Electrical audio signal processing systems and devices – Hearing aids – electrical
Reexamination Certificate
2000-07-26
2004-05-04
Le, Huyen (Department: 2643)
Electrical audio signal processing systems and devices
Hearing aids, electrical
C381S321000, C381S102000, C381S123000
Reexamination Certificate
active
06731768
ABSTRACT:
BACKGROUND OF THE INVENTION
Generally stated, hearing aids attempt to amplify the wide dynamic range of sounds found in the real world into the limited dynamic range of sounds that the impaired ear can hear. Crude hearing aids accomplish the need for different gain or volume levels by providing a manual volume control that may be manipulated by a user. More sophisticated hearing aids, however, use some form of automatic gain control or automatic volume control (“AGC”).
FIG. 1A
illustrates a typical prior art AGC circuit. The circuit
101
comprises a variable gain amplifier
103
for adjusting the gain, an attack time constant resistor R
a
105
, a diode
107
, a release or recovery time constant resistor R
r
109
, and a storage capacitor C
S
111
. In operation, as V
out
increases, representing louder sounds, the diode
107
conducts, charging the storage capacitor C
S
111
. Charging of capacitor C
S
111
in turn causes the variable gain amplifier
103
to reduce gain so V
out
returns towards a fixed level. As V
out
decreases, representing quieter sounds, the diode
107
no longer conducts, causing capacitor C
S
111
to discharge through recovery resistor R
r
109
. This, in turn, causes the variable gain amplifier
103
to increase gain so again V
out
returns toward a fixed level. In other words, the ACG
101
increases gain for soft sounds or in the absence of sounds, and decreases gain for loud sounds.
While such a configuration was an improvement over manual volume/gain controls, it suffered from its own problems. For example, most hearing aids having an AGC circuit similar to that shown in
FIG. 1A
utilize fairly fast (e.g., approximately 10 mS) attack times and moderately fast (e.g., approximately 100-300 mS) release times. Shorter release times create noticeable distortion. If a longer release time is used, however, whole sentences may be missed following a sudden loud noise, such as, for example, a slamming door. Further, when the gain increases dramatically during quiet periods, the increased gain frequently causes feedback.
In addition, because of the very wide dynamic range of sounds in the real world and the very limited dynamic range of the impaired ear, high compression ratios are desirable. When combined with typical attack/release times mentioned above, however, high compression ratios cause a phenomenon known as “pumping.” More specifically, the background noise is amplified to a near normal level during brief pauses in speech, resulting in the user hearing word
oise/word
oise, etc. Prior art designs attempted to reduce the pumping effect by adjusting the threshold knee higher (above which the AGC is active), but resulted in mediocre, at best, results. In any case, such a prior art AGC design necessitates compromises among all the possible settings of compression ratio, attack time, release time and threshold knee.
One prior art attempt to improve over typical AGC designs, such as that shown in
FIG. 1A
, is to use two separate AGC circuits in series. The first AGC circuit is given fairly slow (e.g., approximately 1 second) attack/release times, and the second is given fairly fast (e.g., approximately 10 mS) attack/release times for signals at some level above the current level of the first AGC circuit. While this circuit is an improvement over typical AGC designs, it still suffers from the “pumping” effect, just more slowly.
Another prior art attempt to improve over typical AGC designs is shown in FIG.
1
B. As can be seen, the circuit
113
of
FIG. 1B
includes the same components of
FIG. 1A
, but adds a transistor Q
1
115
in series with recovery resistor R
r
109
. The input of the circuit
113
is connected to the base of the transistor Q
1
115
through a preamplifier
117
. In this configuration, the transistor Q
1
115
acts as a switch with the base-emitter voltage as the reference. The base of transistor Q
1
115
is driven by preamplifier
117
whose gain is a function of a feedback resistor R
r
119
and input resistor R
i
121
. The values of these resistors are chosen to establish an output level of the preamplifier
117
equal to the base-emitter voltage of transistor Q
1
115
when the input equals the desired threshold, typically 65 dB peak instantaneous.
In operation, for signals whose instantaneous amplified level exceeds the base-emitter voltage of the transistor Q
1
115
(e.g., signals above 65 dB SPL), the transistor is on and the release or recovery time constant resistor R
r
109
is connected to ground. In this case, i.e., during the time the input is instantaneously above 65 dB SPL, for example, the circuit
113
of
FIG. 1B
acts like the conventional AGC circuit
101
of FIG.
1
A. During the time that the amplified level is below the base-emitter voltage of the transistor Q
1
115
(e.g., below 65 dB SPL), however, the release or recovery time constant resistor R
r
109
is disconnected from ground, causing the gain to be maintained at the most recent setting.
While the circuit
113
of
FIG. 1B
may arguably be an improvement over the circuit
101
of
FIG. 1A
, it still suffers from many of its own problems. For example, because the base-emitter forward bias voltage of the transistor Q
1
115
performs the decision function, the threshold is ill defined. More specifically, the threshold is not a clear on/off characteristic, but rather an on/mostly-on/partly-on/partly-off/mostly-off/off characteristic. In other words, the circuit has multiple release time constants. Because the transistor is not fully on, recovery to soft speech is much slower than desired. Also, because the transistor is not fully off, undesired recovery occurs over a period of seconds. In other words, the transistor has a “slushy” threshold and requires large amounts of preamplifier gain to reach that threshold.
In addition, the very fast attack time and “slushy” recovery to soft speech creates an undesirable effect if a loud noise (e.g., door close, book slam, etc.) occurs in an environment of soft conversation. More specifically, the loud noise and fast attack time cause immediate and complete gain reduction, while a soft voice enables gain expansion only occasionally, causing the user to miss much of what is spoken.
It is therefore an object of the present invention to provide an improved AGC circuit for hearing aid and other related applications.
Other objects of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
BRIEF SUMMARY OF THE INVENTION
An improved automatic gain control circuit for hearing aid and other related applications is provided. In one embodiment, the automatic gain control circuit or system is contained in a hearing aid. The hearing aid has an input transducer for converting sound energy into an electric signal. The hearing aid also includes control circuitry that receives the electrical signal and controls the gain of the electrical signal. In controlling the gain, the control circuitry uses only two release time constants, one representative of a gain control mode and the other representative of a gain adjust mode. A switch, which may be part of the control circuitry, is also included for switching between only the two time constants. The switch may, for example, select a short time constant if the amplitude of the electrical signal is greater than a predetermined threshold, and a relatively longer time constant if the amplitude of the electrical signal is less than the predetermined threshold. While this is the general case, selection of the longer release time constant may be delayed (i.e., the shorter release time constant may be retained) for a given period of time after the amplitude of the electrical signal falls below the predetermined threshold. Alternatively, the shorter release time constant may be selected for a period of time after the amplitude of the electrical signal rises above the predetermined threshold even if the amplitude of the electrical signal falls below the predetermined threshold during that p
Etymotic Research Inc.
Le Huyen
McAndrews Held & Malloy Ltd.
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