Showing papers on "Code-excited linear prediction published in 1991"

Method and system for CELP speech coding and codebook for use therewith

Abstract: Apparatus and method for encoding speech using a codebook excited linear predictive (CELP) speech processor and an algebraic codebook for use therewith The CELP speech processor receives a digital speech input representative of human speech and performs linear predictive code analysis and perceptual weighting filtering to produce a short term speech information and a long term speech information The CELP speech processor utilizes an organized, non-overlapping, algebraic codebook containing a predetermined number of vectors, uniformly distributed over a multi-dimensional sphere to generate a remaining speech residual The short term speech information, long term speech information and remaining speech residual are combinable to form a quality reproduction of the digital speech input

Method and apparatus for low-delay celp speech coding and decoding

Abstract: A low-bitrate (typically 8 kbit/s or less), low-delay digital coder and decoder based on Code Excited Linear Prediction for speech and similar signals features backward adaptive adjustment for codebook gain and short-term synthesis filter parameters and forward adaptive adjustment of long-term (pitch) synthesis filter parameters. A highly efficient, low delay pitch parameter derivation and quantization permits overall delay which is a fraction of prior coding delays for equivalent speech quality at low bitrates.

16 kbps wideband speech coding technique based on algebraic CELP

Abstract: The application of algebraic code excited linear prediction (ACELP) coding to wideband speech is presented An algebraic codebook with a 20 bit address can be used without any storage requirements and, more importantly, with a very efficient search procedure which allows for real-time implementation The authors describe an efficient procedure for searching such a large codebook deploying a focused search strategy, where less than 01% of the codebook is searched with performance very close to that of a full search High-quality speech at a bit rate of 13 kbps was obtained >

Vector Sum Excited Linear Prediction (VSELP)

Abstract: Vector Sum Excited Linear Prediction falls into the class of speech coders known as Code Excited Linear Prediction (CELP) (also called Vector Excited or Stochastically Excited) [1,4,5]. The VSELP speech coder was designed to achieve the highest possible speech quality with reasonable computational complexity while providing robustness to channel errors. These goals are essential for wide acceptance of low data rate (4.8-8 kbps) speech coding for telecommunications applications.

The Dod 4.8 Kbps Standard (Proposed Federal Standard 1016)

Abstract: In 1984, the U.S. DoD launched a program to develop a third-generation secure telephone unit (STU-III) capable of providing secure voice communications to all segments of the Federal Government and its contractors. In 1988, the DoD conducted a survey of 4800 bit per second (bps) voice coders to select a standard for use in an upgrade of the STU-III to supplement its 2400 bps LPC-10e vocoder. A code excited linear predictive (CELP) coder, jointly developed by the DoD and ATT it’s even comparable to 32,000 bps continuously variable slope delta modulation (CVSD) and is robust in acoustic noise, channel errors, and tandem coding conditions.

Secure digital conferencing system

Abstract: A secure narrowband digital conferencing system is capable of handing multiple speakers simultaneously and in a full duplex mode. The system uses a multipulse or a code-excited linear predictive (CELP) speech processing algorithm for colding the speech signals of the respective participants. A conference director receives the multipluse or CELP encrypted voice signal streams over normal telephone links, decrypts them, then synthesizes a composite speech signal and uses an analysis-by-synthesis algorithm to compress them, and then encrypts the composite signal and transmits it back to all the participants.

Speech coding and decoding system

Abstract: A speech coding and decoding system, the system is operated under a known code-excited linear prediction (CELP) coding method. The CELP coding is achieved by selecting an optimum pitch vector P from an adaptive codebook and the corresponding first gain, and at the same time, selecting an optimum code vector from a stochastic codebook and the corresponding second gain. The system of the present invention is featured by a weighted orthogonalization transforming unit introduced therein. The perceptually weighted code vector AC is not used as is, as usual, but after the transformation thereof into a perceptually weighted code vector AC' by the above unit; the vector AC' being made orthogonal to the optimum perceptually weighted pitch vector AP.

On the use of pitch predictors with high temporal resolution

Abstract: The use of pitch predictors in linear predictive coding systems is an efficient way to represent periodicity in the speech signal. Typically, the predictor is described by one parameter representing the delay in samples and 1 to 3 predictor coefficients can provide interpolation for periodicities that are not a multiple of the sampling interval and allow for a frequency-dependent gain. The authors describe a first-order pitch predictor whose delay is specified as an integer number of samples plus a fraction of a sample at the current sampling frequency. This realization has a better performance than conventional multiple coefficient predictors and leads to more efficient coding of the predictor parameters. >

Constrained-Stochastic Excitation Coding of Speech at 4.8 kb/s

Abstract: In the last few years, Code-Excited Linear Predictive (CELP) coding has emerged as the most prominent technique for digital speech communication at rates of 8 Kb/s and below, and it is now considered the best candidate coder for digital mobile telephony and secure speech communication. While the CELP coder is able to provide fairly good-quality speech at 8 Kb/s, its performance at 4.8 Kb/s is yet unsatisfactory for many applications. The novelty in the CELP coding concept, namely, the stochastic excitation of a linear filter, also constitutes a weakness of this method: the excitation contains a noisy component which does not contribute to the speech synthesis process and can not be completely removed by the filter. It is a common opinion among speech communication researchers that new forms of excitations need to be studied in order to improve the CELP performance at low bit rates.

The application of the IMBE speech coder to mobile communications

Abstract: A 6.4 kb/s improved multiband excitation (IMBE) speech coder is presented. This speech coder combines high speech quality with a robustness to channel impairments which is necessary for successful operation in a mobile communication environment. MOS (mean opinion score) results for the IMBE speech coder are compared against those of four 6.4-kb/s CELP (code excited linear prediction) based speech coders which were tested as part of the INMARSAT-M voice codec evaluation. The IMBE system yielded the best performance of the systems tested. It received an MOS score of 3.4 at both 0% and 1% bit error rate. The test results show that the IMBE system is a viable alternative to CELP based speech coders. >

Pitch sharpening for perceptually improved CELP, and the sparse-delta codebook for reduced computation

Abstract: The authors introduce two techniques for improving low-bit-rate CELP (code excited linear prediction) speech coders. The sparse-delta stochastic codebook is a recursive codebook design which can be searched using roughly 5% of the computational load required to search a full Gaussian codebook. Pitch sharpening is a class of algorithms which attempt to improve the perceptual quality of CELP by limiting the feedback of low-amplitude noiselike information to the adaptive codebook. Simulation results are presented for sparse-delta, ternary sparse-delta, and simplified-search sparse-delta coders, and for reduced-gain and sparse-adaptive-codebook pitch sharpening algorithms. >

Low-delay code-excited linear-predictive coding of wideband speech at 32 kbps

Abstract: The authors report on the use of the codebook-excited linear-predictive (CELP) algorithm for 32 kb/s low-delay (LD-CELP) coding of wideband speech. The main problem associated with wideband coding, namely, spectral noise weighting, is discussed. The authors propose an enhanced noise weighting technique and demonstrate its efficiency via subjective listening tests. In these tests, involving 20 listeners and 8 test sentences, the average rating for the proposed 32 kb/s LD-CELP was essentially equal to that of the 65 kb/s standard (G.722) CCITT wideband coder. >

Embedded CELP coding for variable bit-rate between 6.4 and 9.6 kbit/s

Abstract: The authors consider the design of a variable-bit-rate CELP (code-excited linear prediction) coder which incorporates the facility of producing an embedded bit stream. This characteristic is particularly attractive for packet transmission where some packets can be lost or rejected whenever they are not received within the maximum allowed delay. The basic scheme used for the investigations is a CELP coder in which the innovation signal is split into three separate contributions. The sum of all contributions, together with the side information, determines the operating bit rate of 9.6 kb/s. The reduced bit rates of 8 and 6.4 kb/s can be achieved, dropping respectively the information relevant to one or two contributions to the innovation signal. >

A method of, and system for, coding analogue signals

Abstract: In a Code Excited Linear Prediction (CELP) analogue signal coding system sequences from a master codebook (40), which may be a one dimensional codebook, are filtered (42) and then stored in slave codebooks (70, 72). Input analogue signals (20) are filtered (34, 36) and compared orthogonally (66, 78, 80) with sequences from the slave codebooks and an optimum pair of sequences are selected. Since the comparisons are orthogonal, sequences can be selected from the codebooks (70, 72) and compared (78, 80) with the filtered incoming analogue signals entirely independently. Reduced length sequences from the master codebook may be compared with orthogonalised analogue signals since orthogonalised signals contain some redundancy. The master codebook may not need to be orthogonalised in some circumstances. Various means of orthogonalisation of the sequences are possible including separation into odd and even sequences. Further orthogonalisations are possible, for example to give four comparisons.

Low-delay code-excited linear-predictive coding of wideband speech at 32kbits/sec

Abstract: An improved digital communication system, e.g., a CELP code/decoder based system, is improved for use with a wide-band signal such as a high-quality speech signal by modifying the noise weighting filter used in such systems to include a filter section which affects primarily the spectral tilt of the weighting filter in addition to a filter component reflecting formant frequency information in the input signal. Alternatively, the weighting is modified to reflect perceptual transform techniques.

Efficient codebook search for CELP vocoders

Abstract: A new way of determining correlation coefficients for stochastic codebook vectors for CELP coding of speech takes advantage of the sparsely populated nature of stochastic codebook vectors. N valued input signals (e.g., convolution vectors) to be correlated with N valued codebook vectors are fed to an N by N multiplexer or other selection means and the signal values either passed to an accumulator or not according to the state of N select inputs or other identification means determined from a memory store (e.g., an EPROM) whose entries correspond to the non-zero values of the codebook vectors. The accumulator output is the correlation of the codebook vector with the input signal. A sequencer steps through the entire codebook to provide correlation values for each vectors. The results are used to determine the optimum stochastic codebook vector for replicating the particular speech frame being analyzed.

CELP vocoder with efficient adaptive codebook search

Abstract: A new method for Code Excited Linear Predictive (CELP) coding of speech reduces the computational complexity by removing a convolution operation from a recursive loop used to poll the adaptive code book vectors. In a preferred embodiment, an impulse function of a short term perceptually weighted filter is first convolved with perceptual weighted target speech and the result cross-correlated with each vector in the codebook to produce an error function. The vector having the minimum error function is chosen to represent the particular speech frame being examined.

Wideband CELP speech coding at 16 kbits/sec

Abstract: The authors investigate the use of CELP (code excited linear prediction) in coding wideband speech signals at an operating rate of 16 kb/s. The wideband signals under consideration are bandlimited to 7500 Hz and sampled at 16 kHz. In order to achieve a low operating rate, the coding places more emphasis on the lower frequencies (0 to 4 kHz), while the higher frequencies are coded less precisely, but with little perceived degradation. To this effect, the basic CELP model is modified to operate in a split-band mode. For the operating rate of 16 kb/s, subjective tests showed that the wideband speech coder was preferred to a high-quality narrowband coder. This wideband speech coder offers an attractive alternative to conventional narrowband codes at rates near 16 kb/s for many applications. >

Binary Pulse Excitation: A Novel Approach to Low Complexity CELP Coding

Abstract: The last decade has witnessed an acceleration in the evolution of speech coding. Near-toll quality speech coders are now available at bit rates from 4.8 to 8 kbit/s. These low bit rate speech coders are becoming increasingly needed for many future applications such as digital mobile radio telephony, mobile satellite links, and the emerging ISDN service. High quality speech at these low bit rates has become possible with the introduction of a new generation of speech coding techniques known as analysis-by-synthesis predictive coding. The structure of this new generation of speech coders first appeared with the introduction of multi-pulse excited linear prediction coder (MPE-LPC) by Atal in 1982 [1]. Since its introduction, the MPE has received much attention from researchers, and as a result, several analysis-by-synthesis coders have been developed in the bit rate range from 4.8 to 16 kbit/s with different levels of complexity [2]. These systems include the regular-pulse excited LPC (RPE-LPC) [3], the code-excited LPC (CELP) [4], and the self-excited LPC [5] (or Backward Excitation Recovery (BER) [6]). All the above mentioned coders exhibit the same structure as the originally proposed MPE-LPC [1] in which the excitation signal is optimized by minimizing the perceptually weighted error between the original and synthesized speech. They differ only in the way the excitation signal is defined and coded.

High quality multiband LPC coding of speech at 2.4 kbit/s

Abstract: Until recently good quality digital speech transmission was not possible below about 9.6 kbit/s. With the development of code excited linear prediction (CELP) and multiband excitation (MBE) vocoders,2 high quality digital speech transmission became possible at bit rates as low as 4.8 kbit/s. Below 4.8 kbit/s however, vocoders such as LPC-103 and the channel vocoder4 which can only produce synthetic and unnatural speech are still in use. A new coding scheme termed the multiband LPC (MB-LPC) vocoder is proposed, which produces good and natural quality speech at 2.4 kbit/s. Subjective performance of speech at 2.4kbit/s produced by the CELP is very close to MBE coded speech at 4.8 kbit/s. Informal listening tests have shown that in most cases people could not tell the difference between the new 2.4kbit/s MB-LPC coder and the 4.8 kbit/s MBE vocoder.

Backward Adaptive Configurations for Low-Delay Vector Excitation Coding

Abstract: Many of the advances in speech coding in the past decade at rates of 48–16 kbit/s have been based on excitation coding by means of analysis-by-synthesis Excitation coding schemes have a decoder structure consisting of an excitation signal applied to a time-varying synthesis filter to produce the reconstructed, or “synthesized,” output speech In addition to other tasks, the encoder must determine a suitable excitation signal and transmit data that specifies this excitation In the analysis-by-synthesis technique, the excitation is selected by a closed-loop search procedure where a candidate excitation signal segment is applied to the synthesis filter, the synthesized waveform is compared with the original speech segment, the distortion is measured, and the process is repeated for all excitation segments stored in an excitation codebook The index of the “best” excitation segment is transmitted to the decoder, which retrieves the excitation segment from a codebook identical to that at the encoder The parameters of the synthesis filter are computed using well-known linear prediction analysis techniques on a frame of buffered input samples and transmitted to the decoder This coding scheme is often called Vector Excitation Coding (VXC) or Code Excited Linear Prediction (CELP) [1,2]

A fixed-point 16 kb/s LD-CELP algorithm

Abstract: The authors describe the algorithm modifications they have made in order to make the LD-CELP (low-delay code-excited linear prediction) algorithm suitable for 16 bit fixed-point implementation. They replaced T.P Barnwell's recursive windowing method (1981) by a novel hybrid which is partially recursive and partially nonrecursive. This method avoided the dynamic range problem and the double-precision arithmetic that would otherwise have been required. A fourth-order LPC filter was cascaded at the output of the original 50th-order LPC filter. This filter effectively reduced the spectral dynamic range of the output of the 50th-order LPC filter, therefore alleviating the ill-conditioning problem of the 50th-order LPC analysis. Although the algorithm of J. LeRoux and C. Gueguen (1979) was believed to be better suited for fixed-point arithmetic than Durbin's recursion (see L.R. Rabiner and R.W. Schafer, 1978), this was not necessarily true for the two-stage cascaded LPC filter used. Fixed-point simulation showed that the speech quality of the modified fixed-point LD-CELP algorithm was essentially the same as that of the original floating-point LD-CELP algorithm. >

Efficient Encoding of the Long-Term Predictor in Vector Excitation Coders

Abstract: Speech coding algorithms based on Code-Excited Linear Prediction (CELP) [1], have been widely studied in the past few years for low bit rate speech coding and many improvements and variations of the basic algorithm have since emerged. We use Vector Excitation Coding (VXC) as a generic name to represent the class of coders using vector quantization (VQ) coding of filtered excitation signals with “closed-loop” codebook search [2, 3].

Speech coding system

Abstract: A speech coding system operated under a known code-excited linear prediction (CELP) coding method. The CELP coding is achieved by selecting an optimum pitch vector P from an adaptive codebook and the corresponding first gain and, at the same time, selecting an optimum code vector from a sparse-stochastic codebook and the corresponding second gain. The system of the present invention is featured by special code vectors to be loaded in the sparse-stochastic codebook, which code vectors are hexagonal lattice code vectors each consisting of a zero vector with one sample set to +1 and another sample set to -1.

Techniques for improving the performance of CELP type speech coders

Abstract: Techniques for improving the performance of CELP (code excited linear prediction) type speech coders while maintaining reasonable computational complexity are explored. A harmonic noise weighting function which enhances the perceptual quality of the processed speech is introduced. The combination of harmonic noise weighting and subsample resolution pitch significantly improves the coder performance for voiced speech. A 6.9 kb/s VSELP speech coder which incorporates subsample resolution pitch and harmonic noise weighting is described. Complexity reduction techniques are discussed which allow the coder to be implemented using a single fixed point digital signal processor. >

Efficient calculation of autocorrelation coefficients for CELP vocoder adaptive codebook

Abstract: A new way of determining autocorrelation coefficients for adaptive codebook vectors for CELP coding of speech simplifies and improves the accuracy of the autocorrelation coefficient determination for the situation where the codebook vector length being analyzed is less than a speech frame length. This is important in synthesizing short pitch period speech. Copy-up of the shortened codebook vector to equal the frame length is not needed and autocorrelation coefficient errors associated with copy-up are avoided. The improved system relies on calculating autocorrelation coefficients of the first (shortest) vector and then obtaining subsequent autocorrelation coefficients for successive vectors of increasing length by a simple end correction technique until the vector length equals the frame length. The autocorrelation coefficients are scaled by multiplying them by the ratio of the frame length to the vector length.

Methods of improving vector-scalar quantization of LPC coefficients

Abstract: Methods of improving vector-scalar quantization of linear predictive coding (LPC) coefficients with 20 to 30 bits per 20 ms speech frame are studied. The first approach used is to couple the vector and scalar quantization stages. The second innovation is the incorporation of a small adaptive codebook to the larger fixed codebook. Frame-to-frame correlation of the LPC coefficients is exploited at no extra cost in bits. It is shown that the performance of vector-scalar quantization of speech with the use of the two new techniques is better than that of scalar coding techniques currently used in LPC coders. >

Phonetic Segmentation for Low Rate Speech Coding

Abstract: Efforts to bridge the gap between waveform coders and vocoders has led to a new class of hybrid speech coders. These coders perform analysis-by-synthesis encoding of an excitation signal and reconstruct speech from the coded excitation signal and a quantized time-varying filter model of speech production. Most notable of these coders are those which use vector quantization to code the excitation signal as a sequence of vectors. The coding technique is called Code Excited Linear Prediction (CELP) [1], or Vector Excitation Coding (VXC) [2]. VXC coders result in coded speech with a waveform approximating the original and are able to achieve a satisfactory, natural-sounding quality at bit rates as low as 4.8 kb/s. When the bitrate is reduced below 4.8 kb/s, the quality of VXC coders degrades rapidly and becomes inferior to the synthetic quality of an LPC vocoder operating at 2.4 kb/s. There remains then the challenging problem to find an algorithm that at 2.4 kb/s (or even at 3.6 kb/s) will achieve the quality that VXC offers at 4.8 kb/s