Equal-ripple delay recursive digital filters

All-pole recursive digital low-pass filters approximating a constant group delay in an equal-ripple sense are described. The equalripple conditions are given by a system of nonlinear equations in the poles of the transfer function. The closed-form solution obtained for a particular value of the cutoff frequency yields good first approximations for the iterative procedure required in the general case. The set of approximated delays and ripples leading to realizable filters is discussed. Numerical examples illustrating the procedure are included.