Accuracy of Determination of Linear Spectral Frequencies

Abstract

The article describes the basic idea of coding a speech signal by the method of linear prediction (Linear Predictive Coding – LPC), which consists in the fact that instead of the parameters of the speech signal, the encoded parameters of a certain filter are transmitted over the communication line, which is, in a sense, an equivalent of the human vocal tract, as well as parameters of the excitation signal of this filter: tone or noise. The essence of the parameters of the synthesizing filter – linear prediction coefficients (LP), calculated in the process of frame-by-frame adaptive filtering – is disclosed. The main advantages of these coefficients are stated, which consists in the ability to completely describe the state of the predictor filter, as well as the main disadvantages obtained as a result of numerous studies that prevent the direct transmission of LPCs over the communication channel due to their sensitivity to quantization errors. The necessity of searching for mathematically equivalent parameters f the reducing filter is substantiated. Alternative parameters of the LSP representation of the vocal tract model, called Linear Spectral Frequencies (LSP), are proposed, and are most often used in low-speed speech codecs at the present time. The main difficulty of the task of calculating the LSP directly from the LSP in real time is described, and it is also investigated that most of the analyzer’s processor time, as a rule, is spent on this task. It is proved that despite the high level of development of digital processors, the problem of calculating the parameters of the vocal tract model in real time remains one of the main difficulties in speech coding. That is why methods that reduce the complexity of this procedure are currently of significant interest. A new method for calculating the LSP and an algorithm based on it are proposed. Its main advantages over the existing ones are described. An assessment of the possible improvement in the quality of speech processing was made, due to the use of the developed method for calculating the LSP.