DFS Ultra also incorporates WhistleControl™ to ensure feedback-free performance in all daily life situations, including when a phone is held up to the hearing aids. Because the feedback cancellation is constrained to not adapt to signals that are very disparate from the feedback pathway model, it is possible for intermittent feedback to occur in situations like this. WhistleControl restores the desired response when feedback is imminent. The combination of DFS Ultra in effect solves the most challenging dilemma of the last decades’ growth in popularity of open fittings – how to provide sufficient, undistorted high frequency gain without feedback.