Two non-linear interpolation techniques are introduced for enhancing speech reproduction in prototype waveform interpolation (PWI) and similar encoders. A temporal differential rate (TDR) vector is used to characterise the non-uniform evolution of pitch cycle temporal structure during interpolation. Experimental results show a clear improvement in the accuracy of decoded pitch cycle lengths and in the reproduction of periodicity in general. It is also shown that waveform reproduction can be significantly improved by vector quantising sets of optimal combination coefficients (OCC) aimed at maximising the similarity between interpolated and target signal segments. Both time domain waveform similarity and frequency domain spectral envelope similarity derived OCC are tested. Subjective assessment suggests a general preference for non-linear interpolation methods and the scheme using frequency domain derived OCC with perceptual weighting provided the best subjective preference.

The feasibility and performance of an embedded regular pulse excited speech coder (ERPE) based on multistage coding is investigated. The simulated ERPE system exhibits a graceful reduction of reconstructed speech quality for bit rates from 14.8 to 6.4 kb/s in 4.2 kb/s steps, and carries a very small signal-to-noise ratio (SNR) penalty compared to its conventional version.

The feasibility and performance of an embedded RPE (ERPE) scheme based on multistage coding is investigated. The coding efficiency of second and subsequent stages depends on the spectral envelope difference between the original speech and the error signal at each stage whereas re-use of LPC parameters derived from the original speech depends on the corresponding LPC spectral difference. Suitable measures of spectral difference are defined and simulation shows that both decrease with the perceptual weighting factor. The ERPE system requires little extra coding complexity and can be simplified further by using a partial phase adaptation procedure with marginal loss of SNR performance. The simulated ERPE system shows graceful reduction of reconstructed speech quality for bit rates from 14.8 to 6.4 kb/s in 4.2 kb/s steps.

Packet communication systems cannot, in general, guarantee accurate and prompt delivery of every packet. The effect of network congestion and transmission impairments on data packets is extended delay; in voice communications these problems lead to lost packets. This paper describes techniques for replacing missing speech with waveform segments from correctly received packets in order to increase the maximum tolerable missing packet rate. After presenting a simple formula for predicting the probability of waveform substitution failure as a function of packet duration and packet loss rate, we introduce two techniques for selecting substitution waveforms. One method is based on pattern matching and the other technique explicitly estimates voicing and pitch. Both approaches achieve substantial improvements in speech quality relative to silence substitution.

Certain properties of periodic signals are defined in terms of the zeros and singularities of associated analytic functions of a complex time variablez. This algebraic approach is a generalization of analytic signal theory, and leads to the conception of hybrid modulation as the superposition of twoz-plane zero-singularity (ZS) patterns associated with amplitude- and angle-modulating signals, respectively. It is shown that important spectral properties of the modulated signal, such as band limitation, are explicit in the resultant pattern. Signal design is then interpreted in terms of ZS manipulation and placement. The theory is applied in a unified approach to compatible singlesideband (CSSB) modulation systems. It is shown that two types of proposed CSSB systems give rise to essentially nonband-limited output signals. The relation between conventional and single-sideband (SSB) angle modulation is also discussed in terms of their characteristic ZS patterns.