Multimedia loudspeakers, widely used in the consumer portable electronic devices, have poor low-frequency reproduction ability, due to the limitation of small cell size and other physical conditions. Traditional methods such as equalizer boosts the energy of bass directly, but this will increase energy consumption and reduce the portability.

To address this problem, virtual bass system (VBS) has been proposed based on the psychoacoustic principles of missing fundamental. Missing fundamental states that human’s ears can reconstruct the fundamental frequency from harmonics even when the fundamental frequency is not physically presented. This is also called virtual pitch theory.

Phase vocoders (PV) and nonlinear devices (NLD) are commonly used for harmonic generation in VBS. PV suffers from the unnatural artifact when processing transient signal. The drawback of NLD is the unwanted nonlinear distortion, especially intermodulation distortion. Recently, a hybrid method has been proposed which combines PV with NLD to avoid those drawbacks.

Researchers from the Institute of Acoustics, Chinese Academy of Sciences have presented an improved VBS based on NLD and adaptive notch filter (ANF). A variable step-size ANF based on the onset detection is used to get both fast convergence rate and low misadjustment. Besides, a single side-band modulation is applied to the system before frequency tracking to improve the numericalstability. A new harmonic generation method is also proposed to get a better bass impression and less impairment.

According to the experiments, the new variable coefficients ANF (VC-ANF) has been shown to provide faster tracking speed by objective evaluations.The results of subjective tests show that new method achieves improved bass impression with reduced distortion (See Fig. 1).

Fig.1 Subjective rating for bass enhancement systems (Image by ZHU).

References:

ZHU Rui, YANG Feiran, YANG Jun. A Variable Coefficients Adaptive IIR Notch Filter for Bass Enhancement. Proceedings of the 21st International Congress on Sound and Vibration, ISSN2329-3675/ISBN 978-83-62652-66-2, International Institute of Acoustics and Vibration (IIAV), R51-924, 2014.

Contact:

ZHU Rui

Key Laboratory of Noise and Vibration Research, Institute of Acoustics, Chinese Academy of Sciences, Beijing 100190, China

E-mail: rzhu89@gmail.com