The approach in Foias et al. (Robust control of Infinite Dimensional: Frequency Domain Methods. Springer: Berlin, 1996) is one of the well-developed methods to design H-infinity controllers for general infinite dimensional systems. This approach is applicable if the plant admits a special coprime-inner/outer factorization. We give the largest class of single-input-single-output (SISO) time-delay systems for which this factorization is possible and factorize the admissible plants. Based on this factorization, we compute the optimal H-infinity performance and eliminate unstable pole-zero cancellations in the optimal H-infinity controller. We extend the results on the finite impulse response structure of optimal H-infinity controllers by showing that this structure appears not only for plants with input/output delays, but also for general SISO time-delay plants.This establishes the connection between an asymptotic chain root of q(s), r q,i and the corresponding root of its asymptotic polynomial p(s), r p,i as r q,i = 0 +j i ⇐⇒ r p,i = e −( 0 +j i )/N .The real part of r q,i is negative, o < 0, if and only if the magnitude of the root of the asymptotic polynomial p(s) is greater than 1, |r p,i | > 1. The assertion follows.Given a neutral quasi-polynomial with infinitely many roots inside C + , its conjugate quasipolynomial may have finitely many roots inside C + . This class of neutral quasi-polynomials plays 984