Digital hearing aid

Difference between digital and analog hearing aids

Analogue hearing aids amplify (makes louder) all the sounds picked up by the microphone. For example, speech and ambient noise will be amplified together. On the other hand, DHA processes the sound using digital technology. Before transmitting the sound to the speaker, the DHA microprocessor processes the digital signal received by the microphone according to the complex mathematical algorithm. This allows amplifying the sounds of certain frequency according to the individual user settings (personal audiogram), and automatically adjusting the work of DHA to various environments (noisy streets, quiet room, concert hall, etc.).

For users with varying degrees of hearing loss it is difficult to perceive the entire frequency range of external sounds. DHA with multi-channel digital processing allows you to "compose" the output sound by fitting a whole spectrum of the input signal into it. This gives the users with limited hearing abilities the opportunity to perceive the whole range of ambient sounds, despite the personal difficulties of perception of certain frequencies. Moreover, even in this "narrow" range the DHA microprocessor is able to emphasize the desired sounds (e.g. speech), weakening the unwanted loud, high etc. sounds at the same time.

A block diagram of a digital hearing aid

The signal processing is performed by the microprocessor in real time and taking into account the individual preferences of the user (for example, increasing bass for better speech perception in noisy environments, or selective amplification of high frequencies for people with reduced sensibility to this range). The microprocessor automatically analyzes the nature of the external background noise and adapts the signal processing to the specific conditions (as well as to its change, for example, when the user goes outside from the building).

Advantages

According to researches DHA have a number of significant advantages (compared to analogue hearing aids):

These advantages of DHA were confirmed by a number of studies, relating to the comparative analysis of digital hearing aids of second and first generations and analog hearing aids.

History

The history of DHA can be divided into three stages. The first stage – the widespread usage of computer simulation for the analysis of systems and algorithms for audio processing. The work was conducted with the help of the "big" computer of that time. Although they could not claim to be a real hearing aids (their performance was not enough for audio processing in real time – not to mention the size), they carried out successful studies of the various hardware circuits and algorithms for processing audio signals. The software package BLODI (stands for Block of Compiled Diagrams) developed by Kelly, Lockbaum and Vysotskiy in 1961 allowed to simulate any sound system provided in the form of a block diagram. With its help a special phone for users with hearing impairments was created. In 1967, Harry Levitt used BLODI to simulate a hearing aid on a digital computer.

Almost ten years later the second step was taken – the creation of "quasi-digital" hearing aid, in which the analog components and digital programmable module was combined into a single compact case. In this device the digital controller not only controlled the analog components (amplifiers, filters and signal limiter), but it could be programmed by connecting an external computer (in the laboratory – with medical supplies of hearing aids).

The concept of quasi-digital device was very successful from a practical point of view because of the low power consumption and compact size. At that time, low-power analog amplifier technology was developed very well – in contrast to the semiconductor chips necessary for a "real" digital camera. The combination of high performance analog components and digital signal processing capability has led to the creation module successful production parts.

The hearing aid of this type was developed by Etymonic Design. A little later, Mangold and Lane created a programmable multi-channel hearing aid. A similar approach was applied by Similarly, Graupe with co-authors for developing of an adaptive noise filter on a single crystal. This relatively small chip had low power consumption and fit in the case of ordinary BTE or ITC hearing aid.

The third stage of development was the appearance of "real" digital hearing aids. In DHA all stages of sound processing are carried out in binary form. To do this, an external sound from a microphone first converted into a binary code, and after the conversion the reverse transformation is carried out (to analog signal transmitted by the ear speaker in the form of sound). The first "real" DHA were models developed by Graup in 1970 on the basis of the 8080 microprocessor, which replaced the analog components (amplifier, limiter and filters). The possibilities of a programmable processor made the device self-adjusting, which opened the prospects for the use of advanced signal processing techniques, noise reduction, etc. Although the 8080th processor was relatively slow and big in size.

Further development of the DHA is associated with the creation of microprocessors with parallel processing of data arrays.As a result, a significant decrease of calculations time gave the opportunity to conduct processing of audio signal in real time. The small size of microchips (as of 1987) allowed creating compact hearing aids not exceeding the dimensions of their analog "predecessors" on their basis. However, for ITC aids these processors were not yet sufficiently compact. In all other respects, "full" DHA of that period was very similar to modern models.

Main manufacturers

Digital hearing aids are produced by various manufacturers, including the recognized leaders in the electronics market. These include Siemens (Germany), Oticon (Denmark), Bernafon (Switzerland), GN ReSound (Denmark), Widex (Denmark), Unitron (Canada), and others. The Swiss company Phonak is known for its compact ITE devices which are best adapted to the individual user characteristics and provides high comfort. The technologies of production of digital hearing aids are constantly being improved. Large companies regularly introduce new products design, so it is very difficult to choose the definitive market leader. Basically, the modern trends in DHA are aimed at:

As an alternative to foreign models the digital hearing aids by Russian manufacturers can be considered: “Slukhovye apparaty I tekhnika”, “Medservice”, hearing aids factory "Rhythm", Sonata, Istok-Audio, Videx and others. The price of local production pieces is usually below their foreign counterparts.

Difference between PSAP and digital hearing aids

Personal Sound Amplification Products (abbreviated PSAP) are classified by the FDA as "personal sound amplification devices." These compact electronic devices are designed for people without hearing loss. Unlike hearing aids (which FDA classifies as devices to compensate for hearing impairment) use of PSAP does not require medical prescription. Such devices are used by hunters, naturalists (for audio observation of animals or birds), ordinary people (for example, to increase the volume of the TV in a quiet room), etc. PSAP models differ significantly in price and functionality. Some devices simply amplify sound. Others contain directional microphones, equalizers to adjust the audio signal gain and filter noise. Among the most well-known manufacturers of PSAP are Soundhawk, Etymotic, and others.

"Hearing aid" applications for smartphones / tablets

Computer programs allowing to create a hearing aid on the basis of a PC, tablet or smartphone currently gain popularity. Modern mobile devices have all the necessary components to implement this: hardware (an ordinary microphone and headphones may be used) and a high-performance microprocessor that carries digital sound processing according to a given algorithm. Application configuration is carried out by the user himself in accordance with the individual features of his hearing ability. The computational power of modern mobile devices is sufficient to produce the best sound quality. This, coupled with software application settings (for example, profile selection according to a sound environment) provides for high comfort and convenience of use. In comparison with the digital hearing aid, mobile applications have the following advantages:

It should be clearly understood that "hearing aid" application for smartphone / tablet cannot be considered a complete substitution of a digital hearing aid, since the latter:

Functionality of hearing aid applications may involve hearing test (in situ audiometry) too. However, the results of the test are used only to adjust the device for comfortable working with the application. The procedure of hearing testing in any way cannot claim to replace audiometry test carried by medical specialist, cannot be a basis for diagnosis.

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