LPC Synthesizer Research Articles

Improvement of the excitation source in the narrow-band linear prediction vocoder

The major weakness of the current narrow-band 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 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 additional data. Diagnostic acceptability measure (DAM) tests show an increase of up to five points in overall speech quality with the implementation of each of these improvements. These modifications can also improve the speech quality of LPC-based speech synthesizers.

A Reading Machine Using Bar Coded Allophone Stringing Technique

Abstract-This paper introduces a reading learning aid which is designed to be a low cost consumer product. The design of the machine required the marriage of the recent advancements in both speech and optical technologies. An inexpensive optical wand is used to read bar coded allophone strings from the printed book. Natural sounding speech is then constructed by an 8-bit microcomputer using an LPC synthesizer. The system is self- contained with the flexibility of producing songs, sound effects, and speech. All the speech related data are embodied in the printed material along with games and activities to make the reading learning process fun and rewarding.

Text-To-Speech Using LPC Allophone Stringing

A low cost voice response system is presented, which performs text-to-speech conversion of any English text. The system is built around an LPC synthesizer chip and a microprocessor. Text-to-allophone rules are used to convert an input string of ASCII characters into allophonic codes. LPC parameters are then drawn from an allophone library, which takes very little storage space, and concatenated using a fast and simple algorithm to produce natural sounding speech.

On reducing the buzz in LPC synthesis

A method for reducing the characteristic buzz of LPC synthetic speech is presented. The method consists of the use of a nonimpulse source for exciting the LPC synthesizer during voiced sounds. One novel feature is that the temporal parameters of the source are kept in fixed proportion to the pitch period. An extensive perceptual experiment has shown that the resulting quality of the synthesis is significantly preferred over the quality of the standard LPC synthesis.

Tandem Connections of Wideband and Narrowband Speech Communications Systems: Part 1-Narrowband-to-Wideband Link

The performance of a tandem connection of narrowband and wideband speech communication systems is evaluated. Specifically, the narrowband system consists of a conventional linear Predictive Coding (lpc) vocoder operating at a bit-rate of 2.4 kb/s and the wideband system consists of a Continuously Variable Slope Delta modulator cvsd operating at a bit rate of 16 kb/s. In Part 1 of this paper the properties of the narrowband-to-wideband link are investigated and in Part 2 the properties of the wideband-to-narrowband link are investigated. In part 1 the snr (signal-to-quantizing noise ratio) of the cvsd coder is analyzed over a 50-dB variation of the input signal levels and for a variety of source excitations for the lpc synthesizer. It is shown that snr improvements in the cvsd coder of 2 to 2.5 dB are possible in the slope overload region of the coder by modifying the source excitation of the lpc synthesizer and by preprocessing the input signal to the coder with an allpass filter. Both methods aid in reducing the peak factor (peak-to-rms level) of the input speech to the coder. Subjectively, however, only slight improvements in quality, if any, were observed with these modifications.

Analysis of the performance of a tandem connection of a 2.4-kB/sec LPC vocoder to a 16-kB/sec CVSD coder

The performance of a tandem connection of narrow-band and wide-band speech communication systems is evaluated. Specifically, the narrow-band system consists of a conventional LPC vocoder operating at a bit rate of 2.4 Kb/sec and the wideband system consists of a CVSD (continuously variable slope delta modulator) operating at a bit rate of 16 Kb/sec. This paper describes the properties of the narrowband to wideband link. The SNR (signal-to-quantizing noise ratio) of the CVSD coder is analyzed over a 50-dB variation of the input signal levels and for a variety of source excitations for the LPC synthesizer. It is shown that SNR improvements in the CVSD coder of 2–2.5 dB are possible in the slope overload region of the coder by modifying the source excitation of the LPC synthesizer and by preprocessing the input signal to the coder with an allpass filter. Both methods aid in reducing the peak factor (peak to rms level) of the input speech to the coder. Subjectively, however, only slight improvements in quality, if any, were observed with these modifications. The weakest link in terms of quality in the tandem connection appears to be that of the CVSD coder.