Second Order Goertzel Algorithm 2

Second order Goertzel algorithm that calculates two outputs at a time.

C-------------------------------------------------------
C GOERTZEL'S  DFT  ALGORITHM,  Second order
C Input inorder, output by twos;  C.S. Burrus, SEPT 1991
C-------------------------------------------------------
    SUBROUTINE DFT(X,Y,A,B,N)
    REAL X(260), Y(260), A(260), B(260)
    Q = 6.283185307179586/N
    DO 20 J = 1, N/2 + 1
       C  = COS(Q*(J-1))
       S  = SIN(Q*(J-1))
       CC = 2*C
       A2 = 0
       B2 = 0
       A1 = X(1)
       B1 = Y(1)
       DO 30 I = 2, N
          T  = A1
          A1 = CC*A1 - A2 + X(I)
          A2 = T
          T  = B1
          B1 = CC*B1 - B2 + Y(I)
          B2 = T
30     CONTINUE
       A2  = C*A1 - A2
       T   = S*B1
       A(J)     = A2 - T
       A(N-J+2) = A2 + T
       B2  = C*B1 - B2
       T   = S*A1
       B(J)     = B2 + T
       B(N-J+2) = B2 - T
20  CONTINUE
    RETURN
    END
 
Figure.  Second Order Goertzel Calculating Two Outputs at a Time