Mixed Excitation Linear Prediction Coder Research Articles

Pitch and Fourier magnitude based steganography for hiding 2.4 kbps MELP bitstream

In this study, the authors present a new steganographic technique called random least significant bits of pitch and Fourier magnitude steganography (RLPFS). It is based on hiding a secret speech coded by mixed excitation linear prediction (MELP) speech coder in speech bitstream (cover signal), which is also encoded by MELP coder. First, the RLPFS leaks the hidden speech in the following modes: pitch-based steganography, Fourier magnitude-based steganography or both. These modes are selected randomly. Second, during transmission, the stego speech, the mode number, and the number of embedded bits would be transmitted either through a covert channel created in the transmission protocol or through the cover speech. In this work, the authors have dealt with the challenge of embedding a secret speech into a cover speech coded by a very low bit rate speech coder while maintaining a reasonable level of speech quality. They have shown that RLPFS was able to create hidden channels with maximum steganographic bandwidths up to 266.64 bit/s at the cost of a steganographic noise between 0.031 and 0.62 mean opinion score. Also, this study takes into account the security of the parameters, the synchronisation of the receiver to deal with a packet loss during transmission and the resistance of the proposed method against steganalysis.

Bit rate reduction of mixed excitation linear prediction coder by Lempel-Ziv segment quantization

Bit rate reduction of mixed excitation linear prediction coder by Lempel-Ziv segment quantization

Strategies to improve the performance of very low bit rate speech coders and application to a variable rate 1.2 kb/s codec

This paper presents several strategies to improve the performance of very low bit rate speech coders and describes a speech codec that incorporates these strategies and operates at an average bit rate of 1.2 kb/s. The encoding algorithm is based on several improvements in a mixed multiband excitation (MMBE) linear predictive coding (LPC) structure. A switched-predictive vector quantiser technique that outperforms previously reported schemes is adopted to encode the LSF parameters. Spectral and sound specific low rate models are used in order to achieve high quality speech at low rates. An MMBE approach with three sub-bands is employed to encode voiced frames, while fricatives and stops modelling and synthesis techniques are used for unvoiced frames. This strategy is shown to provide good quality synthesised speech, at a bit rate of only 0.4 kb/s for unvoiced frames. To reduce coding noise and improve decoded speech, spectral envelope restoration combined with noise reduction (SERNR) postfilter is used. The contributions of the techniques described in this paper are separately assessed and then combined in the design of a low bit rate codec that is evaluated against the North American Mixed Excitation Linear Prediction (MELP) coder. The performance assessment is carried out in terms of the spectral distortion of LSF quantisation, mean opinion score (MOS), A/B comparison tests and the ITU-T P.862 perceptual evaluation of speech quality (PESQ) standard. Assessment results show that the improved methods for LSF quantisation, sound specific modelling and synthesis and the new postfiltering approach can significantly outperform previously reported techniques. Further results also indicate that a system combining the proposed improvements and operating at 1.2 kb/s, is comparable (slightly outperforming) a MELP coder operating at 2.4 kb/s. For tandem connection situations, the proposed system is clearly superior to the MELP coder.

An intelligibility enhancement for the mixed excitation linear prediction speech coder

We recently proposed a technique to increase the robustness of perceptually important acoustic cues in speech, based on modification of the phoneme timing structure. We now apply the technique as a preprocessor to improve the intelligibility of the mixed excitation linear prediction (MELP) coder. The enhancement strategy, however, requires adaptations to minimize the introduction of unnatural speech characteristics that are introduced and intensified by parametric coding schemes. The enhanced 2.4 kb/s fixed-point MELP coder achieves a level of speech intelligibility comparable to the 8 kb/s G.729 Annex A coder.

An 8.0-/8.4-kbps wideband speech coder based on mixed excitation linear prediction

A new method based on the Mixed Excitation Linear Prediction (MELP) model for coding wideband speech signals at medium bit rates is proposed. Modifications are made to the voiced/unvoiced strength analysis, pitch estimation and linear predictive (LP) analysis/synthesis of the Federal MELP Standard. An 8.0-/8.4-kbps wideband MELP coder is developed based on the modifications. Both the objective and subjective tests show that the subjective quality of the synthesized clean speech coded by the proposed 8.4-kbps wideband MELP coder is comparable to that of the ITU G.722 (P. Mermelstein, G. 722: a new CCITT coding standard for digital transmission of wideband audio signals, IEEE Comm. Mag. 26 (1) (1988) 8–15.) coder coding at 48 kbps. For the operating bit rate of 8 kbps, the performance of the coder is slightly better than that of MPEG4 CELP (Code-Excited Linear Prediction) (J.D. Johnson, S.R. Quackenbush, J. Herre, B. Grill, Review of MPEG-4 general audio coding, in: Multimedia Systems, Standards and Networks, A. Puri, T. Chen (Eds.), Marcel Dekker, New York, 2000 (Chapter 5)) coder operating at 14.4 kbps.