Why does the filter block not have an input port when it is created using FDA Toolbox 3.2 (R14SP2) with quantization parameters?

I created the following quantized filter using the Filter design toolbox and also created a filter block using the "Realize Model" option in FDA Toolbox 3.2 (R14SP2).

The filter block thus created does not have an input port. This happens only in FDA Toolbox 3.2 (R14SP2) when FDATOOL generates filter blocks with quantized filter coefficients.

See the attached fwizdef.fda session file.

Answer 1

[MathWorks Support Team]

The missing input port is a consequence of the optimization for zero gains. If you notice, the first scale value is quantized to zero so you will not get anything useful out of this filter. So this is not a BUG in FDA Toolbox but a problem with the filter design specifications.

More so, there are fundamental problems with this filter design. The poles of the reference filter being very close to the unit circle, it is very challenging to implement the filter with only 8 bits of precision for the coefficients. In this case, the poles actually move right on the unit circle and the filter becomes unstable.

Reasonable results can be obtained by increasing the coefficients word length, for example to 16 bits of precision (See the fwizdef_16.fda session file).