[0001] This invention belongs to the field of electromagnetic wave transmission and is in
particular related to the wireless transmission of audible information by inductive
devices.
[0002] Wireless communication receivers of audible information have already been known.
One well-known device is a wireless miniature communication receiver called "Phonito®",
developed and distributed by the Applicant. Such receivers are inductively coupled
wireless earphones. Users who do not want to have a wire or an acoustic tube going
to the ear, or users where such a wire or tube cannot be tolerated, can attach an
inductive loop that may be worn around the neck under a garment such as jacket or
uniform, or the inductive loop or coil can be integrated into the clothing. The inductive
loop sends the information to be heard that has been transformed into the appropriate
inductive transmission form, to the earphone where it is received and re-transformed
into acoustic waves. The receiver worn in connection with the inductive loop and which
provides speech information may be a mobile radio such as e.g. a portable citizen's
band (CB) receiver, a portable telephone or any other information source that is capable
of receiving wireless or broadcast information.
[0003] The miniaturized ITE (in-the-ear) devices and/or BTE (behind-the-ear) devices considered
in this document are being used since several years in the field of security and police,
in acoustic studios, for sport applications, and in any other field where the presence
of an acoustic communication should be kept discreet or secret. These devices are
working based on the principle of inductive signal transmission and reception.
[0004] An audio signal U(t) that is generated at the output of a sound device such as a
mobile radio receiver, a portable or fixed telephone, etc., is transmitted via an
appropriate amplifier into a closed wire loop or an inductive coil. Of course, if
a coil is used, it will preferably be miniaturized. This loop or coil that is typically
worn by the user will generate a magnetic field whose field intensity H(t) is correlated
to the original signal U(t). In most cases, this correlation is a proportional one
or a simple mathematical function of the original signal. There are also devices based
on inductive transmissions that are digital or frequency modulated.
[0005] Common features of all these devices and appliances based on inductive transmission
is the fact that their transmission distance is relatively short, and that their signal-to-noise
ratio, where the noise comes mainly from external sources, is low. These features
make the devices relatively insensitive to noise fields and interception in comparison
to RF (radio frequency) receivers.
[0006] In spite of the short interference and noise distance that is only some meters distant
from a source of interference and noise fields, the number of these sources has considerably
increased within the last years so that they become more and more a problem even for
short-distant inductive transmissions. For example, such new and increasing interference
sources are low frequency devices such as aerial conduct lines of electric trains,
tramways and trolley buses; low voltage transformers for lighting purposes and their
connecting and power lines that typically carry high amperages; computer monitors;
inductive security systems such as tags, theft warning installations, security warn
systems in casinos, banks, postal offices, etc.; electronic systems in automotive
vehicles; and mobile telephones which produce the so-called 300 Hz noise. All these
low frequency noise generators directly interfere with the reception field of the
inductor. The consequence of this interference is a hum or still another noise that
has a detrimental effect on the integrity of the signal and thus on the quality of
the displayed audio signal.
[0007] This problem of low frequency interference has already been addressed by introducing
correction systems based on AGC (Automatic Gain Control) and squelch management. Furthermore,
since the frequencies to be transmitted nearly exclusively are speech (voice) frequencies,
it is possible to improve the quality of the transmission, i.e. remove noise (hum),
by low pass and/or high pass filters in order to limit the spectrum of useful frequencies
in the low and high ranges.
[0008] These solutions do bring about significant improvements with respect to inductors
devoid of any interference and noise diminution or suppression. In particular, the
squelch function avoids the reception and generation of audible disturbances when
no voice signal is transmitted, i.e. in speaking pauses.
[0009] Nevertheless, the systems now available, even the very sophisticated ones, do not
produce a satisfactory signal quality in particularly severe environments.
[0010] Accordingly, the invention aims at providing a new and useful solution to the problem
of low frequency disturbances, interference and noise in inductive transmission systems
of the kind described above. The invention therefore concerns a method for noise suppression
in inductive receivers.
[0011] This objective is attained by the method of the invention that is defined in the
first independent claim. A system for implementing the method of the invention is
the subject of the second independent claim. Further preferred embodiments follow
moreover from the dependent claims and from the description.
[0012] It should be stated here that, besides the method according to the invention, the
present invention also relates to a system for carrying out the method.
[0013] In particular the objects are achieved through the invention in that an inductive
receiver comprises a receiver coil, wherein audio information is transmitted from
a transmitter coil or loop in a wireless manner to said receiver coil, wherein the
signal received by said receiver coil is analyzed by digital signal processing in
the hearing frequency range and/or the speaking time range for discrete noise signals
and other sound artifacts, and said noise signals and other sound artifacts are removed
from said signal. A DSP (Digital Signal Processing) module can e.g. manipulate analog
information, such as e.g. sound that has been converted into a digital form. DSP can
e.g. also comprise the use of a data compression technique. A DSP element can comprise
a digital signal processor, i.e. a specialized type of processor designed for performing
the mathematics involved in DSP. The DSPs can be programmable, which means that they
can be used for manipulating different types of information, including e.g. sound,
images, and video. Said noise can e.g. be a low frequency noise in the range of from
50Hz to 4-8 kHz. An audio signal is restored after the removal of said noise signals
and other sound artifacts. The analysis of the received signal and/or the restoration
of said signal and/or the removal of said noise signals and other sound artifacts
can be accomplished by the use of at least one algorithm of acoustic speech processing.
Said at least one algorithm of acoustic speech processing can be selected from algorithms
based on the principles of speech theory. Said algorithm can e.g. comprise a Wiener
filtering method.
[0014] Embodiment variants of the present invention will be described in the following with
reference to examples.
Figure 1 shows a spectrum illustrating schematically an excitation signal of a human
vocal and articulation tract. The properties of resonance of the articulation tract
affect the excitation signal in such a way that certain frequency ranges are passed
and/or amplified and others are suppressed. The passed frequencies F1, F2, F3 and F4 are usually called formant frequencies. Formants F1, F2, F3 and F4 are important to create the voiced sounds. The formants F1, F2, F3 and F4 can have a great importance in relation to noise suppression. Figure 1 shows the
voice spectrum of a spoken /i/.
Figure 2 shows another spectrum illustrating schematically an excitation signal of
a human vocal and articulation tract. The formants here are F1, F2, F3, F4 and F5, showing the voice spectrum of a spoken /a/.
Figure 3 shows the energy spectrum for voiced (a) and unvoiced (b) sounds of the human
voice. The fundamental frequency of men's voices is typically around 80 Hz, whereas
children's voices typically show a fundamental frequency of around 330 Hz.
Figure 4 shows a schematic diagram which illustrates an architecture which may be
used for carrying out the invention. The reference numeral 10 is an inductive receiver,
11 is a loud speaker, 12 is a DSP (Digital Signal Processing) module, 13 is an inductive
element such as e.g. a coil and 14 is a cavity of the inductive receiver 10 to comprise
the mentioned elements.
[0015] Thus, the method of the invention uses algorithms to implement the restoration of
the original speech signals by analyzing the spectrum and the dynamics of the received,
noisy or disturbed signal according to principles based on the theory of speech.
[0016] Acoustic noise cancellation methods, as far as they are already known, are based
on several algorithms or treatment processes.
[0017] Examples of such methods for Audio Noise Reduction are: (1) noise reduction by spectral
weighting, (2) noise reduction using coherent properties, (3) noise cancellation using
Wiener filtering in the frequency domain, (4) directive beam forming, and still others.
[0018] In method (1), different spectral regions of the mixed signal of speech and noise
are attenuated with different factors. One obtains an audio signal that contains less
noise than the original one. The spectral weighting is usually performed in a transformation
domain; a common transformation is the Fourier transformation which provides an equidistant
frequency resolution. Alternatively, one may use the wavelet transform that yields
a non-equidistant spectral resolution. Details are known to the one skilled in the
art of audio frequency correction techniques.
[0019] In method (2), acoustic noise is reduced by a Generalized Sidelobe Canceller (GSC)
that reasonably suppresses the coherent noise components while a Wiener filter (see
below) is designed to suppress the spatial incoherent noise components.
[0020] According to method (3), the Wiener filtering method is based on minimizing the mean
square error between the speech S(f) and the estimate Y(f) where the fact may be used
that speech and noise are statistically not correlated.
[0021] Method (4), directive beam forming, refers to speech recognition and/or extraction
form noise and uses a microphone array of which the different microphones can e.g.
detect the phase difference and/or time of flight (tof) of the incoming signal. The
beamforming can e.g. seek to exploit the phase differences between microphone signals
before combining the outputs of the microphones to restore the original speech.
[0022] Still more proposals have been made to treat noise (i.e. to reduce, minimize or suppress
it) in audio signals, i.e. analog, amplitude modulated waveforms. An application to
the noise reduction as provided by the invention has not become known so far. The
invention is not limited to a special processing algorithm.
[0023] The method of the invention may be implemented in using electronic components already
known per se. The received signal is digitized in an A/D converter, is analyzed e.g.
by algorithms being adapted to speech patterns and/or by comparison with stored speech
patterns, discrete noise signals and clearly distinguishable artifacts are filtered
out using at least one stored algorithm, and the purified digital signal is sent to
the earphone or earplug. If necessary or required, the digital output can be re-transformed
to analog signals.
[0024] According to a particular embodiment of the invention, the method and the corresponding
system are designed for preferably removing low frequency noise, in particular between
50Hz and 4-8 kHz, taking into consideration that a great deal of disturbing and interfering
noise originates from this frequency range. This noise can be e.g. in the form of
sinusoidal signals or set of harmonics that may also be amplitude modulated. As example
can serve the 900 MHz GSM (Global System for Mobile Communications) radio frequency
signal which is switched at a rate of 300Hz.
[0025] The invention allows to drastically improve the quality of wireless speech and voice
transmission by induction. Furthermore, additional application fields are opened to
inductive speech transmission. It is even conceivable to transmit also, additionally
or alternatively, sound and music with an improved quality.
[0026] Numerous modifications, variations and developments are accessible to the one skilled
in the art. Thus, the nature of algorithms to be used and of the required electronic,
inductive and acoustic components is left to the best knowledge of those ordinarily
skilled in the art.
1. A method for noise suppression in inductive receivers comprising a receiver coil,
wherein audio information is transmitted from a transmitter coil or loop in a wireless
manner to said receiver coil, the method being characterized in that the signal received by said receiver coil is analyzed by digital signal processing
in the hearing frequency range and/or the speaking time range for discrete noise signals
and other sound artifacts, and said noise signals and other sound artifacts are removed
from said signal.
2. The method according to claim 1, wherein said noise is a low frequency noise in the
range of from 50Hz to 4-8 kHz.
3. The method according to claim 1, wherein an audible signal is restored after the removal
of said noise signals and other sound artifacts.
4. The method according to one of the claims 1 or 3, wherein said analysis of the received
signal and/or the restoration of the received signal and/or the removal of said noise
signals and other sound artifacts is accomplished by the use of at least one algorithm
of acoustic speech processing.
5. The method according to claim 4, wherein said at least one algorithm of acoustic speech
processing is selected from algorithms based on the principles of speech theory.
6. The method according to claim 4, wherein said algorithm is a Wiener filtering method.
7. A system for noise suppression in wireless receivers comprising a receiver coil, wherein
audible information is transmitted from a transmitter coil or loop in a wireless manner
to said receiver coil, characterized by the fact that the system comprises means for analyzing the received signal by digital
signal processing, means for identifying discrete noise signals and other sound artifacts,
means for removing said discrete noise signals and other sound artifacts, and means
for restoring the undisturbed original speech.
8. The system according to claim 7, wherein said means for analyzing the received signal
are adapted to analyze the signal in the hearing frequency range and/or the speaking
time range.
9. The system according to one of the claims 7 or 8, further comprising means for storing
algorithms for analyzing, removing and restoring.