AID TO HEARING SPEECH IN A NOISY ENVIRONMENT

TECHNICAL MELD

The invention relates to the field of audio processing devices for improving intelligibility of speech in noisy environments.

BACKGROUND

Speech intelligibility can be reduced by background noises, which include loud, confusing, or distracting sounds. Hearing impaired persons often have particular difficulty discerning speech in noisy environments, but people without any hearing disorder can experience similar difficulties in environments with high noise levels.

Audio processing devices have used a variety of techniques for suppressing unwanted noise. One commonly used technique attenuates large amplitude audio signals for protecting against the reproduction of excessively loud noises. Another technique attenuates low frequencies of sound to help prevent a so-called "upward spread of masking" by low frequency noises, which reduces intelligibility of the higher frequency sounds.

For example, U.S. Patent 4,061,875 to Freifeld et al. discloses an audio processor that incorporates an adjustable high pass filter to reduce low frequency noise components of an audio signal. The cut-off frequency of the high pass filter can be adjusted in steps from .25 to 1.5 kilohertz, and the rate of attenuation of the filter (i.e., the roll-off rate) can be adjusted at each cut-off frequency in steps of 6, 12, and 18 decibels per octave. Together,

these two adjustments are used to discriminate against particular noises.

U.S. Patent 4,792,977 to Anderson et al. discloses a hearing aid circuit having a series of state variable filters for controlling frequency response characteristics. The pass band of the filter series can be adjusted to attenuate predetermined low frequencies of noise. The state variable ilters are implemented in an integrated circuit using capacitor loaded operational transconductance amplifiers and include separate external controls for varying respective outputs of a high pass filter, a low pass filter, and a variable slope filter. The high and low pass filters are both fourth order filters (e.g., four pole filters) made up of two cascaded second order filters. The external controls set frequency response characteristics by adjusting the cut-off frequencies of the high and low pass filters without substantially changing the respective shapes ("Q") of their frequency response curves.

Although a predetermined amount of attenuation of particular low frequencies of sound can help to prevent certain kinds of noise from masking higher frequencies that are more important to speech intelligibility, the amount of predetermined attenuation can be more or less than that required for optimally attenuating the noise. For example, if too little attenuation is provided, some masking remains. However, if too much attenuation is provided, the perceived sound quality is unnecessarily reduced. In the absence of masking noise, attenuation of the low frequencies also reduces intelligibility.

Audio processing devices have also been designed to attenuate low frequencies of sound as a function of noise energy. For example, U.S. Patent 4,490,585 to Tanaka discloses a hearing aid in which a low frequency component of ambient sound is used to shift a cut-off frequency of a high pass filter. An increasing level of the low frequency sound

is used to shift the cut-off frequency up to 1.5 kilohertz for attenuating loud noises within the low frequency spectrum. However, important speech information is also conveyed at frequencies much less than 1.5 kilohertz, and shifting the cut-off frequency of the high pass filter through this region reduces speech intelligibility as well as noise.

U.S. Patent 3,927,279 to Nakamura et al. discloses a hearing aid in which both lower and higher frequency components of the acoustic spectrum are attenuated in response to the detection of sound energy at frequencies considered above and below frequencies required for speech. A band-rejection filter is used to isolate frequencies below 300 hertz and above 3000 hertz, and the energy content of the isolated bands is detected to form a control signal. Response characteristics of both a high pass filter and a low pass filter are varied by the control signal to attenuate high and low frequency noises.

However, the hearing aid of Nakamura et al., like the hearing aid of Tanaka, also attenuates frequencies that convey important speech information. For example, the hearing aid of Nakamura et al. attenuates to some degree the entire range of frequencies between 300 and 3000 hertz, which includes frequencies containing crucial information for identifying both consonants and vowels.

SUMMARY OFINVENΠON

Our invention is directed to suppressing noise while preserving sounds that are important to speech intelligibility. In the absence of noise, low frequencies of sound can be preserved to maintain a perceived quality of sound. However, upon detection of noise, the low frequencies are attenuated as a continuous function of their energy content.

For example, our invention can be arranged as a signal processor having a high pass filtering circuit that exhibits a variable response curve. A controlling circuit of the processor varies a slope of the response curve as a function of the energy content of the low frequencies. The response curve is varied in slope below a cut-off frequency that is below a range of frequencies that convey a majority of second for ant transitions between consonants and vowels. Frequencies below the cut-off frequency are progressively attenuated in accordance with the slope of the response curve. In other words, frequencies closer to the cut-off frequency are attenuated less than frequencies farther from the cut-off frequency, and this difference is accentuated by an increase in the slope of the response curve.

The second formant transitions of speech are crucial or the accurate identification of many consonant sounds. In addition, second formant transitions help to identify the underlying vowel sounds that produce the second formants in transition. The cut-off frequency of the response curve is positioned to preserve at least a majority of the second formant transitions, and frequencies below the cut-of frequency are attenuated by varying the slope of the response curve below the cut-off frequency to minimize attenuation of any remaining second formant transitions. In . this way, noise in the low frequency spectrum is attenuated while minimizing any loss of sound that is important for speech intelligibility. Together, the attenuation of noise and the preservation of second formant transitions can significantly improve speech intelligibility in noisy environments.

Our signal processor can also be arranged to closely relate the frequencies that are monitored for detecting noise with the frequencies that are attenuated as a function of the detected noise. For example, a low pass iltering circuit can be used to detect the low frequency noises. The low pass filtering circuit at least partially

attenuates frequencies above the cut-off frequency of the high pass filtering circuit and at least partially transmits frequencies just below the same cut-off frequency.

The attenuation of frequencies by the low pass filtering circuit just above the cut-off frequency of the high pass filtering circuit helps to prevent frequencies of noise outside the range of frequencies that are variably attenuated by the high pass filtering circuit from inducing the variable attenuation, which could reduce perceived sound quality and intelligibility without reducing the noise. The transmission of frequencies of the low pass filtering circuit just below the cut-off frequency of the high pass filtering circuit helps to prevent the variable attenuation of frequencies by the high pass filtering circuit that are outside the range of frequencies that are monitored for noise by the low pass filtering circuit, which could unnecessarily reduce intelligibility along with the desired reduction in noise.

Preferably, the low pass filtering circuit has a cut-off frequency that is above a range of frequencies that convey a majority of first formants of speech to detect particularly obfuscating background noises such as the din of speech chatter. Also, the low pass filtering circuit preferably has a high roll-off rate to enable the cut-off frequencies of the low pass and the high pass filtering circuits to be positioned closely together in frequency.

DRAWINGS

FIG. 1 is a block diagram of an audio reproducing device having a signal processor for suppressing noise while preserving distinctive features of speech.

FIG. 2 is a graph depicting a simplified asymptotic representation of a response curve exhibited by a low pass filtering circuit shown in FIG. 1.

FIG. 3 is a graph similarly depicting three of a family of possible response curves exhibited by a variable high pass filtering circuit shown in FIG. 1.

FIG. 4 is a graph in which one of the response curves of FIG. 3 is superimposed on the response curve of FIG. 2.

FIG. 5 is a circuit diagram of a building block of the variable high pass filtering circuit as a biquadratic filter structure.

FIG. 6 is a block diagram showing two biquadratic filter structures connected in series for constructing the variable high pass filtering circuit.

DETAILED DESCRIPTION

An example of our invention as a signal processor incorporated into an audio reproducing device is shown in FIG. 1. The device, which could be mounted in a headset or hearing aid, is intended to improve speech intelligibility in noisy environments.

A microphone 10 converts ambient sound energy into electrical energy as an audio signal "A" conveying a frequency range covering the range of most voices. A signal "B" is split from the signal "A" for controlling reproduction of signal "A" by the audio reproducing device.

The signal "B" is processed by a low pass filtering circuit 12 as a part of a detecting circuit, including a level detector 14, for determining the energy content of a

low frequency band of the signal. The low pass filtering circuit 12 exhibits a response curve expressible in decibels over a domain of frequencies. FIG. 2 depicts the response curve in a simplified form as piecewise curve 16 composed of two interconnected asymptotes of the actual response curve. A cut-off frequency 18 (approximately 750 hertz) along the response curve 16 separates the audio signal "B" into a band of low frequencies (below 750 hertz) that are substantially transmitted and a band of high frequencies (above 750 hertz) that are substantially attenuated.

The low pass filtering circuit 12 works in conjunction with microphone 10 to transmit frequencies containing particularly obfuscating noises but little speech information. For example, the cut-off frequency of the low pass filtering circuit 12 is positioned above the range of frequencies conveying the majority of first formants of speech (i.e., above 600 hertz) to transmit a band of frequencies containing the largest amount of long term speech energy. This band also contains most of the energy associated with background chatter, which can mask higher frequencies conveying more important speech information.

The level detector 14, which can be constructed as a conventional root mean square value detector, determines the energy content of the frequencies transmitted by the low pass filtering circuit and produces an output signal "C" that is proportional to the detected energy content as a measure of noise. The signal "C" takes a form of a control signal that controls operation of a variable high pass filtering circuit 20.

The signal "A" is processed by the variable high pass filtering circuit 20 in parallel with the processing of the signal "B". The variable high pass filtering circuit exhibits a variable response curve that can take a form of any one of a family of response curves. Similar to the depiction of the response curve 16 in FIG. 2, FIG. 3 depicts

three piecewise curves 22, 24, and 26 that are representative of the family of response curves exhibited by the variable high pass filtering circuit 20. A cut-off frequency 28 (approximately 1000 hertz) terminating a common section of the three response curves 22, 24, and 26 separates the audio signal "A" into a band of low frequencies (below 1000 hertz) that are substantially attenuated and a band of high frequencies (above 1000 hertz) that are substantially transmitted.

The amount of attenuation of the low frequencies is controlled by the particular response curve exhibited by the variable high pass filtering circuit. For example, response curve 22 produces little or no attenuation, whereas response curves 24 and 26 produce progressively more attenuation. The response curves differ by varying in slope below the cut-off frequency 28. The control signal "C" determines which among the family of response curves are exhibited by the variable high pass filtering circuit. In other words, the control signal "C" has the effect of varying the slope of the variable response curve exhibited by the variable high pass filtering circuit.

The cut-off frequency 28 is positioned below the range of frequencies conveying the majority of second formant transitions between consonants and vowels (i.e., below 1500 hertz) . Frequencies below the cut-off frequency are progressively attenuated in accordance with the slope of the response curve. This reduces attenuation of any second formant transitions below the cut-off frequency while increasing attenuation of lower frequencies that convey less speech information. The second formant transitions can be further preserved by positioning the cut-off frequency below a range of requencies conveying a larger percentage of the transitions. For example, the cut-off frequency 28 is positioned at 1000 hertz.

The slope of the response curve is increased in proportion to the value of the control signal "C" to attenuate disproportionately large amounts of sound energy in the low frequency spectrum monitored by the detecting circuit. However, the slope of the response curve is preferably limited to a maximum roll-off of 24 decibels per octave to further limit attenuation of frequencies close to the cut-off frequency.

Once a desired level of attenuation is reached in the direction of roll-off along the variable response curve, the slope of the variable response curve preferably levels off (i.e., returns to zero slope) to attenuate the remaining low frequencies by substantially the same amount. For example, response curve 24 has a corner frequency 30 that limits attenuation of frequencies below 400 hertz to a constant 20 decibels. This prevents unnecessarily high attenuation of certain low frequencies, including some of the first formants of speech, in response to relatively low levels of undesirable sound energy. In other words, the desired amount of attenuation is achieved with a minimum effect on perceived sound quality.

Also, the cut-off frequency preferably remains constant while the slope of the response curve is varied. Significant shifts in the cut-off frequency would undesirably attenuate frequencies containing important speech information. The range of frequencies conveying the majority of second formant transitions of speech is preferably attenuated by no more than 5 decibels. In particular, attenuation at 1000 hertz is preferably limited to no more than 5 decibels while attenuation at 250 hertz is preferably at least 35 decibels.

FIG. 4 shows a relationship between the cut-off frequency 18 of the low pass filtering circuit 12 and the cut-off frequency 28 of the variable high pass filtering circuit 20 such that the band of low frequencies (e.g.,

frequencies below 750 hertz) that are substantially transmitted by the low pass filtering circuit 12 approximately corresponds to the band of low frequencies (e.g., frequencies below 1000 hertz) that are attenuated by the variable high pass filter. The low pass filtering circuit at least partially attenuates high frequencies above the cut-off frequency 28 (e.g., frequencies above 1000 hertz) and at least partially transmits frequencies below the cut-off frequency 28 (e.g., frequencies below 1000 hertz).

Furthermore, the cut-off frequency 28 of the variable high pass filtering circuit is preferably higher than the cut-off frequency 18 of the low pass filtering circuit so that the band of low frequencies (e.g., frequencies below 750 hertz) that are substantially transmitted by the low pass filtering circuit do not include frequencies that are within the band of high frequencies (e.g., frequencies above 1000 hertz) that are substantially transmitted by the variable high pass filtering circuit. This limitation helps to assure that the high pass filtering circuit 20 does not attempt to attenuate noise occurring at frequencies beyond the range of frequencies that can be attenuated by the high pass filtering circuit.

Conversely, the cut-off frequency 18 of the low pass filtering circuit is preferably not more than one-half octave lower than the cut-off frequency 28 of the variable high pass filtering circuit so that the band of low frequencies (e.g., frequencies below 1000 hertz) that are substantially attenuated by the variable high pass filtering circuit include only a limited range of frequencies (e.g., frequencies between 750 and 1000 hertz) that are above the band of low frequencies (i.e. frequencies below 750 hertz) that are substantially transmitted by the low pass filtering circuit. This limitation helps to assure that frequencies above those monitored for noise are not unnecessarily attenuated along with the frequencies containing the monitored noise.

The low pass filtering circuit 12 is preferably constructed as a high order filter (e.g., a four pole filter) having a roll-off rate of at least 24 decibels per octave. The high roll-off rate maximizes the attenuation of frequencies that are above the cut-off frequency 28 of the variable high pass filtering circuit and allows for the respective cut-off frequencies 18 and 28 of the low pass filtering circuit and variable high pass filtering circuit to be positioned close together.

FIG. 5 depicts details of one of two identical biquadratic structures that are shown in FIG. 6 cascaded together in series to produce a variable fourth order filter. Each biquadratic structure exhibits a general transfer function "H(s)" as follows:

s ² + (W _z /Q _z ) s + W _z 2

H(s) = s ^2' + (Wp/Qp) s + _p 2

where "s" is an angular frequency equal to j [2 pi f] (with "j" being an imaginary number equal to the square root of -1, with "pi" being the ratio of the circumference of a circle to its diameter, and with "f" being frequency measured in hertz) ; " _z " is a corner frequency (in angular measure) associated with a "zero" of the function; " p" is a corner frequency (also in angular measure) associated with a "pole" of the function; and "Q ₂ " and "Qp" are terms referred to as "quality factors" or "inverse damping factors".

The particular biquadratic filter structure illustrated includes six operational transconductance amplifiers labeled "q _ml " , "g^", "gm3 ^M ι " -flrt", ^π gn5a"r and "9m5 _b "« Each transconductance amplifier includes two inputs that produce a differential voltage, which is multiplied by a transconductance gain of the amplifiers to produce an output current. The output of each transconductance amplifiers is

connected to ground through one of capacitors "Ci" and "C ₂ " or resistor "R".

The output of the circuit as a model of the transfer function H(s) is given below:

V ₀ (Ci C ₂ ) s ² + (C _X g _m ) s + (g _m2 g _m 5)

H(s) =

Vi (C _X C ₂ ) s- + (C g _m3 ) s + (g _ml g _m2 )

where "Vo" and "Vi" are the respective output and input voltages shown in FIG. 5; "g _m ι", " m2" "~ ^~ m3 ⁿ r "<3m4" ^{are tne} transconductance gains of the amplifiers labeled the same; "9m5" i ^s th effective transconductance gain of the two amplifiers labeled "g _m 5a" ^an d "gm5b" ^an( * "Ci" and "C2" are the respective capacitances of the like-labeled capacitors. The two biquadratic filter structures 36 and 38 produce in series a fourth order transfer function obtained by squaring the above transfer function of a single biquadratic filter structure.

Relating the particular transfer function of the circuit shown in FIG. 5 to the general transfer function of a biquadratic filter yields the following equations for the corner frequencies " _z " and "Wp" and quality factors "Q _z " and.

⁽ Sm2 gm5) ⁰ ' ^{5 W} _Z = (C _X c ₂ ) ⁰ - ⁵

⁽ 9ml gm2 ⁾⁰ ' ⁵ p = (C _l C ₂ )0.5

1 (gm2 gm5 C ₂ ) ⁰ - ^{5 Q} z = gm (ci) ⁰ - ⁵

l (g i 2 ~2 ) ⁰ - ⁵

Q _P = gm3 (Ci) ⁰ ' ⁵

Since the values of the two corner frequencies and two quality factors are determined by a total of five variables, the corner frequencies and quality factors can be independently set. For example, the corner frequency "Wp", representing the pole of the function, is set to produce the desired cut-off frequency 28 of the variable high pass filtering circuit. The quality factors "Q _z " and "Qp" are both preferably set equal to approximately 0.707 to provide for maximum change in curvature at the corner frequency "Wp" without producing a peak. The corner frequency "W _z ", representing the zero of the function, is controlled to vary the slope of the variable response curve.

The corner frequency "W _z " appears along response curves 24 and 26 as the respective corner frequencies 30 and 32. Thus, the change in corner frequency "W _z " produces not only a change in slope of the variable response curve but also changes the maximum attenuation of the variable response curve.

The corner frequency "W _z " is varied by changing the value of "g _m 5"« However, an isolated change in "g _m5 " would also have the undesirable effect of changing the quality factor "Q _z ". Accordingly, "gms" s varied by a first factor that is a square of a second factor for simultaneously varying "g _m4 "• Proportional currents "Ii" and "I ₂ " are controlled by a control circuit 34 to produce this effect.

The transconductance gain of the amplifier "g _m4 " is proportional to the control current "I ". However the total transconductance gain by the two amplifiers "gm5a" ^anc "gm5b" connected in series is proportional to the square of the control current "I ". Accordingly, the proportional control signals "Ii" and "I " cooperate with the two amplifiers

"gm _S a" ^and "g _S b" ^to provide the necessary filter control logic to vary the corner frequency ^M W _Z ^M without varying the corner frequency " p" or quality factors "Q _z " and "Qp".

FIG. 6 illustrates two identical biquadratic filter structures cascaded together in series to construct the variable high pass filtering circuit 20. The control circuit 34 controls currents to both biquadratic filters to vary the slope of the variable response curve in response to the control signal "C" from level detector 14. Output signal "D" from the high pass filtering circuit drives speaker 40 (see FIG. 1) for reproducing signal "A" in a clarified form optimum for discerning important speech information.

Although the slope of the variable response curve is preferably varied by moving the corner frequency ^H W _Z ^H representing the zero of the biquadratic transfer function, similar effects can be achieved by varying the pole corner frequency "Wp" or the quality factors "Qp ^M and "Qz". However, three cascaded biquadratic filter structures may be needed to achieve the similar effects with the alternative variables.

At low noise levels monitored by the detecting circuit, the variable high pass filtering circuit 20 does not attenuate any significant portion of the audio signal "A" to preserve the perceived quality of sound reproduced by speaker 40. For example, the variable high pass filtering circuit 20 exhibits the flat response curve 22 up to a predetermined threshold level of noise, which is preferably within 50 to 75 decibels sound pressure level. The actual threshold level can be set to accommodate application environments or user needs. Once the threshold is exceeded, the variable response curve is varied to attenuate the low frequency portion of the signal "A" proportional to the increase in noise level above the threshold.