Noise filter in a hearing aid

The present invention relates to a method for Filtering of interference signals with largely known equal maintaining the repetition rate according to the preamble of To claim 1 and an arrangement for filtering Noise.

The filtering of example RF signals occurs at the current hearing aids exclusively by appropriate Construction and passive RF filters. This is to be achieved, that RF signals - such as those of a GSM mobile phone generated - either are not coupled and / or through the filter structure of any kept nonlinearities within the circuit and thus will not be demodulated.

Solutions such as those in US 6,205,190 (for CDMA, code Division Multiple Access) or in EP 0876717 (for GSM, Global System for Mobile communications) proposed will act on the phone side by radiation prevent signals or the amplitude modulated (AM) at least reduce. In contrast, we in EP 1104645 B1 presents a solution directly in the hearing aid is realized. This solution changes the System clock frequency of the hearing aid, which present in the Invention is not intended.

The increasing intensification of the regulations regarding 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 make calls using a mobile phone, while the Hearing aid is turned on, brings great difficulties with it. The powerful transmitter of the mobile phone For example, according to the GSM standard, a used Time division multiplexing (Time Division Multiple Access, TDMA) scheme (Rate frame) with a frame rate of approximately 213 Hz and a Duty cycle (duty cycle) of frames of 1/8, 2/8 or 4/8 (Depending on the protocol).

Under the duty cycle is one proportion of time spent understood frame duration during which significantly Perturbation to the hearing device radiates. This by induced switching on and off of the transmission signal Amplitude modulation of the radiation power is determined by the Wires and circuit traces on the circuit board in the hearing aid collected and at various points in the hearing aid coupled. Particularly serious are the Coupling in the vicinity of the microphone, because there on the one hand the very small signals and on the other hand subsequent gain is 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.

It is therefore an object of the present invention, the Coupling of the mentioned interference to hearing aids at least partially suppress or prevent.

According to the invention the object is by means of a Method according to the wording of claim 1 and a Arrangement according to the wording of claim dissolved 10th

The invention consists in the fact that the interference is not is filtered in the RF range, but that its Demodulation is taken by non-linearities in buying and the noise components removed until Busisband will. For this, in the digital part of the signal processing installed a detector by means of which signals known repetition and optionally with a known duty cycle can be detected. For example, it may be involve a detector for GSM signals. detects this typical frequency patterns such as GSM-frequency patterns, it activates a specific Filter which the interfering signal at least partially away.

The detection of the signals, such as GSM signals happens eg on the duty cycle and the Fundamental frequency of the interference signal. Is there an example pulsating signal with a duty cycle of 1/8, 2/8, or 4/8 with a fundamental frequency of about 213 Hz before, then be assumed that it is a GSM signal concerns.

In other words, each of the time of Noise precisely known, then right in each known time domain to work and corresponding samples can dynamically adequately are attenuated. That is a first pulse packet with detected corresponding period and duty cycle, so is the probability of acceptance with a signal known frequency, such as a GSM signal, increased, and after a further period of time the pre-emptive Signal to preferably exact timing of the expected next pulse suppressed by a certain amount.

Was by the detector actually another pulse this time is detected, increases the Probability of acceptance and at the time of turn next pulse can be more suppressed. so is an advantageous compromise between unnecessarily dynamic signal reduction and optimal Noise reduction, such as GSM interference noise suppression, achieved. After a few such detected pulses is the probability of acceptance on almost 100% increase. Thus, the suppression is to each timing of a next pulse, as For example, the GSM pulse, set maximum. Will Finally, no more pulses detected, then the dynamic signal suppression quickly reduced to 0.

The dynamic suppression of the signal is such that shortly before the expected next pulse, the audio signal preferably gently, ie not from a sample to 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 becomes. For a sufficient time after the pulse, such as in turn, for example, the GSM pulse, the gain is the signal again preferably gently on the Original gain increases.

The inventively proposed suppression of therefore glitches is especially possible because has shown that such a dynamic signal suppression within a few milliseconds, as described above, for example, using speech signals not psycho-acoustically perceptible, ie, the human brain fills the gap signal sense, ie from the existing signal and forward Extrapolating backward, out. is a result of the signal gap of sitting there glitch, such as the GSM interference pulse, but effectively suppressed and therefore not perceived.

With reference to the accompanying drawings, the principle of the present invention will be briefly explained.

They show:<dl tsize="7"><dt>figure 1</dt><dd>the exemplary timing of a glitch, 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 aid with a GSM signal detector which either on the gain in the DSP, or alternatively acts directly on the analog output signal.</dd></dl>

In the accompanying Figure 1 schematically a glitch, such as illustrated a GSM pulse. Based on known detected pulse repetition frequency and the Duty cycles can now be defined when the lowered amplification of the original signal (original gain) must be to suppress the glitch. there done the lowering of the original signal, as shown in Figure 1 shown, not abruptly but gently and well done the gain will increase again gently. The by dynamic signal suppression resulting gap of is the original signal in the millisecond range, as already mentioned above, invisible to the human brain or the human brain fills the gap signal sense out, so that no interruption of the original signal is perceptible.

In Figure 2 is based on a simplified block diagram a GSM signal detector shown. Here, as already described earlier, a signal is detected which first it is determined whether it is, for example, a GSM signal concerns. If a known interference signal is present, is a Another pulse to the next waiting time detected, and if in fact a further pulse at this time occurs, increasing the adoption probability that actually corresponding glitches present. Now, the suppression of noise pulses are triggered, as described previously with reference to FIG. 1

In addition to the measures mentioned above are more Measures in the frequency domain to imagine: The glitches, such as GSM glitches own after Demodulation by parasitic nonlinearities a characteristic spectrum. The gain in those Frequency bands, which are particularly strong from noise are concerned, be compared to the less affected bands turn dynamically and smoothly by a few dB lowered to correspondingly turn on after the pulse the original gain to be raised.

In place of a processing in the frequency domain of course, a matched filter in the time domain imagine that a spectrally shaped suppression during the time duration of the disturbing pulse, such as of the GSM interfering pulse, performs.

Although the above-mentioned measures and proposals are specifically mentioned in the context of GSM interference pulses, it should be noted that the method presented or the inventive measures mutatis mutandis to other glitches can be used. requirement is to be understood that the interference pulses with known constant repetition occur and also preferably the duty cyle is known. Specifically, the inventively proposed process to other Mobile phone standards applicable, as for the in North America conventional CDMA standard, which is a different kind disturbance produced. ie, with the aid of a suitable detector the presence of characteristic noise detected or noise and this after a suitable filter temporal and spectral effort from the Audio signal filtered out. Furthermore is to be performed that knowledge of the repetition and only during the operation of the hearing aid can be determined, ie, is detected as a result of signal analysis that a specific pulse repetition patterns is not a desired signal, but belongs to an interference signal, and should accordingly be removed in such a way so that the Fault is no longer perceptible. Filtering out the Glitches can in hearing aids, as shown schematically in Figure 3 shown, both in digital Signal processing section done using digital filters, as well as by means of analog filters, which For example, in the area between the digital Signal processing (digital signal process began, DSP) or the digital / analog converter (digital / analog converter, DAC) and disposed the speaker. Figure 3 shows a simplified block diagram of a hearing aid with a GSM signal detector, which is either on the gain in the DSP or alternatively directly to the analog output signal acts (this ADC stands for analog / digital converter, Analog / Digital converter).

It should be noted that the interference in both the Microphone path, as well as in an analog or digital, wired or wireless transmission link between two hearing or a hearing aid and eingespiesen another device into the hearing system can be, and the measures described independently are applicable from the entry point.

The proposed invention provides a preferred Possible solution for increasing the immunity of Hearing aids. It is of course possible that the inventively proposed measures are not alone are sufficient, but it is possible for example in Interaction with other typical measures, such as For example, ferrite beads, RC filters, etc. a to obtain additional noise suppression.