Electrical audio signal processing systems and devices – Hearing aids – electrical – Wideband gain control
Reexamination Certificate
2000-10-16
2003-09-30
Kuntz, Curtis (Department: 2643)
Electrical audio signal processing systems and devices
Hearing aids, electrical
Wideband gain control
C381S107000
Reexamination Certificate
active
06628795
ABSTRACT:
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 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 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 release-time is variable in response to changes in said received 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 and, normally also of frequency. With the illustrated hearing impairment, the perception of sounds below a certain level K
4
is significantly reduced and at a threshold level K
3
the sound disappears completely.
For sound levels above the threshold level K
4
, the sound 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 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 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.
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 curve
5
, or a variable characteristic as exemplified by the solid curve
6
composed of straight line segments with knee points at sound levels K
2
, K
4
and K
5
.
A linear, constant gain characteristic as illustrated by the curve
4
provides a natural sound perception, when the gain 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 frequently not adjusted to an optimum sound perception for the actual listening situation.
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 K
2
, 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 K
2
through the knee point K
3
, which represent the threshold for the hearing loss, up to the knee point K
4
, a compressor characteristic is provided causing decreasing amplification of received sound levels above knee point K
2
up to knee point K
4
, 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 K
4
up to a knee point K
5
, 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 K
5
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 K
5
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 +/−2 dB from the amplified steady-state output level.
The release or recovery time is defined in the above-mentioned 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 +/−2 dB from the lower steady-state output level.
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 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 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 attack time to prevent the user from receiving uncomfortably high sound levels and a l
Ensey B
Kuntz Curtis
Sughrue & Mion, PLLC
Widex A/S
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