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Digital Hearing Aids


ODHHS Information and Technical Assistance Series
 

Digital Hearing Aids
 


 
The Widex Breakthrough
 
 
 
 
The Widex researchers developed a digital signal processing integrated circuit that uses very little battery power and is totally digital, with no analog circuits. The circuit samples the raw output of the microphone at a million samples per second and then processes those samples 32 thousand times a second in three separate frequency bands.

The digital signal processor statistically analyses the signals to automatically regulate each channel to maximize the user´s listening experience. The system compensates in each of the channels for the differences in loudness perception, known as "recruitment," experienced by most hearing impaired people ("I can´t hear you ... stop shouting").




 
 
This loudness mapping involves a large number of compressors and varying time windows to avoid any sudden audible changes or distortion. After the signal processing is complete, the circuits convert the 20 bit wide data stream into a single pulse, direction-coded (+ or -) signal that is presented directly to the output transducer without any digital to analog conversion. The noise frequencies are above 200 k Hertz and are ignored by the output transducer.

Widex also found through statistical analysis of the various frequency bands, that it was possible to detect how much speech and how much noise was present in a particular frequency band. This gave them the
possibility of enhancing speech in noise.

 
 
Widex SensoTM

 




 
 
The resulting hearing aid will run for about 165 hours on a single battery. It makes 40 million calculations per second; it automatically controls acoustic feedback; and it has automatic volume adjustment and recruitment compensation.
 


 


 
 
 
The fitting process requires the use of a fairly large sound-proof room (such as is found at The Hearing and Speech Center) to adjust the circuitry to the user´s ear and to the exact hearing aid shape. This allows the hearing aid to be part of the hearing test process, compensating for ear canal resonances due to different ear shapes. The SensoTM can also detect feedback caused by changes in the ear canal shape and correct its signal to reduce or eliminate the problem.

The SensoTM BTE and ITE models offer two microphones for increased directionality, as an option. The signals from the two microphones are processed in the dsp chip to enhance the sounds coming from the front. This similar to the Phonak AudioZoomTM hearing aids, except that in the case of the SensoTM, the effect is not switchable. That is, you cannot turn the effect off in the directional SensoTM model.

 
Oticon DigiFocusTM
 
 
 
 
The DigiFocusTM aid is very similar to Oticon´s MultiFocusTM hearing aid, substituting digital signal processing for the analog circuits in their older aid. Digitized input signals, sampled at about 16 kHz, are divided into seven frequency bands from 125 to 6000 Hz, where circuits apply frequency shaping to match the users ears.
 
 
 

 
 
The signals are then sent to two digital signal processors, one for high frequencies and the other for low frequencies. This allows the hearing aid to apply different compression and amplification parameters to the signals, using syllabic compression at the low frequencies and adaptive gain at the high frequencies.


 
Micro-Tech and Philips
 

 
Micro-Tech and Philips joined forces and developed new digital signal processing (dsp) hearing aids based on an "Open Architecture" dsp chip that allows each of them to use their own software in the processors. They also jointly developed a remote control to use with their hearing aids.

 
These dsp hearing aids are on the market now, and are available in two case styles, BTE and ITE. These aids have the ability to have four different listening programs (such as: quiet, crowd noise, concert hall, etc.), which can be changed from the remote control.

 
These two brands of dsp hearing aids offer the possibility that the software could be "upgraded" as technology improves in the coming years. This might be useful in repairing software errors and for adding different listening programs.

 
Starkey Cetera
 

 
The Starkey Cetera dsp hearing aid is in the final testing phases of development and should be available soon. The Cetera uses real ear measurements (with a microphone probe down inside the ear canal) to make fine adjustments to the frequency and phase response of the hearing aid to minimize the effects of placing a hearing aid in the ear. This is designed to make the hearing aid "acoustically transparent".

It has 15 frequency channels that are adjusted during the fitting process to match the user´s hearing loss and ear shape. It uses output compression limiting and has an algorithm to identify and minimized feedback frequencies. The Cetera´s microphone is automatically turned off in very quiet situations to prevent microphone noise from bothering the user. The microphone is turned back on in less than a millisecond when signals above the noise floor occur.

 
The Cetera hearing aids are designed to be worn binaurally (hearing aids in both ears) and come with a remote control to adjust the volume in both ears simultaneously to maintain the appropriate acoustic balance between the ears. The Cetera will be initially available in ITE (in the ear) and ITC (in the canal) configurations.


 
Claims
 

 
The analog Oticon MultiFocusTM has been on the market for many years and has been shown, (in clinical studies using subjects with mild, moderate and severe hearing losses) to improve speech audibility in quiet conditions, to cause less annoyance from unwanted environmental sounds (like newspapers rustling), to have less likelihood of acoustical feedback, and to have higher overall user preference than the other analog hearing aids used in the studies.

 
The SensoTM hearing aid has been entered into a study similar to the one run on the Phonak "Audio Zoom" hearing aid. The results of the study should confirm the preliminary data that shows that the SensoTM digital hearing aid will significantly improve the signal to noise ratio for the listener.

 
The SensoTM aid has no audible microphone or amplifier noise, and uses a high sampling frequency. The digital signal processor is designed to reduce steady state noises such as air conditioners, cafeteria "babble", engines, and road noises. One user reported that the background noise of the airplane he was riding in almost completely disappeared (which gave him quite a scare) when the signal processor circuit turned on.

 
Availability
 

 
The Widex SensoTM can be purchased now in completely-in-the-canal (CIC), canal (ITC), half-shell (ITE), and behind the ear (BTE) packages. The Oticon DigiFocusTM is available in BTE, ITE, and ITC packages. The Micro-Tech and Philips dsp aids are available now in BTE and ITE configurations. The Starkey Cetera has been formally announced (week of April 3rd, 1998), and should become available as audiologists get the special equipment and training necessary to fit them.

Three other hearing aid manufacturers have announced that they have joined together to develop a digital signal processing circuit that could be used by each of them in future hearing aids.