U.S. patent number 3,928,733 [Application Number 05/525,552] was granted by the patent office on 1975-12-23 for hearing aid control circuit for suppressing background noise.
This patent grant is currently assigned to Viennatone Gesellschaft m.b.H.. Invention is credited to Fritz Hueber.
United States Patent |
3,928,733 |
Hueber |
December 23, 1975 |
Hearing aid control circuit for suppressing background noise
Abstract
In a control circuit for a hearing aid, a threshold switch is
connected to the low-frequency channel which transmits an input
signal from a microphone to an earphone. This switch is arranged to
block transmission of the signal below a set threshold level of the
signal. The low-frequency signal in the channel controls the
switch.
Inventors: |
Hueber; Fritz (Vienna,
OE) |
Assignee: |
Viennatone Gesellschaft m.b.H.
(Vienna, OE)
|
Family
ID: |
3616995 |
Appl.
No.: |
05/525,552 |
Filed: |
November 20, 1974 |
Foreign Application Priority Data
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|
|
|
|
Nov 21, 1973 [OE] |
|
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9757/73 |
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Current U.S.
Class: |
381/317;
381/94.5 |
Current CPC
Class: |
H04R
25/502 (20130101) |
Current International
Class: |
H04R
25/00 (20060101); H04R 025/00 () |
Field of
Search: |
;179/1P,17R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Stellar; George G.
Attorney, Agent or Firm: Kelman; Kurt
Claims
What is claimed is:
1. In a control circuit connected between a microphone and an
emitter of a hearing aid, the microphone receiving an input signal
and the circuit being arranged to transmit the input signal to the
emitter, and the circuit including a low-frequency channel
comprising a signal connected to the microphone and an output stage
amplifying transistor connected to the emitter, the improvement of
a threshold switching means connected to the low-frequency channel
and arranged to block transmission of the input signal from the
microphone to the emitter below a predetermined threshold level of
the signal, the low-frequency signal in the low-frequency channel
controlling the switching means.
2. In the hearing aid control circuit of claim 1, means for
adjusting the threshold level of the signal.
3. In the hearing aid control circuit of claim 1, the low-frequency
channel comprising at least one transistor and the threshold
switching means being connected to the transistor.
4. In the hearing aid control circuit of claim 3, the transistor
being the amplifying transistor connected to the emitter of the
hearing aid.
5. In the hearing aid control circuit of claim 1, the threshold
switching means comprising a switching transistor having a
collector-emitter path, and the low-frequency channel comprising at
least one transistor having a base contact, the collector-emitter
path being connected to the base contact.
6. In the hearing aid control circuit of claim 5, the transistor in
the low-frequency channel being the amplifying transistor.
7. In the hearing aid control circuit of claim 5, the switching
transistor and the low-frequency channel transistor connected
thereto constituting a bistable flip-flop stage.
8. In the hearing aid control circuit of claim 5, the amplifier
having an output, a further amplifying transistor having an input
and an output, the output of the amplifier transmitting alternating
circuit to the input of the further amplifying transistor, and the
output of the further amplifying transistor being connected to the
switching transistor.
9. In the hearing aid control circuit of claim 8, a decible
adjusting means connected between the amplifier and the further
amplifying transistor, the high point of the decible adjusting
means transmitting alternating current to the input of the further
amplifying transistor.
10. In the hearing aid control circuit of claim 8, a resistance
connected between the further amplifying transistor and the
switching transistor for transmitting a control voltage to the
switching transistor, and means for adjusting the control voltage
whereby the threshold value may be adjusted.
Description
The present invention relates to improvements in a control circuit
connected between a microphone and an emitter of a hearing aid. The
microphone receives an input signal and the circuit is arranged to
transmit the input signal to an emitter, the circuit including a
low-frequency channel comprising an initial amplifier connected to
the microphone and a final amplifying transistor connected to the
emitter.
Many hard-of-hearing persons, particularly those using hearing aids
for the first time, complain about the annoying audibility of
background noises. People of normal hearing capacity and with two
healthy ears have the ability to "tune out" diffuse noises and to
concentrate on the desired audible signal, i.e. speech or music.
The hard-of-hearing person, however, who normally is provided with
a hearing aid in one ear only, has lost this capacity to a large
extent. As a result, many people using hearing aids subjectively
receive background noise at a high and annoying level and, for this
reason, persons who are hard of hearing and would benefit from the
use of a hearing aid refuse it entirely.
It is the object of this invention to provide a hearing aid wherein
undesired background noises are suppressed by normally blocking
their transmission from the microphone to the emitter of the
hearing aid. The signal transmission is opened only when the input
signal has reached a predetermined threshold level significantly
higher than the level of the usual background noises.
Threshold value switching means are well known in the radio art to
provide noise attenuation, quiet tuning, squelch, etc. Obviously,
however, all of these circuits use the high-frequency carrier of
the received signal as controlling criterion for switching the
low-frequency transmission path in and out. However, since there is
no high-frequency carrier in a hearing aid circuit, the control
signal must come from the low-frequency signal itself. Up to now,
circuits of this type could not be used in hearing aids because of
the required circuit components and the low voltages used in
hearing aids.
In accordance with the invention, this difficulty has been overcome
in the type of circuit described hereinabove by connecting a
threshold switching means to the low-frequency channel and
arranging it to block transmission of the input signal from the
microphone to the emitter below a predetermined threshold level of
the signal, the low-frequency signal in the low-frequency channel
controlling the switching means.
The above and other objects, advantages and feature of the present
invention will become more apparent from the following detailed
description of a now preferred embodiment thereof, taken in
conjunction with the accompanying drawing wherein
FIG. 1 is a graph illustrating the input-output characteristics of
a desirable hearing aid, and
FIG. 2 is a circuit diagram illustrating the invention.
Referring now to FIG. 1, while there is a linear ratio between the
input decible level p.sub.i and the output decible level p.sub.o in
conventional hearing aids even when the input noise level is low
(see broken line), the control circuit illustrated in FIG. 2 will
suppress any output signal p'.sub.o if the input noise level is
smaller than threshold level p'.sub.i. Once the threshold value has
been reached, the sound amplification will increase linearly (see
full line) until the output decible level has reached its
saturation value at level p'.sub.o.
Referring now to the circuit diagram for a hearing aid illustrated
in FIG. 2, there is shown microphone 1 connected to the base
contact of output stage amplifying transistor T.sub.3, condenser
C.sub.2, potentiometer 3 and amplifier 2 being interposed between
transistor T.sub.3 and microphone 1. The collector of the
transistor is connected to the positive terminal of a battery,
which provides the voltage source for the hearing aid, earphone or
emitter 4 being interposed between the transistor collector and the
battery terminal, while the transistor emitter is directly
connected to the negative terminal of the battery. Amplifier 2 and
transistor T.sub.3 form a low-frequency channel.
The collector-emitter path of threshold switching transistor
T.sub.2 is connected between the base contact of output stage
transistor T.sub.3 and the positive terminal of the battery,
resistance R.sub.6 being interposed between the base contact and
the connector emitter path of transistor T.sub.2. Condenser C.sub.4
is connected in parallel to transistor T.sub.2.
The output of amplifier 2 is connected to the base contact of
amplifying transistor T.sub.1, condenser C.sub.1 being interposed
between the amplifier output and the transistor base contact.
Potentiometer 5, with two slide contacts 7, 8, connects the
collector of transistor T.sub.1 to the positive battery terminal
while the emitter of transistor T.sub.1 is connected directly to
the negative battery terminal.
The base contact of switching transistor T.sub.2 may be connected
either to first slide contact 7 by a first pair of terminals of
switch unit 6 or to second slide contact 8 by a second pair of
terminals of the switch unit, condenser C.sub.3 being interposed
between the base contact of transistor T.sub.2 and the switch unit.
Furthermore, the base contact of transistor T.sub.2 may be
connected to the negative battery terminal by a third pair of
terminals of switch unit 6, resistance R.sub.2 being interposed
between the switch unit and the negative battery terminal.
The base contact of amplifying transistor T.sub.1 is connected to
the connecting point between resistances R.sub.3 and R.sub.4 which
are connected in series between the collector of transistor T.sub.3
and the negative battery terminal, resistance R.sub.1 being
interposed between the base contact of transistor T.sub.1 and the
connecting point between resistances R.sub.3 and R.sub.4.
Resistance R.sub.5 is connected between this connecting point and
the base contact of switching transistor T.sub.2.
The above-described circuit operates in the following manner:
The low-frequency signal received by microphone 1 is amplified at
2, the loudness of the signal being individually adjustable by
potentiometer 3, and the adjusted, amplified signal is transmitted
through condenser C.sub.2 to the base contact of output stage
transistor T.sub.3. At rest, when the input signal to the
microphone is negligible, i.e. caused primarily or exclusively by
ambient noises, the voltage transmitted to switching transistor
T.sub.2 via voltage dividers R.sub.3, R.sub.4 and resistance
r.sub.5 is such that the switching transistor is blocked. At the
same time, the base contact of output stage transistor T.sub.3 also
receives no appreciable voltage via resistance R.sub.6 and is also
blocked. Therefore, earphone or emitter 4 produces no output
signal. Point 9 of the circuit is approximately at full working
voltage.
Amplifying transistor T.sub.1 as well as amplifier 2 are constantly
in operation, independently of the level of the input signal to the
microphone. The low-frequency signal at the base contact of
transistor T.sub.1 is practically independent of the adjustment of
potentiometer 3. The low-frequency signal coming from transistor
T.sub.1 is transmitted through operating resistance 5 by the first
pair of terminals of switch unit 6 and condenser C.sub.3 to the
base contact of switching transistor T.sub.2. This signal is
rectified in the base contact-emitter path of this transistor and
opens the transistor as soon as the signal has reached a
predetermined amplitude. A small input signal will not reach this
amplitude.
When the input signal reaches a threshold level of loudness
exceeding a predetermined decible value, a higher low-frequency
voltage will be produced at the output of amplifier 2, this voltage
being further amplified by transistor T.sub.1 and being sufficient
to make switching transistor T.sub.2 partially conductive. This
will cause a current to flow from transistor T.sub.2 through
resistance R.sub.6 to the base contact of output stage transistor
T.sub.3, which makes this transistor partially conductive, too. The
resultant voltage drop at receiver 4 will make the voltage at
circuit point 9 more negative, thus causing transistor T.sub.2 to
be opened more widely and placing output stage transistor T.sub.3
into full operation. The connection of resistance R.sub.4 to
circuit point 9 causes the transition from the blocked to the
conductive condition of the transistors to arise like an avalanche,
the circuit operating like a flip-flop circuit.
When the low-frequency signal falls below the threshold level, the
recitified signal voltage at the base contact of switching
transistor T.sub.2 will be reduced again. The direct current
transmitted through resistance R.sub.5 cannot keep the transistor
T.sub.2 open, the transistor becomes highly resistant again and the
circuit returns or flips back into the blocked condition. Condenser
C.sub.4 prevents the low-frequency signals to pass via transistors
T.sub.1, T.sub.2 to transistor T.sub.3 and also somewhat delays the
flip-flop action so that vey short error signals will not switch on
the circuit.
The level of the input signal causing the flip-flop circuit to
become conductive depends on the amplitude of the low-frequency
signal at the base contact of switching transistor T.sub.2. Since
switching unit 6 may be operated by the hard-of-hearing person, the
operator can adjust the threshold of the circuit to the ambient
acoustical conditions. In very noisy surroundings, the operator
will set the threshold at a higher level since the operating noise
(speech) will then be louder, too, the automatic reaction of a
speaker being to adjust his voice to the surrounding noise so as to
be intelligible.
When switching unit 6 is adjusted to the illustrated position to
make the uppermost pair of terminals operative, operating
resistance 5 will make only a small portion of the amplified signal
voltage effective at switching transistor T.sub.2. In this
position, the input signal must have a very high level to open
transistor T.sub.2. On the other hand, the entire signal voltage
will be transmitted by resistance 5 in the central position so that
transistor T.sub.2 will become conductive even when the level of
the input signal is relatively low. When the lowest pair of
terminals of switching unit 6 is in operation, the voltage coming
from resistance R.sub.2 will keep transistor T.sub.2 constantly
open, i.e. the threshold switch is inoperative. In this position of
the switching unit, the circuit operates with a conventional
hearing aid with linear amplification.
The described circuit does not essentially differ in its
requirement for components from other hearing aid circuits, for
instance conventional hearing aids with automatic adjustments of
noise level, and it may readily be built into hearing aids of
conventional size.
While the threshold value switch has been illustrated in connection
with the circuit of a conventional hearing aid, it could also be
combined with other types of control circuit for hearing aids. A
hearing aid with the control circuit of the present invention
operates very economically since the output stage, which normally
consumes about 60 to 75% of the entire current requirements, is
switched on only when an input signal of a predetermined threshold
value is present.
* * * * *