Electrical audio signal processing systems and devices – Hearing aids – electrical – Noise compensation circuit
Reexamination Certificate
1992-07-29
2003-05-13
Le, Huyen (Department: 2643)
Electrical audio signal processing systems and devices
Hearing aids, electrical
Noise compensation circuit
C381S321000, C381S083000
Reexamination Certificate
active
06563931
ABSTRACT:
TECHNICAL FIELD
The present invention relates to auditory prostheses and, more particularly, to auditory prostheses for adaptively filtering a selected auditory component from an auditory input signal, and methods for doing the same.
BACKGROUND OF THE INVENTION
Auditory prostheses, particularly hearing aids, are utilized by persons having impaired hearing or by persons who want to improve their hearing acuity. While such auditory prostheses are often extremely beneficial in quiet environments, they are usually of more limited benefit in environments which are noisy.
Environmental noise is often tolerated by persons with unimpaired hearing with no more discomfort than annoyance at the existence of such noise and the reduced ability to understand speech in the presence of such noise. However, for persons with impaired hearing fitted with an auditory prostheses or a hearing aid having a fixed frequency response, environmental noise is often disturbing, often interferes with their ability to understand speech, and is sometimes physically painful.
Environmental noise can be classified as follows:
(1) relatively short duration noise such as the clicking of shoes during walking or of dishes during stacking, i.e., so-called “punctate noise”;
(2) relatively long duration noises having near-stationary spectral characteristics such as the noise associated with passing cars, trains and airplanes or running fans or machinery, i.e., so-called “constant background noise”; and
(3) relatively long duration noises that lack stationary spectral characteristics such as a background conversation.
The latter class of noise noted above may partially mask speech preventing its being understood by a hearing aid user and is disturbing for this reason. However, this type of noise generally does not assault the impaired user's ear as much as do the “punctate noise” and “constant background noise”.
A relatively satisfactory solution to the problem of punctate noises is obtained by incorporating automatic gain control (AGC) into the circuitry of the hearing aid. Such circuitry responds to a sudden, high intensity click, by automatically reducing the volume for the duration of the click. This reduces not only the intensity of the sound of the click, but also reduces the intensity of the sound of any intelligence occurring simultaneously with click. Little loss of intelligibility of speech occurs, however, because of the short duration of the gain reduction and the ability of the ear, in cooperation with the brain, to fill in the relatively short information gap depending on the attack and release times of the AGC circuitry.
In contrast however, constant background noise generally contains much the same frequency spectrum as the desirable speech signal. Therefore, schemes to remove constant background noise must avoid diminishing the intelligibility of the speech signal.
In general, two methods of constant background noise removal have been employed in prior art hearing aid devices.
In one technique used in some auditory prostheses, or hearing aids, a single microphone is used to receive both wanted and unwanted parts of the auditory signal and the total auditory signal is processed to de-emphasize the unwanted part, i.e., the noise, relative to the wanted part, i.e., the speech. For example, a good deal of unwanted noise usually exists in the low frequency bands of speech and can actually mask some of the desired high frequency parts of speech. (This is called the upward spread of masking.) By de-emphasizing the lower frequency parts of the signal, i.e ., attenuating the frequencies between 50 and 500 Hertz, for example, the unwanted noise signal is decreased (along with some of the wanted speech signal) making the higher frequency parts of the speech discernible. The overall effect can be to increase the intelligibility of speech in the presence of noise.
One variation of the single microphone technique is to provide a directionality to the microphone so that the wearer (user) can optimize the wanted part of the signal, the speech, while decreasing any unwanted part of the signal, the noise, which is not directionally coincident with the speech signal.
In either case, however, these techniques suffer from the fact that both the wanted and unwanted parts of the auditory signal are picked received by the single microphone. Since there is very little spectral difference between the signals, the ability to separate them is limited.
Another technique for auditory prostheses, or hearing aids, uses two microphones. One microphone is used to receive the total auditory signal (including the wanted speech and unwanted noise parts of the auditory signal). A second microphone is used to receive the unwanted noise part of the auditory signal. The unwanted noise signal from the second microphone is then “subtracted” from the total auditory signal from the first microphone to provide “noise-free” sound. This technique depends on positioning the second microphone so that it receives only the unwanted part of the auditory signal, i.e., the noise, for optimum operation. This generally is not possible in a hearing aid because both microphones have to be carried on the user, making it very difficult to position the second microphone so that it picks up only the unwanted part of the auditory signal. Thus, the second microphone usually picks up some of the wanted speech signal as well as unwanted noise signal. This results in some cancellation of the wanted speech signal as well as the unwanted noise signal.
Yet another background noise filtering technique is illustrated in the system disclosed in U.S. Pat. No. 4,025,721, Graupe et al, Method of and Means For Adaptively Filtering Near-Stationary Noise From Speech. The Graupe et al '721 patent discloses a single microphone hearing aid system having a noise filter between the microphone and the amplifier section of the hearing aid. The filter is designed to filter out the constant background noise present in the user's environment. The filter includes means for continuously adjusting itself in response to the prevailing noise conditions. The system disclosed in the Graupe et al '721 patent includes circuitry which attempts to identify pauses in speech when, presumably, only unwanted noise is present at the microphone. When the system detects what it believes to be a pause, it activates the filter to cause it to adapt its noise filtering characteristics to filter out the sounds present at the microphone at that time. During intervals when the system detects the presence of speech, the characteristics of the filter remain fixed at the last setting. The system disclosed in the Graupe et al '721 patent, thus, attempts to avoid cancellation of the speech component of the input signal to the hearing aid by changing its filtering characteristics only when it believes no speech is present in the environment.
One problem with the system disclosed in the Graupe et al '721 patent is that the repeated adaption of the filter during what the system detects as pauses in speech creates a “pumping” sound audible by the user of the hearing aid. This pumping sound is believed to result from the relatively abrupt, i.e., rapid, reconfiguration of the frequency response of the hearing aid as the characteristics of the noise filter are changed. To the hearing aid user these abrupt and repeated adjustments make it seem as though the gain of the hearing aid is being continuously turned up and down, i.e., continuously changing the quality of the sound heard by the user. This “pumping” can be annoying and the system may not be accepted by some users.
Another problem with the system disclosed in the Graupe et al '721 patent is that it assumes that the sound occurring during pauses in speech constitutes unwanted noise. While this is true where the signal of interest is speech, there are other circumstances in which the signal of interest is of a different character. For example, if the hearing aid user were an automobile mechanic, the signal of interest might be the sound
Fravel Ralph P.
Soli Sigfrid D.
K/S HIMPP
Le Huyen
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