Jan Linden

iLBC - a linear predictive coder with robustness to packet losses

In this paper, we discuss the internet low bit rate codec (iLBC) with an emphasis on the frame-independent long-term prediction. The frame-independent long-term prediction is a method to exploit pitch-lag correlations in the encoding of speech without suffering multiple-frame speech degradation in connection with transmission loss. We present mean opinion scores for the iLBC codec and show by means of signal examples how the nature of degradation in a predictive codec based on frame-independent long-term prediction differs from that of traditional CELP codecs.

Predictive VQ for noisy channel spectrum coding: AR or MA?

In this paper, the performance of different predictive vector quantization (PVQ) structures is studied and compared for different degrees of channel noise. Predictive quantization schemes with an auto-regressive (AR) decoder structure are compared with schemes that employ a moving average (MA) decoder. For noisy channels MA prediction performs better than AR. It is shown here that a combination of a PVQ scheme (AR or MA) and a memoryless VQ outperforms both types of traditional predictive quantizer schemes in noiseless as well as noisy channels.

Exploiting interframe correlation in spectral quantization: a study of different memory VQ schemes

This paper addresses the problem of efficient transmission of the LSF parameters in speech coding using vector quantization (VQ). By performing a comparison of several memory VQ methods on the same database, we investigate what gains can be achieved by exploiting interframe correlation. The memory VQ methods studied are finite-state VQ and linear predictive VQ. By combining the memory VQ with a fixed memoryless VQ, called the safety-net, further improvements in performance can be obtained. It is found that memory VQ can improve the performance with 3-5 bits compared to memoryless VQ for error-free transmission. The best method in this study is a safety-net extended predictive VQ. For noisy channels, most memory methods perform worse than memoryless VQ, but the safety-net predictive VQ outperforms memoryless VQ for all tested channel error rates, with 4 bits less.

Pitch adaptive windows for improved excitation coding in low-rate CELP coders

A novel paradigm based on pitch-adaptive windows is proposed for solving the problem of encoding the fixed codebook excitation in low bit-rate CELP coders. In this method, the nonzero excitation in the fixed codebook is substantially localized to a set of time intervals called windows. The positions of the windows are adaptive to the pitch peaks in the linear prediction residual signal. Thus, high coding efficiency is achieved by allocating most of the available FCB bits to the perceptually important segments of the excitation signal. The pitch-adaptive method is adopted in the design of a novel multimode variable-rate speech coder applicable to CDMA-based cellular telephony. Results demonstrate that the adaptive windows method yields excellent voice quality and intelligibility at average bit-rates in the range of 2.5-4.0 kbps.

Channel optimized predictive vector quantization

The paper investigates how channel optimization techniques can be applied to predictive vector quantizers. In particular, an efficient encoder search procedure and two design methods are derived. The design methods proposed here, one sample iterative and one block iterative, simultaneously optimize the predictor and the codebook. Extensive simulations show the advantage of this quantization method compared to other memory based quantization schemes as well as memoryless VQ. We also demonstrate that the design methods can be used to obtain index assignments that are advantageous to that obtained by post process index assignment algorithms.

Voice over IP: Speech Transmission over Packet Networks

The emergence of packet networks for both data and voice traffic has introduced new challenges for speech transmission designs that differ significantly from those encountered and handled in traditional circuit-switched telephone networks, such as the public switched telephone network (PSTN). In this chapter, we present the many aspects that affect speech quality in a voice over IP (VoIP) conversation. We also present design techniques for coding systems that aim to overcome the deficiencies of the packet channel. By properly utilizing speech codecs tailored for packet networks, VoIP can in fact produce a quality higher than that possible with PSTN.

Low Rate Speech Coding using a Glottal Pulse Codebook

In this paper, we describe a new method for speech coding at low rates using a glottal pulse codebook, populated by typical glottal jlow derivative waveforms originating from recorded speech, Voiced speech is produced by periodizing a codebook entry for a given pitch value. All parameters in the coder are estimated utilizing an analysis-by-synthesis procedure.

Channel optimization of predictive VQ for spectrum coding

In this paper, design algorithms for channel optimized predictive vector quantizers are presented. Traditional methods for memoryless quantizers can be applied directly to the prediction error codebook. However, the predictor must be redesigned to obtain good performance. We present a new and more general method that simultaneously optimizes the predictor and the codebook. We also show that this design method can be used to obtain index assignments for standard vector quantizers that are advantageous to what is obtained by post process index assignment algorithms.

Channel optimized predictive VQ

Combined source-channel coding is considered for the case of predictive vector quantization. A design algorithm for channel optimized predictive vector quantizers is proposed. Under reasonable assumptions, the optimal encoder is presented and a sample iterative design method that simultaneously optimizes the predictor and the codebook is derived. We also demonstrate that this design method can be used to obtain index assignments that are advantageous to what is obtained by post process index assignment algorithms. Results are presented for a correlated Gauss-Markov process and for speech LSF parameters.