Distortion suppression in hearing aids with AGC

In a method and an amplifier circuit for harmonic distortion suppression in hearing aids with signal-dependent gain control (AGC), a number of decay times of different length are prescribed for the gain control, and a switch is made between the decay times, and the selection of the respective decay time ensues dependent on a status change of the signal to be processed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to a method as well as an amplifier 
circuit for distortion suppression of regulated signal amplification in a 
hearing aid. 
2. Description of the Prior Art 
Currently, hearing aids usually have an amplifier with gain control (AGC) 
that is dependent on the signal level. This gain control makes it possible 
to amplify the signals in such a way that they are clearly audible to the 
user while still maintaining the sound sensation pleasant for the user 
given high input signals. 
Given an amplifier with gain control that is dependent on signal level, 
harmonic distortion that the user considers disturbing occurs due to the 
control function. This can be clearly seen given a constant sine input 
signal, whereby a constant re-adjustment is required even in the steady 
state. The re-adjustment intensity is dependent on the decay time of the 
amplifier circuit. The harmonic distortion is directly dependent on the 
re-adjustment intensity, and thus is indirectly dependent on the decay 
time. Longer decay times are required in order to achieve low harmonic 
distortion. Given the minimal decay times that are currently standard and 
desirable in hearing aids, the harmonic distortion can only be kept within 
a tolerable degree with considerable technical outlay. As a result, 
standards for hearing aids are partly difficult to meet or cannot be met 
at all in certain cases. 
German OS 23 53 696 discloses a hearing aid with an automatic gain control 
for which a number of different response/decay times are prescribed 
between which changes optionally can be made. The setting ensues via 
adjustment means that are manually actuated. 
Further, German OS 19 27 848 discloses a hearing aid that has a multi-stage 
transistor amplifier with an input to which a microphone is connected via 
two signal lines, and a stage in the proximity of the amplifier output 
containing a gain control circuit that acts automatically from its output 
onto its input, and the input of the stage contains a voltage divider in 
the first signal line. This voltage divider is composed of a fixed 
resistor and a resistor branch composed of two diodes connected with 
opposite polarity in terms of alternating current that is variable 
dependent on the output signal. As a result, the delays appearing in 
corresponding circuits can be avoided since the d.c. circuit of the 
amplifier itself remains uninfluenced. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method and an amplifier 
circuit available that enable a reduction of the harmonic distortion of 
the automatic gain control in a hearing aid. 
In accordance with the invention, the above object is achieved in a method 
and an amplifier circuit wherein a number of decay times of the gain 
control that differ in length are prescribed for the gain control, and a 
selection of the respective decay time ensues dependent on a status change 
of the signal to be processed. An amplification that is undertaken is 
maintained with weak readjustments as a result of the long decay time. Low 
harmonic distortion consequently occurs because of the long decay time. 
Insofar as the status of the signal changes, for example it becomes 
smaller and the amplification must therefore be increased, a switch is 
made to the shorter decay time. The harmonic distortion of the gain 
control can be substantially minimized as a result. Moreover, the 
invention enables a realization of shorter decay times without an increase 
in the distortion factor of the gain control. The distortion suppression 
that can be achieved roughly corresponds to the relationship of long decay 
time to short decay time. 
The activation of the desired decay time can ensue by indirect or direct 
acquisition of the change in the signal level. 
According to an embodiment of the invention that is simple to realize, such 
an acquisition can be undertaken by evaluating regulating pulses of the 
gain control that are caused by the output signal. 
Preferably, the regulating pulses for this purpose tapped are taken at the 
output of the amplifier circuit. As needed, however, it is possible to 
undertake a signal evaluation at a different or at a number of tap points 
of the signal path. 
According to a further embodiment of the invention, the response duration 
of the switching event between longer and shorter decay times is longer 
than the regulating pulses generated by the signal in order to avoid 
reactive switching. The switching event as such is thereby not degraded 
due to regulating pulses as a result of the signal processing. 
The response duration of the switching event is preferably defined by the 
lowest signal frequency and/or highest cycle duration. For example, the 
time constant of the switch means in a single-wave rectifier must be 
longer than the oscillation duration of the lowest frequency. Additional, 
irregular, stronger regulating pulses caused by activation of the 
switching event can thus be avoided. Such regulating pulses can lead to 
output signal distortions. 
In another embodiment of the inventive method, two different decay times 
t.sub.1 and t.sub.2 are prescribed. For example, the decay times can 
differ by a factor in the range of about 10. 
Another embodiment of the invention is directed to the possibility or 
making at least one of the prescribed decay times variable for adaptation 
to different auditory situations. This creates the possibility of matching 
the decay characteristic for harmonic distortion correction to 
specifically occurring auditory situations (ambient car noise, party 
noise, etc.). This matching can be expediently undertaken at the hearing 
aid by the user when the hearing aid is equipped with a corresponding 
setting element. 
The invention is also advantageous by allowing the setting of the decay 
times of the gain control to be accomplished by digital signal processing 
in a simple technical realization. 
An amplifier circuit for harmonic distortion suppression has means for 
prescribing at least one decay time for the gain control (in addition to a 
settable or preset decay time), whereby the decay times are different, and 
means for switching between the different decay times, whereby the 
switching ensues dependent on a status change of the signal, particularly 
a level change of the signal. 
The acquisition of the level change can ensue in a variety of ways. A 
technically simple solution uses a regulating detector therefor that is 
connected to a switch for switching between the decay times. 
The acquisition of the status change of the signal can ensue at different 
locations. Expediently, the acquisition of the status change of the signal 
is undertaken at the output side of the gain control amplifier, however, 
this status change can also be identified at other locations. 
Expediently, the time constant t.sub.M for switching between the individual 
decay times is longer than the time intervals of the regulating pulses, 
i.e. than the clocking thereof, so that unintended switching between the 
individual decay times is avoided. 
For example, decay times of different length can be achieved in a simple 
way by a combination of dynamic and static elements. To this end, a 
capacitor can be provided as a dynamic element and an ohmic load can be 
provided as a static element. 
In another embodiment of the inventive amplifier circuit at least one of 
the different decay times is variable. If desired, the user can also adapt 
the decay behavior of the gain control to different ambient auditory 
situations by actuating a setting element. 
The invention is also particularly suited for use in hearing aids with 
digital signal processing, whereby different decay times are calculated by 
suitable signal processing algorithms and applied. 
Moreover, it is possible to define different decay times for different 
auditory situations to be fetchable in a memory means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An amplifier circuit has an amplifier 2 for amplification of an input 
signal from a hearing aid acousto-electrical transducer (such as a 
microphone) dependent on its input level (AGC). The output signals of the 
amplifier 2, to the hearing aid's electro-acoustical transducer (such as 
an earphone), are tapped by a regulating detector 3 (for example, a 
comparator). A level-dependent acquisition of the output signals ensues 
with the regulating detector 3. 
The voltage across a capacitor 7 controls the gain factor of the amplifier 
2. 
The regulating detector 3 is in communication with a regulating converter 4 
that determines the rise rate of the voltage across the capacitor 7, and 
thus determines the response time of the gain control (also referred to as 
"attack" time). 
The determination of the decay time (also called "release" time) ensues 
with a resistor 5 connected to the regulating converter 4 that discharges 
the capacitor 7 at a predetermined time t.sub.2 produced by an RC element 
10. Accordingly, the RC element 10 defines the time at which the gain is 
again increased. 
In addition to the RC element 10, the amplifier circuit 1 has a further RC 
element 11 formed by a parallel circuit with a further resistor 6, whereby 
the RC element 11 defines a release time t'.sub.1. 
The release time t'.sub.1 of the RC element 11 is much shorter than the 
release time t.sub.2 of the parallel RC element 10. For example, the 
release time t'.sub.1 amounts to 50 ms and the release time t.sub.2 
amounts to 500 ms. 
The resistor 6 lies at a switch 8 that is held open by a monoflop 9 having 
a time constant t.sub.M. When the regulating detector 3 responds due to 
the level of the output signals, the gain of the amplifier 2 is decreased 
by the feedback via the RC element 10. The slow decay time t.sub.2 ensures 
that regulating pulses that are always weak and defined by the output 
signal occur as long as the input signal does not decrease. The monoflop 9 
is also simultaneously set with every regulating pulse. 
So that no additional regulating pulses are initiated by the monoflop 9 as 
long as the input signal does not decrease, the time constant t.sub.M of 
the monoflop should be greater than the time intervals of the regulating 
pulses that are caused by the signal. The defining factor is the lowest 
signal frequency and/or the longest cycle duration. For example, the time 
constant t.sub.M of the monoflop 9 must be longer than the oscillation 
duration of the lowest frequency given a half-wave rectifier and must be 
greater than half the cycle duration given a full-wave rectifier. 
When the input signal at the amplifier 2 decreases, the amplifier circuit 1 
automatically switches to a shorter decay time. The regulating detector 3 
does not emit any regulating pulses when the input signal decreases. As a 
result, the switch 8 is closed after the expiration of the time constant 
t.sub.M of the monoflop 9, and the gain is thereby boosted with a shorter 
decay time t.sub.1 as a result. As soon as the input signal rises again, 
i.e. regulating pulses are again emitted by the regulating detector 3, the 
monoflop 9 is set again, causing the switch 8 to be opened and the 
amplifier circuit 1 drops back to the decay time t.sub.2. 
The overall decay time t.sub.1 given the amplifier circuit 1 shown in the 
FIGURE is composed, according to the following equation, of the time 
constant t.sub.M of the monoflop 9 and the parallel circuit of the ohmic 
resistors 5 and 6 with the respective release times t'.sub.1 and t.sub.2 : 
##EQU1## 
with t.sub.2 &gt;&gt;t'.sub.1 :t.sub.1 .apprxeq.t.sub.M +t'.sub.1. 
As a result of the inventive circuit, a harmonic distortion suppression 
occurs in the ratio of the decay times t.sub.2 /t'.sub.1. 
Only minimal decay times of about 40-50 ms have previously been possible in 
practical application in the audiological field since the harmonic 
distortion becomes too great otherwise. Considerably shorter decay times 
with the same or even lower harmonic distortion can be realized with the 
inventive circuit and method. The shortest decay time that can be achieved 
is, with t'.sub.1 .ident.0, 
##EQU2## 
Since only signal frequencies with more than 100 Hz occur in hearing aids, 
the minimum decay time that can be realized amounts to about 10 ms for a 
half-wave rectifier and 5 ms for a full-wave rectifier. 
Although modifications and changes may be suggested by those skilled in the 
art, it is the intention of the inventor to embody within the patent 
warranted hereon all changes and modifications as reasonably and properly 
come within the scope of his contribution to the art.