hearing aid compression technology

hearing aid compression technology

Compression Limit

1. Auditory dynamic range
The auditory dynamic range refers to the range of sound intensity from the hearing threshold to the discomfort threshold of the human ear.
As shown in Figure 5-5, the hearing threshold of normal people is about lodB, the discomfort threshold is about 100dB, and the dynamic range is 90dB. For a sensorineural hearing loss patient with a threshold of 50dB, if his discomfort threshold is still around loodB, the dynamic range is reduced, only

The previous hearing aid used a linear amplifier circuit, △ input / △ output is 1, that is, each time the input sound pressure level increases by 1dB, the output sound pressure level increases by ldB, and the hearing aid gain is unchanged. Hearing aids for most deaf people, because he
Their hearing dynamic range is narrowed, and often accompanied by re-vibration. The maximum sound output of the hearing aid may exceed the patient’s discomfort threshold.
Causes patient discomfort and even hearing loss. This requires limiting the maximum sound output of the hearing aid.
Peak clipping is one of the earliest sound limiting techniques used and is mostly used in linear amplifier circuits (see Figure 5-6, Figure 5-7). due to
Peak clipping has the disadvantage of large harmonic distortion, so another kind of limiting method was born – automatic gain control (AGC).
(l) Principle of maximum output control
In order to avoid the hearing aid output amplitude exceeding the discomfort threshold of the hearing impaired when loud, thereby causing discomfort and hearing of the hearing impaired
Damage, the maximum output amplitude of the hearing aid must be controlled. The control principle is to control the maximum output close to the hearing impaired
Discomfort threshold. Current hearing aids have maximum output control.

(2) Type of maximum output control
① Peak clipping circuit (PC). In the linear amplification circuit (that is, the gain does not change with the change of the input sound pressure level), the maximum output control mainly uses a peak clipping circuit. The peak clipping circuit is the simplest output limiting method, which refers to cutting out the peak portion of the output signal that exceeds the limit, as shown in Figure 5-8.

Figure 5-8 Peak clipping output limiting mode
30 40 50 60 70 80 90 100 input (dB SPL)
Figure 5-9 Input-output of the peak clipping circuit

Figure 5-9 shows the corresponding output curves of the peak clipping circuit at different input sound pressure levels. As can be seen from the figure, the hearing aid is linearly amplified at small input sound pressure levels; peak clipping occurs only at large input sound pressure levels. The output amplitude does not change when clipping.
Some hearing aids now use soft peak clipping technology, which is different from previous peak clipping technology (called hard peak clipping) in that hard peak clipping is a technology that completely removes signals that exceed a predetermined output intensity, while soft peak clipping is a step-by-step process for this part of the signal Removed, the latter has less distortion than the former.
② Compression amplifier circuit. Because the peak clipping circuit inevitably causes signal loss, a limiting method that can achieve the limiting purpose without losing the signal is needed. When the processing method of the compression amplifier circuit is a small input sound pressure level, the hearing aid is linearly amplified;
At a large input sound pressure level, the gain gradually decreases, and the maximum output does not exceed a certain limit, that is, a large range of input signals is compressed to a small range of output. It is widely used in the Philip linear amplification circuit (that is, the gain varies with the input sound pressure level)
in. It is worth noting that the shape of the output signal and input signal has changed at this time, as shown in Figure 5-10 and Figure 5-11.
The first and most widely used compression and amplification method is automatic gain control (AGC), which uses linear amplification for small and medium sounds, and only uses compression amplification for sounds above 65dB SPL (medium) and the volume is large. The compression is relatively large, generally greater than

Figure 5-12 Input-output of compression limit Figure 5-13 Input-gain of compression limit
The characteristics of automatic gain control include: linear amplification of small sounds to avoid signal distortion caused by compression; prevention of uncomfortableness caused by the maximum sound output exceeding the discomfort threshold; and reduction of distortion produced by medium and large input sound peak clipping.

There are two types of automatic gain control: output compression (AGC-O) and input compression (AGC-I). The feedback monitoring loop of the AGC-O is located after the power amplifier and volume control. The volume control does not affect the maximum sound output of the hearing aid. AGC-I feedback monitoring loop
The circuit is located before the power amplifier and volume control, which directly affects the maximum sound output of the hearing aid.
In addition, AGC-I will reduce the signal-to-noise ratio, because the signal in the AGC-I line is compressed and amplified, while the noise is weaker, linear amplification is used, and the intensity difference between the signal and noise after output is reduced. As long as AGC-O reduces the volume, the signal and noise are linear.
Zoom in, and the signal-to-noise ratio will not decrease.
The compression limiting method mostly uses output automatic gain control (AGC-O).

3.2 Wide Dynamic Compression (WDRC)

However, Li, hearing aid light with compression limiting cannot meet the needs of deaf patients. Because, patients often complain that they can’t hear a small sound a little further away. If the hearing aid volume is turned on too loudly, then loud sounds will feel too loud again. Let’s introduce
The auditory response of normal hearing and sensorineural hearing loss to speech signals of different intensities.
1. aim of design
The intensity range of speech is generally 50 ~ 80dB SPL. The intensity of whispering is 50dB SPL, the intensity of medium speech is 60-70dB SPL, and the intensity of loud speech is 80dB SPL. For normal hearing, speech intensity is in dynamic range, medium speech intensity
Is the optimal threshold; for patients with sensorineural hearing loss with a hearing threshold of 50dB, whispering is almost inaudible, middle sounds are lighter, and only loud sounds are comfortable; and for sensorineural hearing loss patients with a hearing threshold of 70dB, low, medium No sound can be heard, only loud.
Common linear amplification hearing aids provide the same gain for all sound levels, and peak clipping is used only above the discomfort threshold.

In this way, for a sensorineural deaf patient with a hearing threshold of 50dB, if the amplification is 25dB, the whisper feels comfortable, and the middle sound is already loud; for a sensorineural deaf patient with a hearing threshold of 70dB, if the amplification is 30dB, the whisper feels lighter, medium It sounds too loud and loud
The distortion is also large.
In order to make the output dynamics of the hearing aid in the dynamic range of sensorineural hearing loss, the hearing aid can adapt to various acoustic environments, compression amplification technology is used. Compression amplification is to increase the magnification of small sounds and small the magnification of loud sounds, compressing useful signals to
Deaf patients narrow their dynamic range. It compresses the entire speech dynamic range proportionally to the patient’s dynamic range.
The compression threshold of analog hearing aids can generally start from 45dB, and the compression threshold of digital hearing aids can generally start from 30dB.

It can be seen from Figure 5-14 and Figure 5-15 that the wide dynamic compression is linear amplification below the inflection point, the gain of the compression section decreases with the increase of the input sound intensity, and the compression limit is AGC-O or peak clipping. In order to obtain a larger output, the compression ratio should generally be less than

At high input sounds, wide dynamic compression is not much different from compression limiter; at low input sounds, wide dynamic compression has a lower compression threshold and a greater gain than compression limiter, so there is generally better speech Intelligibility.
Due to the wide dynamic compression, the output is small at mid-range. For patients with severe hearing loss, they prefer a larger compression limit at midrange.

3.3 Compression technology characteristics

1. Static compression characteristics
Static compression characteristics refer to time-independent parameters, that is, compression threshold, compression range, and compression ratio. These parameters can be reflected in the input-output curve and the input-gain curve.
(1) Gain
Because the gain changes with the input sound intensity, to avoid circuit saturation, the gain is generally measured at a 50dB SPL input sound pressure level. In order to better describe the gain characteristics of the compression circuit, ANSI-1987 uses the input-output curve, ANSI-1992
And IEC118-2 uses a set of gain frequency response curves measured at different sound pressure levels. As shown in Figure 5-15, the input is below 40dB SPL and the gain is 44dB; the input is 60dB SPL and the gain is 35dB; the input is 80dB SPL and the gain is 26dB.
(2) Compression threshold
The compression threshold (CK) refers to the input sound pressure level when the hearing aid has just switched from linear amplification to compression amplification, and is called the ” knee ” . In Figure 5-15, the inflection point is 40dB SPL. The definition of the inflection point in IEC118-2 is: the gain of the booster is reduced relative to the linear amplification gain
(2 ± 0.5) dB, the corresponding input sound pressure level.
(3) Compression range
The input range where compression occurs is called the compression range, which is the upper compression limit minus the compression threshold. The compression range in Figure 5-15 is 86-40 = 46 (dB SPL).
(4) Compression ratio
In the compressed state, the compression ratio (CR)-△ input sound pressure level / △ output sound pressure level. In Figure 5-14, the compression ratio one (86-40) / (108-84) = 1.9.
2. Dynamic compression characteristics
The compression circuit monitors the voltage or current of the signal through a feedback loop, determines whether the signal exceeds the compression threshold, and determines whether the hearing aid should enter the compression amplification state.
It takes time to start and recover the working state of the compression circuit, which is called start-up time and recovery time.
(l) Startup time
The start-up time is also called rising time. It is defined in IEC118-2 as the moment when the sound pressure level of the input signal suddenly increases to the specified number of decibels. Instantaneous time deviation within ± 2dB
Interval. It reflects the response speed of the compression circuit to the increase in signal strength.
(2) Recovery time
In IEC118-2, the recovery time is defined as: the moment when the input signal suddenly drops from the specified sound pressure level to a lower sound pressure level, until the sound pressure at the hearing aid output stabilizes again to a lower steady-state sound pressure level, which Instantaneous time interval with deviation within ± 2dB.
It reflects how quickly the compression circuit responds to a decrease in signal strength.
(3) Setting of compression start time and recovery time
How to set the start-up time and recovery time has always been a question discussed by audiology researchers and hearing aid manufacturers. After a long period of research, most scholars believe that a short startup time (startup time of 5 to 10 ms) is a better choice.

However, the pros and cons of recovery time, and the ever-changing signal, do not yet have an optimal choice. This is discussed in detail below.
1) Long recovery time
When the syllable contains a vowel and a consonant, and the time interval between the vowel and the consonant is short, only a few tens of milliseconds, because the vowel sound pressure level is higher, the consonant sound pressure level is lower, and the hearing aid pairs are compressed The sound acts as a compression amplifier. If recovery time is greater than
The time interval between vowels and consonants is still compressed and amplified, which will cause the amplified consonants to be inaudible, resulting in a decrease in speech resolution, which is worse than a linear amplified hearing aid. With short recovery times, compression hearing aids compress the vowels.
Amplification, linear amplification of consonants, because the linear amplification has a higher magnification, both vowels and consonants can be heard clearly, and speech resolution is improved. However, the difference between the vowel and consonant sound pressure levels of the compressed hearing aid is smaller than when the original signal is input, which may cause some
Speech intelligibility declines. Long recovery time is suitable for short-term, high-intensity signals, generally large i 50ms.
2) Short recovery time
When the syllable interval is relatively long, the compression hearing aid quickly restores the linear amplification state after compressing and amplifying the vowel. Generally speaking, the gain at this time is larger than that of the linear amplification hearing aid, so the original speech base signal between syllables cannot be heard.
Feels like breathing or noise. There are two solutions: one is to reduce the overall gain, and the other is to increase the recovery time. Clinically, the startup time and recovery time of general hearing aids are not adjustable, and that only reduces the overall gain. Short recovery time for long duration, medium
The signal of equal intensity is generally less than 50ms.
3) adaptive recovery time
Due to the advantages and disadvantages of the recovery time in various environments, in order to obtain better compression results, adaptive recovery time was born. Its recovery time varies with input signal strength and time history. Most acoustic signals in daily life need to be long
Recovery Time.

3．Effective compression ratio
There are many factors that affect the effective compression ratio, such as the gain of the linear amplification region, the peak-to-valley ratio in the input signal, the average sound pressure level of the input signal, the start-up and recovery time, and the interval between the peak signals. The static compression ratio test is based on pure tone signals
Yes, the pure tone signal is a stable signal, but the speech signal is different from the pure tone signal, and it has fluctuations in intensity, so in real life, it is not always possible to reflect the change in gain displayed by static compression. In general, the compression ratio produced by speech signals will
Xiaoyu generates the compression ratio according to the definition of pure tone signal. This makes the effective compression ratio different from the static compression ratio, even for compression circuits with adaptive recovery time. Market research shows that the effective compression ratio of adaptive recovery time is close to 1. This
It is related to the long recovery time of most acoustic signals in daily life.

4．Peak and average detection
All compression systems require intensity detectors, typically peak detection and average detection methods, to detect the ambient sound around the listener and adjust the system’s gain. The intensity detection circuit generally changes the gain by testing a varying voltage. It usually
Including signal rectification and smoothing. The onset and recovery time of the system is related to this smoothing. The detectors for the startup time and recovery time mentioned above are peak detectors, and the startup time and recovery time are based on the peak value of the signal. Hearing aid now
The detector uses the average detector, which responds to the root mean square value of the signal amplitude. As can be seen in Figure 5-16, the compression amplifier circuit using peak detection, because the signal with relatively large fluctuations (such as speech signals), the gain changes quickly, so the peak detection voltage
The scaled output signal has less fluctuation, which increases signal distortion. And the compression circuit using average detection, because the signal with relatively large fluctuations (such as speech signals), the gain changes slowly, so the average signal compression amplified amplification output fluctuation
Large, similar to the input acoustic signal, which is conducive to the improvement of speech comprehension. In the average detection circuit, there is a control voltage generated by detecting the root mean square value of the signal, which is completed by the voltage detected by the resistance-capacitance low-pass filter. Its effect and release
Time is not independent of each other and is related to some parameters of the smoothing circuit.

So far, average detection is a better detection method in compression circuits.

3.4 Multi-channel compression
Due to the diversity of hearing loss curves, single-channel compression is difficult to accurately match the patient’s hearing loss at various frequencies, and multi-channel compression is more targeted.
In single channel compression, as long as the intensity of a certain frequency in the entire frequency range of the signal exceeds the compression threshold, compression amplification is started, and the difference in loudness of different frequency components in the signal does not change. Multi-channel compression divides the signal into several frequency bands. When the intensity of a frequency band exceeds
Compression threshold. Only compression for this channel is enabled, while signals in other frequency bands are still linearly amplified. Therefore, the intensity difference between the frequency band signal and other frequency band signals is changed. Therefore, when multi-channel compression is not set properly, the patient’s speech intelligibility is better than that of single-channel compression.
Could be worse.
3.5 Filters in compression circuits
As shown in Figure 5-17, in compression hearing aids, filters can be divided into three types: front filter, post filter, and inner filter according to the position of the filter in the compression circuit.

1. Front filter
The front filter is placed before the feedback loop, and its frequency response can affect the start of the feedback loop. If the filter is a high-pass filter, the input sound must be at a high level of compression before it is activated because the low frequencies are attenuated. Therefore, the compression threshold for low frequencies increases
Now, the compression threshold of high frequency is unchanged.
2. Post filter
After the post-filter is in the feedback loop, its frequency response characteristics will not affect the start of the feedback loop. The compression threshold for high and low frequencies will vary.
3．Internal filter
The internal filter is in the feedback loop and will only function when the feedback loop is activated. Its actual compression threshold is the same as the previous filter.
It can be seen that the different positions of the filter in the compression circuit will cause the compression characteristics of the hearing aid to change, thereby affecting the input and output characteristics of the hearing aid. Therefore, the actual dynamic characteristics of the hearing aid cannot depend on the input and output characteristics given in the technical indicators of the hearing aid, but a set of input and output curves must be measured under multiple input sound intensities to understand the dynamic characteristics of the hearing aid. 3.6 Advantages and disadvantages of compression technology to amplify hearing aids and applicability
1. Advantages of compression hearing aids
Benefits of compression-amplified hearing aids include:
④ The biggest advantage of compression hearing aids is that they are comfortable to wear. It avoids the discomfort of the linear amplification hearing aid when it is loud, and also reduces the harmonic distortion generated by the linear amplification hearing aid when it is loud, so patients are more willing to use it.
② Compression amplifying hearing aid provides greater gain at low and medium sounds than linear amplifying hearing aid, and speech intelligibility is improved. In order to avoid discomfort when loud, the linear amplification hearing aid was forced to reduce the gain, and the gain of the middle and small voices including the speech signal was also reduced below the required gain. And the gain of the compression amplifying hearing aid in small, medium and loud can be adjusted flexibly, which is suitable for the hearing loss of patients, and the speech intelligibility is naturally greater than that of the linear amplification hearing aid. This has been clinically proven.
2. Disadvantages of compression hearing aids
The disadvantages of compression amplification hearing aids include:
④ Although the harmonic distortion of the compression-amplified hearing aid is small, it changes the waveform and brings another kind of distortion, especially when the compression technical parameters are not set properly. Although the linear amplification hearing aid has a large distortion when it is loud, it has a small distortion when it is medium and small, and it will not cause signal distortion due to compression and reduce speech intelligibility.
② The output of the compression amplifying hearing aid is small. For patients with severe deafness, linear amplification of the sound quality of the high output hearing aid is generally preferred.
3．Suitability of compression amplification hearing aids
The applicability of compression hearing aids includes:
① Compression limiting is more suitable for patients with severe deafness or patients who are used to linearly amplifying the sound quality of hearing aids.
② Wide dynamic compression hearing aids are more suitable for patients with mild to moderate deafness.
③Wide dynamic compression combined with compression limiting or peak clipping can better apply to patients with different hearing loss. Wide dynamic compression is used for small and medium sounds, compression limit is used for loud sounds (generally above 80dB), and peak noise or digital is used for noise above loodB.
Clipping.
4 Frequency-shift hearing aid technology
Frequency-shift hearing aid technology refers to the technology of moving a high-frequency part of a certain bandwidth of a speech signal to a low-frequency part, and is suitable for patients with a high-frequency cochlear dead zone.
4.1 Defects and causes of traditional hearing aids
1. Defects of traditional hearing aids
The Speech Understanding Index (SII) shows that 27% of important information in daily communication is distributed above 3150Hz, and even nonsense speech content, the information in the high frequency part can be as high as 31% (1997). Taking the / s / tone as an example, the lowest spectral peak of the / s / tone is usually distributed at 2900 to 8900 Hz due to the gender difference of the speaker. Therefore, if people with moderate to severe hearing loss want high-quality hearing / s / tones, then the hearing aids they choose must give higher hearing compensation between 5000 and 9000 Hz. In fact, the demand completely exceeds that of traditional hearing aids. Available bandwidth.