Not understanding PDM 2 PCM

Hello,

I am trying to use the PDM 2 PCM library to read in a MEMS microphone and am having some difficulty understanding the code below.

If I wanted simply output the converted PCM data over a UART, what would need to be done to do this?

My assumption is the PCM data is MidBuffer which is a uint16 bit array of 16 PCM samples?

I think the callbacks are also confusing me. What is going on during these callbacks?

Thanks for any information.

 while (1)
	 {
	 if (rxstate==1)
	 {
 
	 	PDM_Filter(&pdmRxBuf[0],&MidBuffer[0], &PDM1_filter_handler);
 
	 	for (int i=0; i<16;i++)
	 	{
	 		FifoWrite(MidBuffer[i]);
	 	}
	 	if (fifo_w_ptr-fifo_r_ptr > 128) fifo_read_enabled=1;
	 	rxstate=0;
 
 
	 }
 
	 if (rxstate==2)
	 {
 
	 	PDM_Filter(&pdmRxBuf[64],&MidBuffer[0], &PDM1_filter_handler);
	 	for (int i=0; i<16;i++)
	 	{
	 		FifoWrite(MidBuffer[i]);
 
	 	}
	 	rxstate=0;
 
	 }
 
	 if (txstate==1) {
	 	if (fifo_read_enabled==1)
	 	{
			for (int i=0; i<64;i=i+4)	//64
				{
					uint16_t data = FifoRead();
 
					txBuf[i] = data;
					txBuf[i+2] = data;
 
				}
	 	}
	 	txstate=0;
	 }
 
	 if (txstate==2)
	 {
	 	if (fifo_read_enabled==1)
	 	{
 
				for (int i=64; i<128;i=i+4)
				{
					uint16_t data = FifoRead();
					txBuf[i] = data;
					txBuf[i+2] = data;
				}
 
			}
	 	txstate=0;
	 }

void HAL_I2S_TxHalfCpltCallback (I2S_HandleTypeDef *hi2s) {
	txstate = 1;
}
 
void HAL_I2S_TxCpltCallback (I2S_HandleTypeDef *hi2s) {
	txstate = 2;
}
 
void HAL_I2S_RxHalfCpltCallback (I2S_HandleTypeDef *hi2s) {
	rxstate = 1;
}
 
void HAL_I2S_RxCpltCallback (I2S_HandleTypeDef *hi2s) {
	rxstate = 2;
}