FIR filter LPF, HPF, BPF using Windowing Method

FIR filter  LPF, HPF, BPF using Windowing Method

Problem Statement : Design an FIR filter from given specifications using windowing method ( Program should work for different types of filter specifications i.e LPF, HPF, BPF etc and all window sequences. Plot the frequency response for different frequency terms i.e. analog and DT frequency).

1:       // FIR  
2:  #include<stdio.h>  
3:  #include<conio.h>  
4:  #include<math.h>  
5:  void main()  
6:  {  
7:     float wc,tou,M,hd[50],h[50],wn,pi,n;  
8:     int choice,p;  
9:     char HPF;  
10:     clrscr();  
11:     printf("\t\tFIR filter design using windows\n\n");  
12:     printf("Enter the length (number of coefficients) of the filter M = ");  
13:     scanf("%f",&M);             // length(M) of the filter  
14:     p = (int)M;  
15:     printf("\nEnter the discrete time cutoff frequency wc = ");  
16:     scanf("%f",&wc);     // cutoff frequency of the filter i.e. wc  
17:     printf("\nif you want highpass filter, enter HPF = y, otherwise "  
18:              "perss any key\ndefault is lowpass filter\nHPF = ");  
19:     HPF = getch(); printf("%c",HPF);      // enter for HPF or LPF  
20:     tou = (M-1)/2;                  // delay in hd(n)  
21:     pi = 22.0/7.0;                   // value of pi  
22:  //  Next part of the program calculates hd(n)  
23:     for(n = 0; n <= M-1; n++)  // this loop calculates hd(n) for LPF  
24:     {              //            (by default)  
25:      hd[n] = (sin(wc*(n-tou)))/(pi*(n-tou));     // hd(n) for LPF  
26:      if((n==tou)&&((p/2)*2 != p)) hd[n] = wc/pi; // hd(n) at n = tou  
27:     }  
28:     if(HPF =='y')   // this loop recalculates hd(n) for HPF if user  
29:     {         //             wants to design HPF  
30:      for(n = 0; n <= M-1; n++)  
31:      {                        // hd(n) for HPF  
32:        hd[n] = (sin(pi*(n-tou)) - sin(wc*(n-tou)))/(pi*(n-tou));  
33:        if((n==tou)&&((p/2)*2 != p)) hd[n] = 1-(wc/pi);  
34:      }                      // hd(n) at n = tou  
35:     }  
36:  //  Next part of the program performs windowing depending upon  
37:  //                           choice of window  
38:     printf("\n\nEnter the window you want to use");// choice of window  
39:     printf("\nrectangular window (Enter 1)");      //rectangular  
40:     printf("\ntriangular(bartlett) window (Enter 2)");  //triangular  
41:     printf("\nhamming window (Enter 3)");          //hamming  
42:     printf("\nhanning window (Enter 4)\nchoice = ");     //hanning  
43:     scanf("%d",&choice);                // enter choice  
44:     switch(choice)  
45:     {       // h(n) = hd(n) * w(n) depending upon type of window  
46:      case 1 : for(n = 0; n <= M-1; n++)        // rectangular  
47:            {  
48:             wn = 1;          //wn for rectangular window  
49:             h[n] = hd[n] * wn;       // windowing operation  
50:            } break;  
51:      case 2 : for(n = 0; n <= M-1; n++)         // triangular  
52:            {  
53:             wn = 1 - (2*abs(n-tou)/(M-1));  // calculation of wn  
54:             h[n] = hd[n] * wn;       // windowing operation  
55:            } break;  
56:      case 3 : for(n = 0; n <= M-1; n++)          // hamming  
57:            {  
58:             wn = 0.54 - 0.46*cos((2*pi*n)/(M-1));      // wn  
59:             h[n] = hd[n] * wn;       // windowing operation  
60:            } break;  
61:      case 4 : for(n = 0; n <= M-1; n++)          // hanning  
62:            {  
63:             wn = (1-cos((2*pi*n)/(M-1)))/2; // calculation of wn  
64:             h[n] = hd[n] * wn;       // windowing operation  
65:            } break;  
66:     }  
67:  //  Next part of the program displays coefficients on the screen  
68:     if(HPF == 'y') printf("\ncoefficients of highpass FIR filter"  
69:                                 " are as follows...");  
70:     else printf("\ncoefficients of lowpass FIR filter"  
71:                                 " are as follows...");  
72:     for(n = 0; n <= M-1; n++)  
73:     {  
74:      printf("\nh%1.0f = %f",n,h[n]);  
75:     }  
76:       getch();  
77:  }