Low-delay Code-excited Linear Prediction Research Articles

Low Delay Sparse and Mixed Excitation CELP Coders for Wideband Speech Coding

Code Excited Linear Prediction (CELP) algorithms are proposed for compression of speech in 8 kHz band at switched or variable bit rate and algorithmic delay not exceeding 2 msec. Two structures of Low-Delay CELP coders are analyzed: Low-delay sparse excitation and mixed excitation CELP. Sparse excitation is based on MP-MLQ and multilayer models. Mixed excitation CELP algorithm stems from the narrowband G.728 standard. As opposed to G.728 LD-CELP coder, mixed excitation codebook consists of pseudorandom vectors and sequences obtained with Long-Term Prediction (LTP). Variable rate coding consists in maximizing vector dimension while keeping the required speech quality. Good speech quality (MOS=3.9 according to PESQ algorithm) is obtained at average bit rate 33.5 kbit/sec.

Substitution of G.728 vocoder’s codebook search module with SOM array trained by PSO-optimized supervised algorithm

Low delay-code excited linear prediction (LD-CELP) is an attractive algorithm in implementing vocoders in voice over Internet protocol networks. This algorithm has been proposed for the coding of speech at 16 kbps with toll quality. However, operation at transmission rates lower than 16 kbps is desirable, so that traffic can be accommodated during system overload conditions. In this paper, an array of self-organizing maps (SOMs) is employed instead of traditional codebook search module, recommended in ITU-T G.728, to determine the optimum index value of shape codebook. It is noted that a modified supervised training algorithm is used for SOMs in which some of the training parameters are optimized using particle swarm optimization (PSO) algorithm. Based on the occurrence frequency characteristics of codevectors, six bits for shape codebook and two bits for gain codebook are used in this work to produce a vocoder with lower bit rate as compared with traditional ITU-T G.728 vocoder. The performance comparison of the proposed SOM array trained by PSO-optimized supervised algorithm as the codebook search module in the structure of LD-CELP with a conventional implementation of LD-CELP coder shows that execution time of the algorithm is reduced up to 44 %. However, the degradation of voice quality in terms of mean opinion score, perceived evaluation of speech quality and segmental signal-to-noise ratio (SNRseg) is acceptable.

REAL-TIME SPEECH COMPRESSION BY USING CODE EXCITED LINEAR PREDICTION ALGORITHM

A lot of effort has been spent over the last few years in the development of digital speech coding methods and their subsequent standardization. Algorithms have evolved which provide good quality speech at sub 8 kbps bit rates although at a much computational expense. Speech compression is proposed based on code excited linear prediction algorithm and implementation in DSP algorithm. Algorithm based on three-stage technique which involves simulate, evaluate, debug and implementation in G.723 low delay code excited linear prediction (LD-CELP)[4] algorithm. First stage, the algorithm is evaluated via simulation to determine whether it meets the design criterion. Then, it is implemented in real-time based an object oriented approach. After the algorithm is thoroughly tested, it is further refined to obtain tighter and faster coding. This technique can be applied to other real-time DSP algorithms. A simulation result shows that better speech quality is obtained. The techniques described in this paper are applicable to any other speech codec.

Impact of network routing on the end-to-end transmission quality of cellular and PCS connections

Long-distance and especially international calls involve an increasing number of multilink circuits of cellular, personal communications, mobile satellite, and public switched telephone network (PSTN)-type connections incorporating a variety of speech coding devices. In particular, the rapid growth of cellular communications and the impeding emergence of personal communication systems (PCSs) has highlighted the need to characterize the quality of end-to-end connections when cellular and PCS terminals are connected over the PSTN. At the same time, the nonlinear nature of low-rate parametric speech coding has rendered unsuitable known analytical methods for estimating end-to-end quality of interconnected networks. Instead, quantification of the transmission performance requires direct subjective evaluation of the pertinent connections of interest. In this paper, the quality of North American, Japanese, and European digital cellular and PCS terminals is quantified when these are interconnected using 16-kb/s low-delay code-excited linear prediction (LD-CELP) and 32-kb/s adaptive differential pulse code modulation (ADPCM) facilities. From these assessments, it can be concluded that digital cellular networks may meet end-to-end transmission planning criteria when interconnected with the switched network as long as calls are terminated to a wired terminal.

Voice coding communication system and apparatus therefor

A low-delay code excited linear prediction (LD-CELP) voice coding systemapplied to a scheme, in which the transmitting side interrupts transmission during the voice-nonactive period and the receiving side generates and outputs a comfort noise during the voice-nonactive period. At the transmitting side, when voice non activity is detected by the voice activity detector, the CN flag indicating the interruption of transmission is sent from the CN flag generator and shortly thereafter a background noise from the LD-CELP encoder is sent, followed by the interruption of transmission. At the receiving side, when the background noise following the CN flag is decoded by the LD-CELP decoder, the internal gain and synthesis filter coefficients are held, which is subsequently used to decode the input from the pseudo-noise generator.

A non bit-exact approach for implementation verification of the CCITT LD-CELP speech coder

This paper presents the design methodology behind the floating point verification procedure for the Low-Delay Code-Excited Linear Prediction speech coder, recently selected by the CCITT as Recommendation G.728. This procedure is based on a non bit-exact specification, which is different from previous CCITT speech coder verification procedures. This approach gives additional freedom for the implementor of the algorithm, and will allow for more efficient implementations on various kinds of hardware. However, this flexibility also means that different implementations will respond slightly different to verification test signals. To cope with this, explicit objective measurements of such deviations are used in the verification process. These measurements are simple weighted and unweighted signal-to-noise ratios (WSNR and SNR). In addition to the objective measurements, certain restrictions had to be placed on the test sequence design. In spite of the input restrictions, a set of test sequences giving reasonably good coverage of the LD-CELP algorithm and state space has been found. Evaluation experiments are reported, showing that these sequences have a satisfactory error detecting capability. A final discussion concludes that the chosen verification approach is indeed feasible as an implementor s tool.

A low-delay CELP coder for the CCITT 16 kb/s speech coding standard

A low-delay code-excited linear prediction (LD-CELP) speech coder which is expected to be standardized in 1992 as a CCITT G Series Recommendation for universal applications of speech coding at 16 kb/s is presented. The coder achieves a one-way coding delay of less than 2 ms by making both the LPC predictor and the excitation gain backward-adaptive and by using a small excitation vector size of five samples. The official CCITT laboratory tests revealed that the speech quality of this 16 kb/s LD-CELP coder is either equivalent to or better than that of the CCITT G.721 standard 32-kb/s ADPCM coder for almost all conditions tested. A description of the LD-CELP algorithm, its implementation on the DSP32C for CCITT testing, and performance results from these tests are presented. >