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.

Variable rate speech coding based on subband measures of spectral flatness

Abstract: The development of digital multiple-access schemes for telephony and personal communications networks, in addition to the rapid standardization of digital audio-video delivery techniques, has placed a renewed emphasis on efficient compression of speech. Code-excited linear prediction (CELP) is the predominant coding methodology for medium-rate speech coding due to the adoption of several international and U.S. fixed-rate standards. With the standardization of a CELP-based variable-rate coder for digital spread-spectrum telephony, many variable-rate techniques which had not been previously studied have become an area of significant research interest. Variable-rate coding of speech is composed of two distinct components: voice-activity detection (VAD) and coding of active speech. In this research, we focus on new techniques to perform VAD and to efficiently encode time-varying parameters of active speech. These techniques are based on subband measures of spectral shape. This approach produces a departure from the usual methods used for rate allocation which are based on (subband) energy. Here, we use the concept of spectral flatness to modulate the allocation of rate and to determine coding procedures. We also make use of some highly speech-specific redundancy (previously untested in standard approaches) to efficiently encode spectral parameters. We test these techniques in the context of a multi-mode, variable-rate CELP coder and demonstrate that with appropriate assumptions regarding voice-activity, our CELP coder can maintain communications quality in the encoded speech at an average rate below 2000 bits/sec.

Predictive coding of speech using microphone/speaker adaptation and vector quantization

Abstract: A problem with speech coders based on trained quantizers is their lack of robustness against variations in the microphone and input filter response. In this paper a simple backward adaptive prefiltering technique is proposed as a means of improving the robustness and quality of a speech coder at no cost in bit rate. The technique is particularly useful in conjunction with vector quantization (VQ) of the linear predictive coding (LPC) parameters. The performance of the prefilter, denoted a microphone and speaker adaptation (MSA) filter, has been evaluated in terms of prediction gain and spectral distortion, together with objective and subjective quality of a 7.5 kbit/s CELP speech coder. In this coder a 10-bit direct VQ of the LPC parameters using the residual energy distortion measure has been applied. This is consistent with the covariance method of LPC analysis. Simulation results illustrate that the MSA filter significantly improves the performance and robustness of the LPC VQ against changes in the input response. The 7.5 kbit/s CELP with a trained excitation codebook and MSA was found to be clearly better (subjectively and objectively) than the one without MSA. The coder with MSA also showed to be practically indistinguishable from the same CELP with unquantized LPC coefficients and a stochastic excitation codebook. >

Multiple descriptions coding in MELP coder for voice over IP

Abstract: In VoIP systems, CELP coders, such as G.729, are commonly used as they offer good speech quality in the absence of packet loss. However, harmonic coders such as MELP may be a good alternative for VoIP due to their higher resilience to packet loss. In this paper we examine the problem of the packet loss in the VoIP application using MELP coders. The proposed packetization scheme based on Multiple Description Coding (MDC) applied to the MELP coder is presented. A packet will contain information on two MELP coders operating at 2.4 and 1.2 Kbps respectively. The packetization is achieved using 135 bits in 22.5 ms corresponding to a total rate of 6 kbps. The results show that under typical VOIP operating conditions, the method performs well and outperforms CELP coders operating without MDC at 8 kbps.

A Variable Low Bit Rate Speech Coder Based on Melp for Wireless Communications

Abstract: This paper proposes a novel variable low bit rate speech coding algorithm based on mixed excitation linear predictive (MELP) coder. This coder use a superframe structure and joint quantization methods to reduce the inter-frame redundancy of the parameters. In each superframe, the similarity of the linear prediction filter denoted by LSF parameters is imposed to reduce the bit rate by variable rate speech coding. A new distance measurement for LSF is proposed to describe the difference between two sets of LSF parameter. And the coding rate is decided by the distance measures of different LSF parameters in every superframe. Objective test results of synthesized speech quality show that the proposed speech coder is capable of maintaining acceptable speech quality at the rates 560 bps and 800 bps.