Auto DFS: The Next Best Thing

TStenderTammara Stender, Au.D.
Senior Audiologist, GN ReSound Global Audiology

It is 4:30 PM. You have just received a ReSound hearing aid back from repair in the mail. Your secretary calls the patient to schedule a return visit to have the hearing aid refitted and DFS recalibrated. The patient is very excited about receiving the hearing aid, but indicates she is going out of town on business tomorrow, and will be gone for the next 3 weeks. She only has time to stop by to pick up the devices. Your schedule is also booked through the end of the day. What do you do?

This scenario is just one of many real-world scenarios in which what you need to do is at odds with the time you have available. In the past, your options to meet the needs of this patient were limited. You could return the hearing aid to her without feedback suppression, running the risk that the patient would experience feedback. You could turn on feedback suppression using a “fake” calibration, such as one done with the hearing aid sitting on the table. Your final option would be to apologize and say she needed to come back after her trip to be properly fitted. None of these options would provide protection from feedback while returning the hearing aid to the patient in her desired timeframe.

With the introduction of Auto DFS with the ReSound Verso product family, this real-life dilemma is resolved. Auto DFS was designed precisely with these kinds of situations in mind. It is not merely a “standard” DFS calibration imported into the existing algorithm. Auto DFS is a unique feature that is the next-best thing to calibrated DFS Ultra II.

To understand how Auto DFS works, it is helpful to re-examine how DFS Ultra II works. DFS Ultra II consists of a static and a fast-adapting cancellation filter. The calibration procedure sets up the static filter that serves as a baseline for the system. In other words, the static filter contains a “picture” of what feedback looks like in the individual ear, based on the loop gain measurement obtained by the calibration tone. The static filter is the reference for the fast-adapting cancellation filter, and constraints are established around this baseline to account for small variations from the baseline during weartime. For instance, if the patient inserts the device slightly differently from one use to the next, thereby altering the baseline, the fast-acting cancellation filter can still eliminate feedback by virtue of these adaptive constraints. Whistle Control is an added layer of protection from feedback which quickly and temporarily applies frequency-specific gain reduction in difficult, dynamic situations such as when a phone is brought up to the ear. Whistle Control relies on the calibration of the static filter to establish a baseline for its functionality.

Auto DFS also consists of two cancellation filters; one is slow-adapting the other is fast-adapting. The fast-adapting filter estimates the feedback path for a few seconds every time the hearing aid is turned on. This updates the slow-acting filter, which then continues to monitor the feedback path based on the estimation of the fast-acting filter. When the hearing aid is turned off, the slow-adapting filter stores the estimation obtained over the course of the weartime. When the hearing aid is turned on again, the slow-acting filter has a more accurate “picture” of the feedback path in that particular ear than it did in the previous use.

The advantages of Auto DFS are notable. First and foremost, you have the option to provide active feedback suppression without having to perform the calibration procedure, resolving real-world dilemmas such as the one stated above. Second, the slow-adapting filter has the ability to modify itself according to gradual changes in the feedback path, for example when cerumen accumulates over time in the ear canal. This continual adaptation to the feedback path in the ear will allow for better performance over time, although in most cases this will not be noticeable to the patient as it will work adequately from the first use. (Some patients, however, may experience temporary feedback, generally lasting a few seconds, when initially fitted with Auto DFS.) Internal testing revealed roughly equivalent performance for Auto DFS when compared to DFS Ultra in typical, static situations.

Although Auto DFS has its advantages in terms of convenience, DFS Ultra II is still the best option for controlling feedback, especially in dynamic situations. Since Auto DFS does not have a static filter, Whistle Control is not available. Users fitted with Auto DFS will not benefit from the added feedback protection provided by Whistle Control in difficult, feedback-prone situations, such as when a phone is brought to the ear. It is for this reason that the highly-evolved DFS Ultra II remains the “gold standard” for feedback suppression, both in Verso hearing instruments as well as in the hearing instrument industry.

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