Code-excited linear prediction

About: Code-excited linear prediction is a research topic. Over the lifetime, 2025 publications have been published within this topic receiving 28633 citations. The topic is also known as: CELP.

C.E.L.P. Vocoder

Abstract: A high quality low bit rate audio codec having a reproduced voice quality that is comparable to that of a full rate codec compresses audio data sampled at 8 Khz, e.g., 64 Kbps PCM, to 4.2 Kbps or decompresses it back to the original audio or both. The accompanying degradation in voice quality is comparable to the standard 8.0 Kbps voice codes. This is accomplished by using the same parametric model used in traditional CELP coders but determining, quantizing, encoding, and updating these parameters differently. The low bit rate audio decoder is like most CELP decoders except that it operates in two modes depending on the received mode bit. Both pitch prefiltering and global postfiltering are employed for enhancement of the synthesized audio. In addition, built-in error detection and error recovery schemes are used that help mitigate the effects of any uncorrectable transmission errors.

AMR Steganalysis Based on Second-Order Difference of Pitch Delay

Abstract: This paper presents a novel steganalysis scheme for the detection of adaptive multi-rate (AMR) audio steganography. AMR audio codec is used widely in mobile communication and mobile Internet system. Due to the modifiability of pitch delay, several AMR steganography schemes based on pitch delay modulation have emerged gradually and have high capacity and good imperceptibility. Based on the difference between cover and stego AMR audios on the continuity of adjacent pitch delay, this paper proposes the matrix of the second-order difference of pitch delay (MSDPD) steganalysis features by calculating the Markov transition probability MSDPD, and uses the calibration method to estimate the cover’s features to get the calibrated MSDPD features to improve the accuracy of the scheme. Support vector machine is used as the steganalyzer to test the performance of our proposed scheme. The experimental results show that the correct detection rate of our proposed method is more than 85% when the embedding bit rate is 30% or above, and can reach above 85% for cover audios. The results of contrast experiment show that the performance of our proposed method is better than the existing method, especially on low embedding rate. The method can be extended to other CELP codec, such as G.723.1 and G.729.

Variable rate speech and channel coding for mobile communication

Abstract: Although the mobile communication channels are time-varying, most systems allocate the combined rate between the speech coder and error correction coder according to a nominal channel condition. This generally leads to a pessimistic design and consequently an inefficient utilization of the available resources, such as bandwidth and power. This paper describes an adaptive coding system that adjusts the rate allocation according to actual channel conditions. Two types of variable rate speech coders are considered : the embedded coders and the multimode coders and both are based on code excited linear prediction (CELP). On the other hand, the variable rate channel coders are based on the rate compatible punctured convolutional codes (RCPC). A channel estimator is used at the receiver to track both the short term and the long term fading condition in the channel. The estimated channel state information is then used to vary the rate allocation between the speech and the channel coder, on a frame by frame basis. This is achieved by sending an appropriate rate adjustment command through a feedback channel. Experimental results show that the objective and the subjective speech quality of the adaptive coders are superior than their non-adaptive counterparts. Improvements of up to 1.35 dB in SEGSNR of the speech signal and up to 0.9 in informal MOS for a combined rate of 12.8 kbit/s have been found. In addition, we found that the multimode coders perform better than their embedded counterparts. >

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.