2 IIR7
         Name: IIR7.ASM
         Type: Assembler Macro
      Version: 1.0
 Date Entered:  15-Jul-87
  Last Change:  15-Jul-87

  Description: Cascaded Biquad Filters


 Cascaded biquad filters are typically used when  the  transfer
 function  can  be  decomposed  into  a product of second order
 polynomials.  The magnitude of the coefficients of the  second
 order  section  are  guaranteed to be less than 2.0 if the filter
 is stable and minimum phase.  Since  the DSP56000 scaling modes
 can implement 1 bit scaling directly, implementing cascaded second
 order sections can be done efficiently.  The difference equation
 for a second order section is:

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

  with z transform:
                         -1       -2    
       Y(z)     1 + b(1)z  + b(2)z
     ------- = ----------------------
       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
    b(n)  = filter coefficient n
      T   = sample period

 When these sections are cascaded together, the z transform  of
 the overall system is:
                                                   
                 M                 -1          -2                    
       R(z)     ---   1 + b[n](1)z  + b[n](2)z
     ------- =  | |  -------------------------
       S(z)     | |               -1          -2
                n=1   1 - a[n](1)z  - a[n](2)z

 Where M is the total number of sections and [n] refers to  the
 section  number.  A cascaded biquad filter with M=2 is shown below.

 s(n) Input      w[1](n)                       w[2](n)           Output
 >-->(+)-----------------------(+)-(+)-----------------------(+)--> r(n)
      ^            |            ^   ^            |            ^    
      |           1/z           |   |           1/z           |    
     (+)<-a[1](1)--|--b[1](1)->(+) (+)<-a[2](1)--|--b[2](1)->(+)   
      |           1/z           |   |           1/z           |    
     (+)<-a[1](2)--|--b[1](2)->(+) (+)<-a[2](2)--|--b[2](2)->(+)   
                                                                         
             Cascaded Second Order Biquad Filter
             


 For the filter kernel, the input sample  is  in  register
 A  and  the  output  after filtering of a section is also
 in register A.  This allows the next filter section to  be
 executed  by  putting  the  filter kernel  inside  of a DO
 loop.  The memory map for the cascaded biquad filter is shown
 below.  Note in particular  the way  that  the filter states
 are stored.  The first element is the second filter state 
 and the second element  is  the  first filter  state.   Also
 note the method of coefficient storage. The first element is
 a(2) then a(1) followed by b(2) and b(1).

          r0
          |
          v
      -----------------------------------------
 X:   |         |         |         |         |
      |w[1](n-1)|w[1](n-2)|w[2](n-2)|w[2](n-1)| Filter States
      -----------------------------------------
      <-    section 1   ->|<-   section 2    ->

          r4
          |                    
          v
      -------------------------------------
 Y:   |  a(2)  |  a(1)  |  b(2)  |  b(1)  | Section 1
      | .711/2 | -1.29/2|   -.5  |  0.0   | Filter Coefficients
      -------------------------------------

      -------------------------------------
      |  a(2)  |  a(1)  |  b(2)  |  b(1)  | Section 2
      | .807/2 | -1.64/2|   -.5  |  0.0   | Filter Coefficients
      -------------------------------------

           Memory Map for the Biquad Filter

 Since 1 bit scaling is  used,  all  coefficients  are  the
 actual value for the filter divided by two. The symbol "nsec"
 refers to the number of biquad sections in the overall filter.

 For an example of how to use this filter see the test
 program IIR7T.ASM