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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to an apparatus for the transmission of
speech, and in particular to such an apparatus suitable for use in a
hearing aid including an acoustic input transducer and an acoustic output
transducer, and a threshold circuit interconnected therebetween.
2. Description of the Prior Art
German Patent No. 2,452,998 discloses a hearing aid including a broadband
threshold switch, which acts on the output signal of the microphone
(acoustic input transducer) so that the output signal is attenuated during
pauses in speech, and is transmitted broadband in the presence of speech.
This known circuit has the disadvantage that superimposed unwanted signals
(for example, noise) are transmitted broadband in all frequency ranges
when speech signals are present. As a result, the unwanted signals are
audible in those ranges in which they are not covered by spectral
components of the speech (for example, high-frequency unwanted signals
remain clearly audible given low-frequency vowels, or the lower-frequency
unwanted signals continue to be audible as well given higher-frequency
sibilants).
A partially multi-channel circuit for suppressing unwanted signals, which
functions without threshold switches, is disclosed in U.S. Pat. No.
4,185,168, and is also described in the article "Clinical Results Of
Hearing Aid With Noise-Level-Controlled Selective Amplification," Hiroshi
Ono et al, Audiology Vol. 22, 1983, Pages 494-515. A similar multi-channel
hearing aid circuit is described in U.S. Pat. No. 4,508,940, which also
operates without a threshold circuit.
German OS No. 30 27 953 discloses a hearing aid having a microphone, an
amplifier stage with a modulator, and a receiver. A series of filter
circuits which form frequency-selecting channels are connected in parallel
to the output of the amplifier stage. A threshold circuit can be
integrated into the evaluation circuits of the frequency-selecting
channels. The output of the filter circuits is combined at a demodulator,
from which the sum of the output signals is supplied to the receiver via a
final amplifier, which may contain a sound diaphragm.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an apparatus for the
transmission of speech signals including threshold switches, wherein
unwanted components are suppressed in those regions in which these
unwanted signals are not covered by spectral components of the speech.
The above object is achieved in an apparatus for the transmission of speech
which has an acoustic input transducer and an acoustic output transducer,
with a threshold circuit connected therebetween. The input transducer is
followed by a broadband controlled-gain amplifier, which is in turn
followed by a parallel circuit including a plurality of
frequency-selecting channels. The frequency-selecting channels divide the
broadband output signal of the controlled-gain amplifier into a selected
number of frequency channels. A threshold switch is included in each
frequency-selecting channel, and the acoustic output transducer is
supplied with the sum of the output signals of the individual threshold
switches. All of the threshold switches are constructed using a
controlled-gain amplifier, and one of the threshold switches is
supplemented by an automatic gain controlled amplifier circuit. The
control signals supplied to the respective inputs of the controlled-gain
amplifiers forming the threshold switches are derived from the input
signal of the automatic gain controlled amplifier circuit.
The division of the broadband input signal into a plurality of
frequency-selecting channels, and the use of a threshold switch in each
selecting channel, achieves the result that only those frequency channels
in which spectral components of the speech are present are opened or
conducting, whereas those channels wherein such frequency components are
not present remain closed, or non-conducting. The result is that unwanted
signals are only co-transmitted in those frequency-selecting channels
wherein louder speech is present. Unwanted signals in the
frequency-selecting channels wherein no speech components are present are
completely suppressed. Because the unwanted signals are transmitted only
in those frequency channels wherein louder speech signals are
simultaneously present, the unwanted signals are always covered by the
louder speech signals, and the unwanted signals are thus no longer
audible. A noticeable improvement in the speech intelligibility is thereby
achieved.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of a circuit for speech transmission
constructed in accordance with the principles of the present invention.
FIG. 2 is a graph showing the functioning of the broadband controlled-gain
amplifier in the circuit of FIG. 1.
FIG. 3 is a graph derived from FIG. 2 showing the gain of the
controlled-gain amplifier in FIG. 1 dependent on the level of the input
signal.
FIG. 4 is a graph related to the functioning of the threshold switches in
the circuit of FIG. 1 for each frequency-selecting channel.
FIG. 5 is a block circuit diagram of a threshold switch in a
frequency-selecting channel in the circuit diagram of FIG. 1.
FIGS. 6A-6D show various signals plotted over time at various locations in
the threshold switch circuit shown in FIG. 5.
FIG. 7 is a more detailed embodiment of the comparator and a pulse-shaping
circuits used in the threshold switch shown in FIG. 5.
FIG. 8 is a schematic block diagram of a combination circuit including the
threshold switch of FIG. 5 together with a controlled-gain amplifier
circuit.
FIG. 9 is a graph showing the operating characteristics of the circuit of
FIG. 8.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A fundamental circuit diagram is shown in FIG. 1 for a circuit for the
transmission of speech. The circuit can be used in any type of device
wherein speech transmission is desired, however, the circuit is
particularly suitable for use in a hearing aid.
The circuit shown in FIG. 1 includes an input microphone 1 (acoustic input
transducer), a pre-amplifier 2, a controlled-gain amplifier 3, (automatic
gain control, AGC circuit). The AGC circuit includes a gain control stage
4 and a rectification and smoothing feedback element 5. The remainder of
the overall circuit includes a setting potentiometer 6, a parallel circuit
7 having a plurality of frequency-selecting channels 8a . . . n having
outputs connected to a summing element 9, an output amplifier 10, and a
receiver 11 (acoustic output transducer).
Each frequency-selecting channel 8a . . . n includes an input frequency
filter 12a . . . n which, in cooperation with each other, divide the
frequency range of the speech into different frequency bands. The
frequency channels 8a . . . n further include respective threshold
switches 13a . . . n following the frequency filters 12a . . . n. The
output signals of the threshold switches 13a . . . n are supplied via gain
control stages 14a . . . n to the summing element 9, with variable
amplitude.
The controlled gain amplifier 3 has an operating characteristic for
L.sub.a1k as a function of L.sub.e1 (wherein L.sub.a1 is the output
voltage level in dB, and L.sub.e1 is the input voltage level in dB) as
shown in FIG. 2. Thus, when the input voltage level L.sub.e1 is below a
value L.sub.e1k, the output voltage level L.sub.a1 linearly follows the
input voltage level according to branch I of the characteristic curve.
When the input voltage level L.sub.a1 exceeds the value L.sub.e1k (which
corresponds to the output voltage level L.sub.a1k) the voltage output
level L.sub.a1 again linearly follows the input voltage level L.sub.e1,
but with less of a slope, as shown in branch II of the characteristic
curve.
The characteristic curve from which the curve for the gain V in dB is
derived dependent on the input level L.sub.e1, is shown in FIG. 3. This
curve is thus derived from the characteristics of the curves shown in FIG.
2. The setting of the operating points of the circuit arrangement is
selected so that, in the absence of speech, the input signal results in an
operating point Al of the controlled-gain amplifier 3, below the point
L.sub.e1k. The fundamental gain in branch I is then a constant V.sub.0.
Given the presence of speech, the input level L.sub.e1 rises above the
value L.sub.e1k. The operating point of the controlled-gain amplifier 3 is
now in the range II, at the point A2, wherein the gain decreases with
increasing input signal level L.sub.e1, as shown in FIG. 3. The unwanted
noise which remains constant in level is thus attenuated, and falls below
the threshold L.sub.e2s of the respective threshold switch 13a . . . n in
those frequency-selecting channels 8a . . . n in which no speech is
present, as shown in FIG. 4. The unwanted noise is thus suppressed in
those channels. The frequency-selecting channels having loud speech
components continue to contain unwanted noise components, however, these
noise components are substantially inaudible because of the masking
characteristics of hearing the speech.
As shown in FIG. 5, each threshold switch 13a . . . n is constructed of a
controlled-gain amplifier 15, driven by a control voltage U.sub.STa . . .
n which is acquired in the threshold switch. The control voltage is
acquired from the input signals L.sub.e2a . . . n rectification in a
rectifier 16, preferably a full-wave rectifier, with subsequent smoothing
in a low-pass filter 17. The output of the filter 17 is compared in a
comparator 18 to a variable comparison voltage U.sub.Sa . . . n. The
output of the comparator 18 is supplied to a pulse-shaping element 19
which "rounds" the edges of the pulse. The switching threshold L.sub.e2Sa
. . . n for each individual threshold switch 13a . . . n is individually
adjustable via the respective comparison voltages U.sub.Sa . . . n. The
rounding of the rectangular output signals of the comparator 18 avoids
unwanted switching "clicks" in the signal supplied for the gain control of
the controlled-gain amplifier 15.
An exemplary embodiment 20 of a circuit consisting of a comparator and a
pulse-shaping element is shown in FIG. 7. The comparator 18 is a
differential amplifier having an output in the form of a square-wave. The
square-wave signals are rounded by ohmic resistors R1, R2 and R3, and
capacitors C1 and C2. The capacitor C1 defines the "roundness" of the
trailing edge, and the capacitor C2 defines the "roundness" of the leading
edge. The degree of rounding of each of the edges of the pulse is thus
independently adjustable.
FIGS. 6A-6D show signal curves within a selected channel i (with reference
to FIG. 5) given an input signal mix to the overall system of speech and
noise. The suppression of the unwanted signal during the pauses in
speaking in channel i can clearly be seen by comparing the output signals
L.sub.a2i to the input signal L.sub.e2i.
FIG. 8 shows an expansion of the system by additional AGC circuits 21 in
each, or in selected ones of, the frequency-selecting channels 13a . . .
n. The AGC circuit 21 includes a controlled-gain amplifier 22, and a
rectification and smoothing feedback element 23. A frequency dependent
compensation of the impaired hearing of the user is thus possible on the
basis of the threshold switches, in addition to the noise-suppressing
effect. The characteristics obtainable using the circuit of FIG. 8 are
shown in FIG. 9. In principle, these individual channel AGC circuits 21
have the same properties as the AGC circuit 3 of FIG. 1. The various
characteristic curves of FIG. 9 can be set by varying the cut-in points of
the AGC circuits 21.
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.
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