  We show that many delay-based reservoir computers considered in the
literature can be characterized by a universal master memory function (MMF).
  Once computed for two independent parameters, this function provides linear
memory capacity for any delay-based single-variable reservoir with small
inputs. Moreover, we propose an analytical description of the MMF that enables
its efficient and fast computation.
  Our approach can be applied not only to reservoirs governed by known
dynamical rules such as Mackey-Glass or Ikeda-like systems but also to
reservoirs whose dynamical model is not available. We also present results
comparing the performance of the reservoir computer and the memory capacity
given by the MMF.
