EM interference canceller in an audio amplifier

A problem in hearing aids or other audio/acoustic amplifier circuits is that external sources of EM energy may be coupled into the electronics of the hearing aid so as to contribute to the acoustic output. The invention provides a circuit for removing the effects of EM interference. A separate reference generator is used to detect the external EM energy. This is fed into an interference canceller which may be adaptive, which effectively removes the unwanted component in the hearing aid signal, leaving only a signal representative of the desired acoustic output.

FIELD OF THE INVENTION 
The invention relates to a system for cancelling RF interference in audio 
amplifiers. 
BACKGROUND OF THE INVENTION 
The function of an audio amplifier is to take an input audio signal, 
amplify and process it as necessary, and produce an output audio signal. 
Radiated EM (electro-magnetic) signals, such as those from nearby wireless 
equipment, having a transmitted power envelope with frequency components 
in the audio band, may be picked up at some point in the audio equipment. 
This interference can be inadvertently demodulated into audioband 
components in the audio amplifier circuitry (for example by a FET in an 
electret microphone) and added to the desired signal. These interference 
signals may then be output along with the desired signal resulting in an 
undesired noise component in the output signal. 
Acoustic amplifiers include audio amplifier circuitry and thus are 
susceptible to the above-described problem of EM interference. In an 
acoustic amplifier, an input acoustic signal is converted to an audio 
signal which is input to the audio amplifier circuitry where it is 
amplified and processed. The output of the audio amplifier circuitry is 
reconverted into an amplified output acoustic signal. 
EM interference can be a serious problem in hearing aids in which 
amplifiers with a large gain and amplitude compression are usually 
employed. The most important input to a hearing aid is a desired acoustic 
input, and the most important output of a hearing aid is a processed and 
amplified acoustic output. The desired acoustic signal is transduced into 
an electrical signal, processed and amplified by electronic components in 
the hearing aid, and converted back or transduced into the output acoustic 
signal. Depending on the frequency characteristics and power envelope of 
any interfering EM signals, these can be transduced along with the desired 
electrical signal to produce an audible interference component in the 
amplified sound produced by the hearing aid. 
Typically, for an EM source to cause interference in a hearing aid, the 
source must be quite close to the hearing aid, and must possess certain EM 
characteristics such as a non-constant envelope. For example EM radiation 
from television sets, computer monitors, and neon lighting systems can 
interfere with hearing aid operation. More recently, digital cellular 
telephony, whose signals meet these conditions has become a problem in 
this area. With the increasingly widespread use of digital cellular 
telephones, a technique for eliminating their interference effects upon 
hearing aids is desired. 
It is common in many types of audio equipment to employ techniques for 
reducing or cancelling noise or interference. In contrast to the above 
described situation in which an inadvertently received EM signal 
interferes with an internally generated audio signal, existing systems 
deal with interfering signals which are received in the same physical 
manner as the desired signals. For example, in hearing aids which have a 
microphone and a speaker portion, acoustic feedback from the speaker into 
the microphone may exist, and adaptive equalization may be employed in the 
hearing aid to reduce or minimize the negative effects of the feedback 
upon the operation of the hearing aid. 
Three existing systems which employ such a technique for reducing acoustic 
feedback are disclosed in U.S. Pat. No. 5,412,735 by Engebretson et al. 
which issued May 2, 1995 entitled "Electric Filter Hearing Aids and 
Methods", U.S. Pat. No. 5,475,759 by Engebretson et al. which issued Dec. 
12, 1995 entitled "Adaptive Noise Reduction Circuit for a Sound 
Reproduction System" and U.S. Pat. No. 5,402,496 by Soli et al. which 
issued Mar. 28, 1995 entitled "Auditory Prosthesis Noise Suppression 
Apparatus and Feedback Suppression Apparatus Having Focused Adaptive 
Filtering". 
As another example, noise cancellation systems exist for the purpose of 
cancelling acoustic background noise. These systems employ a main 
microphone near the desired sound source, and a noise reference microphone 
near the source of the noise, for example, a vent fan. The main microphone 
will s till pick up unwanted noise from the fan. The inputs from the main 
microphone and the noise microphone are combined so as to remove from the 
main microphone the effects of the ventilation noise. The performance of 
such active noise cancellation systems is also compromised when the noise 
reference microphone can pick up some of the desired sound signal as well 
as the acoustic noise signal. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an audio amplifier with 
increased immunity to interference from nearby EM sources. 
According to a first broad aspect, the invention provides an interference 
canceller circuit for use in an audio amplifier, the audio amplifier 
having electronic circuitry which generates an electric signal which 
includes a desired audio signal component and which may include a 
component due to externally generated EM energy inadvertently coupled into 
the electronic circuitry, the interference canceller circuit comprising: 
an EM reference signal generator for generating a reference EM signal 
representative of the externally generated EM energy; and an interference 
canceller network connected to receive the reference EM signal and to 
cancel from the electric signal the component due to the externally 
generated EM energy. 
According to a second broad aspect, the invention provides an audio 
amplifier comprising electronic circuitry for amplifying and processing an 
electrical signal and an interference canceller circuit for reducing the 
effect of spurious externally generated EM energy being coupled into the 
electronic circuitry; the interference canceller circuit comprising an EM 
reference signal generator for generating a reference EM signal 
representative of the externally generated EM energy; and an interference 
canceller network connected to receive the reference EM signal and to 
cancel from the electrical signal the component due to the externally 
generated EM energy. 
According to a third broad aspect, the invention provides an acoustic 
signal amplifier comprising an input transducer for converting an acoustic 
signal into an electrical signal, electronic circuitry for amplifying and 
processing the electrical signal, an output transducer connected to the 
electronic circuit to derive an amplified acoustic signal, an interference 
canceller circuit for reducing the effect upon the amplified acoustic 
signal of spuriously generated EM energy inadvertently coupled into the 
electronic circuitry, the interference canceller circuit comprising: an EM 
reference signal generator for generating a reference EM signal 
representative of the externally generated EM energy; and an interference 
canceller network connected to receive the reference EM signal and to 
cancel from the electric signal the component due to the externally 
generated EM energy.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a functional block diagram of an acoustic amplifier such as a 
hearing aid, generally indicated by 10, in the context of an environment 
containing an EM interference signal 12 generated by an EM source 14 which 
is nearby, and also containing desired acoustic signals 16 generated by an 
acoustic source 18. The EM source 14 may be a wireless handset, for 
example, while the acoustic source 18 may be a person speaking, for 
example. The hearing aid 10 includes the functionality of a conventional 
hearing aid, generally indicated by 20, and an interference canceller 
circuit according to the invention, generally indicated by 22. 
The conventional hearing aid functionality 20 includes an input transducer 
such as a microphone 24 for receiving acoustic signals 16 produced by the 
acoustic source 18 and converting the acoustic signals 16 to electrical 
signals. The conventional hearing aid functionality further includes an 
input amplifier 26, an NLP (non-linear processing block) 28 followed by an 
output amplifier 32, and produces an output acoustic signal with an output 
transducer such as a speaker 35. The NLP block 28 may include signal-level 
dependent equalization and compression functions, for example. The input 
amplifier 26, NLP 28 and output amplifier 32 are all realized with 
electronics forming part of the hearing aid 10. In conventional hearing 
aids, the output of the input amplifier 26 is connected directly to the 
NLP 28 as indicated by dotted line 35. 
The source of EM energy 14 is producing an EM signal labelled EM.sub.-- 
SOURCE having a signal envelope equal to EM.sub.-- SOURCE Env. The printed 
circuit traces and electronics within the hearing aid 10 may behave like 
an antenna so as to receive components of the EM signal generated by the 
EM source 14. These received EM signals may be inadvertently demodulated 
by the hearing aid electronics so as to contribute to the acoustic output 
of the speaker in the form of unwanted acoustic noise. 
According to the invention, the hearing aid is equipped with an 
interference canceller circuit 22. In place of the direct connection 35 
between the input amplifier 26 and the NLP block 28, an interference 
canceller network 36 forming part of the interference canceller circuit 22 
is connected to receive an input from the input amplifier 26 and to pass 
an output to the NLP block 28. The input to and output from the 
interference canceller network 36 are labelled AIC.sub.-- in and 
AIC.sub.-- out respectively. An EM reference generator 38, also forming 
part of the interference canceller circuit 22 and shown connected to the 
interference canceller network 36, is used to generate a "reference" or 
model of the interfering EM field power envelope EM.sub.-- SOURCE.sub.-- 
Env for use by the interference canceller network. The reference generated 
by the EM reference generator 38 is labelled EM.sub.-- Ref. 
Referring now to FIG. 2, a signal flow diagram for part of the hearing aid 
of FIG. 1 is shown. As indicated by an adder symbol 40, the signal 
AIC.sub.-- in is the sum of two components the first of which is an 
"ideal" audio signal, labelled Rcv, which is the electrical signal which 
would be produced at the output of the input amplifier 26 due to the 
acoustic signal 16 in the absence of any interfering EM signals. The 
second component of the signal AIC.sub.-- in is due to the interfering EM 
signal 12 having a signal envelope equal to EM.sub.-- SOURCE.sub.-- Env. 
The EM signal envelope EM.sub.-- SOURCE.sub.-- Env is not added directly 
to the desired signal Rcv at the input to the interference canceller 
network 36, but is modified by the electronics in the hearing aid. The 
effects of the hearing aid electronics upon the EM signal envelope may be 
modelled as a transfer function. The transfer function between EM.sub.-- 
SOURCE.sub.-- Env and the input to the interference canceller network 36 
is referred to as the interferer channel response, Hicr() 42. 
Depending on how well the reference signal EM.sub.-- ref matches the 
interference component of AIC.sub.-- in (this being Hicr()*EM.sub.-- 
SOURCE.sub.-- Env) and on the degree of cancellation sought, the 
interference canceller network can be fixed or made adaptive. By way of 
example, it is assumed that the interference canceller network is 
adaptive. 
In this case, the interference canceller network 36 has an adaptive filter 
network having a transfer function Ha() 44 for producing a correction 
signal AF.sub.-- out as a function of the reference signal EM.sub.-- ref 
and the output of the interference canceller AIC.sub.-- out. The 
correction signal AF.sub.-- out is subtracted from AIC.sub.-- in to 
produce AIC.sub.-- out, as indicated by a subtraction symbol 46. The 
output of the interference canceller network 36 may be written as: 
EQU AIC.sub.-- out=Rcv+(Hicr()*EM.sub.-- SOURCE.sub.-- Env-Ha()*EM.sub.-- Ref) 
In a well designed system, EM.sub.-- Ref will be a good approximation of 
EM.sub.-- SOURCE.sub.-- Env, and the transfer function of the adaptive 
filter, Ha(), when converged, will be a good approximation of Hicr(). 
Substituting these approximations into the above equation yields: 
EQU AIC.sub.-- out.gtoreq.Rcv+(Hicr()*EM.sub.-- SOURCE.sub.-- Env-Hicr( 
)*EM.sub.-- SOURCE.sub.-- Env).ident.Rcv 
which is the desired result, since it does not contain any effects of the 
interfering signal, EM.sub.-- SOURCE.sub.-- Env. 
The interference canceller network 36 is a classic interference or "noise" 
canceller design. The adaptive filter may use a LMS (least mean square) 
algorithm or other adaptation control schemes. The filter transfer 
function Ha(s) 44 is adapted so as to minimize the correlation between the 
output AIC.sub.-- out of the interference canceller circuit 22 and the 
interfering signal approximated by EM.sub.-- Ref. It is important that the 
adaptive filter have a convergence speed which is sufficient to keep up 
with changes in the interference channel response, Hicr() which are not 
matched by the EM.sub.-- ref generator 38. In this example, these changes 
may result from the relative position of the EM source changing as a 
function of the hearing aid user's position and head orientation. 
The adaptive interference canceller network may be implemented using a 
sampled data system, for example. By way of example, two possible 
realizations include switched capacitor or digital. FIG. 3a is a signal 
flow diagram similar to FIG. 2 for a switched capacitor implementation and 
FIG. 3b is a signal flow diagram similar to FIG. 3a for a digital signal 
processing implementation. Both of these approaches require AAFs 
(anti-aliasing filters) 50 before sampling and RFCs (reconstruction 
filters) 52 after sampling. The digital implementation also requires A/D 
(analog-to-digital) converters 54 and a D/A (digital-to analog) converter 
56. 
The interfering EM signal may be generated by a handset which is being used 
by the user of the hearing aid, or may be generated by another source 
unrelated to the hearing aid user. The EM reference signal generator may 
be tailored to specifically deal with EM signals generated by the hearing 
aid user's handset, or may be designed to handle all EM signals. 
In a first option for generating the reference signal EM.sub.-- Ref, the 
reference signal generator is a simple AM-type power detector which simply 
detects the envelope of radiated EM power. An example of this is shown in 
FIG. 6 in which an antenna 82 and AM demodulator 84 are shown. In a 
preferred implementation a detector which models the interference pickup 
mechanism in the acoustic amplifier/audio amplifier/hearing aid is used. 
For a hearing aid this mechanism would typically be the microphone circuit 
(an electret with a FET device). A reference generator circuit which 
matches the circuit picking up the interference (including similar circuit 
layout topology and the microphone itself with the acoustic pickup 
disconnected) would provide an output similar to the interference signal. 
This would simplify the adaptive interference canceller's task and would 
even permit a limited amount of cancellation by simply subtracting this 
reference from the input AIC.sub.-- in the interference canceller circuit 
without the requirement for an adaptive filter. In this case, interfering 
signals generated by the user's handset will be treated the same as 
interfering signals generated by other sources. 
For interfering signals which are periodic in nature, such as TDMA (time 
division multiple access) signals generated by mobile handsets or base 
stations, the spectrum of the interfering noise is centred around a 
particular frequency. In this case, a second option for generating the 
reference signal EM.sub.-- Ref exists in which the reference signal is 
frequency-locked to the input to the reference generator (AIC.sub.-- in) 
with a PLL (phase-locked loop). A block diagram of an EM reference 
generator using a PLL is shown in FIG. 4. The input signal is AIC.sub.-- 
in rather than a separately detected signal. It is fed through a BPF (band 
pass filter) 70, a PLL 72 and a narrow pulse generator 76. A local 
frequency reference 74 provides a reference frequency input to the PLL 72 
with a frequency set to approximate the interference power envelope 
frequency. This assumes the interfering signal frequency is known and has 
a periodic envelope. 
An option for generating the reference signal EM.sub.-- Ref specifically 
applicable to the situation where the EM interference source is the user's 
handset is to use an infrared link to directly supply a reference signal 
from the handset to the hearing aid. An example of this is shown in FIG. 5 
which shows an infrared connection 80 between the EM interference source 
14 and the interference canceller circuit 22. 
In the cases of the PLL-based and infrared-linked-based reference 
generators, the reference signal produced can only model the frequency of 
the interfering signal. In these cases, an adaptive interference canceller 
network must be used and the EM.sub.-- ref signal produced is a broadband 
audio signal, rich in all harmonics of the interference signal envelope 
frequency. For example, this is the function of the narrow pulse generator 
in the PLL-based reference signal generator. 
It is contemplated that new hearing aids may be designed with the 
interference cancellation mechanism according to the invention built in, 
and that existing hearing aids may be retro-fitted with the interference 
cancellation mechanism. 
Numerous modifications and variations of the present invention are possible 
in light of the above teachings. It is therefore to be understood that 
within the scope of the appended claims, the invention may be practised 
otherwise than as specifically described herein. 
While the invention has been described with reference to application in a 
hearing aid, it may be applied in any acoustic amplifying application. 
Furthermore, while an audio amplifier application has been described, and 
more particularly an audio amplifier forming part of a hearing aid, it is 
to be understood that the invention can also be applied to other audio 
amplifier applications where there is no direct acoustic input, for 
example CD players and the like. In this case, there are no microphone and 
speaker components, and the input and output signals are electrical 
signals, perhaps originating from another component.