Hearing aid acoustic processing technology  

Hearing aid acoustic processing technology

    Hearing aid circuit can be divided into linear hearing aid and non-linear hearing aid based on the input and output characteristics of the amplifier; it can be divided into non-programmable hearing aid and programmable hearing aid according to the adjustment method; it can be divided into analog based on the signal processing method of the amplifier. Hearing aids and digital hearing aids.

1 Analog hearing aid circuit
    Analog hearing aids refer to the hearing aid amplifier’s signal processor using analog components, mainly integrated operational amplifiers composed of transistors, resistors, capacitors and other components.

1.1 Linear analog hearing aids
    1. Linear analog non-programmable hearing aid
    The main amplification characteristics of linear hearing aids and non-linear hearing aids are that the former △ input / △ output is fixed, and the latter has a variable change ratio. Non-programmable hearing aid means that the adjustment of the hearing aid (volume, tone, etc.) is controlled by an external potentiometer. However, because hearing aids are small and can only install individual external components, non-programmable hearing aids have fewer adjustable items, and the frequency response of the hearing aid is more difficult to fully meet the hearing compensation requirements of the hearing impaired, and external components are also more easily damaged.
 
    The basic circuit of a linear analog non-programmable hearing aid (see Figure 5-1) is: a microphone output is connected in series with a low-frequency filter (using a resistance-capacitor filter, the degree of low-frequency attenuation can be adjusted by changing the resistance and capacitance values), and the electrical signal is input into the amplifier. Then output to the output limiter controller (the output limiter generally uses the peak clipping method, and the maximum output can be changed by adjusting the feedback resistance between the amplifier and the output limiter controller). After high-frequency filtering, it is finally input to the receiver. A high-frequency filter is implemented by connecting a resistor-capacitor filter in series between the input end of the receiver and the negative electrode of the power supply to bypass the high-frequency. The degree of high-frequency attenuation can be adjusted by changing the resistance and capacitance values.
 
    2. Linear analog programmable hearing aid
 
    Programmable hearing aids use digital adjusters [use analog / digital (A / D) converters for adjusting potentiometers], so that the adjustment of hearing aids can be achieved by a computer, so there can be more adjustment items without occupying volume, or The frequency response of the hearing aid is better suited to the hearing compensation requirements of the hearing impaired. It generally only has a volume potentiometer, which reduces the chance of damage.
 
    Figure 5-2 Basic circuit of linear analog programmable assistant
 
    The circuit of a linear analog programmable hearing aid is basically the same as that of a linear analog non-programmable hearing aid, except that digital technology is used for adjustment (see Figure 5-2). Some hearing aid output clips use compression technology.
 
    These two types of hearing aids are low-grade hearing aids, with simpler lines and poor adaptability to various complex environments. However, the price is low, and the power can be made larger, which is suitable for the hearing impaired with low sound quality requirements and limited economic conditions.
     1.2 Non-linear simulation aid
    1. Non-linear analog non-programmable hearing aid
    Compared with the linear analog non-programmable hearing aid circuit, the difference lies in the input and output characteristics, that is, the former circuit is more complicated. In addition to the filtering, amplification, and limiting circuits, there is also a signal analysis circuit – the size of the monitoring signal and the length of the time course. To decide whether to enable compression, compression start time, release time, etc.
    The most typical non-linear analog non-programmable hearing aid circuit is the standard K-AMP circuit, which is an adaptive amplifier circuit. It was introduced to the market by Etymotic in 1990. The designer Mead Killion, Ph.D’s intention is to increase speech intelligibility, because most sensorineural hearing loss is more obvious in the high frequency part. Gain) input compression line, TILL line has the effect of increasing high frequency at low sound pressure level, the loud characteristic of WDRC line can be automatically changed according to the input sound pressure level, it uses Cla. . D amplifier output to improve speech intelligibility.
The standard K-AMP line hearing aid has three basic adjustment knobs: VC (volume control), LFC (low frequency control) and CK (knee threshold). VC controls the overall gain of the hearing aid. When LFC is at a minimum, VC plays a role of high frequency boost.
CK controls the amount of amplification of high-frequency sound. Its compression inflection point is about 40dB SPL, which gives sufficient amplification to soft sounds, and for strong sounds above 90dB SPL, the hearing aid does almost nothing. The compression ratio of K-AMP lines at low frequencies is 1.3: 1; the compression ratio at high frequencies is 2.1: 1. After development, the programmable K-AMP lines can achieve compression ratios at high frequencies of 3: 1. Therefore, K-AMP line hearing aids are very suitable for patients with mild and moderate sensorineural hearing loss. The frequency band of the K-AMP line is very wide, from 100 to 14000 Hz, covering the high-frequency speech in our daily speech. Although these high-frequency speech energy is low, it is very important for language discrimination. The wideband characteristics of K-AMP circuit, coupled with the WDRC feature, can better improve the language’s discrimination ability.
    Although this kind of circuit has made great progress and is widely used in the era of analog hearing aids, with the rapid development of digital signal processing, it has been rarely used at present.
 
    2. Non-linear analog programmable hearing aid
    Non-linear analog programmable hearing aid is a good hearing aid circuit. It not only has the adaptive function to the input signal, allowing patients to hear comfortable, low-distortion sound, but also has the advantage of being programmable, which can be adjusted flexibly to make the hearing aid. The frequency response is more suitable for the hearing loss of the hearing impaired, so it is widely used. However, with the development of digital hearing aid technology, such hearing aids have tended to be eliminated.
 
    ⑤ Output automatic gain control (AGC-O) and soft clipping 5
    ⑥Directional microphone, microphone performance is automatically matched;
    ⑦Multiple memories;
    ⑧Wide range high and low frequency filter;
    ⑨ Low voltage, memory conversion sound.
    Because digital hearing aids need to be adjusted, they are basically programmable. Figure 5-4 shows the circuit diagram of a digital signal processor.

    3．Digital / analog converter (D / A)
    The final signal must be digitally encoded into an acoustic signal. This work is performed by a digital / analog converter combined with a hearing aid receiver. The digital / analog converter converts the digital signal into an electric signal and transmits it to different types of receivers, and finally completes the conversion of sound.
    In order to minimize energy consumption and increase battery life, digital hearing aids generally use another solution similar to Class D receivers, that is, using digital / analog converters. Directly drive a zero-biased receiver through a digital / analog converter.
     2.2 Advantages and prospects of digital hearing aids
    The advantages of digital technology include: greater accuracy than analog hearing aids, less internal noise, less distortion, longer life, and the ability to perform more complex calculations without increasing power consumption. These advantages allow digital hearing aids to
Stronger adaptive capabilities in complex environments.
    Hearing-impaired people’s biggest complaint about hearing aids is that they cannot hear clearly and feel comfortable in various environments. The linear line has the worst signal recognition and processing ability, and the self-adaptability is the worst. Although the non-linear analog programmable circuit has the automatic signal recognition and processing ability, it cannot complete too complicated calculations in a small hearing aid chip. The processing power is exactly the advantage of digital hearing aids, so it is the most adaptive. However, early digital hearing aids, due to the small chip memory, slow operation speed, simple circuit, and performance were not as good as high-end non-linear analog programmable hearing aids.
 
With the continuous development of audiology, digital technology and computer technology, digital hearing aids will have more powerful signal processing capabilities and adaptive capabilities. Digital hearing aids are the direction of the hearing aid industry.
However, digital hearing aids also have disadvantages, that is, complex adjustment items require a higher professional level of the adjuster, and the adjustment is not good.
Not only cannot the advantages of digital hearing aids be realized, it may even be worse than linear hearing aids. So with hearing aid technology
With the continuous development of technology, the technical level of practitioners also needs to be continuously improved.