High quality audio coding

Lin, Xiao (1993) High quality audio coding. University of Southampton, Doctoral Thesis.

Abstract

Analysis-by-synthesis linear prediction coding (AS-LPC) of wideband (15kHz) audio signals is examined, and the resulting novel codecs are used for digital audio broadcasting (DAB) of high fidelity (Hi-Fi) audio signals over mobile radio channels. In recent years, AS-LPC has been successfully used in narrow band speech coding and is able to produce toll quality speech with fewer than 2 bits/sample. However, when AS-LPC is used for encoding audio signals about 3 bits/sample are needed to maintain Hi-Fi quality. A cardinal aim of this study is to decrease the encoded bit rate, maintain low equipment complexity, while retaining Hi-Fi quality.

A fixed-pulse excitation (FPE) LPC codec is introduced which is capable of encoding audio signal with less than 3 bits/sample while providing Hi-Fi audio. This codec has the same audio quality as a multi-pulse excitation (MPE) codec, but with much less complexity. By introducing a novel multi-mode excitation sequence for the LPC filter the resulting codec, called multi-mode FPE (MFP), Hi-Fi quality is achieved at a rate of 2.8 bits/sample with less complexity compared to MPE. Additional exploitation of the human hearing mechanism yields a subband technique combined with multi-mode MPE (SB-MMPE). An adaptive bit allocation strategy is conceived to the SB-MMPE according to the signal power and a perceptual hearing model. This codec has a compression gain of 0.5 bits/sample over MFP, while retaining its low complexity.

A DAB scheme is proposed that provides high fidelity audio quality transmissions over mobile radio channels using our SB-MMPE codec. In addition, an embedded Bose-Chaudhuri-Hocquenghem (BCH) channel codec and a star quadrature amplitude modulator (StQAM) is employed. (DX178,589)

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