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 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^-1/2are 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 values by the characteristics of the "earlier" inputs of the modulators. The dc stability and adjustment are considered, and a dc control loop is suggested for stabilization.