Abstract
A harmonic coder extracts the harmonic components of a signal and represents them efficiently using a few parameters. The principles of harmonic coding have become quite successful and several standardized speech and audio coders are based on it. One of the key issues in harmonic coder design is in the quantization of harmonic magnitudes, where many propositions have appeared in the literature. The objective of this paper is to provide a survey of the various techniques that have appeared in the literature for vector quantization of harmonic magnitudes, with emphasis on those adopted by the major speech coding standards; these include constant magnitude approximation, partial quantization, dimension conversion, and variable-dimension vector quantization (VDVQ). In addition, a refined VDVQ technique is proposed where experimental data are provided to demonstrate its effectiveness.
Highlights
A signal is said to be harmonic if it is generated by a series of sine waves or harmonic components where the frequency of each component is an integer multiple of some fundamental frequency
This section summarizes the experimental results in regard to variable-dimension vector quantization (VDVQ) as applied to harmonic magnitudes quantization
The technique of VDVQ is studied, and an enhanced version is proposed where interpolation is performed among the elements of the codebook
Summary
A signal is said to be harmonic if it is generated by a series of sine waves or harmonic components where the frequency of each component is an integer multiple of some fundamental frequency. Many signals in nature—including certain classes of speech and music—obey the harmonic model and can be specified by three sets of parameters: fundamental frequency, magnitude of each harmonic component, and phase of each harmonic component. One of the fundamental issues in the incorporation of harmonic modeling in coding applications lies in the quantization of the magnitudes of the harmonic components, or harmonic magnitudes; many techniques have been developed for this purpose and are the subjects of this paper. The term harmonic coding was probably first introduced by Almeida and Tribolet [1], where a speech coder operating at a bit rate of 4.8 kbps is described. For the purpose of this paper we define a harmonic coder as any coding scheme that explicitly transmits the fundamental frequency and harmonic magnitudes as part of the encoded bit stream. We use the term harmonic analysis to signify the procedure in which the fundamental frequency and harmonic magnitudes are extracted from a given signal
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