 0;
            SDL_Delay(10);
            for(i = 0; i < NUM_NOTES; i++)
                notes[i] /= 1.1;
        }
    }
}