Patent Application
20050256594
The invention provides a programmable, power-efficient, hearing aid and cell phone-compatible system for selectively filtering continuous mechanical noise which includes a programmable, user-adjustable DSP comprising a multiband equalizer, noise reduction and anti-feedback function, and advanced gain control (AGC) volume control.
Hearing loss is generally associated with a loss of hearing sensitivity, which is a function of frequency. The most common type of sensitivity loss is an increasing function of frequency. Sensitivity is typically a function of speech level as well. Hence, loud sounds should be amplified less than soft sounds. It has been long known that a hearing aid should treat the various frequency components of speech differently to render them intelligible to a hearing impaired person.
In many cases the response of the ear of a hearing impaired person will be substantially different in terms of sensitivity and frequency response from that of a normal person. In a sensorineural hearing impairment, soft sounds are rendered inaudible, while loud sounds may be subjectively just as loud as for a normal person. Conversational levels may be very soft or even inaudible for a person with sensorineural hearing impairment. Consequently, if linear amplification is used to assist such a person, loudness relationships are perceived as distorted, and loud sounds may be uncomfortably loud. It is necessary in such cases to raise the amplitude of soft speech cues to the level of audibility. Beyond this, further improvements may be had by reestablishing loudness relationships. In many instances, a hearing impaired person will only experience a hearing loss at high frequencies and at low levels. For such a person it is desirable to provide a device, which amplifies sound only at low levels and high frequencies.
Individuals with impaired hearing often experience difficulty understanding conversational speech in background noise. The majority of daily conversations occur in background noise of one form or another. In some cases, the background noise may be more intense than the target speech, resulting in a severe signal-to-noise ratio problem.
Background mechanical noise (e.g., noise emitted by air conditioners, fans, air dryers, cars, trains, and machines) is usually continuous, with noise frequency and amplitude usually not varying by more than 5% or so. Such continuous mechanical noise therefore essentially reflects a spectrum of fixed amplitude sinusoidal signals that are subject to destructive interference when combined with a 180° off (half phase off) destructive signal. (Under such conditions, the “peak” of the fixed amplitude sinusoidal signal is in phase with the “valley” of the destructive wave and reduces the fixed amplitude sinusoidal signal to zero amplitude.)
In a conventional hearing aid, a microphone converts incident sound waves into an analog electrical signal, which is then processed to filter out unwanted noise, amplified, and coupled to a receiver or speaker which converts the electrical signal back to sound waves. The electrical signal processor may be an analog processor, which operates directly upon an analog electrical signal. Known analog hearing aids use relatively simple methods to alter their frequency shaping and dynamic range compression to mitigate the loss in hearing sensitivity for frequency and level. Alternatively, the analog signal may be converted to a digital signal and processed by a digital signal processor (DSP). Various hearing aids and related systems, including noise reduction systems, are described in U.S. Pat. Nos. 6,292,571; 6,236,731; 6,240,192; and 6,104,822.
General-purpose DSP chips provide an architecture that is flexible and that may be used in hearing aid-related systems. A generic DSP device consists of various functional blocks including accumulators, adder/subtractors, multipliers, registers, data memory, and program memory. These functional blocks are multiplexed to minimize the number of blocks needed. When two numbers need to be multiplied, the numbers are retrieved from memory and latched into the input registers of the multiplier. The output of the multiplier contains the result, which is then stored in another register or in memory.
The design of digital hearing aids involves numerous trade-offs between processing capability, flexibility, power consumption and size. Minimizing both chip size and power consumption are important design considerations for integrated circuits such as DSP's used in hearing aids. Fully programmable implementations of digital hearing aids (i.e., those that use a software-controlled digital signal processor) provide the most flexibility. However, with current technology, a fully programmable DSP chip or core consumes a relatively large amount of power. An application specific processor (typically implemented using an application specific integrated circuit or ASIC) will consume less power and chip-area than a fully programmable, general-purpose DSP core for equivalent processing capabilities, but is less flexible and adaptable.
Many hearing aid instruments have been designed to compress the dynamic range of the input sound signal so that it more nearly fits into the residual dynamic range of the recruited ear. The ratio of input dynamic range in dB to compressor output dynamic range in dB is called the compression ratio. To adequately specify the compressor, the compression ratio needs to be accompanied by a static gain value. This static gain value will determine at which input power level the system delivers a specified fixed gain. For example the static gain may be set so that at 80 dB SPL input power, the system delivers unity gain. If the compressor is set to a 2:1 compression ratio, then at 60 dB SPL input power the system will produce a 70 dB SPL output, that is a gain of 10 dB, and at 100 dB SPL input power the system will produce a 90 dB SPL output, that is a gain of −10 dB.
Usually the compression ratio is not constant over the entire input power range. A low-level compression knee may be defined. For input powers below this low-level compression knee, the compression ratio may be 1:1, that is, a fixed linear gain may be applied. The designated compression ratio (e.g. 2:1) may take affect only for input power levels above this low-level compression knee. A high level or limiting knee may also be defined. For input power levels above this high level knee, the compression ratio may increase or even become infinite, or it may be that the output level is fixed regardless of increase in input level. A system which has only a high-level compression knee below which the compression ratio is 1:1 (linear gain) is called a limiter. A system which has a low-level compression knee positioned at 40-50 dB SPL is termed a full range compressor.
The compression process requires a means for measuring the power of the input signal and generating a dynamically varying gain as a function of this input power. This gain is then applied to the signal which is delivered to the ear. When the input power is low, this gain will generally be high so that soft sounds are made louder. When the input power is high, this gain will generally be low so that loud sounds are not made too loud. The measure of input power requires averaging over time. The time span of the averaging defines a compression time constant. If the time span is very long then the compressor will react slowly to changes in input power level. This is sometimes referred to as Automatic Gain Control (AGC) where time constants of one to two seconds are typical. When the time span of the averaging is short the compressor will react quickly to changes in input power level. With a time span of approximately five to fifty milliseconds, the compressor may be referred to as a syllabic rate compressor. A syllabic rate compressor will limit the gain of a loud vowel sound while amplifying a soft consonant which immediately follows it.
In most designs there is both an attack and release compressor time constant. The attack time constant determines the time it takes for the compressor to react at the onset of a loud sound. That is, the time it takes to turn down the gain. The release time constant determines the time it takes for the system to turn up the gain again after the loud sound has terminated. Most often the attack time is quite short (<5 milliseconds) with the release time being longer (anywhere from 15 to 100 s of milliseconds).
To match the variability of recruitment with frequency, a compressor is often designed to perform differently in different frequency bands. A multi-band compressor divides the input signal into multiple frequency bands and then measures power in each band and compresses each band separately with possibly different compression ratios and time constants in the different bands. For example a properly designed two band compressor can make soft high frequency consonants audible while suppressing low frequency competing noises occurring simultaneously. The outer hair cells of the cochlea, when functioning normally, are often thought to perform compression function in overlapping frequency bands called critical bands. These frequency bands are spaced linearly at intervals of approximately 100 Hz at frequencies below about 500 Hz, and are spaced logarithmically at approximately third octave intervals above 500 Hz. Thus, the outer hair cells behave as a biological critical band compressor. The time constant associated with this compressor has been approximated to be about 1 ms.
As the number of compression bands increases, each with its own compression ratio and static gain, it is possible to view the compressor as having an almost continuously varying compression ratio as a function of frequency. In this case the system may, be represented as a set of frequency dependent gain curves. Each gain curve applies at a certain input power level. For input between these power levels, the system interpolates between gain curves.
The process of adjusting the compression ratios or gain curves of a compressor is central to the hearing aid fitting process. One approach to doing this is to attempt to adjust the compressor so that for all input levels and all frequencies the hearing impaired listener has the same impression of loudness that a normal listener would have. Loudness is a perceptual quantity which can under certain constraints be plotted as a function of input power level. The loudness growth curve may be measured by presenting a number of input signals at different levels and asking the listener to subjectively rate these on a perceptual scale (e.g. 1 to 10). By measuring the loudness growth curves of an impaired listener at different frequencies and comparing these to the loudness growth curves of an average of normal listeners, a loudness matching compression fitting can be attempted. To accurately match loudness growth curves, the hearing instrument would permit continuously variable compression ratio over input level. In this case it is more useful to think in terms of continuously variable input/output power curves. The system described above with low and high-level compression knees is able to implement only three segment piecewise input output curves.
The need still exists for a system that filters a broad range of continuous mechanical noise to facilitate the use of auditory-related apparatus such as hearing aids, auditory protection devices, and portable phones (e.g., cell phones). A power-efficient, cell phone-compatible system that could be employed in a hearing aid or auditory protection device to selectively filter continuous mechanical noise in accordance with a user's particular auditory profile would benefit not only to the hearing impaired, but also those of ordinary hearing that desire to filter out distracting or dangerous mechanical noise.
The invention provides a power-efficient, hearing aid and cell phone-compatible system for selectively filtering continuous mechanical noise. Systems of the invention include a programmable, user-adjustable DSP comprising a multiband equalizer, noise reduction and anti-feedback function and advanced gain control (AGC) volume control. Systems of the invention can be used, e.g., in hearing aids; in auditory protective devices used to protect against the dangers, discomforts, and inconveniences of continuous mechanical noise; and with cell phones to filter unwanted background noise.
In one embodiment, systems and methods of the invention facilitate cell phone usage by serving as a noise-reducing ear-piece component that does not create unwanted interference. In another embodiment, systems and methods of the invention are used in combined hearing aid-cell phone applications.
Systems of the invention may be calibrated through a novel calibration routine by programming the DSP multiband equalizer to: select one or more of at least around twenty-three different bandwidths based on a user's auditory profile; and filter continuous mechanical noise.
Significantly, systems and methods of the invention enable a user to self-calibrate, either manually or through interconnection with a PC program, a multiband equalizer in accordance with the user's particular auditory frequency responses. Further, the DSP used in the invention does not interfere with the electromagnetic impulse of portable phones such as cellular phones.
An auditory protection system of the invention comprises:
In a preferred embodiment: the DSP is programmed through use of a personal computer to provide a frequency response that is optimized for a particular user and that is selected from at least around twenty-three bandwidths ranging from 50 Hz to 8,000 Hz. In preferred embodiments, the DSP noise reduction function generates, in intervals of between about 1 to about 7 seconds, signals having a frequency of about 50 Hz to about 8,000 Hz and amplitudes of about 70 dB to about 130 dB. These signals achieve substantially complete destructive interference of continuous mechanical noise in the range of 0 to about 15 dB. In a preferred embodiment, the DSP output signal is defined by a frequency response of around 2,000 Hz to around 2,500 Hz; and the volume of the DSP output signal is about 130 dB.
The invention also provides systems and methods for calibration of a system for processing digital signals in a hearing aid and/or auditory protection device. Such calibration comprises:
A hearing aid that employs systems of the invention (collectively, the “unit”) offers numerous advantages over, and differs from, traditional BTH (behind the ear)/ITH (in the ear)/ITC (in the canal) type hearing aids. For example, the unit battery and PCBA compartment may be separated from the microphone and or earphone; the unit can use a battery that is rechargeable; the unit is connectable to a mobile phone and may be used with a hands-free microphone and earphone set while the input signal is processed through the unit and heard by the user; the unit's multiband equalizer can be adjusted by the user either by a built-in user self-calibration function or through a PC link program; mechanical noise reduction strength can be selected by the user according to the environment; and the unit itself will be styled like a fashion electronic appliance such as MP3 or mobile phone, contributing to a completely different appearance than that of a traditional hearing aid.
Further advantages offered by the instant invention include the following:
These and other aspects of the invention are described in further detail in the following detailed description.
The embodiments of the invention described herein are purely illustrative. Other embodiments of the invention will be recognized by those of ordinary skill in the art.
Feedback is generated when amplified sound from hearing aid speakers 31 leaks back to the microphones 5, 7, and 9 and is re-amplified. The anti-feedback function 21 separates the signal which leaks from the hearing aid amplifier and normal input sound and then eliminates the leakage sound. AGC volume control 25 limits output volume to a specific dB level.
DSP 19 can comprise a single integrated circuit (e.g., integrated circuits SSCM 2167 (Analog Devices), Paragon® Digital filter (Gennum), or Paragon® Digital GB3215 (Gennum)), that has been programmed to include a noise reduction and anti-feedback function 21, multiband equalizer 23 with a variable compression ratio of approximately from about 4:1 to about 1:1 and a bandwidth of approximately from about 50 Hz to 8,000 Hz, and AGC volume control 25. In a preferred embodiment, DSP 19 is an integrated circuit that is programmed by interconnection to personal computer 33 or in accordance with a calibration routine for user adjustment of the multiband equalizer 23. The input signal to DSP 19 and the output signal from DSP 19 are transmitted to speakers 31. DSP 19 output signal transmitted to speakers 31 is defined by a frequency response at approximately 2,000 Hz; and the volume of the DSP 19 output signal is 130 dB.
Calibration of DSP 19 in accordance with one embodiment of the instant invention is illustrated in
This application claims priority from U.S. Provisional Patent Application No. 60/566,298, filed Apr. 29, 2004.
| Number | Date | Country | |
|---|---|---|---|
| 60566298 | Apr 2004 | US |
