Digital Signal Processing Hearing Aid History
Researchers have been intrigued by the promise of digital hearing aids for many years. The things that can be done with digital signal processing are truly astounding, and the techniques are used for everything from finding enemy submarines to creating the latest hit recording.
The main problem in using digital signal processing for hearing aids has been the size of the processors and the large amount of power used. Around 1989, an unsuccessful attempt was made to market a digital hearing aid that required a large signal processor and battery pack attached to the user's belt. Even though it had significant signal processing advantages over other hearing aids on the market at that time, it was not commercially successful. People just would not tolerate the inconvenience of the large size.
Other uses of digital techniques in hearing aids include digital control of analog circuits, allowing multiple channels, control of gain by ambient noise, and remote controls.
In 1996 two companies, Widex and Oticon, introduced true digital signal processing hearing aids that could be worn in the ear just like regular hearing aids. Now, several years later, true digital signal processing hearing aids are available from more than a half dozen companies with a variety of case styles and circuit features.
The Widex Senso Breakthrough
The Widex researchers developed a digital signal processing integrated circuit that used very little battery power and was totally digital, with no analog circuits. The circuit sampled the raw output of the microphone at a million samples per second and then processed those samples 32 thousand times a second in three separate frequency bands.
The digital signal processor in that first Senso statistically analysed the signals to automatically regulate each channel to maximize the user's listening experience. The system compensated 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, 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.
The resulting hearing aid ran for about 165 hours on a single battery. It made 40 million calculations per second; it automatically controled acoustic feedback; and it had automatic volume adjustment and recruitment compensation
Widex Senso™ Models
The fitting process requires the use of a fairly large soundproof room 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 Senso™ can also detect feedback caused by changes in the ear canal shape and correct its signal to reduce or eliminate the problem.
The Senso™ 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 AudioZoom™ hearing aids, except that in the case of the Senso™, the effect is not switchable, that is, you cannot turn the effect off in the directional Senso™ model. The Senso™ BTE is also available in a high power model for extremely hard of hearing people.
The Senso™ 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.
The DigiFocus™ aid is very similar to Oticon's MultiFocus™ 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.
The analog Oticon MultiFocus™ 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.
Micro-Tech, Philips, (and Lori and Telex)
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 were available in three case styles, BTE, ITE, and ITC. 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 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.
Lori and Telex went on to join the Micro-Tech and Philips team to offer the Open Architecture dsp hearing aids as well.
Bernafon has a dsp hearing aid that has two channels and a volume control. The two channels allow the aid to process the high and low frequencies independently. The volume control is a knob on the case that allows the user a 25dB range of control. This volume control is not available on the CIC case style.
The Siemens Prisma dsp hearing aid is a four channel circuit that allows the audiologist a tremendous range in fitting control. The overall circuit contains a preamplifier with high level compression followed by a 23 bit analog-to-digital converter that yields a 138dB dynamic range. The digital signal processor chip operates at 150 million operations per second and is followed by an all-digital output stage to drive the output transducer (loud speaker) directly.
The four channels can use different compression algorithms to chose the temporal, frequency and amplitude characteristics to best suit your hearing loss. The channels can be cross coupled to allow the detection of speech information in the higher frequency channels to lower the volume in the lower frequency channels to reduce low frequency masking of the speech.
The Prisma offers two microphones (TwinMic™) to give directional listening in the BTE and ITE case styles. The Prisma also has two programs that can be selected by push button in all case styles except the CIC. The two programs allow the listener to switch between two different listening environments, such as speech versus music, or perhaps quiet conversation versus noisy environment.
ReSound digital 5000
The ReSound digital 5000 is a true digital signal processing hearing aid that uses the Fast Fourier Transform to process multiple overlapping segments of the signal using a 14 band compression scheme. The processor detects the presence of noise in each of the 14 bands (by measuring dynamic ratios) and applies noise attenuation in bands where noise is present, preserving the speech information in the other bands.
The ReSound digital 5000 also uses dual microphones to provide improved signal to noise ratios for subjects in front of the listener, and has an active feedback reduction system to help reduce feedback. See the detailed article on the ReSound digital 5000.
In 2001, ReSound introduced the Canta7 digital signal processing hearing aid, which uses some of the ideas first introduced in the digital 5000, but adds many new ideas and refinements. And in 2003, the digital 5000 was discontinued.
Sonic Innovations Natura and Natura 2SE
The Natura digital signal processing hearing aid was first introduced in the fall of 1998, and has been an immediate success. The first models introduced were the nearly invisible Completely in the Canal (CIC) and the In the Canal (ITC). Although these are the models that many people prefer, it hampered demonstrating the instrument since you had to get a custom hearing aid manufactured for your ear before you could properly listen to it. Sonic Innovations released their Behind the Ear (BTE) model in 1999, and now you can get demonstrations without waiting.
The Phonak Corporation has a digital signal processing hearing aid that incorporates 20 channels of noise reduction and signal shaping. It's Digital Perception Processing™ circuits are based on psychoacoustic models of the ear. Read the detailed article for more information elsewhere in the section.
AHS has announced the availability of true digital hearing aids. Some of these are repackaging of aids that are already on the market, such as the new offerings by Lori and Telex of the aids brought out by Philips and Micro-Tech, but this does not mean they should be ignored. Some of the "me-too" aids may have lower prices or more case styles.
The Starkey Aries is a nine channel hearing instrument that features a very small digital signal processing circuit. The Aries is one of the smallest hearing aids available, and should be considered if your ears are too small for other completely-in-the-canal hearing aids. Even the behind-the-ear case style is dramatically smaller than usual.
Each of the nine channels is adjusted to match your hearing loss, both for amplitude and compression, yielding an instrument that sounds quiet but sends speech through clearly.
The Aries features adjustable expansion to prevent over amplification of low level environmental sounds and microphone noise. This applies maximum gain to speech sounds and increasingly less gain to lower level sounds. This can be adjusted in each of the nine frequency bands. Long time hearing aid wearers sometimes object to the lack of distortion and noise in the environment that they remember from their old aids, but they usually can hear and understand much better with these aids.
Not all audiologists will stock all of the brands of dsp aids because of the similarities between them. Most audiologists have a variety of demo aids from a representative sample of companies so you can compare how they sound for you. Typically, the audiologist will test you and discuss your hearing needs and then demonstrate the hearing aid that best meets your requirements. Almost all of the hearing aids mentioned in this historical article are no longer on the market. Even some of the companies have disappeared, but their ideas and technology are embedded in the roots of today's digital aids.
One user finds the Widex Senso™ aids to be useful on a fishing trip!
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