LPC BASED SPEECH SYNTHESIS WITH ADAPTIVE PITCH PREFILTER

Abstract: A linear predictive coding speech synthetizer wherein an excitation signal that includes pitch information is filtered in a pitch enhancement filter (201) prior to provision of the excitation signal to an LPC filter (106). In one embodiment, the speech synthesizer is configured in a radio.

Description: LPC BASED SPEECH SYNTHESIS WITH ADAPTIVE PITCH PREFILTER Technical Field This invention relates generally to speech synthesis, and more particularly to linear predictive coding based speech synthesis. Background of the Invention The synthesis of speech through use of a linear predictive coding (LPC) based platform is known in the art. A prior art radio that embodies such a platform is depicted generally in Fig. 1 by the reference numeral 100. The radio (100) receives a speech coded signal (101) through an appropriate energy transducer (102), such as an antenna. An RF unit (103) converts the received signal (101) to baseband and demodulates the signal to recover the speech coded information. A parameter decoder (105) develops control parameters for various subsequent processes from this information. An excitation source (104) as described above utilizes the parameters provided to it to create an excitation signal and then provides that excitation signal (which excitation signal includes pitch information that has been inserted by a pitch filter) to an LPC filter (106) which in turn provides, at it's output, a synthesized speech signal. This synthesized speech signal is then filtered in an adaptive pitch postfilter (107) and an adaptive spectral postfilter (108), as well as a post emphasis filter (109), to enhance the perception of natural speech and to minimize the impact of various distortions and artifacts introduced in the synthesis process. The enhanced synthesized speech signal is then properly processed in an audio processing unit (111) and rendered audible through an appropriate audio transducer (112). Each of the above generally referred to components is well known and understood in the art. The pitch postfilter (107) serves an important function, in that it provides additional control of the pitch content of the synthesized speech. Without this filter, the resultant synthesized speech product may be rougher and of lower quality. Notwithstanding this important benefit, the pitch postfilter (107) frequently contributes artifacts to the resultant synthesized speech, which artifacts can themselves noticeably disturb the perception of natural speech. Accordingly, a need exists for providing appropriate pitch enhancement filtering in an LPC based speech synthesizing unit that minimizes a concurrent perceptible expression of artifacts in a resultant synthesized speech signal. Summary of the Invention This need and others are substantially met through provision of the LPC platform disclosed herein. According to the invention, the pitch enhancement postfilter is moved from a position of processing the LPC filter output to a position where it processes the excitation input to the LPC filter. The subsequent processing of the LPC filter itself functions to minimize the perceptible effect of any artifacts introduced by the pitch enhancement prefilter in the resultant synthesized speech signal. Brief Description of the Drawinos Fig. 1 comprises a block diagram depiction of a prior art embodiment of a radio receiver having LPC based speech synthesis; and Fig. 2 comprises a block diagram depiction of a radio having LPC 3asθd speech synthesis in accordance with the invention. Best Mode For Carrying Out The Invention In Fig. 2, a radio embodying the invention includes an antenna (102) for receiving a speech coded signal (101 ). An RF unit (103) processes the received signal to recover the speech coded information. This information is provided to a parameter decoder (105) that develops control parameters for various subsequent processes. An excitation source (104) as described above utilizes the parameters provided to it to create an excitation signal. This resultant excitation signal from the excitation source (104) is provided to a pitch prefilter (201) that functions to filter the pitch information contained in the excitation signal. The resultant filtered signal then passes to the LPC filter (106) which yields a SUBSTITUTE SHEET synthesized speech signal in accordance with the coded information. Consistent with known methodology, this resultant signal is then further processed in an adaptive spectral postfilter (108) and post emphasis filter (109) to further enhance the quality of the synthesized speech, and is then processed in an audio processing unit (111 ) and rendered audible by an audio transducer (112). What is claimed is: SUBSTITUTE SHEET

Claims: Claims 1. A method of synthesizing speech, comprising: A) providing an excitation signal that includes pitch information; further characterized by the steps of: B) filtering the excitation signal in a pitch enhancement filter to provide a pitch filtered " excitation signal; C) filtering the pitch filtered excitation signal in a speech synthesis filter to provide a synthesized speech signal. 2. A method of synthesizing speech, comprising: A) receiving a coded information signal; B) using the coded information signal to provide an excitation signal that includes pitch information; further characterized by the steps of: C) filtering the excitation signal in a pitch enhancement filter to provide a pitch filtered excitation signal; D) filtering the pitch filtered excitation signal in a speech synthesis filter to provide a synthesized speech signal. 3. A radio, comprising: A) RF means for receiving a broadcast signal and for recovering a coded information signal included therewith; B) excitation source means operably coupled to the RF means for providing an excitation signal that includes pitch information in response to the coded information signal; wherein the radio is further characterized by: C) pitch enhancement filter means operably coupled to the excitation source means for filtering the pitch information in the excitation signal to provide a filtered excitation signal; D) LPC filter means for receiving the filtered excitation signal and for providing a synthesized speech signal in response thereto.

Inventors: GERSON IRA ALAN, JASIUK MARK ANTONI, GERSON, IRA, ALAN, JASIUK, MARK, ANTONI
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1990
Main classification: G10L5
IPC codes: G10L19