Two Danish companies have worked together within the Esprit Hearmaster project to set up promising techniques of mixed-signal testing for the new generation of hearing aids, using Digital Signal Processing (DSP). Oticon A/S, a hearing-aid manufacturer, founded in 1904, introduced the world's first digital hearing-aid, DigiFocus, in 1995. The test system vendor, MicroLEX Systems A/S, has managed to develop a revolutionary method for qualifying DSP features so the DigiFocus product can adapt better than earlier models to the surrounding sound environment, providing maximum speech recognition and avoiding unpleasant sound distortions.
Oticon designed a special computer chip for hearing instruments, the so-called Digital Audio Processor, which outperforms the conventional hearing aids based on analogue technology because of its high customising properties to individual needs. Indeed, conventional technology only offers a poor compromise between soft audible and loud uncomfortable sounds on the one hand or inaudible soft and audible loud sounds on the other. The DigiFocus hearing aid operates on seven different frequencies instead of the common two or three frequency bands, thus providing the exact power the client wants to assure balance and harmony in the reproduction of sound.
To enhance the speech recognition, two different speech processors have been integrated into the Digital Audio Processor, one controlling the powerful deep vowels and another one to sharpen the soft and fragile high-pitch consonants. This Adaptive Speech Alignment System permits a much greater intelligibility than the sound compression method used before but which caused muddled speech. The whistling effect formed yet another difficulty to overcome. Normally, to eliminate 'whistling', the amplification of all high frequencies must be reduced with loss of speech understanding as a consequence. DigiFocus however has an advanced 4-band Feedback Manager tuning just the whistling frequencies down.
The customer is able to improve his hearing in a three-step-procedure. Initially, the hearing-aid protects him from hearing too many new sounds at once. The next phase provides access to more sound information and in the final stage, the patient's brain is able to process the full sound picture for maximum speech understanding. The hearing care professional connects the instrument to a computer in the clinic for extensive and precise testing as to compare the personal soundmap of the individual with a model based on thousands of other users, in order to find the right setting for his patient's needs.
The MicroLEX test methodology has added significantly in terms of characterising the quality of the advanced DigiFocus hearing aids. At present, people with hearing difficulties who really are a candidate for hearing instruments - in fact, not everyone is, because the benefits of amplification may widely vary! - can count on a more subtle approach of their problem thanks to the computer-controlled Digital Audio Processor which automatically adapts to every imaginable sound environment once it has been programmed according to the Digital Signal Processing testing results.