Research On Signal Predistortion Processing And Modulation Methods For Parametric Loudspeaker

The parametric loudspeaker is a new type of acoustic directional propagation device that uses self-demodulation of ultrasonic signals in the air to produce highly directional audible sound.Due to the nonlinearity of the self-demodulation process,the influence of non-linear factors such as temperature,humidity,pressure,scattering,and heat propagation loss of the air medium,the audible sound has more serious distortion and affects the speaker system sound quality.This dissertation mainly focuses on the problem of self-demodulation signal distortion of parametric array loudspeakers.An improved NLMS adaptive filter predistortion compensation signal processing algorithm is proposed and combines the vestigial sideband modulation algorithm to improve the speaker’s sound quality.According to the basic theory of nonlinear acoustics,the harmonic distortion analysis of modulation signal processing algorithm about the common parametric array loudspeaker is performed based on the Berktay far-field solution theory.The analysis results show that the main factor affecting the harmonic distortion is the audio preprocessing in the signal processing algorithm and signal modulation method.Aiming at the problem of harmonic distortion caused by parametric speaker signal preprocessing mechanism,this paper designs a predistortion compensation algorithm.The algorithm mainly includes two modules: improved NLMS adaptive filter and square root modulation feedback algorithm.The improved NLMS adaptive filter algorithm is studied from the aspect of normalization and variable step length,and an improved adaptive filter simulation is performed on the added-noise frequency signal through Matlab.The result shows that the output signal can restore the original signal without distortion.The square-root modulation feedback algorithm theoretically does not have harmonic distortion,and the actual system cannot provide infinite bandwidth for approximate processing.The experimental results have less harmonic distortion.Then the overall predistortion compensation algorithm is theoretically deduced and simulated by Matlab.The experimental results show that the output signal amplitude changes and the system’s modulation coefficient can be adjusted dynamically to reduce the harmonic distortion of the self-demodulated output audible sound.For the problem of harmonic distortion caused by parametric loudspeaker signal modulation method,the residual sideband modulation algorithm is designed in this paper.The algorithm consists of double-band carrier amplitude modulation and band-pass filter.Mainly to study the vestigial sideband modulation algorithm,design a band-pass filter,and verify the algorithm from several aspects of single-frequency,dual-frequency and wide-band signals,the results can save system spectrum,reduce the impact of other noise frequency components.Based on the research results of the above theories and algorithms,this paper designs and builds a sound distortion test platform for the parametric array speaker system.In a laboratory environment,the modulation signal and the self-demodulation signal are tested respectively.The result shows that the improved predistortion compensation algorithm and the vestigial sideband modulation algorithm of the parametric array loudspeaker reduce the harmonic distortion of the system by 7%,which verifies the feasibility of the algorithm.