Reg :: Issues with Filter Designing in SciLab

[Sumit Adhikari]

Dear All,

I am designing an IIR filter scilab, in which the quantized filter response
is as follows:

###########################################################################
 FLDN :: Designed Filter := Hiir_q

                      2        3        4        5        6        7
 8          9        10       11       12       13       14       15
    139 - 208z + 7920z - 25291z + 55974z - 66935z + 46643z + 26354z - 95388z
+ 136947z - 100619z + 33947z + 39939z - 63176z + 54471z - 24951z
              16

       + 7607z


-----------------------------------------------------------------------------------------------------------------------------------------------

                                      2            3            4
 5            6            7            8             9           10
    14419729 - 2.352D+08z + 1.827D+09z - 8.997D+09z + 3.148D+10z -
8.305D+10z + 1.710D+11z - 2.806D+11z + 3.709D+11z - 3.965D+11z + 3.420D+11z

                   11           12           13           14           15
        16
       - 2.358D+11z + 1.275D+11z - 5.233D+10z + 1.541D+10z - 2.917D+09z +
2.684D+08z

 ###########################################################################


Hiir_q is stable and I tested this filter with quantized input for 16-bits.
So, when I was satisfied I got the filter coefficients like as follows :


-->getArray(invert(a))
 B(01) = 268435456
 B(02) = 1377518686
 B(03) = -1768307133
 B(04) = -786674615
 B(05) = -1379482255
 B(06) = 441572766
 B(07) = -1550825035
 B(08) = -1373807840
 B(09) = 1486413132
 B(10) = -1394174588
 B(11) = -787299224
 B(12) = -1440621730
 B(13) = 1413056441
 B(14) = -406911246
 B(15) = 1826903011
 B(16) = -235150099
 B(17) = 14419729

-->getArray(invert(b))
 B(01) = 7607
 B(02) = -24951
 B(03) = 54471
 B(04) = -63176
 B(05) = 39939
 B(06) = 33947
 B(07) = -100619
 B(08) = 136947
 B(09) = -95388
 B(10) = 26354
 B(11) = 46643
 B(12) = -66935
 B(13) = 55974
 B(14) = -25291
 B(15) = 7920
 B(16) = -208
 B(17) = 139

-->

and then tried to implement it using a double precision C++/SystemC model. I
found that the filter output is unstable. While I was debugging,
as a check I copied and pasted these coefficients in MATLAB so that I can be
sure what is going wrong. With my surprise I found that
the MATLAB is showing completely different response and a completely
unstable filter.

I think I am doing some mistake. Can anybody please point out where ?

Regards,


-- 
Sumit Adhikari,
Institute of Computer Technology,
Faculty of Electrical Engineering,
Vienna University of Technology,
Gußhausstraße 27-29,1040 Vienna
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