Multipulse Excitation Research Articles

Regular-pulse excitation--A novel approach to effective and efficient multipulse coding of speech

This paper describes an effective and efficient time domain speech encoding technique that has an appealing low complexity, and produces toll quality speech at rates below 16 kbits/s. The proposed coder uses linear predictive techniques to remove the short-time correlation in the speech signal. The remaining (residual) information is then modeled by a low bit rate reduced excitation sequence that, when applied to the time-varying model filter, produces a signal that is close to the reference speech signal. The procedure for finding the optimal constrained excitation signal incorporates the solution of a few strongly coupled sets of linear equations and is of moderate complexity compared to competing coding systems such as adaptive transform coding and multipulse excitation coding. The paper describes the novel coding idea and the procedure for finding the excitation sequence. We then show that the coding procedure can be considered as an optimized baseband coder with spectral folding as high-frequency regeneration technique. The effect of various analysis parameters on the quality of the reconstructed speech is investigated using both objective and subjective tests. Further, modifications of the basic algorithm, and their impact on both the quality of the reconstructed speech signal and the complexity of the encoding algorithm, are discussed. Using the generalized baseband coder formulation, we demonstrate that under reasonable assumptions concerning the weighting filter, an attractive low-complexity/high-quality coder can be obtained.

Graphic method of calculating echo-response parameters under conditions of multipulse excitation of quantum systems

Multipulse excitation of systems is widely used in nonlinear spectroscopy. In high-resolution NMR spectroscopy of solids excitation by a succession of many pulses makes it possible to substantially suppress resonance line broadening. Such sequences are used in echo-spectroscopy, making it possible to obtain a more informative or the same response as with a signal of a two-pulse echo. Therefore it is important to predict theoretically the action of multipulse system excitation. This article presents an approach for obtaining faster and more complete information on the generating signals. Comparing the responses with graphically determined elements, by the shape of the latter the author finds formation times, wave vectors, and complex amplitudes of signals; he determines their dependence on reversible T/sub 2/ and irreversible T/sub 1/, T/sub 2/ phase relaxation times, as well as on spectral diffusion processes. By means of the given method the author succeeded in finding pulse sequences leading to response generation with new dependences on cross sections of the exciting pulses.

Periodic repetition of multi-pulse excitation

B. S. Atal and J. R. Remde (Proc. ICASSP 82) described a method for determining the multipulse excitation for LPC speech synthesis. We find that, even for reasonably periodic speech waveforms, the secondary pulses in the multipulse excitation do not show similar periodic behavior. What is the influence of this lack of periodicity in the excitation on the quality of synthetic speech? We have investigated this question by introducing controlled periodicity into the multipulse excitation. The excitation was first segmented into individual pitch periods. For every sequence of n pitch periods, the multipulse pattern from the pitch period in the middle of each sequence was then used to repeat the excitation pattern in the other n − 1 surrounding pitch periods. In this process, the overall amplitude envelope of the utterance was left unchanged. We find that as many as 15 periods may be repeated without introducing significant audible distortion in the synthetic speech signal. However, repetition of a single multipulse pattern across all voiced segments of an utterance produced noticeable coloration in synthetic speech. We will discuss further implications of these results in this paper.

Optimizing pulse amplitudes in multipulse excitation

Multipulse excitation [B. S. Atal and J. R. Remde, Proc. ICASSP 82] is an attractive way of modeling the input to the LPC synthesis filter at low bit rates. In the method described by Atal and Remde, the amplitudes and locations of the pulses are determined in successive stages—one pulse at a time. At each stage, one pulse is determined to achieve maximum reduction in the weighted mean-squared error between the original and synthetic speech waveforms. For closely spaced pulses, successive optimization of individual pulses is inaccurate, and often requires additional pulses to compensate for inaccuracies introduced earlier. This problem can be only partly corrected by reoptimizing all pulse amplitudes at the end. This paper describes a method which locates s pulse at any stage by jointly optimizing its amplitude together with the amplitudes of all pulses located up to that stage. As a result the amplitudes of all pulses are kept optimal at every stage, and very little inaccuracy is introduced even if the pulses are closely spaced. The new method provides significant improvements in the SNR—by as much as 10 dB in some sections of speech.

Changing pitch and duration in LPC synthesized speech using multipulse excitation

Pitch and duration changes are fairly simple to realize in pitch-excited LPC vocoders. Recent work [Atal and Remde, Proc. ICASSP 82, pp. 614–617) has shown that use of multi-pulse excitation provides a significant improvement in the quality of synthetic speech. However, procedures for changing pitch with multi-pulse excitation are not known. We discuss methods for introducing pitch and duration changes in speech synthesized with multi-pulse excitation. Pitch is changed by modifying the length of individual pitch periods. Two methods of adjusting the period length were investigated: one by linear scaling of the time axis of the multi-pulse excitation, the other, by adding or deleting zeros in the excitation. The second method produced very little distortion in the synthetic speech, except in those cases where there was a significant interaction between the first formant and the pitch frequency. The scaling of the time axis produced significantly more distortion. Duration changes are accomplished by adding or removing pitch periods. The LPC area parameters and the amplitude of the major excitmion pulse are interpolated as necessary in creating additional pitch periods.

Real-time determination of multi-phase excitation for LPC speech synthesis

We describe a real-time implementation of a multi-phase excited LPC speech coder using Bell Laboratories DSP chips. A major advantage of multi-pulse excitation is that it produces natural-sounding synthetic speech and avoids both pitch analysis and voiced-voiceless decision. Pulse amplitudes and locations are determined by a noniterative, analysis-by-synthesis procedure which minimizes a perceptual distance criterion. The multi-pulse analysis is performed in fixed-point arithmetic on two DSPs. The analysis yields four excitation pulses for every 5 ms of speech, which are necessary for producing high quality synthetic speech. A companion real-time LPC area-parameter analyzer using DSP chips was described previously by Hanson and Olive [J. Acoust. Soc. Am. Suppl. 1 69, S18 (1981)].