The major weakness of the current narrowband LPC synthesizer lies in the use of a "canned" invariant excitation signal. The use of such an excitation signal is based on three primary assumptions, namely, (1) that the amplitude spectrum of the excitation signal is flat and time-invariant, (2) that the phase spectrum of the voiced excitation signal is a time-invariant function of frequency, and (3) that the probability density function (PDF) of the phase spectrum of the unvoiced excitation signal is also time-invariant. This paper critically examines these assumptions and presents modifications which improve the quality of the synthesized speech without requiring the transmission of any additional data. Diagnostic Acceptability Measure (DAM) tests show an increase of up to 5 points in overall speech quality with the implementation of each of these improvements.

The narrowband linear predictive coder (LPC) is becoming widely used in both civilian and military applications. Yet, in spite of the many improvements made over the years, it is still not universally acceptable to general users. Therefore we present improvements to three aspects of the LPC analysis to increase the quality and intelligibility of the narrowband LPC. They are: (1) frequency spreading of voiceless fricative sound spectra, (2) adaptive analysis-window placement at onsets, and (3) suppression of intraframe pitch interference.