Patent Application
20120321091
The invention relates to a method and a system for improving the signal-to-noise distance of output signals of a microphone arrangement of two or more microphones due to acoustic useful signals occurring at the sides of the microphone arrangement. Such a method and system can be used in hearing instruments, especially in hearing devices worn on the head of a hearing device user. The term side is to be understood here in particular as to the right and left of the head of the wearer of a binaural hearing device arrangement.
Conventional directional effect methods, which are currently used in hearing devices, offer the option of factoring out signals and/or noises, which strike the hearing device wearer from the front or the rear, from the remaining ambient noises in order thus to increase speech intelligibility. They nevertheless do not provide the option of factoring out signals and/or noises from a lateral source, which strike from the left or right.
Previously known hearing devices only provide the option of highlighting such lateral signals such that the signal of the desired side is transmitted to both ears. To this end, audio signals are transmitted from one side of the ear to the other and are played back there. As a result, a mono signal is nevertheless presented to the hearing device wearer which results in signal properties, which render localization of sound sources possible (binaural cues), getting lost. Such signal properties may be interaural level differences for instance, i.e. the level at the ear and/or hearing device facing the noise and/or signal source is greater than at the ear and/or hearing device facing away therefrom.
Calculation of a conventional, differential directional microphone is not a solution which can be used unrestrictedly, since inter alia with signals with high frequency portions on account of the so-called “spatial aliasing”, no differential directional microphone is possible without spatial ambiguities.
Such spatial ambiguities, i.e. the classification of the spatial origin of a signal which is no longer clear, occur if one subtracts a right and left microphone signal of an acoustic source signal from one another. The differential processing by means of subtracting the microphone signals normally allows a targeted sensitivity of the microphone arrangement in a desired direction. If the wavelength of the acoustic source signals is however too small in comparison with the spatial distance of the microphone in the microphone arrangement, the spatial origin of a source signal can still only be determined equivocally.
The object of the invention consists in specifying an improvement in the interference signal-useful signal distance in acoustic signals by taking a spatial direction of the signal source into account.
The invention achieves this object in that it is considered to be a classical interference noise reduction problem. A binaural interference signal and a binaural useful signal are determined and/or estimated in the manner described below, said signals being used as input signals of a suitable filter, e.g. a Wiener filter, in which an amplification factor is preferably calculated and applied per frequency band which is equally large for both sides of the ear. The use of the same amplification factor for both ears achieves the interaural level differences, i.e. the localization of sounds and/or sound sources is enabled.
A basic idea behind the invention consists in processing high and low frequency portions (limit frequency in the region between 700 Hz and 1.5 kHz, e.g. approx. 1 kHz) differently. For low frequency ranges, a filtering takes place, preferably similar to a Wiener filtering, on account of a differential preprocessing with the aid of the calculation of a differential binaural directional microphone, wherein a signal directed to the left and to the right is generated by means of the preprocessing, typically with oppositely directed cardioid characteristc (kidney-shaped direction-dependent sensitivity).
These two signals directed to the left and to the right on the basis of a conventional differential directional microphone are used as a basis for estimating the level of lateral useful and interference sound, wherein these estimations are in turn used as input variables for the filtering, preferably Wiener filtering.
This filtering is then applied separately to each of the microphone signals of the microphone arrangement, and not to the shared differential directional microphone signal of the binaural arrangement, which was calculated as an output signal of the conventional directional microphone.
The advantage, e.g. compared with the use of Omni signals, is that the upstream directional effect artificially generates greater differences between the left and right side, which manifest themselves in increased interference sound suppression of signals, which strike from the direction to be suppressed.
An advantageous development provides to perform, as described above, a prefiltering with the aid of the calculation of a conventional differential directional microphone and subsequent filtering, preferably Wiener filtering in low frequency ranges, and to use the natural shadowing effect of the head as a prefilter for interference and useful sound estimation for a subsequent Wiener filtering in high frequency ranges (limit frequency in the range between 700 Hz and 1.5 kHz, e.g. approx 1 kHz).
The determination of interference and useful sound estimation by using the shadowing effect of the head takes place as follows: the monaural signal facing the desired side is used as a useful signal estimation, the side facing away therefore as an interference sound estimation. This is possible since particularly with higher frequencies (>700 Hz and/or >1 kHz) the shadowing effect of the head brings about a considerable attenuation of the signal on the opposite side.
These two signals directed to the left and to the right on the basis of a signal which is prefiltered by shadowing of the head are used as a basis for the estimation of the level of lateral useful and interference sound, and these estimations are in turn used as input variables for the filtering, preferably Wiener filtering.
This filtering is then applied separately to each of the microphone signals of the microphone arrangement.
The advantage, e.g. compared with the use of Omni signals, is that on account of the upstream directional effect, greater differences are artificially generated between the left and right side, which manifest themselves in an increased interference sound suppression of signals, which strike from the direction to be suppressed.
A signal directed to the left and to the right is generated in each instance for the low and/or high frequency range by the respective preprocessing, usually with oppositely directed cardioid characteristic (kidney-shaped direction-dependent sensitivity). These respectively directed signals are used as a basis for the estimation of respective lateral useful and interference sound levels. The respective useful and interference sound levels are in turn used as input variables for the filtering, preferably Wiener filtering. By combining the respective filtering method for high and for low frequency ranges, a filtering can therefore be achieved above the entire frequency range.
In a further advantageous development, the acoustic signals are broken down into frequency bands, and the filtering, preferably Wiener filtering, is performed specifically for each of the frequency bands.
In a further advantageous development, the filtering, preferably Wiener filtering, is performed in a directionally-dependent manner. The direction-dependent filtering can be performed in a conventional manner.
One or several of the following parameter values is advantageously determined and/or estimated as a useful signal level and/or as an interference signal level: energy, output, amplitude, smoothed amplitude, averaged amplitude, level.
Further advantageous developments and advantages are to be taken from the dependent claims and the subsequent figures plus the description, in which:
If hearing to the left (270°) is now required for instance, the right signal L1 is used as an interference sound signal, the left L2 as a useful sound signal. On the basis of this interference sound and useful sound signal, the input variables can then be estimated for a filtering, e.g. Wiener filtering.
Respective useful signal and interference signal levels are determined and/or estimated from the useful signal and the interference signal for the Wiener filtering. These were used as input variables for a Wiener filtering, in other words:
Wiener filter=useful signal level/(useful signal level+interference signal level)
Compared with the preceding figure, it is apparent that the interaural level differences are retained. Signals from the right side are observed as interference signals and lowered, signals from the left remain unattenuated. The spatial impression, i.e. the signal information from where the signals come in each instance is retained, since the level differences are retained. If signals enter from both sides, there is a drop in the ratio of useful sound and interference sound estimation according to the known Wiener formula.
As previously described, it is proposed to make use of the natural shadowing effect of the head in order to use the signals prefiltered by the shadowing effect of the head as interference and useful signals for determining the input variables of an interference noise elimination approach which is based on a filter, e.g. Wiener filter. Since the shadowing effect of the head is particularly obvious at high frequencies (>700 Hz and/or >1 kHz), but is however reduced further at lower frequencies, this method can be used particularly advantageously for frequencies above 1 kHz.
For low frequencies (<1.5 kHz and/or <1 kHz), the solution explained above cannot be used optimally on account of the shadowing effect of the head. In low frequency ranges, the method described below can be used again, which can also be used separately and exclusively.
Since for low frequencies (<1.5 or <1 kHz), the binaural microphone distance on the head of a hearing device wearer is small enough compared with the wavelength, no spatial ambiguities occur (spatial aliasing). Therefore a conventional differential directional microphone, which “looks” and/or “listens” to the side, can be calculated at low frequencies (<1.5 kHz and/or <1 kHz) of the acoustic source signal with the microphone arrangement of a left and a right microphone and/or microphone arrangement on the head of a hearing device wearer.
The output signal of such a directional microphone could be easily used directly, in order to generate a lateral directional effect at low frequencies. The directed signal determined in this way could then be reproduced identically on both ears and/or hearing devices of the hearing device wearer. This would nevertheless result in the localization ability in this frequency range getting lost, since only a shared output signal would be generated and displayed for both sides of the ear.
Instead, both a signal directed to the left and also to the right is therefore calculated on the basis of a conventional directional microphone and these signals are used according to the desired useful signal direction as interference and/or useful sound signal for a subsequent filtering, preferably with Wiener filter. This filter is then applied separately to each of the microphone signals of the microphone arrangement, and not however to the shared directional microphone signal calculated as an output signal of the conventional directional microphone.
Within the scope of the prefiltering, a conventional differential directional microphone which is directed to the left is initially calculated as a useful signal and as an interference signal directed to the right (continuous line in the Figure). The directed microphone signals have the usual kidney/anti-kidney shaped (cardioid/anticardioid, briefly also card/anticard) direction-dependent sensitivity characteristic.
Useful signal and interference signal levels are determined and/or estimated from the useful signal and interference signal. This was used as an input variable for a Wiener filter, in other words:
Wiener filter=useful signal level/(useful signal level+interference signal level).
Such a Wiener filter was calculated for each frequency range (in Figure therefore 250 Hz and 500 Hz) for all spatial directions and applied individually to each of the directional microphone signals. As a result, a Wiener pre-filtered direction-dependent sensitivity characteristic, shown in Figure by dashed lines L6 and L7, results for each of the directional microphone signals.
The figure shows how a higher attenuation is achieved in the interference signal direction (in other words right, 90°) than in the useful signal direction (in other words left 270°). It is also apparent that the level differences are largely retained (namely a higher level of the left L7 compared with the right microphone signal L6) and thus a spatial assignment of the acoustic source signal largely remains possible for the hearing device wearer.
The previously described filter methods for high and low frequency ranges can be used individually for high or for low frequencies in hearing instruments to be worn on the head for instance. They can however also be used in combination and in this process particularly advantageously extend beyond the entire frequency range of a hearing instrument to be worn on the head.
In step S1, a binaural microphone arrangement receives acoustic signals. Such a microphone arrangement includes at least two microphones, to be worn to the left or right on the head of a hearing device wearer respectively. The respective microphone arrangement may also include several microphones respectively, which can enable a directional effect for localization toward the front and/or rear for instance.
In step S2, a lateral direction is determined, at which the highest sensitivity of the microphone arrangement is to be directed. The direction can be automatically determined as a function of an acoustic analysis of the ambient noises or as a function of a user input. The spatial direction in which the source of the acoustic useful signal lies or presumably lies, is selected as the direction with the highest sensitivity. It is therefore also referred to as useful signal direction. The microphone and/or microphone arrangement disposed in this direction is similarly also currently referred to as useful signal microphone.
In step S3, a lateral direction is defined, in a similar manner to the step mentioned above, in which the lowest sensitivity of the microphone arrangement is to be directed. It is therefore also referred to as interference signal direction and the microphone or microphone arrangement disposed in this direction as an interference signal microphone.
The output signals of the microphone are broken down in step S4 into a frequency range having higher frequencies above a limit frequency of at least 700 Hz, possible also 1 kHz, and a frequency range with low frequencies below a limit frequency of 1.5 kHz, possibly also 1 kHz.
The microphone signals in the high frequency range are further processed in steps S5 to S7. In step S5, a useful signal level is determined and/or estimated as a function of the output signal of the useful signal microphone.
An interference signal level is determined and/or estimated in step S6 as a function of the output signal of the interference signal microphone.
In step S6, a filter, preferably a Wiener filter, is calculated using the useful signal level and interference signal level determined above. The signal level and the filtering can be determined for the complete high frequency range. Nevertheless, a breakdown into frequency bands can take place within the high frequency range, and the filtering can take place individually for each of the frequency bands.
In step S7, the filter calculated previously is applied separately to the respective output signals of the right and left microphone and/or microphone arrangement in the high frequency range.
In steps S8 to S13, the microphone signals of the low frequency range are further processed. In step S8, a conventional differential binaural directional microphone is calculated with high sensitivity in the useful signal direction, as a result of which a second useful signal is obtained.
In step S9, a conventional, differential binaural directional microphone with high sensitivity is calculated in the interference signal direction, as a result of which a second interference signal is obtained.
In step S10, a second useful signal level is determined and/or estimated as a function of the second useful signal.
In step S11, a second interference signal level is determined and/or estimated as a function of the second interference signal.
In step S12, a second filter, preferably Wiener filter, is calculated using the second useful signal level and second interference signal level calculated beforehand. The second signal level and the filtering can be determined for the complete low frequency range. Nevertheless, the frequency bands can be broken down within the low frequency range and the filtering can take place individually for each of the frequency bands.
In step S13, the previously calculated filter is applied separately to the respective output signals of the right and left microphone and/or microphone arrangement in the low frequency range.
In step S14, the filtered output signals of the microphones of both frequency ranges and/or with a further breakdown into frequency ranges of all frequency bands, are combined to form a filtered output signal of the binaural microphone arrangement.
An embodiment variant of the method which is not shown alone in the Figures includes the following detailed steps:
In one development, the output signals of the microphone are broken down into frequency bands, and the amplification factor is determined separately in each instance for one or several of the frequency bands.
In a further development, the amplification factor (Wiener) is determined according to the formula amplification factor (Wiener)=useful signal level/(useful signal level+interference signal level).
In a further development, the useful signal microphone is arranged on a hearing device to be worn on the right by a hearing device wearer and the interference signal microphone is arranged on a hearing device to be worn on the left by the hearing device wearer, or vice versa.
In a further development, one or several of the following is estimated as a useful signal level and/or as an interference signal level: energy, output, amplitude, smoothed amplitude, averaged amplitude, level.
A further development also includes the following steps:
In a further development, the output signals of the microphone are broken down into frequency bands, and the amplification factor is determined in each instance separately for one or several of the frequency bands.
In a further development, the amplification factor (Wiener) is determined according to the formula amplification factor (Wiener)=useful signal level/(useful signal level+interference signal level).
In a further development, the useful signal microphone is arranged on a hearing device to be worn on the right by a hearing device wearer and the interference signal microphone is arranged on a hearing device to be worn on the left and/or vice versa.
In a further development, one or several of the following is estimated as a useful signal level and/or as an interference signal level: energy, output, amplitude, smoothed amplitude, average amplitude, level.
In a further development, an amplification factor is determined in a low frequency range, which includes frequencies of less than 1.5 kHz, as explained in the immediately preceding sections, and an amplification factor is determined in a high frequency range, which includes frequencies of greater than 700 Hz, as specified in the sections introduced in the preceding sections.
The invention can be summarized as follows: the invention relates to a method and a system for improving the signal-to-noise distance in output signals of a microphone arrangement of two or more microphones due to acoustic useful signals occurring at the sides of the microphone system. Such a method and system can be used in hearing instruments, especially in hearing devices worn on the head of a hearing device user. To solve this problem, the invention proposes processing high and low frequency portions (limit frequency in the range between 700 Hz and 1.5 kHz, e.g. approx. 1 kHz). In low frequency ranges, a differential microphone signal directed to the left and to the right is generated in order to determine the level of the lateral useful and interference sound with the aid of these two directional signals. These levels are in turn used for a Wiener filtering and each of the microphone signals is individually subjected to the Wiener filtering. In addition, in high frequency ranges, the natural shadowing effect of the head is used as a prefilter for interference and useful sound estimation for a subsequent Wiener filtering. Each of the microphone signals is then subjected individually to the Wiener filtering. The method can be used for instance in hearing instruments to be worn on the head individually for high or low frequencies, they may however also be used in combination and extend particularly advantageously in this process.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10154096 | Feb 2010 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2010/059690 | 7/7/2010 | WO | 00 | 8/20/2012 |
