FR2635680A1 - HEARING AID - Google Patents

Abstract

Disclosed is a hearing aid which amplifies speech signals more than noise signals. It includes a microphone 12, a variable high pass filter 14, a transducer 16 and a sensor assembly 18. The latter detects the amplitude of a low frequency band of the filtered signal from the microphone and applies a feedback signal to the variable filter. to vary its cutoff frequency according to the characteristics of the filtered signal. The sensor also takes into account the time characteristics of the signal envelope, distinguishing between permanent signals, analogous to noise, and dynamically varying signals, such as those of speech. Field of application: aids for the hearing impaired, etc.

Description

The invention relates to the manufacture of hearing aids. In general,

  Hearing aids receive sound signals and amplify them or

  modify otherwise to help the user hear.

  In particular, the invention relates to hearing aids that attempt to amplify speech signals more than noise signals and thus improve the clarity of the signal that the hearing aid ultimately transmits to the user. Electrical circuits for hearing aids are well known. An example of such a hearing aid circuit is described in United States Patent Application No. 838,924, filed March 12, 1986

  by James Anderson and Richard Brander.

  Since hearing aids are worn on or in the human ear, their circuit and battery must necessarily be physically small. As a result, this circuit must necessarily be

  bulky and must use low energy levels.

  Nevertheless, the circuit needs the complexity to produce an appropriate signal to help the user of the

  hearing aid, taking into account his particular impairment

to hear.

  Many users of hearing aids

  For example, they find it difficult to understand words when background noises are present simultaneously. The circuit of the hearing aids used by these people must attempt to amplify the speech signals more than the noise signals. The ability of the hearing aid circuitry to thereby "discriminate" speech and noise and adjust the amplification levels accordingly significantly improves the efficiency of this process.

  prosthesis and therefore the ability to hear of its user.

  Unfortunately, many of the techniques available for rejecting noise are poorly suited for use in a hearing aid. Many circuits suitable for the implementation of the techniques are physically large or consume large amounts of electrical energy. Other of these circuits are simply expensive to manufacture or perform minimal discrimination between speech and noise signals, or have undesirable side effects on the reproduced sounds. Prior art devices have used the fact that most noise signals contain high levels of low frequency energy and use the presence of this energy to reduce low frequency amplification if a threshold level is exceeded. . Unfortunately, speech also contains a high energy & low frequency that can trigger

  such circuits so as to reduce the gain in low frequency

  quence, even if no background noise is present,

  which mitigates the naturalness of reproduction.

  It would be desirable to take into account the time characteristic of the signal envelope to distinguish between steady-state noise-like signals and varying signals.

  dynamically, such as speech. This is a

  of the present invention.

  Other examples of devices of the prior art are described in US patents

  No. 4,490,585 and No. 4,409,435.

  A main aspect of the invention is a hearing aid that improves the discrimination between speech waves and noise signals. The hearing aid comprises an assembly formed of a microphone, a variable filter, a transducer and a sensor. The microphone receives an audible signal and produces in response to this signal an electrical signal of

  microphone. The microphone signal is characterized by

  characteristics of both frequency and amplitude. The variable filter receives the signal from the microphone and transmits a filtered signal to the transducer. The latter converts the filtered signal into a sound signal for the user of the

hearing aid.

  The sensor assembly detects the amplitude of a low frequency range of the filtered signal and produces, in response thereto, a feedback control signal applied to the variable filter. The control signal to

  The reaction thus varies the characteristics of the filter.

  Accordingly, when the sensor assembly detects that the filtered signal has characteristics indicating that the hearing aid is in an environment of high noise, the. filter stops one more

  large proportion of lower frequency signals.

  Conversely, when the sensor detects a filtered signal having no characteristics indicating that the hearing aid is in a high noise environment, the filter allows passage to the transducer of a

  higher proportion of low frequency signals.

  In another embodiment of the invention, the sensor assembly comprises a filter of

  sensor, a level detector and a smoothing circuit.

  The sensor filter produces an activation signal in response to a low frequency filtered signal provided by the variable filter. The level detector produces only an activation amplitude signal in response to an activation signal having an amplitude exceeding a predetermined level. The activation amplitude signals from the level detector are then applied to the smoothing circuit, which itself applies the signal of

  reaction control to the variable filter.

  In another embodiment of

  the invention, the smoothing circuit responds to the characteristics

  recent markings of the microphone signal. The smoothing circuit defines pickup and pickup times that affect the nature of the feedback control signal that the smoothing circuit applies to the variable filter. As a result of the operation of the sensor assembly, signals in a substantially permanent mode, of greater amplitude and of lower frequency, such as noise, cause the sensor assembly to produce a reaction signal which, in turn, leads to the variable filter blocking a larger proportion of the lower frequency signals produced by the microphone. Conversely, weak signals, higher frequency, filtered and

  highly dynamic, such as speech, produce

  feel a different reaction signal. This feedback signal allows the variable filter to pass a proportion of

  higher level of low frequency signals to the transduc-

  tor. An object of the invention is therefore to propose

  a hearing aid that discriminates more

  noise. Another object of the invention is to provide a smaller hearing aid which provides better discrimination between noise and speech and

  greater amplification of speech than noise.

  Another subject of the invention relates to a hearing aid which detects in a simpler way the

  presence of noise and which consequently adjusts its

  frequency response characteristics. Another object of the invention is to provide a hearing aid that performs better noise discrimination, uses fewer parts and is more cost effective to assemble. Another object of the invention is to provide a hearing aid having improved noise discrimination characteristics, and

  calling less current to the prosthesis battery.

  The invention will be described in more detail with reference to the accompanying drawings by way of non-limiting example and in which: Figure 1 is a simplified block diagram of a preferred embodiment of the invention; FIG. 2A is a first graph showing a hypothetical example, given for illustrative purposes only, of the envelope of a microphone signal produced by the microphone of the hearing aid shown in FIG. 1, while the prosthesis is in the presence of noise; Fig. 2B is a second graph showing an illustrative example of a feedback control signal that the sensor assembly of the hearing aid can produce in response to a filtered signal; Fig. 2C is a third graph showing an illustrative example of a feedback control signal that can be produced by a prior art hearing aid in response to a filtered signal; FIG. 3A is a first graph showing a hypothetical example, purely illustrative, of the envelope of a microphone signal produced by the

  the hearing aid shown in FIG.

  it is in the presence of sounds containing information

  tion; Fig. 3B is a second graph showing an illustrative example of a feedback control signal that the sensor assembly of the hearing aid can produce in response to a filtered signal; Fig. 3C is a third graph showing an illustrative example of a feedback control signal which may be produced by a hearing aid of the prior art in response to a filtered signal; and Figure 4 is a schematic of the preferred form

  embodiment shown in Figure 1.

  With reference to FIG. 1, the preferred embodiment of the invention is shown in the form of a hearing aid 10. This comprises a microphone 12, a variable filter 14, a transducer amplifier 16 and a sensor assembly 18. The microphone 12 receives speech sounds (containing information) and noise (not containing information) and consequently produces an electrical signal. The microphone signal present at the

  a frequency characteristic and a characteristic

  amplitude tick. The variable filter 14 receives the microphone signal and, in response, applies a filtered signal to

1the transducer-amplifier 16.

  The variable filter 14 of the preferred embodiment is a high pass filter having a feedback control input. The cutoff frequency of the high-pass filter varies depending on the control signal received through the control input. The variable filter 14 delivers a filtered high-pass form of the

  microphone signals, which constitutes the filtered signal.

  The transducer amplifier 16 comprises a

  variable main amplifier 22 and receiver 24.

  The amplifier 22 receives the filtered signal and, in response to this signal, applies to the receiver 24 an amplified signal

  having a different amplitude, usually larger.

  The receiver 24 receives the amplified signal from the amplifier

  22 and produces an audible sound for the user of the hearing aid. The amplification performed by the main amplifier 22 can be adjusted to vary the volume of the sound produced by the prosthesis

auditory 10.

  The sensor assembly 18 includes a threshold control, a bandpass filter 26, a level detector 30 and a smoothing circuit 32. The threshold control 25 allows the level below which the sensor assembly 18 to respond. not. The bandpass filter 26 receives the filtered signal and produces & outputs a portion of all

  filtered signal having particular characteristics.

  In the preferred embodiment, the pass-band filter 26 has a center frequency of about 250 hertz. Components of the filtered signal included in the frequency band around 250 hertz are transmitted to the level detector. These components

  are called, for convenience, bandpass signal.

  Level detector detects if a part

  any of the bandpass signal from the pass filter

  band 26 exceeds a particular predetermined amplitude level. If so, the level detector 30

  applies an activation signal to the smoothing circuit 32.

  The smoothing circuit 32 itself applies a feedback signal to the control input of the variable filter. The reaction signal depends on the recent past evolution of the

  filtered signal that the sensor assembly 28 has detected.

  In general, hearing aid users prefer that sounds containing information, such as speech, be amplified more by the prosthesis than the ambient noise. This ambient noise may include, for example, a "hubbub", the cumulative effect of entering a large room occupied by a large number of people all speaking at once. Although the hubbub is made up of different track sounds, the overall effect of all the talk- ing channels is to produce a noise. If this noise is amplified to the same degree as the sound of the path of a person standing beside the user of the hearing aid and speaking with him, this user may have difficulty distinguishing between noise (the cumulative sounds of all channels speaking together) and the particular way that the user

wish to understand.

  It has been noted that the reduction of the low frequency response of a hearing aid in such a situation improves the intelligibility of the signal finally produced for the user of the prosthesis. In the preferred embodiment, desired speech signals, contaminated by the buzzing of a hubbub, can be amplified with a reduced low frequency response compared to microphone signals associated with a noise.

speech free of noise.

  It has been noted that such ambient noise tends to be characterized by microphone signals which are large (high amplitude), of a substantially stabilized regime (having an amplitude envelope of substantially

  constant), low frequency (less than 500 Hz).

  By way of illustration only, an example of a hypothetical signal, which could be produced by the microphone 12 when the hearing aid 10 is in the presence of ambient noise, is represented by the first graph of FIG. 2A. An example of a hypothetical signal that could be produced by the microphone 12 when the hearing aid 10 is in the presence of a sound containing information or speech, is shown by a first graph shown in Figure 3A. The whole amplitude of the signal (the waveform that is obtained when the successive signal peak values of the signal are connected together) is represented along the axis

  vertical, the time being represented by the horizontal axis.

  As shown by the examples of FIGS. 2A and 3A, the noise signal of FIG. 2A and the speech signal of FIG. 3A have the same peak amplitude. For example, it is assumed that the two signals of

  microphone associated with FIGS. 2A and 3A are approximate-

  the same frequency, less than 500 hertz. It has been found that, in the case of the microphone signal shown in FIG. 2A, the sensor assembly 18 must apply a feedback control signal to the control input of the variable filter so that the cutoff frequency of the variable filter 14 is raised to substantially block the lower frequency components of the microphone signal. However, in the case of the signal shown in FIG. 3A, the feedback control signal applied to the control input of the variable filter 14 must cause this variable filter 14 to maintain a substantially lower cutoff frequency; this allows the greater part of the lower frequency signal to pass substantially to the transducer amplifier 16 and to be amplified and produce an audible sound for

  the user of the hearing aid.

  The feedback signal produced by the smoothing circuit 32 is responsive to the "historical" signals previously produced by the filtered signal 12. The smoothing circuit 32 is substantially slow in increasing the feedback signal if, for example, the microphone 12 does not produce first no signal, then begins to send a signal to the variable filter 14. In other words, the smoothing circuit 32 has a slow "attack". However, after a low frequency signal has been generated by the microphone 12 for a significant period of time, the smoothing circuit 32 begins to apply a control signal of

  reaction to the control input of the variable filter 14.

  In the case where the low-frequency component of the microphone signal continues, the smoothing circuit 32 continues to send a feedback control signal to maintain at a higher level the cutoff frequency of the microphone.

  variable filter (high-pass) 14. However, if the

  As the microphone's low frequency dropout drops sharply, the control signal applied to the variable filter 14 drops smoothly. The time for this fall to occur is the "release time" of the smoothing circuit 32 and it is

  short (compared to the attack time).

  The signal of Figure 2A could be seen as having long periods of constant sound level ("A") with relatively short cuts of lesser amplitude ("B"). This signal is said to have a high "duty ratio" envelope. The duty cycle refers to the ratio, in time, of the high amplitude portions to the low amplitude portions of the signal envelope. The signal of Figure 3A has shorter periods of high peak amplitude ("A") and longer periods of lower amplitude ("B"). This signal has a lower duty cycle envelope. It is not necessary that the signals of FIGS. 2A and 3A have periodic and well-defined amplitude variations as shown. Speech and noise have relatively random amplitude and period characteristics, but the signals described above represent the type of envelope difference that can occur and provide a better understanding of the effect of the envelope duty cycle. on the output signal of the amplitude detectors with characteristics of attack time and

different release.

  Such feedback control signals are presented graphically by the curves of FIGS. 2B and 3B. In Fig. 2B, for example, the feedback control signal remains substantially high (close to "A"), keeping the variable switching frequency of the variable filter 14 at a high position. In contrast, in the case shown in Fig. 3B, the feedback control signal is substantially less than the feedback control signal shown in Fig. 2B. As a result, the cutoff frequency is maintained at a substantially lower level. A greater proportion of the low frequency signal associated with the example of FIG. 3B is therefore

  transmitted to the transducer amplifier 16.

  In usual systems & automatic gain control, the attack times are short in comparison with the release times. This essentially results in a peak detection system. If such a conventional gain detection control system was used, the filtered and recovered control voltage would tend to follow the peaks of the microphone signal envelope. This is shown for illustrative purposes only in FIGS. 2C and 3C. However, it has been noted that if the attack time is increased relative to the release time, the control voltage tends to follow more closely the minimum values of the envelope of the voltage of the

  control and to allow the hearing aid 10 to perform

  kill discrimination between speech and noise and realize a greater reduction of low frequencies for noise than for speech, with the same amplitude of

peak as noise.

  As shown in FIG. 4, the variable filter 14 is of a conventional embodiment comprising a

  capacitor 34, a resistor 36 and a transducer 38.

  Transistor 38 is an NPN transistor, connected in reverse polarity to its normal configuration (this produces a lower DC offset voltage across the collector and emitter terminals of transistor 38). Transistor 38 includes a base that defines the input 20 of

reaction control.

  The transistor 38 is connected in parallel with the

  36. Therefore, when the state of

  transistor 38 moves to a conductive state.

  The resistor 36 is short-circuited by the reduced impedance between the collector and the emitter of the transistor 38 and the cut-off frequency of the variable (high-pass) filter 14 is moved upwards. Conversely, when the transistor 38 is blocked, the resistor 36 is not short-circuited and the filter cut-off frequency

  variable 14 is at a lower level.

  The transducer amplifier 16 comprises a source source field effect transistor 40, a gain control resistor 42, a DC power stop capacitor 44, a variable gain main amplifier 46 and the receiver 24. The transistor 40 establishes a buffer impedance between the variable filter 14 and the rest of the circuit. The resistor 42 regulates the gain of the amplifier 46 and the capacitor 44 stops the DC component of the source transistor 40,

  from the main amplifier 46.

  The threshold control 25 of the sensor assembly 18 includes a potentiometer 48 which can be manually adjusted to modify the sensitivity of this set

sensor 18.

  The bandpass filter 26 comprises an inverting amplifier 50 and a first capacitor

  52 and a first resistance 54, and a second

  56 and a second resistor 58. The first capacitor and the first resistor 52, 54 establish the lower cutoff frequency of the bandpass filter 26 and the second capacitor 56 and the second resistor 58 set the upper cutoff frequency of the filter 26. The resistors 58 and 54 set the gain of the bandpass filter in the bandwidth. In the preferred embodiment, the filter 26 has a center frequency of about 250 Hz; signals having a frequency between about 100 and 350 hertz are transmitted through the bandpass filter 26 to constitute the signal

  bandpass applied to the level detector.

  The level detector comprises an amplifier

  inverter 60, and a first resistor 62, a capacitor 64, a power source 66 & 0.9 volt, a second resistor 68 and a Schottky diode 70. The first resistor 62 and the first capacitor 64 apply the pass signal. band at the inverting amplifier 60. The capacitor 64 stops the DC component. When the

  current of the band-pass signal called through the resistor

  In this case, the amplifier 62 exceeds the current supplied by the source of 0.9 volts through the resistor 68, the inverter amplifier 60 applies an activation amplitude signal to the smoothing circuit 32. The Schottky diode 70, mounted between the output and the input of the level detector, blocks the output to less than about 0.3 volts from the input, making the level detector less sensitive to variations in

  battery voltage of the hearing aid 10.

  The smoothing circuit 32 receives an activation signal only when the filtered signal has reached a particular amplitude as set by the level detector. The smoothing circuit 32 comprises an RC circuit

  consisting of a resistor 72 and a capacitor 74.

  The smoothing circuit 32 receives the activation signal and in response applies the feedback signal to the reaction control input 20 of the variable filter 14. The output of the smoothing circuit 32 has an attack time of about 1 second and a release time of about 300 milliseconds. It has been noted that in the preferred embodiment, the release time is

  advantageously less than half the attack time.

  Accordingly, the feedback signal applied to the feedback control input may correspond in a manner

  general to the graphs of Figures 2B and 3B.

  Therefore, the smoothing circuit 32 transmits larger feedback signals to the feedback control input for more stabilized microphone signals (such as a noise signal), as compared to a microphone signal having the same effect. same peak amplitude, but whose amplitude varies more dynamically (such as a speech signal). The larger reaction signal developed for the noise signal with respect to the speech signal results in raising the cutoff frequency of the variable filter 14 for the noise signal, resulting in greater attenuation of the noise signal.

  low frequency response of the hearing aid 10.

  A speech signal having the same peak amplitude,

  undergoes lower attenuation of the low frequencies.

  It goes without saying that many modifications can be made to the hearing aid described and

  shown without departing from the scope of the invention.

Claims (8)

  A hearing aid, characterized in that it comprises a microphone (12) for receiving a sound signal and producing, in response thereto, an electrical microphone signal which has both frequency and frequency characteristics. amplitude, a variable filter (14) defining variable filtering characteristics and a feedback control input (20) for receiving a feedback control signal and   vary, in response to this signal, said characteristics   filtering device, the variable filter producing a filtered signal having both frequency and amplitude characteristics, the hearing aid also having a transducer (16) for receiving the filtered signal and, in response thereto, to produce a sound, and   sensor means (18) responsive to said characteristics   frequency and amplitude of the filtered signal and applying in response thereto said reaction control signal to said feedback control input of the variable filter.   2. Hearing aid according to claim 1, characterized in that it further comprises a reaction filter (26) intended to receive the filtered signal and to produce consequently a second filtered signal having characteristics of both frequency and frequency. amplitude, a level detector (30) for producing an activation signal when the amplitude characteristic of the second filtered signal exceeds a predetermined level, and smoothing means (32) for applying said control signal of reaction to the variable filter in response to   activation signal from the level detector.   Hearing aid according to claim 2, characterized in that the smoothing means define &   both an attack time and a release time.   4. Hearing aid according to claim 3, characterized in that the attack time is greater than the time of release.   Hearing aid according to claim 4, characterized in that the attack time is more than two   times longer than the release time.   6. Hearing prosthesis according to one of the claims   3 and 5, characterized in that the smoothing means   comprise a resistor (72) and a capacitor (74).   7. Hearing prosthesis according to one of the claims   3 and 5, characterized in that the reaction filter   comprises a bandpass filter (26).   Hearing aid according to claim 7, characterized in that the bandpass filter defines a   center frequency of about 250 hertz.