2 IIR1
         Name: IIR1.ASM
         Type: Assembler Macro
      Version: 1.0
 Date Entered:  15-Jul-87
  Last Change:  15-Jul-87

  Description: Direct Form Second Order All Pole Filter

 This macro implements a second order all pole IIR filter.
 The difference equation for the filter is:

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

       with z transform:

       Y(z)                1
      ----- =  -------------------------
       X(z)              -1           -2
               1 - a(1) z     - a(2) z

where:
    x(n)  = input sample at time nT
    y(n)  = output of the filter at time nT
    a(n)  = filter coefficient n
      T   = sample period

An example network to realize this filter is shown below.

    Input
  >----------(+)------------------------> Output
 x(n)         ^              |          y(n)
              |     a(1)    1/z
             (+)<-- 0.8 -----|
              ^              |
              |     a(2)    1/z
             (+)<-- -0.3 ----|

    Second Order Direct Form All Pole IIR Filter

 The coefficients in the example are arbitrary. The memory
  map for the filter is shown below:

           r0
           |
           v
       -------------------
  X:   |        |        |
       | y(n-1) | y(n-2) | Filter States
       -------------------
        y(n-1)    y(n-2)

       --------------------
  Y:   |  a(1)  |   a(2)  |
       |  .8    |   -.3   | Filter Coefficients
       --------------------
           ^
           |
           r4

      Memory Map for the Second Order All Pole Filter

 The modulo registers m0  and  m4  are  set  to  -1  for  linear
 arithmetic. The coefficients in this filter are restricted to
 a magnitude less than one.

 For an example of how to use this macro, see the test 
 program IIR1T.ASM.