Noise filter in a hearing aid

p0001The present invention relates to a method for filtering noise with known constant repetition largely according to the preamble of claim 1 and an arrangement for filtering interference signals.

p0002The filtering of example RF signals is done at the current hearing aids exclusively by suitable construction and passive RF filters. This is to ensure that RF signals - such as those for example produced by a GSM mobile phone - either are not coupled and / or removal by the filter structure of any non-linearities within the circuit and thus will not be demodulated.

p0003Solutions such as for example, in <patcit id="pcit0001" dnum="US6205190B"><text>US 6205190</text></patcit> (For CDMA, Code Division Multiple Access) or in <patcit id="pcit0002" dnum="EP0876717A"><text>EP 0876717</text></patcit> (GSM, Global System for Mobile communications) are proposed to act on the phone side by preventing radiation from the amplitude modulated (AM) signals, or at least reduce. In contrast, in<patcit id="pcit0003" dnum="EP1104645B1"><text>EP 104 645 B1 1</text></patcit> a solution presented, which is implemented directly in the hearing aid. In this solution, in the hearing aid, the frequency of the jamming signal is first determined and then a comb filter according to this detected frequency adjusted so that the noise signal is effectively filtered out. Thus the strongest possible suppression of the interference signal can be achieved at low signal interference, the zeros of the comb filter case must be placed precisely on the harmonics of the detected frequency. To achieve this, the system clock frequency of the hearing aid is derived from the frequency of the interference signal respectively. coupled thereto. In<patcit id="pcit0004" dnum="EP0941015A2"><text>EP 0941015 A2</text></patcit> is also proposed based on a comb filter solution, however, which is based on an adjustable delay element. This results in this case, an effective suppression of the interference signal at low signal interference, the delay has to be set very accurately here. In<patcit id="pcit0005" dnum="US6137888A"><text>US 6,137,888</text></patcit> describes an "interference canceller". Here in the hearing device, a reference signal is generated, which should be as identical as possible to the interference signal. In order optimally to liberate the audio signal from the noise signal, the reference signal, ie, the estimated noise signal from the audio signal is subtracted, so that substantially the useful signal remains. The difficulty with this method lies in the exact best estimate of the interference signal to generate a corresponding user signal. In the<patcit id="pcit0006" dnum="US20040053575A1"><text>US 2004/0053575 A1</text></patcit> the problem is tackled in a simpler manner. With the aid of a high-frequency receiver electromagnetic interference can be detected. Once the HF receiver has detected such a disturbance, it transmits a control signal to a device that could be affected by this noise, which is shut out for the duration of the disruption. Alternatively, the speaker is temporarily silent. A disadvantage of this method is that the detection of the interference signal occurs outside of the audio signal path. This could mean that the device off or muted is due to a disturbance in the HF range, which has no effect on the useful signal in the audio path, eliminating this unnecessary.

p0004The increasing tightening of the requirements concerning interference immunity leads to ever more complicated filter structures both on the integrated circuits (integrated circuit, IC) as well as on the PCB (printed circuit board, printed circuit board). In particular, the requirement to be able to make calls using a mobile phone while the hearing aid is turned on, brings great difficulties. The powerful transmitter of the mobile phone, for example, according to the GSM standard uses a time division multiplexing (Time Division Multiple Access, TDMA) scheme with a frame rate (frame rate) of about 213 Hz and a duty cycle (duty cycle) of frames of 1/8, 2 / 8 or 4/8 (depending on the protocol).

p0005Under the duty cycle one time share the frame duration is understood, while significantly irradiates interference energy on the hearing aid. This caused by the switching on and off of the transmission signal amplitude modulation of the radiation power is absorbed by the wires and circuit traces on the circuit board in the hearing aid and injected at various points in the hearing aid. Particularly serious the couplings are in the vicinity of the microphone, because there on the one hand, the signals on the other hand, the subsequent gain is very small and very large. Filter structures with ferrite bead or RC (resistor-capacitor) filters on the different ICs, Hybrids or PCBs are expensive and make big the circuit.

p0006It is therefore an object of the present invention to suppress the coupling of the aforementioned noise on hearing aids at least partially or prevent.

p0007According to the invention, the stated object by a method according to the wording of claim 1 and an arrangement according to the wording of claim 10 is achieved.

p0008The invention consists in that the interference signal is not filtered in the RF range, but that its demodulation is taken by non-linearities in the purchase and the noise components are removed only in Busisband. For this purpose a detector is installed in the digital part of the signal processing, by means of which signals can be detected with a known repetition rate, and optionally with known duty cycle. For example, it can involve a detector for GSM signals. Detects that the typical frequency patterns such as GSM frequency pattern, then it activates a special filter that the interference signal at least partially removed.

p0009The detection of the signals, such as GSM signals, for example, takes place via the duty-cycle and the fundamental frequency of the interference signal. For example, if before a pulsating signal with a duty cycle of 1/8, 2/8, or 4/8 with a fundamental frequency of about 213 Hz, then can be assumed that it is a GSM signal.

p0010In other words, the time of the interference signals is respectively accurately known, so you can work directly in each known time domain and the corresponding samples can be dynamically adequately damped. Ie, a first pulse package with respective period duration and duty-cycle is detected, then the probability of acceptance of a signal of known frequency, such as a GSM signal, increases, and after a further period preventively the signal for preferably exact timing of a next pulse to a certain amount suppressed.

p0011Was by the detector actually detects at this time another pulse, thus increasing the probability of acceptance and at the time of turn next pulse can be more suppressed. Such an advantageous compromise between unnecessarily dynamic signal reduction and optimal noise reduction, such as GSM noise canceling achieved. After a few such pulses detected acceptance probability will rise to nearly 100%. Thus, the suppression at the time of the expected next pulse, such as the GSM pulse, set a maximum. If finally no more pulses are detected, the dynamic signal suppression is rapidly reduced to 0.

p0012The dynamic suppression of the signal is such that shortly before the expected next pulse the audio signal preferably gently, ie not from one sample to the next, but within a few milliseconds, preferably (0.5-10 ms) to the set suppression, such as 20 dB, in the case of a GSM signal in the amplitude is reduced. For a sufficient time after the pulse, such as again for example, the GSM pulse, the gain of the signal is preferably in turn increases gently to the original gain.

p0013The inventively proposed suppression of interference pulses is particularly possible because it has been shown that such a dynamic signal suppression, as perceptually are not within a few milliseconds, as described above, using speech signals perceptible, ie, the human brain fills the signal gap sense, ie from the existing signal forward and backward extrapolation from. By signal gap is of sitting there glitch, such as the GSM-glitch, but effectively suppressed and therefore not perceived.

p0014Reference to the accompanying figures, the principle of the present invention will be briefly explained.

p0015They show:<dl id="dl0001"><dt>figure 1</dt><dd>the exemplary time course of a noise pulse, such as a GSM pulse,</dd><dt>figure 2</dt><dd>a simplified block diagram of a GSM signal detector, and</dd><dt>figure 3</dt><dd>a simplified block diagram of a hearing device with a GSM signal detector which alternatively acts either on the gain in the DSP or directly to the analog output signal.</dd></dl>

p0016In the accompanying <figref idrefs="f0001">figure 1</figref> schematically an interference pulse, as shown, for example a GSM pulse. Using known detected pulse repetition frequency and the duty cycles can now be determined when the amplification of the original signal (original gain) must be lowered to suppress the glitch. The lowering of the original signal is, as shown in<figref idrefs="f0001">figure 1</figref> shown, not abruptly but gently and as the increase of the gain is again gently. The resulting from the dynamic signal suppression gap of the original signal in the millisecond range is, as mentioned above, not visible to the human brain and the human brain fills the gap signal of sense, so that no interruption of the original signal is perceptible.

p0017In <figref idrefs="f0001">figure 2</figref> is shown using a simplified block diagram of a GSM signal detector. Here, a signal is, as described previously, detects where it is first determined whether it is as a GSM signal. If a known interference signal is present, an additional pulse to the next waiting time is detected, and if in fact a further pulse at this time occurs, increasing the adoption probability that actually exist corresponding glitches. Now, the suppression of interference pulses can be triggered, as previously described with reference to<figref idrefs="f0001">figure 1</figref> described.

p0018Further measures in the frequency domain in addition to the measures mentioned above are conceivable: The glitches, such as GSM glitches own after demodulation by parasitic nonlinearities a characteristic spectrum. The gain in those frequency bands, which are particularly affected by noise will be, compared to the less affected bands turn dynamically lowered and gently by a few dB to be raised again according to the momentum on the original gain.

p0019Instead of processing in the frequency domain is of course a matched filter in the time domain to imagine that performs a spectrally shaped oppression during the period of the interference pulse, such as the GSM interference pulse.

p0020Although the above-mentioned measures and proposals are mentioned specifically in the context of GSM interference pulses, it should be noted that the presented method or the inventive measures can be analogously used for other glitches. The prerequisite is of course that the glitches occur with known constant repetition and also preferably the duty cyle is known. Specifically, the invention method presented here is also applicable for other mobile telephone standards, such as the customary in North America CDMA standard, which produces a different type disorder. That is by means of a suitable detector, the presence is detected by characteristic noise or spurious signals and these then filtered using a suitable filter temporal and spectral effort from the audio signal. Furthermore, it is to be mentioned that the knowledge of the repetition rate may also be determined only during operation of the hearing aid, that is detected as a result of signal analysis that a certain pulse repetition pattern does not belong to a desired signal but a spurious signal, and accordingly, should be removed so so that the fault is no longer perceptible. Filtering out the glitches can in hearing aids, as shown schematically in<figref idrefs="f0001">figure 3</figref> shown, both carried out in the digital signal processing section using digital filters, as well as by means of analog filters, which for example in the range between the digital signal processing (digital signal process began, DSP) and the digital / analog converter (digital / analog converter, DAC) and the arranged speakers. <figref idrefs="f0001">figure 3</figref> shows a simplified block diagram of a hearing device with a GSM signal detector which either the gain in the DSP or alternatively applied directly to the analog output signal (this ADC stands for analog / digital converter, analog / digital converter).

p0021It should be noted that the interference in both the microphone path, as can be fed into the hearing system also for example in an analog or digital, wired or wireless transmission path between two hearing or a hearing aid and a further device, and the measures described apply regardless of the entry point are.

p0022The proposed invention provides a preferred solution option for increasing the immunity of hearing aids. It is of course possible that the inventively proposed measures alone are not sufficient, but it is possible for example in combination with other typical measures, such as ferrite beads, RC filters, etc. to provide additional noise suppression.