Copying from double pointer to int array. Is this correct?

Question

I have a double pointer data->mono_channel, where I would like to cast all the doubles to integers and put them in an array.

int frames = data->audio_info_save->frames;
short buffer[frames];
double* p;
int i = 0;
for (p = &(data->mono_channel)[0]; p < &(data->mono_channel)[frames]; p++) {
  buffer[i] = (int) *p;
  i++;
}

The purpose is that ALSA takes an integer array of samples, and my samples are in doubles.

if ((err = snd_pcm_writei(playback_handle, buffer, frames)) != frames) {
  fprintf(stderr, "write to audio interface failed (%s)\n", snd_strerror(err));
  exit(1);
}

So somehow I need my doubles to be casted to integers.

I don't hear any sound. So is the casting done correctly?

Answer 1

The samples probably need to be scaled from [-1,1] to [min, max], where min and max are minimum and maximum integer values for integer samples.

Answer 2

If data->mono_channel is an array of doubles, the casting is correct (although your code is overly complicated -- you should just use index i as the control variable in the loop, and use it to index the input array of doubles as well as the resulting array of ints).

The resulting silence may be due to the input doubles not being in the right range (i.e., you might need some scaling!). What do you see if you print out some of those doubles?

Answer 3

It looks okay, it'd be easier to just:

for (i = 0; i < frames; i++) {
  buffer[i] = (int)pdata->mono_channel[i];
}

Is the data PCM encoded? That might be your problem.

Answer 4

Your buffer is of type short[]. Have you configured ALSA PCM to expect 16bit values? Have a look at this example from ALSA project.

Answer 5

You probably have to scale and clip the input values, clipping can be quite slow, but there's a trick. So if your input is in range[-1,1], then you may use the following code:

double fclip( double value, double start, double end ) {
  return 0.5 * ( fabs(value-start) + (start+end) - fabs(value-end) );
}
...
for( i = 0; i < frames; i++ ) {
  buffer[i] = (short)fclip( 32767.0 * pdata->mono_channel[i], -32767, +32768 );
}