Latest digital hearing aids design challenges and related considerations

The latest hearing aids are digitally programmable, which means that although they have an analog signal processing functions, but by audiologists may adjust the number of parameters to control processing.

Digital hearing aids in the design, engineer will encounter which challenge? how to deal with them? this article will provide you an interpretation. Design challenges hearing aids designers have strict technical requirements. Hearing aids must be small enough to place the human ear or ears, ultra-low power must run, and no noise or distortion. To meet these requirements, existing hearing aid device consumes power to less than 1mA, operating voltage to 1V, utilization of chip area less than 10mm2, this usually means that two or three devices are stacked. A typical analog hearing aids with non-linear input/output functions and frequency dependent gain amplifier. But this simulation processing relies on custom circuit, compared with digital processing, lack of programmability and more costly. The latest digital equipment with its corresponding simulation equipment, reducing equipment cost, reduced power consumption. Digital equipment's biggest advantage is its improved processing power and programmability, allows custom hearing aids to apply to a specific hearing impairment and the environment. Replace it with a simple voice amplification and adjustable frequency compensation for more complex processing strategies to improve provided to damage the ear sound quality. But such policies require DSP can offer extremely complex processing functions. In General, there are two types of hearing loss: conductive hearing loss and sensorineural hearing loss (SNHL). When the patient's ear or middle ear sound conduction anomalies will occur conductive hearing loss, and when the cochlear auditory sensory cells or higher in the problem of neural mechanisms of sensorineural hearing loss can occur. If a conductive hearing loss, you cannot normally through the middle ear or tympanic transfer sound. Because voice is mainly composed of conductive losses decay, so you only need to enlarge the sound can be recovered nearly normal hearing. Does not need any special signal processing, traditional analog hearing aids to good work. However, only 5% suffered from some hearing loss are attributable to the conductive losses. Another type of hearing loss is SNHL. It includes and old age-related hearing loss, and noise-induced hearing loss and taking up on auditory system harmful drugs cause of hearing loss. Most SNHL is caused by cochlear fault. SNHL is considered by the inner hair cells and outer hair cells, or a combination of the damage caused. However, the underlying physiology are extremely complex. Different people have different pathological, this means listening to the same patient plan does not necessarily have the same type of hearing loss. Furthermore, patients or even on a different frequency range damage varies. SNHL results often leads to: 1) in some frequency channel has no input, 2) lack of sensitivity, and 3) auditory filter. In turn these results to a large extent on the injury the audience sound sleep. And compared to the normal audience, listeners who SNHL among other difficulties, most frequently encountered loudness revitalization (compared with normal, comfortable listening level range is compressed) and frequency resolution loss. Sound awareness of these changes significantly affected the ability of the audience to understand speech. Because SNHL just sound transmission problem is so simple, but in fact is the voice, through a simple zoom cannot treat this kind of loss-making vague voices greater cannot make them clearer. Therefore, to help a patient may SNHL effective ways is through preprocessing signal to enhance complex tone mode to compensate for hearing loss. By the same best treatment cannot cure SNHL manifestations. Handling sound enables speech easier to understand. However, the best processing algorithms vary by individual, even in different listening conditions (such as quiet and noisy Stadium) for individuals. Accommodate these differences lies in hearing aid of flexibility. Traditionally, hearing aid has been mounted on the right for the end user of custom ear moulds the amplifier. Hearing aid system includes a microphone, an amplifier, a zinc-air battery and a receiver/speakers. Most amplifiers employ some compression, it is non-linear input/output relationship used to compensate for loudness revitalization. You can also adjust the gain of different bands, bands number varies, but usually two or three bands. Many of the latest hearing aids are digitally programmable, which means that although they have an analog signal processing functions, but by audiologists may adjust the number of parameters to control processing. In addition, some analog hearing aids for different listening environments with multiple "program", or a parameter set. Some digital hearing aids on the market is with programmable coefficient of ASIC. The ASIC provides algorithm sets and multiple frequency band, this is the typical simulation equipment is not possible. For example, digital hearing aids have the following functions: 2 to 14 with adjustable crossover frequency band, a microphone, directional hearing dual microphones, background noise reduction, automatic gain control (AGC), speech enhancement, feedback and noise abatement. In short, to complete the processing of amazing, especially the simulations of hearing aids in traditional processing functions, and even more so. Design example based on DSP hearing aids scalable software control features, including the frequency shaping, feedback reduction, noise reduction, Binaural processing, ear shells and ear canal filtering, reverberation elimination as well as the provision from a digital telephone, television or other audio device direct digital input. Programmable DSP also means hearing algorithm/features can be customized or without changing hardware change. Hearing professionals almost in real time for economic use of the available algorithms. You can even be a user-selectable program is used to switch to hearing difficult cases are highly-processed sound, or return to the quiet environment of traditional, less distorted sound.

Figure 1: block diagram of a hearing aid DSP.

The above block diagram shows the DSP based on the main elements of a digital hearing aids.

Typical digital hearing aid consists of three mutually stacking of semiconductor bare chip: EEPROM or non-volatile memory, a digital deviceAnd an analog device. Latest allows those modules integrated into two or even a semiconductor bare chip. Because the battery voltage range 1.35V to 0.9V, these devices are designed to operate with 0.9V voltage. Some implementations use power management to monitor the battery voltage, and warn the user when the battery, the voltage drop too low, and close the system in a timely manner. Analog devices usually including Σ-Δ ADC with compressed input restrictions feature microphone preamplifier, remote control data decoder, clock oscillator and voltage regulators. Σ-Δ A/D frequency range usually 20kHz, a resolution of 16 bits (14-bit linear). Digital devices including DSP, logical support features, programming interfaces and output level. The output stage is typically a fully digital, use use speaker impedance of pulse width modulation (PWM) output and class d amplifiers perform analog-digital conversion. In short, current analog and digital hearing aids-power roughly equal. Analog devices total current consumption approximately 0.7mA-1.0mA, and digital equipment consumes 0.5mA-0.7mA. Section about providing 30mAh-65mAh and 50uA since discharge current of zinc-air battery power for this system. End-of-life approximately 0.9V voltage. As a result of the processing in digital hearing aid increased direct comparison between digital and analog hearing aids of power is not entirely fair. And analogue hearing aids equivalent with handling features of digital hearing aids consumes power even lower. Featured product AIC111: low power audio codec AIC111 is a low power DSP or microcontroller-compatible audio codec for personal medical equipment (hearing, auditory processing and low power headphones), as well as provide high-performance analog interface solutions. AIC111 support a 1.3V CMOS Digital SPI interface, and includes an external microphone power and bias power and low battery monitor and indicator.