Eliminating feedback - FeedbackStopper

Acoustic feedback is one of the most negative aspects associated with hearing instruments. In many cases, the annoyance and embarrassment caused by feedback may even outweigh an individual’s perceived benefit of amplification. Feedback occurs when amplified sound from the receiver leaks out of the ear canal (e.g. through a vent), is picked up by the microphone, and in turn amplified again. The route the amplified signal travels back to the microphone is called the feedback path. The goal, therefore, is to break this feedback path. With BestSound™ Technology, Siemens introduces the new FeedbackStopper which provides the most efficient protection against acoustic feedback, even in the most critical situations. Effective feedback suppression is so fundamental and essential in a hearing instrument that the new FeedbackStopper is featured in all products equipped with BestSound Technology.

While most currently available feedback management systems are able to account for individual ear and fitting differences, and counteract the static feedback path, the challenge lies in rapidly changing feedback paths. Even for the same person, no matter how well an earmold or in-the-ear hearing instrument fits in the canal, as the user goes about in his daily life, the feedback path changes when he talks or chews, when he leans his head back against the couch, or when he hugs a loved one. Changes in the feedback path, such as those just mentioned, occur very quickly. In order to eliminate feedback in these situations before it becomes disturbing to the user, it is essential that the feedback canceler acts within milliseconds. A potential downside, however, is that fast-acting feedback cancelation systems often mistake tonal stimuli in the environment such as music for feedback, and thereby create artifacts and distortions in such acoustic situations. In summary then, a good feedback cancelation must fulfill three requirements: suppress feedback, no artifacts, fast reaction.

Siemens’ powerful FeedbackStopper is an adaptive phase cancelation system combined with Transient Frequency Shift. The adaptive phase cancellation filter effectively cancels feedback by continuously estimating the feedback path and generating a corresponding out-of-phase signal. Thus, feedback is suppressed without reducing gain for external signals like speech, music, and environmental sounds. This adaptation process can be achieved extremely quickly due to transient frequency shifting. Briefly shifting the entire output of the amplifier by 25 Hz breaks the feedback loop and therefore helps to suppress feedback. But even more importantly, the frequency shift helps to avoid artifacts. This is because the adaptive filter is more likely to generate artifacts if external signals are similar to the feedback signal. By shifting the frequency of the feedback signal, it becomes less similar to the external signal so that the adaptive filter can react extremely quickly without generating artifacts. This small frequency shift may be perceived by some listeners as a slight “roughness” in sound quality, however, the brief roughness is much less audible or annoying than the whistling of feedback.

Absolute freedom from artifacts can only be achieved if the adaptation is stopped and if the frequency shifting is switched off. To maximize listening comfort, therefore, FeedbackStopper completely halts adaptation and frequency shifting whenever there is no feedback risk. To distinguish between feedback situations and non-feedback situations, FeedbackStopper employs the Siemens-patented Acoustic Fingerprint Technology (AFT). Similar technologies are used in audio coding and referred to as “digital watermarking.” With AFT, the amplified signal that leaves the hearing instrument receiver is slightly phase-modulated and this “tag” or “fingerprint” is used to identify signals that have already been amplified by the hearing instrument (i.e. feedback potential). It has been demonstrated in psychoacoustic experiments that this phase modulation tag is inaudible to the human ear. It can, however, be identified by a technical modulation detector and used as a critical piece of information which tells the FeedbackStopper when to activate frequency shift and adaptation.

The figure above animates how the new FeedbackStopper works. Once the Acoustic Fingerprint is detected (e.g., feedback is likely to occur), FeedbackStopper briefly shifts the entire output of the amplifier by 25 Hz. As soon as feedback is suppressed and the fingerprint is no longer detected in the incoming signal, the FeedbackStopper switches off frequency shifting and completely freezes the adaptation in order to avoid artifacts.

FeedbackStopper with transient frequency shifting and fast phase cancellation provides ~25 dB added stable gain in dynamic situations.