Nonlinear Compensation Algorithm Of Parametric Loudspeaker Based On Volterra Filter

Parametric loudspeaker systems have been widely used for projecting high directional audible sound beams. Due to the parametric acoustic array effect in air, the input audible signal of a parametric loudspeaker system can be reproduced with high directivity at the target region. However, the reproduced audible signal suffers from harmonic distortion, which is the by-product of nonlinear interaction between the primary waves. The harmonics combined with the primary waves always distort the desired signal and degrade the sound quality.A pth-order nonlinear compensating technique based on the Volterra filter is present in this dissertation. The result of simulation and experiment conducted that this technique can overcome the problem of inaccurate nonlinear model used in the conventional Parametric loudspeaker compensating modulation algorithms. Then the derivation and the implementing process of the compensating technique is present in this dissertation, and, in the end, this article conducts the validity of the compensating technique thorough experiment.In order to investigate this inherent nonlinear phenomenon and then, compensate the harmonic distortion effect, a baseband distortion model is developed from nonlinear system identification using an adaptive Volterra filter. By using a truncated Volterra series up to the 3rd-order kernel, measurement date are conducted to develop a system model with one group of parameters. And for the purpose of examining the validity of nonlinear effect by comparing with the classic theories, experimental measurements, which take into account of emitter’s response, are carried out to verify the modeling result and evaluate the model performance. With the Volterra system model, the sound pressure level and the harmonic distortion can be accurately predicted.Based on the Volterra model, a pth-order inverse system is derived to compensate the harmonic distortions present in the demodulated signal, which is referred as the compensating model. With the result of the compensating model, a 1st-order compensating system which implemented with FPGA platform is present. Measurement results demonstrate that the harmonic distortion can be greatly reduced, especially a 10 dB level reduction shown in the 5~10k bandwidth. Furthermore, an analytical comparison between the 2~5th-order compensating system implemented with preprocessing technique and the conventional Parametric loudspeaker compensating modulation algorithms is present. Measurement results demonstrate that the harmonic distortion can be effectively reduced to an acceptable level when the compensating system is introduced with a suitable recursive order and it shows a better performance compared with the conventional ones. Therefore, the pth-order nonlinear compensating technique can be employed as an alternative for reducing the harmonic distortion present in parametric loudspeaker systems.