Compute the output of a general delta-sigma modulator with input u,

a structure described by ABCD, an initial state x0 (default zero) and

a quantizer with a number of levels specified by nlev.

Multiple quantizers are implied by making nlev an array,

and multiple inputs are implied by the number of rows in u.



Alternatively, the modulator may be described by an NTF.

The NTF is zpk object. (The STF is assumed to be 1.)