TECHNICAL FIED

This invention relates to driver circuits for use with induction loops. In particular, the invention relates to an improved driver circuit for hearing aid induction loops.

BACKGROUND

Hearing aid induction loop systems are an assistive listening technology for individuals with reduced ranges of hearing. They are particularly useful in situations where there may be a large amount of ambient noise, for example when carrying out transactions in a commercial setting, for example, in a bank, or where an individual is remote from the sound source, for example in a cinema or concert hall. Typical installation sites of these assisted listening systems include concert halls, ticket kiosks at a railway or bus station, high traffic public buildings like train stations, lecture theatres, courtrooms, meeting rooms and homes.

The induction loop consists of a physical loop of cable or wire or an array of looped cables or wires placed in or around a designated area. The size and shape of the loop used in a given area will be dependent upon the size and shape of that area. Typically, hearing aid induction loops are driven by baseband audio-frequency currents, typically in the frequency range 100Hz to 5kHz, i.e. no carrier signal is used. The current in the cable generates a magnetic field throughout the looped space which can be picked up by hearing aids equipped, for example, with telecoils or Cochlear implant processors.

Induction loops have a complex impedance comprising mainly inductance and resistance and the inductance, and therefore the impedance, is higher at higher frequencies than lower frequencies. As a result the loop response at higher frequencies is less than that at lower frequencies. This is particularly problematic in induction loop hearing aid systems because it is most commonly in the upper range of the audio frequencies that hearing impairment arises. It is therefore desirable that the frequency response of an induction loop system should not fall off at such higher frequencies. Further, the impedance of an induction loop and therefore its frequency response will be dependent upon its size and shape. Accordingly, there is a problem in designing a driver for an induction loop hearing aid system which will provide the required relatively flat frequency response over the required frequency range, typically 100 Hz to 5 kHz and for a range of different impedances that may arise in different installations because of because of the different sizes and/or shapes that different induction loops may have.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a driver for an induction loop hearing aid system in which the transfer function of the driver can be adjusted and set to a condition which may compensate at least partly for the differing impedances of different induction loops.

In a second aspect, the invention provides a driver for hearing aid induction loop system having a setup module which is operable for adjusting the frequency response of the driver.

In a third aspect, the invention provides a driver for hearing aid induction loop system which includes a test signal generator for generating one or more test signals for testing the characteristics of an induction loop, to enable the driver input audio signals to be processed in a manner which compensates at least partly for the characteristics of the loop.

In a fourth aspect, the invention provides a driver circuit which includes a class D amplifier for providing an output audio signal to an induction loop, and a signal processor which processes input audio signals prior to application to the class D amplifier. In a fifth aspect, the invention provides a driver for an induction loop hearing aid system, comprising a processor whose transfer function can be set at a selected one of a plurality of different transfer functions.

In a sixth aspect, the invention provides a driver circuit for an induction loop hearing aid system which, upon start-up or power up, performs a setup process which adjusts the characteristics of the driver circuit to compensate at least partially for the characteristics of the particular induction loop that it is to drive.

In a seventh aspect, the invention provides a driver for an induction loop hearing aid system which provides a constant current to the induction loop via a class D amplifier, without using a real-time feedback loop to control the frequency response or transfer function of the driver.

DETAILED DESCRIPTION

The invention is described further, by way of example, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic perspective view of an audio frequency induction loop hearing aid system according to an embodiment of the invention, installed in an auditorium;

Figure 2 is a functional block diagram of the system shown in figure 1 ;

Figure 3 is a set of curves illustrating transfer functions of an adjustable filter arrangement included in the system shown in figure 2;

Figure 4 is a functional block diagram of part of the system of figure 2 illustrating in more detail the components which are used for adjusting the transfer function of the adjustable filter shown in figure 2; Figure 5 is a flowchart illustrating a setup procedure which is performed by the system of figure 2, upon power up, in order to set the transfer function of the adjustable filter; and

Figure 6 is a functional block diagram of a suitable adjustable filter. Overview

Figure 1 shows an audio frequency induction loop hearing aid system 100 which comprises an induction loop 103 which, in this example, extends around the perimeter 102 of an auditorium (not otherwise shown), a driver unit 104 for the loop 103 and a microphone 105 for supplying audio signals to the unit 104 representing speech (or other sounds) which the microphone picks up from the voice of a person 107. The loop 103 generates a varying magnetic field which is sensed by well-known hearing aid devices 109, for example by means of a telecoil in the device, which may be worn by people (such as person 108) in the auditorium having impaired hearing. In other examples, the loop 103 may be significantly larger or significantly smaller than required in an auditorium. For example, large induction loops may be provided in a sports stadium and small induction loops may be provided in conference rooms, individual offices or in the region of a desk such as a cashier's desk.

As explained above, loops such as loop 103 have a complex impedance (typically in the range 1 to 100 ohms at 1 kHz) comprising mainly inductance and resistance and the inductance, and therefore the impedance, is higher at higher frequencies than lower frequencies. As a result the loop response at higher frequencies is less than that at lower frequencies. Since it is most commonly in the upper range of the audio frequencies that hearing impairment arises, it is desirable that the frequency response of an induction loop system should not fall off at such higher frequencies. Further, the frequency response of a given loop will be dependent upon its size and shape. As will be explained in detail below, the driver unit 104 is operable to process the signal from the microphone 105 so that the loop may be driven by a substantially constant current throughout the frequency bandwidth of the system (typically 100Hz to 5kHz), thereby to compensate, at least to some extent, for the frequency response of the particular loop 103 which it is used to drive.

Driver Unit 104

With reference to figure 2, driver unit 104 comprises an automatic gain control (AGC) module 206 which receives the audio signal from the microphone 105 and outputs it to an adjustable filter 208 via terminal 231 of a switch 230. The output of filter 208 is supplied, via a low pass filter 209, to the input of a variable gain amplifier (VGA) 212. The gain of the VGA 212 is controllable by a control signal supplied on line 220 to control input 213 of VGA 212. The output of the VGA 212 is applied to the input of a class D amplifier 210. The loop 103 is driven by the output of the class D amplifier 210 via a filter 240 which converts the pulse width modulated output of the class D amplifier to an audio frequency current that corresponds to the audio signal from the microphone. The unit 104 includes a controller 202 which controls the processes performed by the unit 104.

The driver unit 104 also includes a current sensor 224 which senses the current in the coil 103 when being driven by the output of the class D amplifier 210. The current sensor 224 supplies the RMS value of the sensed current to the controller 202.

A control panel 204 of the unit 104 includes user actuable input devices (not shown as they may be conventional), such as switches or one or more touchscreens to enable the user to input, in known manner, instructions for the operation of the unit 104. Indicators such as electronic displays or LEDs (not shown as they may be conventional) may be provided on the control panel 204, for example for indicating the status of the driver unit. The AGC module 206, adjustable filter 208, low pass filter 209, VGA 212, controller 202 and switch 230 may be, and preferably are, implemented by an appropriately programmed microprocessor.

Controller 202

In order to compensate for different frequency responses of different coils 103, or different arrangements of a given coil 103, the transfer function (frequency response) of filter 208 can be adjusted by the controller 202. In particular, in this embodiment, the transfer function of filter 208 can be selected so that the transfer function of the combination of the filters 208 and 209 may correspond to any one of thirty-two different curves CI to C32 as diagrammatically represented in fig 3, in which the horizontal axis represents frequency and the vertical axis represents dB. These curves are not to scale but are merely to illustrate different shapes of transfer function to which the combination of filters 208 and 209 can be set, and for clarity only curves CI, C2, C19, C25 and C32 are labelled in figure 3.

Thus, for example, curves CI represents a transfer function in which the response of the filter 208 is low at 100 Hz and rises relatively steeply with frequency towards 5 kHz whereas curve C32 represents a response which the transfer function of the filter 208 is high at 100 Hz and relatively flat over the range 100 Hz to 5 kHz. Inspection of figure 3 shows that if a given transfer function is represented by curve C(n) it has a higher frequency response at 100 Hz than C(n-l) but a lower rate of increase with frequency than C(n-1). Hence, the responses represented by curves CI to C32 are spaced apart at about 100 Hz but converge towards 5 kHz. The spacing at 100 Hz may be approximately 1 dB, so that the difference between CI and C32 is approximately 42 dB at 100 Hz. These figures are given by way of example, and different figures may be used in different embodiments.

The selection of the appropriate one of the transfer function curves CI to C32 is, in this embodiment, performed automatically under control of the controller 202 each time the driver unit 104 is powered up.

As shown in figure 4, the controller 202 comprises a management module 410 which manages the operations performed by the driver unit 104 and a setup module 420 which is activated each time the unit 104 is powered up in order to select the appropriate one of the 32 transfer function curves diagrammatically illustrated in figure 3. For this purpose, the setup module 420 comprises 5 kHz and 100 Hz signal generators 422 and 424 for generating test signals at these frequencies which will be applied, at different times during the setup process, to the input of filter 208, a gain controller module 426 for controlling the gain of VGA 212, a comparator

434 for comparing the RMS value received from the current sensor 224 with a required RMS current value, which is typically 1 amp, and a look up table 428 storing sets of coefficient values for application to the filter 208, each set causing the filter 208 to adopt a respective one of the transfer function curves CI to C32.

Setup Process

The setup process is carried out in two phases, which will be referred to as phase I and phase II. In phase I, the gain of VGA 212 is adjusted to a value which causes the driver unit

104 to produce a current of value 1 amp RMS in the loop 103 in response to the 5 kHz test signal. Phase I is performed by initially setting the gain of VGA 212 to a low value and gradually increasing it until the current in the loop is at the required 1 amp RMS value. At this point, the gain of VGA 212 is fixed.

In phase II, the transfer function of filter 208 is selected so that the driver unit 104 also produces a current value of 1 amp RMS in the loop 103 in response to the test signal of 100 Hz with the gain of the VGA 212 remaining at the value set in phase I. Phase II is performed, after phase I has been completed and with the gain of VGA 212 fixed at the level selected in phase I, by initially setting the transfer function of filter 208 to correspond to curve CI and stepping through curves C2, C3 et cetera in turn until a current of 1 amp RMS is again produced in the loop 103. The transfer function which gives this result is then set in the filter 208. The system may then be set into normal use.

The setup process is illustrated in more detail in the flowchart of figure 5. In this, phase I is represented by steps 601, 602, 604, 606, 608 and 610. Phase II is represented by steps 611, 612, 614, 616, 618, 622 and 620. Since the legends in the blocks of figure 5 describe each of these steps, it is not necessary to further elaborate on them in this description.

Filter 208

As shown in figure 6, the filter 208 in this embodiment comprises a multiplier 640 to which the signal input to the filter 208 is directly applied, a delay module 642, having a delay of one sample period, through which the input signal is supplied to a second multiplier 644 and an adder 646 which sums the outputs of the multipliers 640 and 644 and outputs the result as the output of the filter 208. The multipliers 640 and 644 have associated with them respective coefficient stores 648 and 650 for storing coefficients by which the inputs to the multipliers 640 and 644 are multiplied respectively.

The values of the coefficients stored in the stores 648, 650 determine which of the transfer functions CI to C32 is applied by the filter 208. As will be apparent, these values are selected during the setup process from values stored in the look-up table 428 shown in figure 4

The coefficients of the adjustable filter 208 may be calculated using the following equations: z ₀ + 1 (Equation 1)

-1 (Equation 2)

where Z ₀ is the first coefficient and Z is the second coefficient, S is the sample frequency (in Hz) and F is the desired "corner" frequency (in Hz). The corner frequency is the frequency where the response of the filter transitions from being relatively flat to rising relatively steeply.

A numerical example of a set of filter coefficients for producing a set of transfer functions CI to C 32 as illustrated in figure 3 is given in Table 1 below.

Table 1

Response Curve Coefficient Zo Coefficient Zi Corner Frequency (Hz)

CI 1.004149378 -0.995850622 100

C2 1.004930598 -0.995069402 118.92071 15

C3 1.005858038 -0.994141962 141.4213562

C4 1.006958707 -0.993041293 168.1792831

C5 1.008264463 -0.991735537 200

C6 1.009812814 -0.990187186 237.841423

C7 1.011647842 -0.988352158 282.8427125

C8 1.013821236 -0.986178764 336.3585661

C9 1.016393443 -0.983606557 400

C 10 1.019434916 -0.980565084 475.682846

Cl l 1.023027464 -0.976972536 565.6854249

C12 1.027265628 -0.972734372 672.7171322

C13 1.032258065 -0.967741935 800 C14 1.038128802 -0.961871198 951.365692

C15 1.045018270 -0.954981730 1131.37085

C16 1.053083891 -0.946916109 1345.434264

C17 1.062500000 -0.937500000 1600

C18 1.073456786 -0.926543214 1902.731384

C19 1.086157863 -0.913842137 2262.7417

C20 1.100816072 -0.899183928 2690.868529

C21 1.117647059 -0.882352941 3200

C22 1.136860257 -0.863139743 3805.462768

C23 1.158647036 -0.841352964 4525.4834

C24 1.183166062 -0.816833938 5381.737058

C25 1.210526316 -0.789473684 6400

C26 1.240768829 -0.759231171 7610.925536

C27 1.273848776 -0.726151224 9050.966799

C28 1.309620209 -0.690379791 10763.47412

C29 1.347826087 -0.652173913 12800

C30 1.388096193 -0.611903807 15221.85107

C31 1.429954923 -0.570045077 18101.9336

C32 1.472839693 -0.527160307 21526.94823

With these sets of values of the coefficients of the filter 208, a set of transfer functions of the combination of the filter 208 and filter 209 may be produced as represented by curves CI to C32 illustrated diagrammatically in figure 3.

As can be seen from the above table, and as diagrammatically represented in figure 3, each successive transfer function in the series CI to C32 has a corner frequency higher than the preceding transfer function in the series. At frequencies above the corner frequency, each transfer function rises less steeply than the preceding transfer function in the series, whereby the curves CI to C32 converge with increasing frequency towards 5 kHz.

It may be noted from the above table that the corner frequency of curves C24 to C32 is higher than the frequency range of interest (100 Hz to 5 kHz). To reduce or eliminate unwanted frequencies above this range, the fixed low pass filter 209 is provided. By way of example, this may have a cut-off frequency of 7 kHz, specifically, at 7 kHz its frequency response has dropped by approximately 3 dB. In fact, the 32 different transfer functions in the series CI to C32 are the inverse of 32 different frequency responses that may be possessed by 32 different loops or arrangements of loops. Thus, the corner frequency corresponds to the point in the respective frequency response at which the response begins to fall off significantly. The inventor has verified that such a set of transfer functions is adequate to provide a flat frequency response loops of a wide range of different impedances.

Variations and Modifications

In the present embodiment, a set of 32 filters has been found to be sufficient for use with many different induction loops, whether the induction loop is the size of a small room or an induction loop that is installed in a sports stadium or anything in between, like the lecture hall illustrated in Fig. 1. However, this number of filters (transfer functions) is not essential and more or less may be used, for instance there may be between 20 and 40 filters (transfer functions) or less than 20 or more than 40. The choice is dependent on the level of accuracy required and the range of different induction loop impedances that the driver is to capable of driving with a substantially constant current.

Although the test signals used in the embodiment described with reference to the drawings have been 5 kHz and 100 Hz respectively, other values may be used. However, the high-frequency test signal should preferably have a value which corresponds to, or is near to, the upper end of the bandwidth of the system and is preferably a frequency at which the transfer functions (responses) of the filter converge. The low-frequency test signal is preferably close to the lower end of the bandwidth of the system.

Although in the embodiment described, the setup process involves selecting selecting, from the transfer function CI to C32, a transfer function which produces approximately 1 amp RMS in the induction loop, it is possible for other values of current to be used. For example, in other embodiments, the setup process may be such that the current to be produced in the induction loop in that process is 0.5 amps RMS or 1.5 amps RMS. A low value, however, is desirable since, during the setup process, the current in the induction loop may be detectable in hearing aids and therefore cause disturbance to nearby people with hearing aids. However, the value should not be so low that it would fall out of the range of the measurement circuitry and/or be susceptible to external magnetic noise. The level of the test signals (100 Hz and 5 kHz in the embodiment described) will be selected to provide the required current level in the induction loop in the setup process.

Although figure 2 shows the 5 kHz test signal being supplied to the input of the VGA 212 via the filter 208, it could alternatively be applied directly to the VGA 212.

Although it has been indicated above that some components, at least, of the driver may be implemented by a programmed microprocessor, this is not essential. The device may be a hardwired digital device or it could be implemented by analogue circuits. In this case, the filter 208 may be constituted by a bank of analog filters.

Although the filter shown in figure 6 is a simple circuit requiring only two coefficients, more complex filters requiring more coefficients may be employed. However, the simple filter shown in figure 6 has been found to be suitable.

Although in the embodiment shown in the drawings, the audio signal applied to the driver is shown as generated from the microphone, other signal sources may be used such as from radio broadcasts or sound recordings.

Further Aspects and Features of the Invention

Further aspects and features of the invention are defined in the following clauses:

A. A driver for an induction loop hearing aid system comprising:

(a) an input for receiving an input audio signal; (b) a processor for processing the audio input signal to produce an output audio signal, the processor having an adjustable transfer function;

(c) an output for outputting the output audio signal to an induction loop; and

(d) a set up module operable to perform a setup process in which:

(i) a plurality of test input signals at spaced apart frequencies are processed by the processor to produce test output signals applied to the induction loop,

(ii) the current produced in the induction loop by the test output signals is detected,

(iii) the transfer function of the processor is adjusted to identify a transfer function which provides substantially the same current in the induction loop at said spaced apart frequencies; and

(iv) the identified transfer function is selected as the transfer function of the processor for processing the input audio signal.

B. A driver according to clause A in which the setup process comprises:

(a) causing the processor to process a said first test signal at a relatively high frequency,

(b) setting the gain of the transfer function such that, at said relatively high frequency, the current in the induction loop has a predetermined value,

(c) causing the processor to process a second said test signal at a relatively low frequency,

(d) identifying a shape of the transfer function which is such that the current in the induction loop has substantially said predetermined value both when processing the first said test signal and when processing said second test signal, and

(e) selecting the transfer function of said identified shape as the transfer function of the processor for processing the input audio signal.

C. A driver according to clause B, in which the processor includes a filter having an adjustable frequency response and an amplifier having variable gain and the setup process comprises:

(a) amplifying said first test signal by said variable gain amplifier, adjusting the gain of the amplifier until said gain is at a level at which the current in the induction loop has substantially said predetermined value and setting the gain at said level; and thereafter

(b) filtering said second test signal by said filter, amplifying the filtered second test signal by said variable gain amplifier with the gain set at said level, adjusting the frequency response of the filter until the frequency response has a characteristic such that the current in the induction loop has substantially said predetermined level, and setting the filter to have said frequency response characteristic,

the selected transfer function of said processor thereby being determined by said frequency response characteristic and said gain level.

D. A driver according to clause C, in which said filter has a plurality of discrete frequency response characteristics each determined by a respective one of a plurality of sets of filter coefficients, the adjusting of the frequency response of the filter being performed by applying different ones of said sets of coefficients to said filter.

E. A driver according to clause D, in which said sets of coefficients are pre- calculated and prestored in said processor.

F. A driver according to clause E, in which said sets of coefficients are stored in a look up table.

G. A driver according to any of clauses D to F, in which each set of coefficients consists of two coefficients.

H. A driver according to any of clauses D to G, in which the filter has from 20 to 40 discrete frequency response characteristics. I. A driver according to any of clauses D to G, in which the filter has between 30 and 34 discrete frequency response characteristics.

J. A driver according to any of clauses D to G, in which the filter has 32 discrete frequency response characteristics

K. A driver according to any of clauses B to J, in which the first test signal has a frequency of 5 kHz ± 10% and the second test signal as a frequency of 100 Hz ±10%.

L. A driver according to any of clauses B to J, in which the first test signal has a frequency of 5 kHz and the second test signal has a frequency of 100 Hz.

M. A driver according to any of clauses A to L, including a class D amplifier arranged to amplify the output audio signal from the processor for providing an amplified output audio signal to the induction loop.

N. A driver according to any of clauses A to M, wherein said set up module is arranged to perform said setup process in response to powering up the driver.

O. A driver for an induction loop hearing aid system comprising:

an input for receiving an input audio signal;

a processor for processing the audio input signal to produce an output audio signal; and

an output for outputting the output audio signal to an induction loop;

wherein the processor is operable in a setup process in which, utilising test signals at spaced apart frequencies, the transfer function of the processor is selected to provide substantially the same current in the loop in response to each said test signal.