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DYNAMIC AUTOMATIC GAIN CONTROL IN
A HEARING AID
BACKGROUND OF THE INVENTION
The invention relates to a method for automatic, gain control in a hearing aid comprising at least one input signal transducer, a signal processor including at least one processing channel and an output signal transducer, said method comprising the steps of detecting an input signal from said input signal transducer and/or an output signal from said 10 signal processor and adapting, within an operational range of said automatic gain control, said output sound level supplied by said output signal transducer in response to said detected sound level by controlling the gain of said signal processor towards an actual desired value of said output 15 sound level, said gain control being effected at increases and decreases, respectively, of said input sound level by adjusting the gain towards said actual desired value with an attack time and a release time, respectively, whereby said releasetime is variable in response to changes in said received 20 sound level.
In FIG. 1 of the accompanying drawings, the dashed line 1 illustrates the sound volume perception of a person having normal hearing as a function of the sound level received by the ear in the form of a straight line indicating sound perception with the same volume as the received sound.
The solid curve 2 illustrates a typical example of the sound volume perception for a person having a hearing impairment. The hearing loss is dependant of the sound level 3Q and, normally also of frequency. With the illustrated hearing impairment, the perception of sounds below a certain level K4 is significantly reduced and at a threshold level K3 the sound disappears completely.
For sound levels above the threshold level K4, the sound 35 perception approaches normal hearing with a certain damping.
Complete compensation of a hearing impairment as illustrated by the curve 2, to make the sound perception of the hearing impaired person equal to that of a normal hearing 40 person, would in theory require a transfer function from the sound received at the ear to the sound perceived by the ear as illustrated by the dotted curve 3. A theoretical compensation of this kind would not be desirable in practice, however, since amplification of sound would be effected 45 also in quiet sound environments having a low sound intensity without any real sound information, in which amplified sound would be perceived as noise. Such a theoretical compensation would further require a hearing aid having a very high gain and a low noise. 50
Therefore, compensation of a hearing impairment as illustrated by the curve 2 has been implemented in practice by means of hearing aids having a constant gain up to a cut-off limit as illustrated by the dashed curve 4, or hearing aids having a compressor characteristic as illustrated by the 55 curve 5, or a variable characteristic as exemplified by the solid curve 6 composed of straight line segments with knee points at sound levels K2, K4 and K5.
A linear, constant gain characteristic as illustrated by the curve 4 provides a natural sound perception, when the gain 60 is adjusted to the actual listening situation or sound environment, but would require continuously repeated adjustment of the gain to the actual situation, whereby operation of the hearing aid will become complicated and cumbersome. As a result, hearing aids of this type are 65 frequently not adjusted to an optimum sound perception for the actual listening situation.
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Attempts to remedy this disadvantage have involved the use of hearing aids having automatic gain control, e.g., as exemplified by the compressor characteristic illustrated by the curve 5. Whereas such a linear continuous characteristic provides for automatic adaptation to different sound environments and an improved sound perception, in particular at low sound levels, the performance does not provide an ideal approximation to the actual hearing loss as illustrated by the curve 2, but provides only a higher amplification of low sound levels. Since very low sound levels frequently contain noise only, the high amplification may cause a serious discomfort.
An improved hearing loss compensation can be obtained with a variable gain characteristic, e.g., as illustrated by the curve 6 in FIG. 1. This transfer function provides an expansion characteristic at low sound levels with maximum amplification of the received sound level at the knee point K2, whereby sound levels below this knee point are damped with increasing attenuation for decreasing level of the received sound. In the range from knee point K2 through the knee point K3, which represent the threshold for the hearing loss, up to the knee point K4, a compressor characteristic is provided causing decreasing amplification of received sound levels above knee point K2 up to knee point K4, thereby providing a compensation counteracting the hearing loss in this range, which is at the same time a critical range, within which silent speech or other sound may cause problems to hearing impaired persons, who will therefore benefit from this type of compensation approaching an ideal compensation. Above the knee point K4 up to a knee point K5, which represents a pain or discomfort limit, the transfer function will provide a substantially constant gain to provide compensation for the reduction-in sound perception in this range. Above the knee point K5 a compressor characteristic is provided, which may either be determined by the transfer function or result from clipping in the amplifier circuit. Beyond the knee point K5 the sound reproduction will often be selected to prevent Hounds beyond the pain or discomfort limit to reach the ear.
If transfer functions with variable gain as illustrated by curves 5 and 6 in FIG. 1 act momentarily to provide a momentarily implemented nonlinear transfer function, sound will be heavily distorted, and the sound reaching the ear will become unnatural and uncomfortable. As an example, with a transfer function as shown by the curve 5, a sine-wave tone will be changed towards a square wave signal.
This distortion may be avoided and a more natural sound reproduction like the one obtainable with constant linear gain may be obtained by use of automatic gain control (AGC) with a quasi-linear amplification by which the gain will be continuously adapted to the actual received sound level with a smooth adjustment. The adaptation is effected with time delays which according to IEC Standard No. 118-2 from 1983 are defined as an attack time and a release or recovery time.
In this standard, the attack time is defined as the time interval from a sudden increase of the input signal level by a predetermined amount in dB until stabilization of the output level from the hearing aid with AGC within +1-2 dB from the amplified steady-state output level.
The release or recovery time is defined in the abovementioned IEC standard as the time interval from a sudden decrease of the input signal level by a certain amount in dB until stabilization of the output signal level within +1-2 dB from the lower steady-state output level.
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In the following description of the invention, the terms "attack time" and release time" are used primarily as synonyms for the equivalent slope rates measured in dB/sec.
In practice, this form of AGC is implemented by detection of the received sound level or the output sound level and use 5 of this detection to effect a smooth adjustment of the gain with the time delay, attack or release time, to the value desired for the actually detected sound level. The adjustment is effected by means of a compressor function as illustrated by the curve 5 in FIG. 1. In case of an increase of the 1° received sound level compared to what has been earlier detected, gain adjustment is effected with an attack time, and in case of a decrease of the received sound level gain adjustment is effected with a release or recovery time. In practice, the time delays are selected to provide a short 15 attack time to prevent the user from receiving uncomfortably high sound levels and a long release time to prevent pulsation or pumping of the sound level from reaching the ear. However, in case of a compressor function, a release time of long duration for increasing the gain at a decrease in the 20 detected received sound level, has the disadvantage that when the user is exposed to a high sound level caused e.g. by the user shouting at a person situated remotely or a door is slammed nearby, the user will be unable to hear low sound levels during a period thereafter. 25
In conventional hearing aids it is necessary to compromise between reception of an optimum amount of information with ;short adjustment times and avoidance of up/down pumping by using long adjustment times. As a result, prior art designs exhibit a smaller or larger tendency to suppress 30 information and/or allow up/down pumping in some listening situations.
Numerous attempts have therefore been made to distinguish between received sounds and adjust for a decreasing detected input sound level by means of varying release times, in order that a high gain can be reinstated quickly following a short heavy sound pressure.
In connection with these efforts, the parameter or parameters of the input signal that are measured or detected to 4Q determine the detected sound level, are important. In simple designs these parameters may comprise peak value, average value, effective value or the like.
Apeak value detector produces a signal dependant on the peak values of the detected signal and provides a fast 45 adjustment or short attack time at increasing received peak values, but a considerably slower adjustment or a relatively long release time at decreasing received peak values. Use of a peak value detecting circuit in conventional hearing aids having a transfer function as illustrated, e.g., by the curve 5 50 in FIG. 1 provides the advantage of a quick damping of short heavy received sound levels in the form of noise pulses, but also the accompanying disadvantage that in case of speech signals containing high peak values spaced in time the gain will quickly be adjusted towards the peak values of the 55 speech, whereby the speech is smoothed on the basis of the peak values and will attain the same level as received in speech pauses during which the sound is frequently noise.
Average or effective value detectors provide in general a less quick adjustment at suddenly increasing detected 60 values, but compared to peak value detectors they show a smaller tendency to suppress speech signals or suppress the sound reproduced after very short heavy received sound levels.
In practice, use is frequently made of combined circuits to 65 determine or distinguish between received sounds. Such circuits provide short attack time at increasing input level
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and acts like a peak value detector, whereas at stationary or decreasing input level they have a relatively longer release time and acts frequently as an average value detector.
A suitable alternative to conventional detectors are so called percentile detectors as known, e.g., from EP-B1-0 732 036. Generally such percentile detectors serve to determine the value of the detected signal, at which predetermined percentages or percentiles of the detected signal are below or above the selected value, respectively. Such detectors are well suited to determine and separate noise from information signals.
In a hearing aid AGC circuit known from U.S. Pat. No. 5,165,017, as a solution to the disadvantage of a long release time by detection using a peak value detector to provide for the peak value detector a short release time after heavy received sound levels and a long release time after relatively weak received sound levels.
For hearing aid detectors it is further known, e.g. from U.S. Pat. Nos. 4,531,229 and 5,144,675, to combine a peak value detecting circuit providing adjustment with short time delays and an average value detecting circuit providing adjustment with long time delays, whereby the average value detecting circuit can measure the average value of peaks. By this form of adjustment, short heavy sound levels will quickly excite the peak value detecting circuit and provide a quick gain reduction. After the heavy sound level the peak value detecting circuit will provide a fast readjustment of the gain to an amount corresponding to the actually received sound level or an amount, at which the average value detecting circuit takes over the gain adjustment, and at repeated short pulses there will be a pronounced pumping effect. At heavy sound levels of a longer duration the average value detector is excited and takes over the gain adjustment, After disappearance of the heavy sound pressure of longer duration following the taking over by the average value detector, the gain is adjusted slowly as a function of the decreasing mean value and during a time interval thereafter there will be an insufficient amplification of weak signals.
In multiple channel hearing aids, it is known to use separate AGC controls in the individual processing channels, each having attack and release times adapted to the specific frequency band of the channel, such as described e.g. by Brian C. J. More and Brian R. Glasberg in "A comparison of four methods of implementing automatic gain control (AGC) in hearing aids", British Journal of Audiology, 1988, volume 22, pages 93 to 104.
Thus, to compromise between minimization or a pumping or vibrating sound effect of the reproduced sound and avoidance of insufficient amplification of weak sound following heavy received sound levels, it has become known in the art to use a short attack time and different release times.
SUMMARY OF THE INVENTION
Against this background, it is the object of the invention to provide a method for improving the sound reproduction in a hearing aid and minimize the disadvantages of known AGC methods.
According to the invention, this object is attained by a method as defined hereinbefore, which is in that said attack and release times are adjusted in response to said detected sound level to a relatively short duration providing fast gain adjustment at high input and/or output sound levels and to a relatively long duration providing slow gain adjustment at low input and/or output sound levels.
By this method, the sound will be controlled with long attack and release times at low sound levels, at which the
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