A CMOS time-multiplexed digital filter for vocoder applications

In vocoder systems speech is spectrally analysed with filter banks containing 16 to 19 narrowband filters. Digital techniques can be used to realize these filter banks in an economical manner by using single time multiplexed second order filters. A hardware implementation of a fourth and an eighth order filter, realized with one time-shared second order section is described. A software package is developed to simulate the filter performance, and validate the experimentally measured response. The CMOS design of a programmable filter is presented. A second order section is time multiplexed to realize up to 32 poles of filtering. The configuration is mask programmable and the filter coefficients are programmable with a UV-PROM. The bank of contiguous bandpass filters for vocoder use can be implemented with three multi-function filter chips.