Automatic Gain Control for Hearing Aid

1. Field of the Invention

This invention relates to hearing aids and is more particularly directed to the automatic control of hearing aids to enhance the operation under presently difficult conditions encountered by a user as, for example, receiving communication from a source of sound located close to the user but surrounded by a multiplicity of other sources of sound, as in a crowded room with numerous conversations and extraneous "noise" of varying sound levels and occurring at varying distances from the user of a hearing aid.

2. Prior Art

The prior art with which this invention is concerned ranges from relatively simple hearing aid instruments, which may contain what have come to be known as agc, "compression" or automatic gain control features, and which may also require a user to physically manipulate a volume control so as to compensate for variations in the sound field in which a hearing aid instrument is operated or may, as is shown in U.S. Pat. No. 4,461,025 issued July 17, 1984, include complicated circuitry for completely eliminating the background "noise" components of a sound field that are typically characterized as "noise" in the sense that the user of a hearing aid instrument does not desire to receive, or perceive, such components of a signal.

Prior art hearing aid instruments have utilized compression circuits for reducing output distortion as the total input signal exceeds the maximum capability of a hearing aid instrument system and this is accomplished by utilizing a typical form of automatic gain control circuit to eliminate the accompanying distortion when an excessive input signal is encountered.

One such automatic gain control for this purpose is designed to effect a gain reduction during a relatively short time interval, that is known as the "attack time", to prevent discomfort or annoyance to the user of the hearing air instrument due to an excessive signal. On the other hand, when the excessive signal is no longer present or diminished, it may be desirable to have a short or a long release time, dependent upon the environmental conditions in which the hearing aid instrument is used, and the characteristics of the input signal. The fast attack time, typically less than ten milliseconds, has been determined to be reasonable whereas a typical release time may vary from ten milliseconds to much greater intervals of time. The aural perception of the hearing air user is influenced by the release time.

My invention provides a hearing aid apparatus having an automatic gain control signal for controlling the gain of a compression amplifier which provides a uniform, "fast attack" time in response to an input signal exceeding a predetermined maximum value and a variable "release time" responsive to adapt to the various type of signals in the sound fields encountered by a hearing aid user.

Broadly, the principles of my invention are provided through the utilization of a signal characterizing means for providing a uniform "fast attack" characteristic for an automatic gain control signal in response to increases of the input signal over a predetermined maximum value and to provide a "release time" which varies in accordance with the frequency and duration of time intervals in which the input signal exceeds such predetermined value.

It has been observed that the signal to noise ratio is dramatically improved in the class of sound fields encountered by a typical hearing aid instrument user which contain a plurality of sources of sound of an undesired nature, and, in the sense that a sound field contains a plurality of sources of undesirable sound that is typically referred to as "noise" (unintelligible signal) and will contain fewer components of desirable sound that may be conveniently referred to as "intelligible" sound, the perception of the user is enhanced without having to resort to manual manipulation of the volume control in a hearing aid instrument or to experience the perceptible variations in sound intensity previously experienced in hearing aid instruments exhibiting a "fast attack" and "fast release" time regardless of the nature of the sound contained in a sound field.

In a preferred and the illustrated embodiment of my invention, a voltage controlled compression amplifier is configured to receive and respond to an automatic gain control signal in the form of a voltage that is derived from the signal appearing at the output of the amplifier and which will vary in accordance with the amount, or magnitude, of the output signal which exceeds a predetermined compression threshold value. The excessive output signal of the voltage controlled amplifier is supplied to vary the gain of the amplifier through circuit means including at least a pair of capacitors and a resistor which are connected to receive the automatic gain control signal derived from the output of the amplifier in such a manner that an automatic gain control signal of relatively short duration is operative to, in a sense, to directly and immediately reduce and then immediately restore the gain of the amplifier; and an automatic gain control signal of longer duration may be operative to immediately reduce the gain of the amplifier but will gradually restore the gain of the amplifier at a rate that varies directly with the duration and frequency of the received signal and the automatic gain control signal.

In a further embodiment of my invention, the same means for characterizing the AGC signal for controlling the voltage controlled input amplifier means is utilized to receive a plurality of AGC signals, one derived from the output of an input voltage controlled compression pre-amplifier and another derived from the output of a power amplifier to be respesentative of the sound power level (SPL) of the audio signal applied to the auditory sense organs of hearing aid instrument user. Again, for signals of a variable frequency and/or duration that exceed a predetermined value, the release time of the AGC signal controlling the gain of the voltage controlled input pre-amplifier is varied so as to enhance the output signal applied to the auditory sense organs through an increase in the "intelligible" signal-to-"noise" ratio and thereby improve the perception of the user. This may be accomplished through the addition of but a few uncomplicated components which respond to varying automatic gain control signal levels and durations to characterize the signals for controlling the gain of the voltage control input pre-amplifier.

These and further objects and features of my invention will become apparent upon consideration of the following description and drawings in which:

FIG. 1, is a schematic and diagrammatic illustration of a hearing aid embodying the principles of my invention;

FIG. 2, is a family of curves illustrating the operation of the drawing of FIG. 1; and

FIG. 3, is a block diagrammatic and schematic drawing of a hearing aid system illustrating a further embodiment of my invention.

Referring to FIG. 1 of the drawings, a diagrammatic and schematic drawing of a simplified hearing aid instrument includes a voltage controlled pre-amplifier 10, a tone control amplifier 26 and an output power amplifier 28 connected to receive a signal from a microphone through an input conductor 11 connected to voltage controlled amplifier 10, an output conductor 12 connecting the output of voltage controlled gain control amplifier 10 to tone control amplifier 26 and to an AGC signal source, indicated generally by reference character 14, an output conductor 27 connecting the output of tone control amplifier 26 to the input of power amplifier 28 and an output conductor 29 for connecting the output of amplifier 28 to an earphone or receiver.

Voltage controlled variable gain control pre-amplifier 10 further includes a gain control terminal 13 for connection to a source of signal for controlling the gain thereof from automatic gain control signal source 14.

Signal source 14 provides an output signal on conductor 16 which consists of a rectified component of the output signal appearing on output conductor 12, connected to amplifier 10, whenever the output level exceeds a predetermined maximum value. Conductor 16 is connected through terminal 24 on a signal characterizer 20 to gain control terminal 13 on a amplifier 10.

Signal characterizer 20 is shown comprised of capacitor 21 connected intermediate terminal 24 and a ground terminal and a resistor 23 and capacitor 22 connected in series across capacitor 21 and intermediate terminal 24 and the ground terminal. Capacitor 21 and capacitor 22 are typically of substantially different capacitance and resistor 23 may be selected from a range of higher values to provide an increased time constant when charging capacitor 22.

Referring to FIG. 3 of the drawings, a more elaborate representation of a hearing aid instrument is shown including a microphone 30 and a receiver 60 operatively connected through voltage controlled variable gain pre-amplifier 10, tone control amplifier 26 and power amplifier 28.

Gain control amplifier 10 has an input terminal 11, connected to microphone 30 through capacitor 32 and conductor 31 and an output terminal connected to tone control amplifier 26 through an output conductor 12, terminal 40, terminal 48, resistor 50 and capacitor 51. The output of tone control amplifier 26 is shown connected to power amplifier 28 through conductor 27 capacitor 52 and the output conductors 29 of amplifier 28 are connected to terminals 61 and 63 on a receiver 60. Terminal 48 is connected to ground through a resistor 49 and terminal 40 is shown connected to a full wave AGC rectifying means 35 (shown in phantom outline) in voltage controlled variable gain pre-amplifier 10 through capacitor 41, resistor 42, wiper 43 on potentiometer 44 and conductor 45.

Signal characterizer 20 includes the elements described above and is further connected to terminal 64 on output 29 of power amplifier 28 through conductor 74, Schottky diode 73, terminal 75, capacitor 70, resistor 69, wiper 68 on potentiometer 67 and resistor 66. Terminal 64 is also connected to ground through capacitor 65.

Various portions of the amplifiers and circuitry are suitably connected to a source of current (not shown) and such connecting components are not further identified.

FIG. 2 contains a number of curves placed over a grid to illustrate the signals appearing at various portions of the apparatus described above. Curve 80 represents the sound power level of an acoustical input signal; curve 81 represents an AGC signal derived from the output of an amplifier; curve 82 represents a component of the AGC signal in curve 81 as derived from the output of an amplifier; curve 83 represents a compressed acoustical output (SPL) signal as may be applied to the auditory sense organs of a user; and curve 84 represents the perceived signal as sensed by the auditory organs of a hearing aid user.

As indicated under curve 80, the signals are divided into a plurality of discrete portions indicated by the letters G through U (inclusive) applied to the brackets appearing under the signal illustrated as curve 80. The vertical axis represents either voltage, or potential, or sound power level and the horizontal axis represent time during which a signal may occur.

The illustrated signal characterizer 20 is derived from a description contained in the November 1950 issue of Audio Engineering, pages 18, 19, 69 and 70 in an article entitled "Performance and Operation of a New Limiting Amplifier" by G. P. Singer--a description of recently developed broadcast-type limiting amplifier and its characteristics under various input-signal conditions".

The threshold magnitude for initiating operation of the automatic gain control circuit into a compression mode of operation, also known as the "knee" of the compression curve, is determined by the characteristics of a given instrument and the physiological characteristics of a user. This may be determined by designers of hearing aid apparatus and is based upon experience and other data accumulated so that the sum total of the entire hearing aid instrument including the automatic gain control circuit, is satisfactory to present a comfortable, intelligible audio signal for perception by the audio sense organs of a user.

However, a long standing undesirable operation occurs whenever the input signal from the sound field in which the instrument is operable, contains undesireable "noise", such as in a crowded room with many conversations taking place, and, depending upon the time constant or reaction time of a given AGC compression control signal, the user either experiences perceptions of a rapid fluctuation in volume, or SPL, or a continued decrease in sound pressure output so that a severe loss of intelligibility is perceived and, for most practical purposes, the instrument is rendered substantially useless.

By applying the principles of my invention to a hearing aid instrument in controlling the operation of the compression mode of a controlled gain amplifier, relatively short "bursts" or "pulses" of energy are, in effect, substantially reduced and/or eliminated from the reporduced signal applied to the auditory sense organs of a user. "Intelligible" signals, which when combined with the background "noise" result in an overall reduction in gain with respect to the excessive signal received by a microphone but permit a combined signal, having enhanced signal to "noise" ratio, to be amplified within the regime of a permissible sound power level so that a greater proportion of the signal applied to the audio sense organs of a user consists of the "intelligible" component and the previously obtained perception of a loss of intelligibility or other forms of discomfort is eliminated and the user is thereby better able to understand what he is seeking to hear.

In the curves of FIG. 2, an input signal shown on the curve 80 contains a number of components variable about the zero axis and shown in relation to a pair of horizontal dotted lines which indicate the maximum level of energy that may reasonably be accommodated by the electrical amplifiers of a hearing aid without producing distortion.

The initial section, G, represents random or background noise; sections H,J,L,N, and P represent "noise" pulses and section R and T represent intelligible sound energy. Whenever the signal represented by curve 80 exceeds the maximum values indicated by the dotted horizontal lines, the voltage or potential appearing at capacitors 21 and 22 will be affected in the manner shown in curves 81 and 82 to provide a transient AGC signal that produces the output illustrated on curve 83 which is then perceived by a hearing aid user as illustrated on curve 84.

When considering a period of random or background noise in Section G of the curves followed by a pulse of noise of less than 12 milliseconds, the relationships as illustrated by section H on the remainder of the curves are obtained, the AGC signal for controlling the compression amplifier is such that compression to the maximum desired limits as illustrated on curve 83 occurs and the "noise" pulse remains unperceived by the hearing aid user.

When a series of pulses illustrated by Sections J,L,N, and P are received, the characterized control signal appearing thereunder on curve 81 is obtained and the relationships are seen in the remainder of the curves. Similarly, the "intelligible" signal of portions R and T of curve 80 result in the phenomina shown in cruves 81,82,83, and 84.

Referring specifically to Sections J-Q (inclusive) of the curve, it may be seen that when a plurality of relatively closely spaced pulses are received, curve 81 indicates that capacitor 21 acquires a voltage or potential that increases in steps to serve as the compression control signal for the amplifier whereby instead of having a successive plurality of compression control signals as represented in Section H of curve 81, the potential rises in steps that are imperceptible to the hearing aid user and thereby do not present a source of confusion resulting from the successive "noise" pluses of Sections J,L,N, and P of the signal shown on curve 80.

When the relatively longer duration of an "intelligible" signal appears, as in Sections R and T of the curve of FIG. 80, capacitors 21 and 22 are charged as shown on curves 81 and 82 respectively, the output signal of curve 83 is compressed to the desired value and the perception of the hearing aid user is as shown on curve 84.

In an operative embodiment of my invention compression amplifier 10 comprised a commercially available LC/LD No. 502, low distortion, AGC Compression Preamplifier supplied from Linear Technology Inc. of Burlington, Ontario, Canada L7R 3Y3 was utilized in connection with a signal characterizer having values of 0.47 microfarads for capacitor 21, 2.2 microfarads for capacitor 22 and 220K ohms for resistor 23.

In the embodiment of FIG. 3, an AGC-compression component is added from output 29 on power amplifier 28 in which resistor 67 is a 10K ohm potentiometer, resistor 69 is a 3.3K ohm resistor, capacitor 70 is a 2.2 microfarad capacitor; resistors 71 and 72 are 220K ohm resistors and diode 73 is a Schotkey diode exhibiting the characteristics of a low forward voltage drop and rectification of the signal appearing at output 29 representative of the sound pressure level output that is to be applied to the auditory sense organ of a user through microphone 60. While the operation of signal characterizer 20 remains essentially the same, it may be seen that what is known as "input" and "output" compression are utilized to form the signal that appears at terminal 24 on signal characterizer 20 and the operation remains essentially the same as that described above in connection with the curves of FIG. 2.