Showing papers on "Code-excited linear prediction published in 1989"

An expandable error-protected 4800 bps CELP coder (US Federal Standard 4800 bps voice coder)

Abstract: The authors describe a code-excited linear predictive (CELP) coder that was recently selected as the US government standard 4800-b/s voice coder. The coder has the capability for future expansion and is robust to channel errors and noisy environments. Listening tests and DRT (diagnostic rhyme test) and DAM (diagnostic acceptability measure) scores show that the decoder exceeds the performance of all government standard speech coders operating at rates below 16 kb/s and is comparable to 32-kb/s continuously-variable-slope delta modulated coders. >

Method for improving speech quality in code excited linear predictive speech coding

Abstract: By reconciling differences between the estimator and the filter of a code excited linear predictive (CELP) voice coder, higher quality is achieved in the output speech. The pulse amplitudes and pitch tap gain are solved for simultaneously to minimize the estimator bias in the CELP excitation. Increased signal to noise ratio is accomplished by modifying the pitch predictor such that the pitch synthesis filter accurately reflects the estimation procedure used to find the pitch tap gain, and by improving the excitation analysis technique such that the pitch predictor tap gain and codeword gain are solved for simultaneously, rather than sequentially. These modifications do not result in an increased transmission rate or significant increase in complexity of the CELP coding algorithm.

Spectral quantization and interpolation for CELP coders

Abstract: The authors present results on the comparative performance of nonuniform scalar quantizers using three different LPC (linear predictive coding) representations: the arcsine of reflection coefficients, the log area ratios, and the line spectral frequencies. On comparing the spectral distortion introduced by quantizers based on these representations, it was found that the average distortion was very similar for all three, with the arcsine showing fewer large spectral errors. In a parallel study, the performance of the above LPC representations and the autocorrelation coefficients for interpolating the spectrum between adjacent time frames was investigated and revealed only small differences between the different representations. Informal listening tests with a complete 8 kb/s code-excited linear predictive (CELP) coder, incorporating both quantization and interpolation, showed no significant differences between the various LPC representations, suggesting that the random codebook for the excitation is able to compensate for small spectral deviations. >

Phonetically-based vector excitation coding of speech at 3.6 kbps

Abstract: A phonetically based segmentation of speech is performed to classify segments into five classes: onset, unvoiced low-pass voiced, steady-state voiced, and transient voiced. The segment lengths are constrained to an integer multiple of a unit-frame. For each segment class, a distinctive coding scheme based on vector excitation coding (VXC) is used. The maximum bit-rate is 3.6 kb/s, and a moderate coding delay of 45 ms is incurred. Performance is roughly comparable to conventional VXC/CELP (code-excited linear prediction) coding at 4.8 kb/s. >

Robust CELP coders for noisy backgrounds and noisy channels

Abstract: The authors examine the robustness of the code-excited linear predictive (CELP) coder, such as its ability to cope with nonspeech and corrupted speech inputs or to survive errors in the transmission of the coder parameters. They describe how they determined the error sensitivity of each coder parameter and identified the error propagation mechanisms. They find that the coder is robust to many kinds of input signals but is sensitive to channel errors. They show that the effect of bit errors can be reduced significantly with relatively simple measures such as repetition of the coder parameters from the most recent error-free frame. Incorporating these techniques into CELP results in a robust system with an acceptable performance for both burst and random bit errors rates of up to 1%. The clear channel performance degrades slightly (0.2 dB) as a result of these techniques. >

Hybrid switched multi-pulse/stochastic speech coding technique

Abstract: Improved unvoiced speech performance in low-rate multi-pulse coders is achieved by employing a multi-pulse architecture that is simple in implementation but with an output quality comparable to code excited linear predictive (CELP) coding. A hybrid architecture is provided in which a stochastic excitation model that is used during unvoiced speech is also capable of modeling voiced speech by use of random codebook excitation. A modified method for calculating the gain during stochastic excitation is also provided.

A robust low-delay CELP speech coder at 16 kbits/s

Abstract: A candidate algorithm for the new CCITT 16-kb/s speech coding standard is presented. This algorithm is based on backward-adaptive CELP (code-excited linear prediction) where the predictor and the excitation gain are updated by analyzing previously quantized signals. The only information transmitted is the excitation vector with a size as small as five samples so as to achieve a one-way coding delay of less than 2 ms. With a clear channel, this 16-kb/s coder slightly outperformed the CCITT standard 32 kb/s ADPCM (adaptive differential pulse code modulation) (G.721) in speech quality as measured by the mean opinion score (MOS). With noisy channels, the coder scored slightly higher than G.721 fora bit-error rate of 10/sup -2/ and significantly higher (by a margin of 0.5) for bit-error rate of 10/sup -3/. >

Speech coding with time-varying bit allocations to excitation and LPC parameters

Abstract: The authors explore the benefits of time-varying bit allocation to excitation and LPC (linear predictive coding) parameters for the case of codebook-excited LPC. The overall bit rate in the experiment was 4.8, 6.4, or 8.0 kb/s. In each case, permissible bit rates for the LPC component were 0, 24, 36, or 48 bits per frame, one of which was selected for each speech frame using a brute-force search maximum performance. Average SNR gains over conventional time-invariant methods were modest, on the order of 1 to 2 dB, but gains for certain speech segments were as high as 3 to 5 dB. Perceptually, gains due to variable bit allocation were most noticeable in the 6.4 kb/s system, especially with female speakers. However, even in this case, the benefits of flexible bit allocation were somewhat offset by distortions due to other inadequacies in the coding algorithm. >

Method and apparatus for dynamically adapting a vector-quantizing coder codebook

Abstract: A signal vector quantizing coder (CELP) is provided with an adaptive codebook originally loaded with preselected codewords. The codebook is split into a fixed contents portion and a fixed length adaptive contents portion. During coding operations, the codewords dynamically selected for coding the coder input signal are shifted into the fixed length adaptive codebook section for codebook contents updating purposes.

Backward adaptation for low delay vector excitation coding of speech at 16 kbit/s

Abstract: To attain a very-low-delay speech coder at 16 kb/s while maintaining a quality acceptable for the public switched telephone network, low delay vector excitation coding (LD-VXC) is introduced. Backward adaptation is used to track the spectral characteristics of the signal without requiring any buffering of the input speech, thereby allowing a very low delay to be achieved in an analysis-by-synthesis structure. The algorithm differs markedly from conventional VXC or CELP (code-excited linear prediction) coders due to the use of backward adaptive linear prediction for modeling the time-varying short- and long-term correlation of speech. The LD-VXC coder provides very good speech quality at 16 kb/s, moderate complexity, a delay of under 2 ms, and a gentle degradation of quality with transmission errors. The algorithm was submitted to the CCITT as a candidate for a future 16-kb/s speech coding standard. >

Fast CELP coding based on the Barnes-Wall lattice in 16 dimensions

Abstract: The authors present an algebraic code-excited linear prediction (CELP) speech coder where the innovation codebook comes from the first spherical code of the Barnes-Wall lattice in 16 dimensions. Novel fast optimal algorithms for finding the best sequence in this Barnes-Wall shell innovation codebook are described. This algebraic codebook makes it possible to design a CELP coder at 9.6 kb/s with good quality and still implementable on a current digital-signal-processing chip. >

Multimode coding: application to CELP

Abstract: The authors introduce a novel approach to narrow- and medium-band speech coding that can dynamically balance the transmission rate between the excitation and the spectral parameters. The coding algorithm, called multimode coding, operates several coding blocks, each of which has a different bit assignment in parallel, and selects the optimum coding block frame by frame based on an evaluation of the reproduced speech quality. This coding algorithm is applied to 4.8 and 8.0 kb/s CELP coders, and 2.0-2.4 dB of SNRseg improvement is achieved over conventional CELP coders. The spectral distortion measure is added as an evaluation function, improving the subjective speech quality. >

Some experiments of 7 kHz audio coding at 16 kbit/s

Abstract: The problem of coding speech at 7 kHz is considered. A possible application of these codecs could be the videophone, especially for hands-free telephone use. The authors propose a split-band coder structure, where both hands are coded with analysis-by-synthesis techniques in order to take advantage of their high coding gain. Two techniques are considered: multipulse coding and codebook-excited linear prediction. The structures of two possible codecs are described, and indications are given of the considerations that led to their design. Their main characteristic is the use, as far as possible, of excitation model parameters optimized within the analysis by synthesis loop, still maintaining a reasonable computational complexity. Subjective and objective results, obtained with high-quality speech, are reported. They show that the obtained speech quality is close to the original. >

Adaptive code excited linear predictive coder (ACELPC)

Abstract: Code-excited linear predictive coding (CELP) is a recent vector waveform coding technique which permits the encoding of telephone speech with high quality at very low bit rates. The authors show that they can further reduce the computational complexity and the storage requirements of the coder, while improving the perceptual quality of the reconstructed speech. These improvements are achieved by two key factors: the implementation of a noise-shaping effect by alternate estimation of the short-term predictor coefficients, and the use of a fixed/adaptive codebook together with a long-term predictor. >

Code-book driven glottal pulse analysis

Abstract: A code-book approach is developed for glottal-pulse modeling of speech. The authors extend previous methods of glottal pulse analysis, suggesting a practical scheme suitable for speech coding and compression. The scheme is based on using a codebook of glottal pulse signals, thereby allowing for efficient minimization of an objective measure of distortion, consistent with ordinary CELP (code-excited linear prediction) analysis. Subjective and objective quality, as well as analysis complexity, compare favorably with established methods, such as stochastic 1024-CELP coders. Moreover, the proposed coder is robust. >

Robust and fast code-excited linear predictive coding of speech signals

Abstract: A description is given of an efficient code-excited linear predictive (CELP) coder for bit rates between 6 and 16 kb/s, and novel effective algorithms for the selection of the excitation signal. The authors then propose a class of binary codebooks having interesting properties in terms of performance, robustness against transmission errors and flexibility in the choice of the bit rate. Due to the optimal structure of the coder and to the fast algorithms for selecting the excitation signal, a real-time implementation has been made possible on one 32-bit floating-point digital signal processor. >

Cascaded likelihood vector coding of the LPC information

Abstract: Cascade likelihood vector quantization is proposed for coding LPC spectral parameters for application in low-rate code-excited linear prediction systems. The approach is based on representing the LPC all-pole filter as a cascade of two lower-order all-pole filters. The partitioning of the LPC polynomial is done in the root domain by clustering the roots into two distinct groups. The quantizer uses two codebooks to quantize each of the lower-order filters. However, the quantization process is done so as to optimize jointly the performances of the two subsystems. The likelihood-ratio distortion measure is used as a performance criterion in the design and coding processes. Splitting the LPC filter into two subsystems dramatically reduces the coding complexity while the efficiency of using vector quantization is essentially preserved. Experimental results show an average performance of 1.59, 1.41, 1.24, and 1.10 dB log-spectral distortion at the rates of 1.0, 1.1, 1.2, and 1.3 kb/s, respectively. These figures represent good quality at 1.0 kb/s up to essentially transparent quality at 1.3 kb/s. >

Binary code excited linear prediction (BCELP): new approach to CELP coding of speech without codebooks

Abstract: A new analysis-by-synthesis speech coding approach able to produce good quality speech in the vicinity of 4.8 kbit/s is presented. The new approach produces the same speech quality as obtained by CELP codecs without needing any excitation codebook storage. The new coder employs a very simple excitation search procedure and processes an inherent robustness against channel errors. The approach is based on the ternary code excitation CELP introduced previously (see P. Desantis et al. 1986).

A 2 ms delay CELP coder

Abstract: The objective of the described work is to constrain the algorithmic delay of an analysis-by-synthesis coder to be 2.5 ms while maintaining the bit rate at approximately 13 kb/s, which has been shown to be adequate for the Pan European Digital Mobile Radio System. For applications which do not incorporate some redundancy for error-protection purposes, the bit rate of the speech coder increased to 16 kb/s, while the delay was reduced to as little as 2 ms. >

Multistage self-excited linear predictive speech coder

Abstract: We present an approach to low-rate speech coding based on multistage self-excited linear prediction (SELP). The coder has much lower complexity than a code-excited linear prediction (CELP) coder, but yields comparable output speech quality at the same transmission rate. In the proposed multistage SELP coder, the long-term predictor is modified to model better the onset of a voiced section of speech or voice with high pitch.

High temporal resolution in multi-pulse coding

Abstract: A CELP (code-excited linear predictive) coder using code books populated with pulses generated using a higher time resolution than a classical multi-pulse system is proposed. The pulses in the code book are generated as sampled sinc pulses, which are allowed to attain their corresponding maximum values at positions not necessarily equal to sample positions. The proposed code book performs equally well as CELP coders using multipulse code books in terms of robustness toward corrupted speech. However, the speech obtained is smoother, and the number of pulses required per subblock can be reduced without degrading the speech quality. A preliminary estimation of the bit rate required indicates that a reasonable choice lies in the range 8-9 kb/s. >

Improved quality CELP base-band coding of speech at low-bit rates

Abstract: An improved CELP (code-excited linear prediction) baseband (BB) coder was tested using informal subjective listening tests. The overall bit rate of the coder was 4.7 kb/s. The subjective quality of speech scored just over three on a five-point mean opinion scale. Improvements are described that increase the score by about 0.35 over its original form. This means that in terms of quality the CELP BB coder at around 4.8 kb/s can replace the analog ACSSB operating even at 51 dB/Hz because in its original form it had subjective quality equivalent to ACSSB. To further investigate the performance of the coder the authors introduced random channel errors into the bit stream between the encoder and the decoder. At bit error rates of 10/sup -3/ and less, the coder did not need any forward error protection. When the bit error rate was around 10/sup -2/ an unexpected distortion occurred; the output energy level of the coder was very low whenever there were errors in the pitch filter or the optimum vector gain parameters. This was due to the new adaptation scheme used for the optimum vector gain. The authors solved this problem by adapting the optimum gain with the maximum block energy of the pitch filter memory or by simply using the energy of the whole of the pitch filter memory. >

4800 and 7200 bit/sec hybrid codebook multipulse coding

Abstract: The authors describe a low-complexity coding technique that combines multipulse and stochastic excitation. The system, known as hybrid multipulse coding (HMC), provides good quality at 4.8 and 7.2 kb/s. HMC uses efficient pulse excitation for voiced speech and stochastic excitation for unvoiced speech. For the best speech quality, HMC uses an optimization algorithm for simultaneous solution of pitch predictor and excitation parameters, producing a higher signal-to-noise ratio than CELP (code-excited linear prediction) in the 4.8-kb/s configuration. At 7.2 kb/sec, CELP and HMC give diagnostic acceptability measure (DAM) scores close enough so that their standard error limits overlap. In all configurations, HMC requires from 5 to 14 times fewer multiply/accumulate operations than CELP for similarly sized codebooks. >

A comparison of US Government standard voice coders

Abstract: The new US Government standard 4800-b/s code-excited linear predictive (CELP) coder is compared with the 2400-b/s linear prediction (LPC-10e) and 16000-b/s continuously variable slope delta modulation (CVSD) Federal Standard voice coders. This comparison includes speech quality and intelligibility in quiet and noisy environments/channels, speaker recognition, analog and digital implementation requirements, tandem coding, and coding delay. CELP is shown to offer dramatic improvements in speech quality relative to LPC-10e and CVSD. Although CELP requires significantly more computation than CVSD or LPC-10e, a version can be implemented on a single, new-generation, digital-signal-processing (DSP) chip. CELP provides interoperability among various computationally complex implementations, and CELP's robust performance tolerates a wide range of speaker variation, background noise conditions, analog impairments (i.e. nonflat microphones and channels), tandems, and errors. These features make CELP ideal for many speech applications. >

On the performance of CELP algorithms for low rate speech coding

Abstract: The performance of a 4.6 kb/s CELP (code-excited linear prediction) speech coder has been investigated by using almost stationary speech sources. The results show that quantization of voiced phones produces much more audible noise than that of unvoiced ones. Investigations using spectral analysis lead to the conclusion that a considerable part of the noise is caused by degradation of fine structure information. The performance of the long-term prediction is severely influenced by the quantization of the short-term prediction parameters. Some strategies for improving coder performance are discussed. >

Radio channel structure for QPSK digital mobile radio

Abstract: Experiments on QPSK (quadrature phase-shift keying) signal transmission and 8 kb/s TC-WVQ (transform coding with weighted vector quantization) speech coding are presented. Computer simulation of the TC-WVQ speech codec has shown that the subjective quality is comparable to that of 5-bit log PCM (pulse code modulation) at a channel bit error rate of 10/sup -2/. Computation complexity can be greatly reduced compared with that of CELP. A proposed channel structure is presented for implementation in a frequency-efficient digital mobile radio system. With QPSK and 8 kb/s TC-WVQ speech codecs, such a digital mobile radio system is as frequency-efficient existing analog systems. >

Of speech signals

Abstract: This paper describes an efficient Code-Excited Linear Predictive (CELP) Coder for bit rates between 6 and 16 kbills, and new effective algorithms for the selection of the excitorion signor Then we propose a chss of binary codebooks having interesting properties in terms of performance, robustness against transmission errors and &xibili@ in the choice of the bit rate. Due to the optimal structure of the coder and to the fast algorithms fur selecting the excitation signal, a real time implementation has been made possible on one 32 bits floating-point digital signal processor. I. INTRODUCTION In the recent years the analysis-by-synthesis procedure has been shown to be an effective method for selecting the optimum excitation in predictive coders. Given a synthesis model consisting of a long term and a short term synthesis filter in cascade, coder structws have been derived for different models of the excitation signal such as the multi-pulse, the regular-pulse and the codeexcitation. This paper describes a new Code-Excited Linear Predictive (CELP) structure with a minimum number of filtering operations. It is shown that, depending on the way transitioos between adjacent blocks are treated, two different methods are derived for finding the excitation signal. Finally fast algorithms are outlined for a special class of codebooks having an inherent robustness against transmission errors.