• Journal of the Korea Society of Computer and Information
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• v.10 no.6 s.38
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• pp.233-238
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• 2005

In general CELP(Code Excited Linear Prediction) type vocoders provide good speech qualify around 4.8kbps. Among them, G.723.1 developed for Internet Phone and video-conferencing includes two vocoders, 5.3kbps ACELP(Algebraic-CELP) and 6.3kbps MP-MLQ(Multi-Pulse Maximum Likelihood Quantization) In order to improve the speech qualify in CELP vocoder, in this paper. we proposed a new spectrum analysis algorithm with variable window In CELP vocoder, the spectrum of the synthesised speech signal is distorted because the fixed size windows is used for spectrum analysis. So we have measured the spectral leakage and in order to minimize the spectral leakage have adjusted the window size. Applying this method G.723.1 ACELP, we can got SD(Spectral Distortion) reduction 0.084(dB), residual energy reduction 6.3$\%$ and MOS(Mean Opinion Score) improvement 0.1.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.28 no.3C
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• pp.239-247
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• 2003

This paper presents a dual-rate (2.4/4.0 kbps) Improved Harmonic-CELP(IHC) speech coder based on the EHC(Efficient Harmonic-CELP) which was presented by the authors. The proposed IHC employs the harmonic coding for voiced and the CELP for unvoiced segments. In the IHC, an initial voiced/unvoiced estimate is obtained by the pitch gain and energy. Then, the final V/UV mode is decided by using the frame energy contour. A new harmonic estimation combining peak picking and delta adjustment provides a more reliable harmonic estimation than that in the EHC. In addition, a noise mixing scheme in conjunction with an improved band voicing measurement provides the naturalness of the synthesized speech. To demonstrate the performance of the proposed IHC coder, the coder has been implemented and compared with the 2.0/4.0 kbps HVXC(Harmonic excitation Vector Coding) standardized by MPEG-4. Results of subjective evaluation showed that the proposed IHC coder and produce better speech quality than the HVXC, with only 40% complexity of the HVXC.

• The Journal of the Acoustical Society of Korea
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• v.13 no.3
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• pp.33-40
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• 1994

Code Excited Linear Prediction(CELP) speech coders exhibit good performance at data rates below 4.8 kbps. This major drawback of CELP type coders is required much computation. In this paper, we propose a new pitch search method that preserves the quality of the CELP vocoder with reducing complexity. In the pitch searching, we detect the segments of high correlation by a simple preprocessing, and then carry out the pitch searching only for the segments obtained by the preprocessing. By using the proposed method, we can get approximately $77\%$ complexity reduction in the pitch search.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.5 no.1
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• pp.23-33
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• 2001

In this paper we propose a multi-codebook structure which can synthesize high quality speech without increasing of CELP coder's computation. We also design a 4.8kbps CELP speech coder with the proposed codebook structure. The proposed multi-codebook structure is made up of basic codebook and the other codebook which Is formed for strengthen spectrum an4 pitch. Multi-codebook structure can represent accurate gains since it represents excitation signals as summation of two kinds of codebooks and uses different codebook gains respectively. Therefore it can provide better speech quality than other conventional structures. In computer simulation of the 4.8kpbs CELP coder designed with the proposed codebook structure its segSNR was 0.81dB more high than the DoD CELP coder of same transmission rates.

• Journal of the Korean Institute of Telematics and Electronics T
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• v.35T no.1
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• pp.120-124
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• 1998

Tn the analysis of the audio signal characteristices and the codebook indices of LD-CELP, there are many cases of detecting codebook indices that are used previous. LD-CELP algorithm achives good quality of audio because it has analyzed a short term of audio signal. In spite of these advantage, the method has a drawback in which searching time of best codebook index inclose due to a complicated calculation for codebook index. This paper is proposed to decreasing the searching time of codebook index using a searching memory. As a simulation of the proposed method, searching time for codebook index is reduced 3.2%-11.7% as compared with LD-CELP.

• Proceedings of the Acoustical Society of Korea Conference
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• 1994.06c
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• pp.199-204
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• 1994

In this paper, we researched abut CELP speech coding algorithm using efficlent pseudo-stochastic block codes, adaptive-codebook and improved fixed-gain codebook. The pseudo-stochastic block codes refer to stochastically populated block codes in which the adjacent codewords in an innovation codebook are non-independent. The adaptive-codebook was made with previous prediction speech data by storage-shift register. This CELP coding algorithm enables the coding of toll quality speech at bit rates from 4.8kbits/s to 9.6 kbits/s. This algorithm was realized TMS320C30 microprocessor in realtime.

• Proceedings of the Acoustical Society of Korea Conference
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• 1998.08a
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• pp.136-141
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• 1998

In order to enhance the performance of a CELP coder at low bit rates, it would be necessary to make the CELP excitation have the peaky pulse characteristic. In this paper we introduce an excitation signal with peaky pulse characteristic. It is obtained by using a two-tap pitch predictor. Samples of the signal have different gains according to their amplitudes by the predictor. In voiced sound the signal has the desirable peaky pulse characteristic, and its periodicity is well reproduced. Particularly, peaky pulses at voiced onset and a burst of plosive sound are clearly reconstructed.

• Proceedings of the Acoustical Society of Korea Conference
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• 1993.06a
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• pp.280-287
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• 1993

Code Excited Linear Prediction(CELP) speech coders exhibit good performance at data rates as low as 4800bps. The major drawback to CELP type paper, a comparative performance study of three pitch searching algorithms for the CELP vocoder was conducted. For each of the algorithms, a standard pitch searching algorithm was used by the sequential pitch searching algorithm that was implimented in the QCELP vocoder. The algorithms used in this study were 1) using the skip table(TABLE), 2) using the symmetrical property of the autocorrelation(SYMMT), and 3) using the preprocessing autocorrelation(PREPC). Performance scores are presented for each of the three pitch searching algorithms based on computation speed and on pitch prediction error.

• Proceedings of the Acoustical Society of Korea Conference
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• 1994.06c
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• pp.179-182
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• 1994

본 논문에서는 4.8kbps 이하의 낮은 전송률에서 Code-Ecited Linear Prediction 방법에 MBE 방법을 도입하여 장구간 예측 성능을 향상시키는 방법을 제안하였다. 제안된 방법에서는 기존의 CELP 방법으로 장구간 예측을 한 후에도 여전히 남아 있는 주기적 성분들을 다시 한번 다대역 장구간 예측을 한다. 이 때 전체 주파수 영역을 기본 주파수의 하모닉 간격으로 주파수 분할하였고, 주기적 다대역 여기 신호를 각 대역 내의 하모닉들에서 여기 신호 스펙트럼과 근사한 크기를 갖는 정현파의 합으로 표현함으로써 실제 여기 신호의 특성을 잘 반영하도록 하였다. 음질 평가의 비교 기준 대상으로 4.8 kbps DoD CELP를 선정하여 주관적 음질평가를 실시한 결과 4.8 kbps DoD CELP보다 우수한 성능을 보임을 알 수 있었다.

• Proceedings of the IEEK Conference
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• 2000.09a
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• pp.561-564
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• 2000

MPEG-4 CELP의 알고리즘은 전송환경에 따라 선택할 수 있는 다양한 전송률을 지원하며 각종 옵션을 추가함으로써 다양한 기능을 부가할 수 있도록 구성되어 있다. 본 논문에서는 MPEG-4 오디오 CELP 부호화기를 VHDL(Very High Speed Intergrated Circuit Hardware Description Language)을 이용하여 구현하였다. MPEG-4 CELP의 부호화 과정을 크게 3부분으로 나누어 LPC 분석부, 코드북 검색부, 비트열 생성부로 나누어 설계하였으며, 내부 메모리의 크기를 줄이기 위하여 중복적인 성격을 갖거나 계산과정에서 나오는 중간 결과 값들을 저장하지 않도록 알고리즘을 최적화하였다. 또 계산 값의 동적 범위(Dynamic Range)가 크거나 정밀도가 요구되는 부분에 대해서는 배밀도(Double Precision) 연산을 사용하여 값의 오차를 줄였다.