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.

The change of speech signal is formed, but because of the non-proportional frequency conversion method used, the relative relationship between morphemes in the speech signal cannot be stabilized, resulting in the extremely unnatural change of speech sound. Language understanding does not help, but interferes with normal hearing function.
At the end of the 20th century, although hearing aid technology has been able to better meet the needs of the hearing impaired, Davis Penn and Ross (1993) of the Australian National Hearing Laboratory still believe that hearing aid frequency shifting technology can avoid gain restrictions Signal processing strategies in the unused hearing band are still the best solution to fill the gap between ordinary hearing aids and cochlear implants. Some studies at home and abroad also show that frequency-shift hearing aids can significantly improve the speech comprehension ability of people with severe high-frequency hearing loss than non-frequency-shift hearing aids.

In 2003, the American Hearing and Speech Society’s guidance program for fitting hearing aids for infants and young children also clearly stated that, for people with severe high-frequency hearing loss and above, when a traditional hearing aid is not available to hear key speech, frequency-shift hearing aids Is a good choice.

Figure 5-18 Proportional frequency shift compression
(The curves represent the typical spectra of the consonants / sh / and / s / respectively, and the shaded area is the frequency range of the hearing aid)

8s。 As shown in the spectrum shown in Figure 5-19 (a), the horizontal axis is “She sinks instead of swimmlng.” Among them: ① The energy of / sh / in “she” is concentrated above 2000Hz. ② “e” / e / starts at about 150ms. The first phoneme of / e / appears at about 400 Hz, and the second phoneme is at 2600 to 2800 Hz. ③ The formant transition point of / e / in “she” is at the beginning of the first / s / in “sinks”, at about 250ms. ④ The first energy band of / s / in “sinks” is located between 2000 and 3000Hz, and the second energy band is more obvious, and it must be located above 5000Hz.
Then, when the hearing aid wearer has no available hearing above 2000Hz, it can only hear sound signals below 2000Hz. Therefore, what he hears at this time is “_e_in (k) i_ead o_wlmmin.” You can hear the first energy band but you cannot hear the second energy band).
Comparing Fig. 5-19 (b) with Fig. 5-19 (a), it can be seen that the key high-frequency information is already below 2000Hz, and at the same time: ④ The energy of / sh / in “she” is concentrated at 500 ~ 3500Hz. ② The “e” in / e / remains in place, and the conversion between front and back phonemes is not affected, that is, the selectivity of dynamic speech recoding processing.
③ The energy of the first / s / in “sinks” is concentrated at 1100 ~ 3700Hz. 2) Proportional frequency-shift compression hearing aids ImpeCt and Logicom hearing aids from Hyde Hearing Aid Co., Ltd. are proportional frequency-shift compression hearing aids, and the frequency compression control range of clear consonants is 1.5 to 5.0 times, and each 0.25 is a step. There are 15 stops.

These two hearing aids can perform real-time digital analysis based on the input speech signals, and dynamically adjust the frequency, amplitude and time characteristics of speech, that is, based on the traditional amplitude compression amplification hearing aids, the unique hearing aids are similar to the cochlear language. Real-time dynamic speech recoding (DSR), dynamic consonaboost (DCB), frequency compression (FC) digital processing technology, and a large number of standard programmable parameters to improve understanding of speech Important language discrimination. For low-frequency sound signals, automatic gain control (AGC) processing is used to keep it at the optimal threshold (MCL). At the same time, the built-in digital phoneme analyzer follows the dynamic range of the speech to perform DSR control. Once identified, the DSR function is activated immediately, pushing the high-frequency consonant signal within the wearer’s remaining auditory dynamic range. Of course, although more frequency compression can improve speech perception, it will reduce speech discrimination. Therefore, ImpaCt and Logicom use dynamic consonant boost (DCB) and frequency compression (FC) technology to balance speech perception and discrimination Relationship.