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The concept of "frequency shifting techniques" has a long history. As early as the 1950s and 1960s, researchers tried to apply "frequency lowering techniques".

Frequency-shift hearing aid technology

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The concept of "frequency shifting techniques" has a long history. As early as the 1950s and 1960s, researchers tried to apply "frequency lowering techniques".

Chinese name

Frequency-shift hearing aid technology

Foreign name

Frequency Transposition Algorithm in Hearing Instruments

Presentation time

1950s and 1960s

Statistical principle

1000-2000HZ based sound spectrum information

The hearing aid industry is, as always, developing hearing aids for people with hearing impairments to improve their hearing. The technological developments facing audiologists and users are astonishing: digital signal processing hearing aids, multi-microphone hearing aids, FM receivers integrated in hearing aids, disposable hearing aids, etc. Candidates can't help asking: What technology can best improve the patient's hearing? What techniques can best improve the quality of life for patients? How to evaluate the effectiveness of hearing aids? Can hearing-impaired patients hear more clearly in a noisy environment? Are digital hearing aids better than analog ones? Is it easier for patients to understand speech?

Although the above series of technological advances have indeed improved patient comfort, they still cannot solve the problem of understanding language in all auditory environments (except FM). With these advanced technologies, why is improving speech understanding still a challenge? Why is it always a challenge to choose hearing aids for people with severe hearing impairment? Why is it so difficult to test a patient's hearing aid?

Traditional hearing aids regulate the amplitude of the signal to make the amplitude of each frequency larger. This concept meets the needs of some patients. However, some studies suggest that when high-frequency hearing loss is greater than 60dB, amplifying the sound in these frequency bands will not only improve speech recognition, but will actually have a negative effect.

The reasons for the above results are:

Non-frequency-shifting hearing aids use the traditional principle of amplitude compression and amplification. The amplitude amplification and frequency bandwidth are limited by the performance of the microphone and speaker, which makes its gain effect at high frequencies, especially above 2 kHz, limited. According to the classic 1/2 compensation principle, the hearing aid effect of the hearing aid using only the principle of amplitude compression amplification is not enough to compensate the gain demand of patients with high-frequency hearing loss exceeding 60 dB.

It is related to the acoustic and physical characteristics of consonants in language. The center frequency of most of the unvoiced consonants is above 4 kHz, and the sound intensity range in most daily conversation sound spectrum is mainly between 5 ~ 40 dB HL, with an average of 30 dB. Therefore, once a patient's hearing loss above 4 000 Hz exceeds 60 dB, according to the 1/3 compensation principle, it is possible to perceive high-frequency consonants only when they can obtain a high-frequency gain of at least 20 dB. Non-frequency-shifting hearing aids can hardly produce 20 dB gain at frequencies above 4 kHz, so for patients with severe hearing loss and above, non-frequency-shifting hearing aids do not help much in the perception of clear consonants. Because more than 35% of important speech information is distributed above 2 kHz, even if the hearing aid can compensate the patient for the amount of gain needed for high-frequency hearing loss, if the patient's hearing system cannot benefit from these amplified signals, how can a hearing rehabilitation expert make him Users (hearing aid users) improve speech understanding? .

It is related to the acoustic up-diffusion masking characteristics, which determines that low-frequency sounds have more masking properties for high-frequency sounds. In view of this, many hearing aids have adopted a method of reducing the low frequency gain to reduce the noise of the hearing aid. The vowel spectrum of speech phonemes is mostly distributed at low frequencies, and the consonant spectrum is mostly distributed at high frequencies. If the hearing aid has sufficient gain in the low and medium frequency bands and the high frequency gain is insufficient, it will produce amplified sounds. Masking. This is exactly why many foreign studies have proved that when a patient's high-frequency hearing loss exceeds 60 dB, traditional hearing aids have no effect on the patient's speech discrimination, but have a negative impact.

Patients with sensorineural hearing loss need more and better signal-to-noise to achieve normal speech discrimination under normal signal-to-noise conditions. Therefore, a hearing aid device is required to further improve the signal-to-noise ratio of consonant phonemes that are essential for speech recognition and understanding based on traditional noise reduction. And, how can you quickly evaluate the effectiveness of hearing aids for many users in busy daily work?

It is precisely because of the shortcomings of traditional hearing aids that many of the needs of hearing impaired patients cannot be met, so that audiologists from various countries are committed to researching new solutions. Just then, the concept of "frequency shifting techniques" was proposed.

As early as the 1950s and 1960s, researchers tried to use "frequency lowering techniques" for hearing aids and language training (Ben net and Byers, 1967; Beasley et al, 1976). Professor Zhang Jialu, a famous scientist in China, also An article on frequency-shifting technology was published in 1998. The idea was to match the bandwidth of the speech signal to the patient's dynamic range of sensitive residual hearing rather than attempt to respond to the disappearance of high-frequency hearing. There are many methods that can be used for out-of-scale and proportional frequency conversion, however they have not been used for wearable devices or speech training tools for many hours. For example, a frequency conversion device is a method of non-proportional frequency conversion. It causes unnatural changes in speech sounds by constant frequency movement. It does not help language understanding. Because the method of non-proportional frequency conversion cannot protect the corresponding relationship between morphemes in speech signals, it interferes with hearing. Proportional frequency shifting is similar to the slow playback of a recorder, that is, recording at a normal speed and then playing at a slower speed, so this method maintains the corresponding spectral position relationship between the morphemes in the language.

The selection of hearing aids for people with severe hearing impairment has always been a challenge. Common hearing aids not only fail to solve these problems, but also lead to new problems. Candidate technologies or devices to address this challenge are "frequency shifting" technology and the cochlea. Davis Penn and Ross (1993) of the Australian National Hearing Laboratory believe that frequency-shifting hearing fills the gap between ordinary hearing aids and electronic cochlea. "Frequency-shifting" hearing aids are a signal processing strategy that avoids gain restrictions and useless hearing, namely Proportional Frequency Compression. Proportional frequency compression allows the elector to "move" key high-frequency speech information to low-frequency regions with better residual hearing. With this technique, it is possible to "match the bandwidth of the input speech signal to the limited frequency band where the affected ear is most sensitive, rather than attempting to respond to the disappearance of high-frequency hearing."

The concept of "frequency shifting techniques" has a long history: as early as the 1950s and 1960s, researchers tried to apply "frequency lowering techniques" for hearing aid and speech training. (Ben net and Byers, 1967; Beasley et al, 1976), a well-known Chinese professor Zhang Jialu also published an article on frequency shifting technology in 1998. The idea at that time was to match the bandwidth of the speech signal to a patient's better dynamic range of sensitive residual hearing rather than attempt to respond to the disappearance of high-frequency hearing. There are many ways to perform out-of-scale and proportional frequency conversion, however they are not used in wearable devices or speech training tools. For example, the transposition device is a method of frequency conversion that is not proportional. It causes unnatural changes in speech sounds by constant frequency movement. It does not help language understanding. Because the method of non-proportional frequency conversion cannot protect the corresponding relationship between morphemes in speech signals, it interferes with hearing.

Proportional frequency shifting is similar to the slow playback of a recorder, that is, recording at a normal speed and then playing at a slower speed, so this method maintains the corresponding spectral position relationship between the morphemes in the language. However, it is unrealistic to turn a recorder into a wearable device for real-time recording and slow playback.

The United States EMILY has been using a "frequency shifting" device. The principle of the device statistics is based on the sound spectrum information in the range of 1000-2000HZ is very important for language acceptance. The EMILY device can detect the phoneme and energy composition in this range, and then move and copy it in the higher or lower frequency range, making its sound characteristics more easily detectable. This frequency shift allows the selected phonemes to be increased in real time. Although this technology can be used on wearable devices, it shows promise for patients with a wide range of hearing loss. However, relevant clinical data are limited. The early frequency-shifting hearing aid produced by Hyde Company is a kind of extra-high-power hearing aid (1994) that uses slow-play frequency-shifting technology. In order to overcome the limitations of traditional hearing aids, it applies proportional slow-play frequency compression. An electronic tape device that moves and compresses speech signals proportionally in "real time", and also promotes consonant information of weak high-frequency consonants. It moves useful high-frequency signals to the available low-frequency region as much as possible. In theory, moving the high-frequency consonant signals that the patient cannot hear to the audible low-frequency region should overcome the dilemma of ordinary hearing aids being incapable of treating patients with little or no high-frequency residual hearing.

Several literatures in recent years have mentioned the use of proportional frequency compression as a method of improving hearing 7-8. Although there are various proportional compression schemes 7-9, so far, Hyde is the only company that can produce ear-level hearing aids with proportional compression. ImpaCt and Logom hearing aids.

ImpaCt is specially designed to improve the speech intelligibility of deaf people with high-frequency hearing loss. She dynamically analyzes the frequency, amplitude and time characteristics of speech based on the real-time digital analysis of the input speech signal. That is, the traditional amplitude compression amplification Based on the strategy of hearing aids, it adopts unique real-time dynamic speech re-coding (DSR), dynamic consonant boost (DCB), and frequency compression (Proportional Frequency Compression, FC) similar to cochlear language coding. ) Digital processing technology and a large number of standard programmable optional parameters improve the language discrimination ability that is important for speech understanding.Automatic gain control (AGC) processing of low frequency vowel signals to keep them at the optimal threshold (MCL) At the same time, ImpaCt's digital phoneme analyzer follows the dynamic range DSR control of speech. When the high-frequency consonants in speech are detected, the DSR function is activated immediately, pushing the high-frequency consonant signals into the patient's remaining auditory dynamic range. But too much frequency compression, although it can increase awareness, will reduce speech discrimination. The use of DCB and frequency compression (FC) technology effectively balances the relationship between speech perception and discrimination. ImpaCt is especially suitable for patients with normal or mild low frequency hearing loss and severe high frequency hearing loss. These patients are currently the largest group of people with hearing impairment.

Hearing rehabilitation workers should carefully consider the various options available during hearing rehabilitation for high-frequency hearing loss. The current findings support the switch to frequency-shifting hearing aids with frequency compression. If we recognize the importance of high-frequency unvoiced consonants, especially the / s / sound with a lot of grammatical information in English, then we can understand the significance of switching to "frequency shifting" hearing aids. People wearing "frequency-shifting" hearing aids can hear plural sounds, possessives, third-person singulars, and so on. These sounds are of great significance for speech and language development, speech understanding and speech production.

Optional hearing aids are like hearing rehabilitation workers and patients playing a "lucky wheel" game. If we only provide patients with voiced consonants and vowels that are easy to recognize, it will be difficult for patients to solve the problem of speech understanding. If we provide unvoiced consonants, the patient's ability to "solve problems" is greatly increased. When hearing aid professionals choose techniques to provide patients with the perfect sound "picture", they need to reconsider how hearing aids amplify speech; how do amplified sound signals reach the hearing-impaired ears? And how to evaluate the effect of hearing aids? Proportional frequency compression is a tool designed for hearing aid professionals to improve speech discrimination for patients with high-frequency hearing loss.