[A5] Beep sound function.

no-reply@allegro.cc (joecarl) — Thu, 08 Aug 2013 08:26:59 +0000

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:

#SelectExpand
  1void PlaySound(int nota, int time, int octava) {
  int frec=(int) (440.0*pow(2.0,(float) (octava-3.0+(nota-10.0)/12.0)));
  3
  unsigned int samples = float(time)/1000.0 * frec;
  unsigned long sample_size=al_get_channel_count(ALLEGRO_CHANNEL_CONF_1) *al_get_audio_depth_size(ALLEGRO_AUDIO_DEPTH_INT16);
  unsigned long bytes = samples * sample_size;
  void *buff = NULL;
  buff = al_malloc(bytes);
  
  ALLEGRO_SAMPLE *beep = NULL;
  beep = al_create_sample (buff,samples, frec,ALLEGRO_AUDIO_DEPTH_INT16,ALLEGRO_CHANNEL_CONF_1,true);
  unsigned char* ptr;
 
  if(beep!=NULL){
      ptr = (unsigned char*) al_get_sample_data(beep);
      for(int i=0;i<samples;i+=2){
          ptr[i]=100;
          ptr[i+1]=0;
      }
      al_play_sample (beep,1.0,0,1.0,ALLEGRO_PLAYMODE_LOOP,NULL);
      al_rest(time/1000);
      al_destroy_sample(beep);
  }
 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.

no-reply@allegro.cc (Elias) — Thu, 08 Aug 2013 14:59:06 +0000

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

no-reply@allegro.cc (joecarl) — Thu, 08 Aug 2013 15:57:47 +0000

Thanks for your answer.

I replaced this line:
al_rest(time/1000);

with this:
al_rest(float(time)/1000.0);

but I still get no sound.

no-reply@allegro.cc (Elias) — Thu, 08 Aug 2013 17:08:51 +0000

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]

References

http://sourceforge.net/p/alleg/allegro/ci/5.1/tree/examples/ex_audio_timer.c

no-reply@allegro.cc (l j) — Thu, 08 Aug 2013 18:09:10 +0000

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

Also 100 is a small number.

no-reply@allegro.cc (André Silva) — Sat, 10 Aug 2013 17:33:25 +0000

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?

no-reply@allegro.cc (Elias) — Sat, 10 Aug 2013 19:27:37 +0000

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.