A Parallel Analog and Digital Adaptive Feedforward Controller for Active Noise Control

Abstract

Digital adaptive controllers are widely used for feedforward active noise control, especially in headphones. In such applications, the secondary path delay, including the sampling and reconstruction effects, must be shorter than the primary path delay to maintain good broadband performance. A mixed analog and digital adaptive feedforward controller is developed to eliminate the added delay of the sampling and reconstruction. The analog controller is based on a state-filtered adaptive linear combiner, while the digital one uses an adaptive finite-impulseresponse filter. It is shown that both filters can be adapted using the normalized filtered-reference LMS algorithm but with different secondary paths' models. A method to design the analog state-filter based on Padé's approximation is described. The performance of the proposed controller with two analog states, the direct feedthrough and a 0.3 milliseconds delay, is assessed and compared to the separate analog or digital controllers in a controlled environment. The results highlight that adding the analog delay improves the digital controller performance by about 5 dB in this application, regardless of the primary noise direction.

Dynamic Matching of Reconstruction and Anti-Aliasing Filters in Adaptive Active Noise Control

“…The use of FPGAs is an effective way to enhance the processing speed, but the associated programming process is often difficult and inflexible, and the debugging work in such systems is tedious. Some other meaningful works have also been conducted [2], and another effective method is the use of parallel analog and digital adaptive feedforward controllers in a system [28]; however, accurately modifying the analog filter coefficient is often not easy. As the new proposed methods are often restricted by system flexibility and adjustment accuracy, Bai's method is still very attractive.…”