[A5] Beep sound function.

Hi! Im trying to create a function which plays a beep sound (square signal) of a specified frecuency during a specified time.

This code compiles but I hear nothing:

1void PlaySound(int nota, int time, int octava) { 2 int frec=(int) (440.0*pow(2.0,(float) (octava-3.0+(nota-10.0)/12.0))); 3 4 unsigned int samples = float(time)/1000.0 * frec; 5 unsigned long sample_size=al_get_channel_count(ALLEGRO_CHANNEL_CONF_1) *al_get_audio_depth_size(ALLEGRO_AUDIO_DEPTH_INT16); 6 unsigned long bytes = samples * sample_size; 7 void *buff = NULL; 8 buff = al_malloc(bytes); 9 10 ALLEGRO_SAMPLE *beep = NULL; 11 beep = al_create_sample (buff,samples, frec,ALLEGRO_AUDIO_DEPTH_INT16,ALLEGRO_CHANNEL_CONF_1,true); 12 unsigned char* ptr; 13 14 if(beep!=NULL){ 15 ptr = (unsigned char*) al_get_sample_data(beep); 16 for(int i=0;i<samples;i+=2){ 17 ptr[i]=100; 18 ptr[i+1]=0; 19 } 20 al_play_sample (beep,1.0,0,1.0,ALLEGRO_PLAYMODE_LOOP,NULL); 21 al_rest(time/1000); 22 al_destroy_sample(beep); 23 } 24}

What can I do to solve this?

Any help is appreciated, thanks in advance.

BTW: I'm using Allegro 5.1 under MacOSX 10.6.8.


I didn't look closely, but time/1000 is always 0 so that would be one reason that code could never play sound.


Thanks for your answer.

I replaced this line:

with this:

but I still get no sound.


Well, the sample you produce has the first half filled with silence (you set bytes to 0/100) and the second half unmodified. What you probably could try is something like:

int16_t *ptr = al_get_sample_data(beep);
for (i = 0; i < samples; i += 2) {
    ptr[i] = -32768;
    ptr[i + 1] = 32767;

Did you look at ex_audio_timer? [1]


It should be
int16_t *
not unsigned char *
I think.

Also 100 is a small number.

André Silva

Also, I'm not fully sure, but wouldn't 0 -> 100 -> 0 -> 100 -> 0 -> 100 (or -32768 -> 32767 -> ...) just create a triangle wave? A square wave would have to be bottom -> bottom -> top -> top -> ...

Not to mention that, from what I'm understanding in the code, that would generate a wave with a wavelength of two samples. Which would create a triangle wave with the highest pitch/frequency possible. Can humans even hear the output?


He sets the playback frequency really low so it would indeed create a square wave. The more logical way would be to keep the frequency at 48k Hz and create a normal square wave of course... but his way should work.

[Edit] Actually, if interpolation is on you're right, it should end up as a triangle wave instead.

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