code: func FullWaveRectifier(buf *audio.PCMBuffer) error {
  	if buf == nil {
  		return audio.ErrInvalidBuffer
  	}
  	buf.SwitchPrimaryType(audio.Float)
  	for i := 0; i < buf.Len(); i++ {
  		buf.Floats[i] = math.Abs(buf.Floats[i])
  	}
  
  	return nil
  }
docs: // FullWaveRectifier to make all signal positive
  // See https://en.wikipedia.org/wiki/Rectifier#Full-wave_rectification