  It is known that the digital waveguide (DW) method for solving the wave
equation numerically on a grid can be manipulated into the form of the standard
finite-difference time-domain (FDTD) method (also known as the ``leapfrog''
recursion). This paper derives a simple rule for going in the other direction,
that is, converting the state variables of the FDTD recursion to corresponding
wave variables in a DW simulation. Since boundary conditions and initial values
are more intuitively transparent in the DW formulation, the simple means of
converting back and forth can be useful in initializing and constructing
boundaries for FDTD simulations.
