How to convert pcm samples in byte array as floating point numbers in the range -1.0 to 1.0 and back?

Question

The resampling algorithm i use expects float array containing input samples in the range -1.0 to 1.0 . The audio data is 16 bit

Answer 1

16 bit PCM has a range - 32768 to 32767. So, multiply each of your PCM samples by (1.0f/32768.0f) into a new array of floats, and pass that to your resample.

Going back to float after resampling, multiply by 32768.0, saturate (clip anything outside the range - 32768 to 32767), round (or dither as Björn mentioned) and then cast back to short.

Test code that shows conversion forward and back using multiplies with no bit errors:

// PcmConvertTest.cpp : Defines the entry point for the console application.
//

#include 
#include 
#include 
#define SZ 65536
#define MAX(x,y) ((x)>(y)) ? (x) : (y)
#define MIN(x,y) ((x)<(y)) ? (x) : (y)
int main(int argc, char* argv[])
{
  int16_t *pIntBuf1 = new int16_t[SZ];
  int16_t *pIntBuf2 = new int16_t[SZ];
  float   *pFloatBuf = new float[SZ];

  // Create an initial short buffer for testing
  for( int i = 0; i < SZ; i++) {
    pIntBuf1[i] = (int16_t)(-32768 + i);
  }

  // Convert the buffer to floats. (before resampling)
  const float div = (1.0f/32768.0f);
  for( int i = 0; i < SZ; i++) {
    pFloatBuf[i] = div * (float)pIntBuf1[i];
  }

  // Convert back to shorts
  const float mul = (32768.0f);
  for( int i = 0; i < SZ; i++) {
    int32_t tmp = (int32_t)(mul * pFloatBuf[i]);
    tmp = MAX( tmp, -32768 ); // CLIP < 32768
    tmp = MIN( tmp, 32767 );  // CLIP > 32767
    pIntBuf2[i] = tmp;
  }

  // Check that the conversion went int16_t to float and back to int for every PCM value without any errors.
  assert( 0 == memcmp( pIntBuf1, pIntBuf2, sizeof(int16_t) * SZ) );

  delete pIntBuf1;
  delete pIntBuf2;
  delete pFloatBuf;
  return 0;
}