Note: Descriptions are shown in the official language in which they were submitted.
CA 02257461 1999-12-16
HEARING AID WITH IMPROVED PERCENTILE ESTIMATOR
The invention relates to a hearing aid, preferably to a programmable
hearing aid.
Percentile estimators which may also be used in hearing aids, are known
in principle from US-A 4.204.260.
Clinical tests have shown, that the use of correctly fitted hearing aids, i.e.
hearing aids with constant gain, independent of signal-levels, in noisy as
well as
quiet surroundings are superior to hearing aids with an automatic gain
control, with
respect to speech comprehension. However, while linear hearing aids require
the
user to adjust the volume control dependent on the actual listening
environment,
hearing aids with automatic gain control adapt themselves to the environment
and
thereby clearly improve the ease-of-use.
Based on the clinical tests mentioned above, the percentile estimators
have to work very slowly to achieve an almost constant gain for speech
signals.
This works very well if one stays in an environment where the level of sound
does
not vary too much, but the long response times of the system will in some
cases not
adapt fast enough to changes in environment, resulting in phrases not being
heard.
A common problem is the situation where the user of the hearing aid is
yelling a message to a distant person. This will increase the percentile
estimate
and hence reduce the gain in the hearing aid. Since the percentile estimator
works
slowly, the gain stays reduced for a while, and the hearing aid user will not
be able
to hear the distant person answering, because the resulting output of the
hearing
aid will be very low, perhaps even below the user's hearing threshold level.
On the other hand one could let the percentile estimators work fast,
which obviously will make the system adapt faster to changes in environment,
but
the gain can then not be considered constant for the speech signals. The fast
gain
adjustment of the system will cause "pumping" -effects, which can be very
annoying
for the user, especially in noisy surroundings, and may result in loss of
speech
comprehension.
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In an automatic gain control system for hearing aids, percentile
estimators operating on the present signal in one or more channels may be used
for controlling the gain of the electronic signal processors. Such a system is
f.i.
disclosed in WO 95/15668 of applicant.
According to an aspect of the present invention there is provided a
programmable hearing aid having at least one microphone, at least one signal
processor with at least one channel, an output amplifier and an output
transducer,
at least one of the channels containing a signal processing circuit with at
least one
percentile estimator for the continuous determination or calculation of at
least one
percentile value of the input signal from a continuous analysis and evaluation
of the
frequency and/or amplitude distribution of the input signal, whereby the
percentile
values) serve either directly or indirectly as control signals for controlling
the gain
and/or the frequency response of the electronic processing circuit, the
percentile
estimator consisting essentially of a comparator stage with two inputs and two
outputs, the first input being directly or indirectly connected to the input
of the
hearing aid, its two outputs controlling a first control stage the output
signals of
which control a first integrator, the output of which, directly or indirectly,
conveys a
control signal to the signal processing circuit and the second input of the
comparator stage.
It is an object of the present invention to create an improved percentile
estimator, particularly for use in hearing aids of the kind referred to above,
which
makes it possible for the hearing aid to adapt quickly to changes in the
environment, while maintaining a slow response when operating on continuous
signals, e.g. speech signals in a steady environment.
This is achieved in a hearing aid in accordance with the present
invention with a percentile estimator structure having at least a second
control
stage connected to the first control stage, and at least one additional
integrator
controlled by the second control stage, the output of which is connected to a
further
input of the second control stage as well as to a multiplier stage,
interconnected
between the first control stage and the first integrator.
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It is of particular importance that the second control stage supplies a
rectified and scaled version of the predefined parameters of the first control
stage,
generating a positive control signal for the second integrator and a forward
reset
signal to said second integrator for establishing a predefined minimum value
of said
integrator whenever the output signal of the first control stage changes.
Furthermore, in accordance with a second aspect of the present
invention there is provided a hearing aid having at least one microphone for
providing an input signal, at least one signal processing channel receiving
and
processing at least a portion of said input signal to produce at least one
output
signal, an output amplifier for amplifying said at least one output signal,
and an
output transducer responsive to the amplified output signal, said at least one
channel including a signal processing circuit for processing said input signal
portion
in accordance with an output signal from a percentile estimator, said
percentile
estimator including a comparator stage for comparing said input signal portion
to
an integrated value, a first control stage responsive to an output signal from
said
comparator stage for providing an integrator control signal in accordance with
said
comparison, and a first integrator responsive to said integrator control
signal,
providing said integrated value representing a value and a direction of
integration
to said comparator stage, and, as said output signal to said signal processing
circuit, wherein a multiplier stage is interconnected between the output of
said first
control stage and the input of said first integrator for modifying said value
of the
integrator control signal of said first control stage, and a second control
stage is
provided responsive to the integrator control signal of said first control
stage for
controlling a second integrator for providing a modification signal to the
multiplier
stage.
The present invention will be further described with reference to the
drawings in which:
Figure 1 shows a schematic circuit diagram of a multichannel hearing aid
using percentile estimators;
Figure 2 shows a schematic diagram of the principle of a percentile
estimator;
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Figure 3 shows, schematically, an improved percentile estimator for
hearing aids with two levels in accordance with the present invention;
Figure 4 shows, schematically, an improved percentile estimator for
hearing aids with three levels in accordance with the invention; and
Figure 5 shows a diagram of the operation of a traditional percentile
estimator in comparison with the operation of the improved percentile
estimator on
an actual sound example.
Figure 1 shows a principle circuit diagram of a multi-channel hearing aid
with one microphone 1 and preamplifier 2, a band split filter 3 for splitting
the
signals into a number of channels (here 3 are shown), each having a signal
processing circuit 4 consisting of a signal processor 5 and a percentile
estimator 6,
a register 7 for storing parameters related to the basic hearing aid
performance, a
summing circuit 8, an output amplifier 9 and a receiver 10.
Figure 2 shows the principle of a traditional percentile estimator 6. Such
percentile estimators are known from US-A 4.204.260.
The input signal for the specific channel is led into a detector stage 11,
which is not essential for the operation of the percentile estimator, but is
preferably
used. It could include a rectification for determining the envelope of the
input
signal, and also a logarithmic conversion to obtain the envelope on a dB-
scale,
which is commonly used in hearing aids. The output signal from the detector 11
is
supplied to a comparator 12 with its two inputs connected to the output from
the
detector 11 and an integrator 14.
The result of the comparison is supplied to the control stage 13, which
in case of the output of the integrator 14 being greater than the output of
the
detector 11 holds a predefined negative value at its output, causing a
decrease of
the value stored in the integrator 14, and in the opposite case holds a
predefined
positive value at its output, causing an increase of the integrator value.
In this way the value present at the output of the integrator 14 will be
percentile estimate of the input signal of the detector 11 and the signal
processor
5, the percentile value being dependent on the actual predefined values of the
control stage 13.
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The output of the percentile estimator 6 is used for controlling the signal
processor 5. Clearly, it is possible to include more than one percentile
estimator
6 in each channel and let the signal processor be controlled by all of these
in
combination. In that case, a combination and control logic may be used to
combine
the output signals of the different percentile estimators.
Figure 3 shows the principle of an improved percentile estimator in
accordance with the invention.
The traditional percentile estimator is modified with a multiplier 15, with
its output supplying the integrator 14 and its inputs connected to the output
of the
control stage 13 and the output of an integrator 17. The integrator 17 is
controlled
by a control stage 16 which includes a rectifier 21 and a gain block 22# for
rectifying and scaling the predefined parameters of the control stage 13 and
thereby modifying the timing of the increase and decrease of the integrator 14
and
thus the response time of the percentile estimator.
The control stage includes a zero-cross detector 23 which provides a
reset pulse for the integrator, which then resets to a predefined minimum
value
whenever the output from the control stage 13 changes, hence whenever the
input
sound crosses the percentile estimator level.
The control stage 16 further may include a comparator 24 for checking
if the output of the integrator 17 is less than a predefined maximum allowable
value
25, in which case the transmission control 26 passes the output of the gain
block
22 on to the integrator 17, and in the opposite case passes a value of zero or
less
on to the integrator 17 in order to prevent further increase of the integrator
output.
The effect is an "accelerating" percentile estimator. The short term
percentile estimator response time is long, dependent on the minimum value of
the
integrator 17 and will be dominant when the environment is characterized by a
relatively constant sound level, where the input sound level crosses the
percentile
estimate frequently. The long term response time is relatively short because
of the
acceleration, and this effect will be of use in cases where the sound level
changes,
e.g. when communicating with a distant person, as mentioned earlier.
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CA 02257461 1999-12-16
Figure 4 shows an expansion of the improved percentile estimator by
another level by adding a multiplier 18 with its output supplying the
integrator 17
and its inputs connected to the output of the control stage 16 and the output
of an
integrator 20, which again is controlled by a control stage 19 similar to
control stage
16.
Clearly it is possible to expand the number of levels in the improved
percentile estimator even more than the three levels shown.
For the traditional percentile estimator of Figure 2 a percentile level of
p percent is obtained by the following formula:
p = 100 u/( u-d)
where a is the upward integration value (positive)
d is the downward integration value (negative)
Both a and d in the formula above are defined by the predefined values
of the control stage 13.
In the improved percentile estimator, the integration # speeds are time
dependent. The upward integration speed is determined by
t t
a a
uresult a ~ k16 ~ lul. k~9 . ~ul dt......dt
0 0
and the downward integration speed is
td td
dresult d~ klb ~ ~d~' klg ' ~d' dt......dt
0 0
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where k,s and k,9 are the scaling factors in the control stages 16 and 19.
In a "stationary" sound environment, i.e. when the percentile estimate is
stable, we have
lu ~ ~ to = Id ~ ~ td = constant
where t~ and td are the collective time intervals over this stable time period
in which
the integrator integrates upwards and downwards, respectively.
This simplifies the integrations in the formulas above, and yields the
following expressions for the integration speeds of the improved percentile
estimator:
uresult - a ~ k~b. ~u/ ~k~9 ~~u~ .tu......tu
- a ~ k~b ~ k~9 . . . . . , constant
,
dresult - d kt6~~d~~kt9~~d~.td ......td
- d ~ k~b. k~9 ....., constant
Hence, for stationary environments, even though the integration speeds
are time dependent, the percentile level can be obtained by the same formula
as
for the traditional percentile estimator, since a constant multiplied to the
integration
speeds a and d does not change this formula.
Figure 5 shows the function of a 2-level improved 90% percentile
estimator with an increase from a minimum odB/sec growing 207.36 dB/sec2 to a
maximum of 57.6 dB/sec and a decrease from a minimum odB/sec growing 2,56
dB/sec2 to a maximum of 6.4 dB/sec.
This is achieved by using a digital implementation with:
a 32 kHz sampling frequency
an upward integration step of a = 5e - 4 in the control stage 13
a downward integration step of d = - 5e - 5 in the control stage 13
a scaling factor of k,s = 1 in the control stage 16
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a predefined minimum value of 0 of the integrator 17
a predefined maximum allowable value of 4 of the integrator 17
The function is compared with a traditional 90% percentile estimatorwith
an increase of 14.4 dB/sec and a decrease of 1.6 dB/sec.
The comparison is performed on an actual sound example with a
duration of 32 secs. The sound level is stepped down 20 dB after approximately
7 secs to simulate a change of sound environment.
Note that the improved percentile estimator, because of the increasing
integration speed, adapts much faster to change in an environment than the
traditional one, with respect to sound level increases (see the first 2
seconds) as
well as sound level decreases (see the signal behaviour around 7 seconds).
Still, the improved percentile estimator behaves similar to the traditional
one in the time range where the percentile estimation in both cases has become
"stationary", i.e. from approximately 20 secs to 32 secs. This is due to the
signal
crossing the output of the improved percentile estimator, which generates a
frequent reset of the integrator speed, and hereby keeps the response time of
the
percentile estimator long for this signal.
Finally, it may be pointed out that all the parameters of control stages 13
and the scaling factors of control stages 16 and 19 may be preset, may be
programmable or may even be program controlled.
The steady progress in the design of very highly integrated circuits may
lead to an extremely compact design of hearing aids, incorporating not only
the
improved percentile estimators for one or several channels but also the micro-
processor and storage means for the necessary operational tools, such as
algorithms.
Furthermore it is to be understood that the register 7 in Figure 1 should
comprise all necessary control parameters for the control of the transfer
characteristic of the hearing aid, possibly also for various different
programmed or
programmable environmental listening situations.
_g_
