This function creates and initializes a digital linear filter whose the Z-transform is rational and
processing nchann channels of a data type specified by proctype.
The numerator and denominator of the rational expression are specified by respectively two arrays num and
denum containing the coefficients in the ascending order of the 2 polynoms. The number of elements in
each arrays is controlled by num_len and enum_len. denum_len is allowed to be equal to zero as well as
denum is allowed to be NULL. In such case, the denominator will be set to 1. The data type of the values
in num and denum are specified by type.
The proctype and type must be one the following constants:
RTF_FLOAT specifies real single precision (float)
RTF_DOUBLE specifies real double precision (double)
RTF_CFLOAT specifies complex single precision (complexfloat)
RTF_CDOUBLE specifies complex double precision (complexdouble)
The expected data type of the output of the filter has the same precision as the one specified by
proctype and is complex proctype or type specifies a complex type. Said otherwise:
* If proctype is RTF_FLOAT or RTF_CFLOAT then the output data type will have single precision. Otherwise
it will have double precision.
* If proctype or type specifies a complex type, then the output will be complex as well. Otherwise, it
will be real.
rtf_create_filter() can be used to use a filter that has been designed somewhere else. In particular,
this function can be used directly with the output of filter design function of MATLAB. In such case, the
usual B and A arrays returned by the filter design functions corresponds exactly to respectively num and
denum.