Real-Time Detection of Unstable Control Loop Behavior in a Feedback Active Noise Cancellation System for In-Ear Headphones

Author(s)： Sven Höber, Christian Pape, Eduard Reithmeier

ABSTRACT
Active noise controls are used in a wide field of applications to cancel out unwanted surrounding noise. Control systems based on the feedback structure however have the disadvantage that they may become unstable during run-time due to changes in the control path—in this context including the listener’s ear. Especially when applied to active noise cancellation (ANC) headphones, the risk of instability is associated with the risk of harmful influence on the listener’s ear, which is exposed to the speaker in striking distance. This paper discusses several methods to enable the analysis of a feedback ANC system during run-time to immediately detect instability. Finally, a solution is proposed, which identifies the open loop behavior parametrically by means of an adaptive filter to subsequently evaluate the coefficients regarding stability.

1. Introduction
Noise can interfere, adversely affect or even be harmful. In this context, the use of hearing protection or headphones with active noise cancellation (ANC) derives a substantial gain in comfort. Besides pilot headsets, this technology is also increasingly being offered in commercially available headphones to reduce noise especially when traveling or at work. While the current selection of ANC headphones is mainly limited to circumaural models, the concept has been extended to the more compact in-ear headphones in recent years. For the technical implementation of ANC systems based on the feedback structure, the interference signal of noise and speaker sound is measured by an internal microphone and fed back to a controller. The problem with the feedback structure is the risk of instability, which is expressed in an overdrive and eventual destruction of the speakers or even damage of the user’s inner ear. In [1] for example, this problem is overcome by limiting the controller gain. Additionally, practical applications in general limit the actual speaker output or at least deactivate the controller output in case of an overdrive. Thus, conventional solutions either act preemptively or in response to the already unstable state, still exposing the listener to a potential danger. This results in the need to continuously observe the behavior of the control loop during run-time, in order to detect the possibly occurring unstable state at an early stage. In this paper, various methods are compared, which yield an estimation of the control loop state on the basis of the microphone and speaker signals to realize a real-time analysis.

Source:

Journal： Engineering

See also: Comments to Paper