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.

Predictive coding in a homomorphic vocoder

Abstract: Application of a type of predictive coding to the channel signals of a homomorphic vocoder has produced sizable bit rate reductions. With only slight degradation in speech quality, reduction (for the spectral envelope information) from 7800 to 4000 bits/s was achieved. A technique for obtaining the formant frequencies from the predictive coding parameters is described; this approach promises further bit rate reductions. As a by-product of this study of predictive coding, direct and cascade form speech synthesizers are compared on the basis of differing quantization effects.

Excitation signal bandwidth extension

Abstract: An apparatus for generating a high band extension of a low band excitation signal (eLB) defined by parameters representing a CELP encoded audio signal includes the following elements: upsamplers (20) configured to upsample a low band fixed codebook vector (uFCB) and a low band adaptive codebook vector (uACB) to a predetermined sampling frequency. A frequency shift estimator (22) configured to determine a modulation frequency (Ω) from an estimated measure representing a fundamental frequency (F0) of the audio signal. A modulator (24) configured to modulate the upsampled low band adaptive codebook vector (uACB↑) with the determined modulation frequency to form a frequency shifted adaptive codebook vector. A compression factor estimator (28) configured to estimate a compression factor. A compressor (34) configured to attenuate the frequency shifted adaptive codebook vector and the upsampled fixed codebook vector (uFCB↑.) based on the estimated compression factor. A combiner (40) configured to form a high-pass filtered sum of the attenuated frequency shifted adaptive codebook vector and the attenuated up-sampled fixed codebook vector.

Wideband re-synthesis of narrowband CELP-coded speech using multiband excitation model

Abstract: In this paper, a method for improving the quality of narrowband CELP-coded speech is present. The approach is to reduce the hoarse voice in CELP-coded speech by enhancing the pitch periodicity in the reproduction signal and also to reduce the muffing characteristics of narrowband speech by regenerating the highband components of speech spectra from the reproduction signal. In the proposed method, multiband excitation (MBE) analysis is performed on the reproduction speech signal from a CELP decoder and the pitch periodicity is enhanced by resynthesizing the speech signal using a harmonic synthesizer according to the MBE model. The highband magnitude spectra are regenerated by matching to lowband spectra using a trained wideband spectral codebook. Information about the voiced/unvoiced (V/UV) excitation in the highband are derived from a training procedure and then stored alongside with the wideband spectral codebook so that they can be recovered by indexing to the codebook using the matched lowband index. Simulation results indicate that the quality of the wideband resynthesized speech is significantly improved over the narrowband CELP-coded speech.

Pitch adaptive windows for improved excitation coding in low-rate CELP coders

Abstract: A novel paradigm based on pitch-adaptive windows is proposed for solving the problem of encoding the fixed codebook excitation in low bit-rate CELP coders. In this method, the nonzero excitation in the fixed codebook is substantially localized to a set of time intervals called windows. The positions of the windows are adaptive to the pitch peaks in the linear prediction residual signal. Thus, high coding efficiency is achieved by allocating most of the available FCB bits to the perceptually important segments of the excitation signal. The pitch-adaptive method is adopted in the design of a novel multimode variable-rate speech coder applicable to CDMA-based cellular telephony. Results demonstrate that the adaptive windows method yields excellent voice quality and intelligibility at average bit-rates in the range of 2.5-4.0 kbps.