Analog discrete-time filter for speech synthesis

A controllable analog discrete-time filter is described that can act as an all-pole or all-pass filter and is especially useful for predictive speech synthesis. It is based on a piecewise-uniform transmission line structure as in a "wave digital filter." The control parameters are the
\tanh^{-1}
function of the reflection coefficients of this line, also known as "log area ratios." The one-multiplier form of the signal flow graph of one filter stage leads to a simple and inexpensive analog realization. Delay elements z
^-1/2
are represented by sample-hold-circuits which simultaneously operate as adders. Integrated transistor balanced-modulators with differential inputs are used for differencing and multiplying the differences with the reflection coefficients. The latter are recovered from their
\tanh^{-1}
values by the
\tanh
characteristics of the "earlier" inputs of the modulators. The dc stability and adjustment are considered, and a dc control loop is suggested for stabilization.