aliasing.c

本文件囊括了数字信号处理常用滤波器的C语言程序。For each project example, a folder is available that contains appropriate fi

//Aliasing.c illustration of downsampling, aliasing, upsampling 

#include "lp33.cof"	           //lowpass at 1.8 kHz
short flag = 0;                    //toggles for 2x down-sampling
float indly[N],outdly[N];          //antialias and reconst delay lines 
short i;                           //index 
float yn;                          //filter output
short antialiasing = 0;            //init for no antialiasing filter

interrupt void c_int11()	     //ISR
{  

 indly[0]=(float)(input_sample()); //new sample to antialias filter 
 yn = 0.0;                         //initialize downsampled value 
 if (flag == 0)                    //discard input sample value
     flag = 1;                     //don't discard at next sampling 
 else
  {                         
   if (antialiasing == 1)          //if antialiasing filter desired
    {                              //compute downsampled value
     for (i = 0 ; i < N ; i++)     //using LP @ 1.8 kHz filter coeffs
        yn += (h[i]*indly[i]);     //filter is implemented using float
    }
   else                            //if filter is bypassed
        yn = indly[0];             //downsampled value is input value
   flag = 0;                       //next input value will be discarded
  }
  for (i = N-1; i > 0; i--)       
        indly[i] = indly[i-1];     //update input buffer

  outdly[0] = (yn); 		     //input to reconst filter
  yn = 0.0;                        //4 kHz sample values and zeros
  for (i = 0 ; i < N ; i++)        //are filtered at 8 kHz rate
        yn += (h[i]*outdly[i]);    //by reconstruction lowpass filter

  for (i = N-1; i > 0; i--)
        outdly[i] = outdly[i-1];   //update delays
   
  output_sample((short)(yn));      //8 kHz rate sample
  return;                          //return from interrupt
}                           

void main()
{
  comm_intr();                     //init DSK, codec, McBSP
  while(1);                        //infinite loop
}