2 IIR3
         Name: IIR3.ASM
         Type: Assembler Macro
      Version: 1.0
 Date Entered:  15-Jul-87
  Last Change:  15-Jul-87

  Description: Direct Form Arbitrary Order All Pole Filter

  To implement a direct form all pole filter with  an  arbitrary
  number of  coefficients,  looped  code  is used.  An example
  of an all pole IIR filter (N=5) is shown in below.

        Input
      >----------(+)------------------------> Output
     x(n)         ^              |           y(n)
                  |     a(1)    1/z
                 (+)<-- 0.8 -----|
                  ^              |
                  |     a(2)    1/z
                 (+)<-- -0.7 ----|
                  ^              |
                  |     a(3)    1/z
                 (+)<-- 0.6  ----|
                  ^              |
                  |     a(4)    1/z
                 (+)<-- -0.5 ----|
                  ^              |
                  |     a(5)    1/z
                 (+)<--  0.4 ----|

          Direct Form All Pole IIR Filter

  The values for the coefficients are arbitrary.  This
  filter is described by the difference equation:

      y(n)= x(n)  +  a(1)y(n-1)  +  a(2)y(n-2)  +  ...  +  a(r)y(n-r)

 with z transform:
 
   Y(z)                         1
  -------  =  --------------------------------------
    X(z)               -1          -2            -r
              1 - a(1)z    - a(2)z    ... - a(r)z

 where:
    x(n)  = input sample at time nT
    y(n)  = output of the filter at time nT
    a(n)  = filter coefficient n (magnitude less than one)
      T   = sample period
      r   = number of coefficients

  This filter is efficiently implemented  by  using  modulo
  addressing  and multiple address pointers, similar to the IIR1
  macro.

  For the above filter, the  memory  map  for  the filter
  coefficients  and  state  variables (past output samples
  is shown below.


                   r0          m0=ncoef-1  (=4, mod 5)
                   |
                   v
               ------------------------------------
          X:   |y(n)  |y(n-1)|y(n-2)|y(n-3)|y(n-4)| Filter States
               ------------------------------------

               ------------------------------------
          Y:   |  a(1)| a(2) | a(3) | a(4) | a(5) |
               |  .8  | -.7  |  .6  | -.5  |  .4  |
               ------------------------------------
                  ^
                  |
                  r4          m4=ncoef-1  (=4, mod 5)

            Memory Map for the Direct Form Canonic Filter

 For an example of how to use this filter, see the test program IIR3T.ASM