Patent Publication Number: US-7912228-B2

Title: Device and method for operating voice-supported systems in motor vehicles

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and to a device for operating voice-enhancement systems, such as communication and/or intercom/two-way intercom and/or duplex telephony devices in motor vehicles, where voice signals are picked up via a microphone system and routed to at least one loudspeaker. 
     BACKGROUND INFORMATION 
     Methods of this kind are used in motor vehicles for voice-supported duplex telephony or for supporting voice input-controlled electronic or electrical components. The fundamental difficulty that arises is that, depending on the particular operating state, there is background noise in the motor vehicle. This masks the voice commands. Intercom and two-way intercom systems in motor vehicles are mainly advantageously used in large vehicles, minibusses and the like. However, they can also be used in normal passenger cars. When using voice-controlled input units for electrical components in motor vehicles, it is still very important for the background noise to be suppressed or for the voice command to be filtered out. 
     Thus, a voice-recognition device for a motor vehicle is described in European Patent Application No. 0 078 014, where the status of engine operation and/or motor vehicle movement is signaled or fed in, via sensors, to the amplifier system of the voice-recognition device. Based on this, a noise-level control is then used to attempt to filter out the voice command from the background noise. 
     A filtering operation is described in PCT International Published Patent Application No. WO 97/34290, where periodic interfering noise signals are filtered out by determining their periods and by using a generator to interfere with them, so that the voice signal remains. 
     In German Published Patent Application No. 197 05 471, it is described to support a voice recognition with the aid of transversal filtering. 
     In German Published Patent Application No. 41 06 405, a method is described for subtracting noise from the voice signal, a multiplicity of microphones being used. A duplex telephony device having a plurality of microphones is discussed in German Published Patent Application No. 199 58 836. 
     In German Published Patent Application No. 39 25 589, it is described to use a multiple microphone system, where, in motor vehicle applications, one of the microphones is placed in the engine compartment and one other microphone in the passenger compartment. A subtraction of both signals then follows. The disadvantage in this context is that only the engine noise or the actual running noise of the vehicle itself is subtracted from the total signal in the passenger compartment. Specific secondary noises are disregarded in this case. Also lacking is a feedback suppression. Everywhere that microphones and loudspeakers are placed in an acoustically coupleable vicinity, the acoustic signal that is extracted, coupled out or decoupled at the loudspeaker is fed back into the microphone. The result is a so-called feedback, and a subsequent overmodulation. Methods for avoiding such an overmodulation are described in European Published Patent Application No. 1 077 013, PCT International Published Patent Application No. WO 02/069487, and PCT International Published Patent Application No. WO 02/21817. 
     It is an object of the present invention to provide a method and a device that may improve the verbal communication among the occupants of a vehicle. 
     SUMMARY 
     The above and other beneficial objects of the present invention may be achieved by providing a method and a device as described herein. 
     The above object may be attained in that, for the operation of a voice-supported system, such as a communications and/or duplex telephony device in a motor vehicle, using at least one microphone and at least one loudspeaker to reproduce a signal generated by the microphone, as well as using a bandpass filter arranged between the microphone and the loudspeaker, the power of a signal is determined as a function of a frequency, and the bandpass filter is adjusted or set as a function of at least one local maximum of the power of the signal as a function of the frequency. 
     A local maximum of the power of the signal as a function of the frequency may include also the global maximum of the power of the signal as a function of the frequency. 
     In an example embodiment of the present invention, the local maximum of the power of the signal may be determined as a function of a derivative, e.g., the first derivative, of the power of the signal with respect to the frequency. 
     In an example embodiment of the present invention, an edge or slope signal may be formed using the first derivative of the power of the signal with respect to the frequency, which takes on a first binary value when the first derivative of the power of the signal with respect to the frequency is greater than or equal to zero, and which takes on a second binary value when the first derivative of the power of the signal with respect to the frequency is less than zero, the local maximum of the power of the signal being determined as a function of the first derivative of the slope signal. 
     In an example embodiment of the present invention, the presence of a local maximum of the power of the signal may only be assumed if the first derivative of the slope signal is less than zero. 
     The foregoing object may additionally be attained in that, for the operation of a voice-supported system, such as a communications and/or duplex telephony device in a motor vehicle, using at least one microphone and at least one loudspeaker to reproduce a signal generated by the microphone, as well as using a bandpass filter arranged between the microphone and the loudspeaker, the power of a signal may be determined as a function of a frequency, and the bandpass filter may be adjusted as a function of a derivative of the power of the signal with respect to the frequency. 
     In an example embodiment of the present invention, the bandpass filter may be adjusted as a function of at least two local maxima of the power of the signal as a function of the frequency. 
     In an example embodiment of the present invention, the bandpass filter may be adjusted as a function of the first derivative of the power of the signal with respect to the frequency. 
     In an example embodiment of the present invention, a slope signal may be formed using the first derivative of the power of the signal with respect to the frequency, which takes on a first binary value when the first derivative of the power of the signal with respect to the frequency is greater than or equal to zero, and which takes on a second binary value when the first derivative of the power of the signal with respect to the frequency is less than zero, the bandpass filter being adjusted as a function of the slope signal or of the first derivative of the slope signal. 
     In an example embodiment of the present invention, all local maxima may be determined in one frequency range. In an example embodiment of the present invention, the global maximum may be determined in that frequency range. 
     In an example embodiment of the present invention, the bandpass filter may be adjusted so that it blocks the portion of the signal generated by the microphone at a notch frequency only when the ratio:
         at least of the power of the signal generated by the microphone at the frequency at which the power of the signal generated by the microphone is a maximum       

     to
         the average value of the power of the signal generated by the microphone at additional frequencies of the signal generated by the microphone
 
is greater than a feedback-power threshold (ratio threshold, OutGrdRatioThreshold).
       

     In an example embodiment of the present invention, the bandpass filter may be adjusted so that it blocks the portion of the signal generated by the microphone at a notch frequency only when the ratio:
         at least of the power of the signal generated by the microphone at the frequency at which the power of the signal generated by the microphone is a maximum       

     to
         the average value of the power of the signal generated by the microphone at additional frequencies of the signal generated by the microphone
 
is greater than a feedback-power threshold (RatioThreshold, OutGrdRatioThreshold) for longer than a time-ratio threshold (BinRatioTimeThreshold).
       

     In an example embodiment of the present invention, the bandpass filter may be adjusted so that it blocks the portion of the signal generated by the microphone at a notch frequency only when the ratio:
         of the power of the signal generated by the microphone at the frequency at which the power of the signal generated by the microphone is a maximum, plus/or of the power of the signal generated by the microphone at one of the further frequencies of the signal generated by the microphone which are adjacent to the frequency at which the power of the signal generated by the microphone is a maximum       

     to
         the average value of the power of the signal generated by the microphone at additional frequencies of the signal generated by the microphone
 
is greater than a feedback-power threshold (RatioThreshold, OutGrdRatioThreshold).
       

     In an example embodiment of the present invention, the bandpass filter may be adjusted so that it blocks the portion of the signal generated by the microphone at a notch frequency only when the ratio:
         of the power of the signal generated by the microphone at the frequency at which the power of the signal generated by the microphone is a maximum, plus/or of the power of the signal generated by the microphone at one of the further frequencies of the signal generated by the microphone which are adjacent to the frequency at which the power of the signal generated by the microphone is a maximum       

     to
         the average value of the power of the signal generated by the microphone at additional frequencies of the signal generated by the microphone
 
is greater than a feedback-power threshold (RatioThreshold, OutGrdRatioThreshold) for longer than a time-ratio-threshold (BinRatioTimeThreshold).
       

     In an example embodiment of the present invention, the bandpass filter may be adjusted so that it blocks the portion of the signal generated by the microphone at a notch frequency only when the ratio:
         of the power of the signal generated by the microphone at the frequency at which the power of the signal generated by the microphone is a maximum, plus/or the power of the signal generated by the microphone at the frequency of the signal generated by the microphone:
           which is directly adjacent to the frequency at which the power of the signal generated by the microphone is a maximum; and   at which the power is greater than at a frequency which is also directly adjacent to the frequency at which the power of the signal generated by the microphone is a maximum   
               

     to
         the average value of the power of the signal generated by the microphone at additional frequencies of the signal generated by the microphone
 
is greater than a feedback-power threshold (RatioThreshold, OutGrdRatioThreshold).
       

     In an example embodiment of the present invention, the bandpass filter may be adjusted so that it blocks the portion of the signal generated by the microphone at a notch frequency only when the ratio:
         of the power of the signal generated by the microphone at the frequency at which the power of the signal generated by the microphone is a maximum, plus/or the power of the signal generated by the microphone at the frequency of the signal generated by the microphone:
           which is directly adjacent to the frequency at which the power of the signal generated by the microphone is a maximum; and   at which the power is greater than at a frequency which is also directly adjacent to the frequency at which the power of the signal generated by the microphone is a maximum   
               

     to
         the average value of the power of the signal generated by the microphone at additional frequencies of the signal generated by the microphone
 
is greater than a feedback-power threshold (RatioThreshold, OutGrdRatioThreshold) for longer than a time-ratio-threshold (BinRatioTimeThreshold).
       

     In an example embodiment of the present invention, the bandpass filter may be adjusted so that it blocks the portion of the signal generated by the microphone at a notch frequency only when the ratio:
         of the power of the signal generated by the microphone at the frequency at which the power of the signal generated by the microphone is a maximum, plus the power of the signal generated by the microphone at the frequency of the signal generated by the microphone:
           which is directly adjacent to the frequency at which the power of the signal generated by the microphone is a maximum; and   at which the power is greater than at a frequency which is also directly adjacent to the frequency at which the power of the signal generated by the microphone is a maximum   
               

     to
         the average value of the power of the signal generated by the microphone of all, at least essential, additional (investigated) frequencies of the signal generated by the microphone
 
is greater than a feedback-power threshold (RatioThreshold, OutGrdRatioThreshold).
       

     In an example embodiment of the present invention, the bandpass filter may be adjusted so that it blocks the portion of the signal generated by the microphone at a notch frequency only when the ratio:
         of the power of the signal generated by the microphone at the frequency at which the power of the signal generated by the microphone is a maximum, plus the power of the signal generated by the microphone at the frequency of the signal generated by the microphone:
           which is directly adjacent to the frequency at which the power of the signal generated by the microphone is a maximum; and   at which the power is greater than at a frequency which is also directly adjacent to the frequency at which the power of the signal generated by the microphone is a maximum   
               

     to
         the average value of the power of the signal generated by the microphone of all, at least essential, additional (investigated) frequencies of the signal generated by the microphone
 
is greater than a feedback-power threshold (RatioThreshold, OutGrdRatioThreshold) for longer than a time-ratio-threshold (BinRatioTimeThreshold).
       

     In an example embodiment of the present invention, the feedback-power threshold (RatioThreshold, OutGrdRatioThreshold) may be established as a function of an output signal of the bandpass filter. 
     In an example embodiment of the present invention, the feedback-power threshold (RatioThreshold, OutGrdRatioThreshold) may be between 20 and 40. 
     In an example embodiment of the present invention, the bandpass filter may be adjusted so that it blocks the portion of the signal generated by the microphone at a notch frequency only when the ratio:
         of the power of the signal generated by the microphone at the frequency at which the power of the signal generated by the microphone is a maximum       

     to
         the average value of the power of the signal generated by the microphone at further frequencies at which the power of the signal generated by the microphone has a local maximum
 
is greater than an additional power threshold (RichContentThreshold).
       

     In an example embodiment of the present invention, the bandpass filter may be adjusted so that it blocks the portion of the signal generated by the microphone at a notch frequency only when the ratio:
         of the power of the signal generated by the microphone at the frequency at which the power of the signal generated by the microphone is a maximum       

     to
         the average value of the power of the signal generated by the microphone at all further (investigated) frequencies at which the power of the signal generated by the microphone has a local maximum
 
is greater than an additional power threshold (RichContentThreshold).
       

     The power of the signal generated by the microphone at the frequency at which the power of the signal generated by the microphone is a maximum, and/or the power of the signal generated by the microphone at a frequency at which the power of the signal generated by the microphone has a local maximum, in the sense of the foregoing, may include alternatively or additionally also the power that the signal has in response to a closely adjacent frequency of above-named frequency and which (still) has a similar high power, such as the maximum in each case. 
     In an example embodiment of the present invention, the additional power threshold (RichContentThreshold) may be between 20 and 50, e.g., between 30 and 40. 
     In an example embodiment of the present invention, the bandpass filter may be adjusted as a function of its output signal. 
     In an example embodiment of the present invention, the bandpass filter may include a notch filter or a filter bank, e.g., a multifilter, having at least one notch filter. The filter bank may include, for example, 10 notch filters. 
     Further aspects, features and details are set forth below in the following description of exemplary embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a motor vehicle. 
         FIG. 2  schematically illustrates an exemplary embodiment of a device according to the present invention. 
         FIG. 3  schematically illustrates a notch filter. 
         FIG. 4  schematically illustrates a filter bank. 
         FIG. 5  schematically illustrates an exemplary embodiment for a flow diagram implemented in a decision logic. 
         FIG. 6  schematically illustrates an power-frequency diagram. 
         FIG. 7  schematically illustrates an exemplary embodiment of query  41  in  FIG. 5 . 
         FIG. 8  is a schematic power-frequency diagram. 
         FIG. 9  is a schematic power-frequency diagram. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic inside view of a motor vehicle  1  from above. In this context, reference numerals  2  and  3  indicate the front seats, and reference numerals  4 ,  5  and  6  indicate the rear seats of the motor vehicle. Reference numerals  7 ,  8 ,  9 ,  10 ,  11 ,  12 ,  13 ,  14 ,  15 ,  16 ,  17 ,  18 ,  19  and  20  indicate loudspeakers. Reference numerals  21 ,  22 ,  23  and  24  indicate microphones. Loudspeakers  7 ,  8 ,  9 ,  10 ,  11 ,  12 ,  13 ,  14 ,  15 ,  16 ,  17 ,  18 ,  19  and  20  belong, in part, to a music system and, in part, to a communication and/or intercom/two-way intercom device. They may also be used by both systems. 
     In the present exemplary embodiment, loudspeakers  9 ,  17 ,  18 ,  19 ,  20  output a signal generated by microphone  21 , loudspeakers  7 ,  17 ,  18 ,  19 ,  20  output a signal generated by microphone  22 , loudspeakers  7 ,  9 ,  19 ,  20  output a signal generated by microphone  23 , and loudspeakers  7 ,  9 ,  17 ,  18  output a signal generated by microphone  24 . In this manner, the possibility of verbal communication in a motor vehicle is supported. In this context, in principle, the more strongly a signal is amplified between one of microphones  21 ,  22 ,  23 ,  24  and one of loudspeakers  7 ,  9 ,  17 ,  18 ,  19 ,  20 , the better is the communication may be. However, the possibility of implementing such an amplification is limited by possible feedback effects caused by sound radiated by a loudspeaker  7 ,  9 ,  17 ,  18 ,  19 ,  20 , which is received by microphone  21 ,  22 ,  23 ,  24 , and is subsequently amplified and radiated by loudspeaker  7 ,  9 ,  17 ,  18 ,  19 ,  20 . 
     To reduce such a feedback, in accordance with the example embodiment illustrated in  FIG. 2 , a bandpass filter  32  is arranged between a microphone  30 , which may be one of microphones  21 ,  22 ,  23 ,  24 , and a loudspeaker  31 , which may be one of loudspeakers  7 ,  9 ,  17 ,  18 ,  19 ,  20 . This filters a signal S generated by microphone  30  and supplies a filtered signal S′, which has certain frequency ranges filtered out, for which a decision logic  33  had recognized the danger of feedback. To this end, decision logic  33  determines filter parameters f c  and Q, which are used to adjust bandpass filter  32 . 
     To amplify signal S and/or signal S′, amplifiers may be provided. However, the amplifier function may also be provided by the bandpass filter. 
       FIG. 3  illustrates the characteristic curve of a bandpass filter designed as a notch filter, amplification V of the bandpass filter being plotted against frequency f. In this context, f c  indicates the mid-frequency of the bandpass filter and Q its quality. To filter a plurality of frequency ranges, bandpass filter  32  may be arranged as a filter bank, as illustrated in  FIG. 4 . The filter bank may include up to 10 notch filters. 
       FIG. 5  illustrates an exemplary embodiment for a flow diagram implemented in a decision logic  33 . In this context, frequency f of signal S is first analyzed in a step  40 , and, as illustrated exemplarily in  FIG. 6 , power P of signal S is determined at, e.g., 192, different test frequencies f n , f n+1 , f n+2 , f n+3 , f n+4 , f n+5 , f n+6 , f n+7 , f n+8 , which are spaced apart by, e.g., 40 Hz. 
     It may be provided to average over time the power at test frequencies f n , f n+1 , f n+2 , f n+3 , f n+4 , f n+5 , f n+6 , f n+7 , f n+8  i.e., to develop an average over time, and to test this average value over time of the power instead of the current power of signal S at test frequencies f n , f n+1 , f n+2 , f n+3 , f n+4 , f n+5 , f n+6 , f n+7 , f n+8 . The foregoing may consequently also include the average value of the power developed over a certain time period. Furthermore, power in the present context may include the amplitude or its average value over time. In the present context, further modifications of power, amplitude or their average values over time may also be included, such as normalized values. Thus, for instance, by the power of signal S at a test frequency f n  in the present context, the value of the power of signal S at this test frequency f n  divided by the sum of the power of signal S at all test frequencies f n , f n+1 , f n+2 , f n+3 , f n+4 , f n+5 , f n+6 , f n+7 , f n+8  may be understood. 
     Step  40  is followed by interrogation  41 , e.g., whether the danger of feedback exists at a test frequency f n , f n+1 , f n+2 , f n+3 , f n+4 , f n+5 , f n+6 , f n+7 , f n+8 . Details pertaining to this query are explained with respect to  FIG. 7 . Provided there is no danger of feedback for any test frequency f n , f n+1 , f n+2 , f n+3 , f n+4 , f n+5 , f n+6 , f n+7 , f n+8 , step  40  follows interrogation  41 . If, however, the danger of feedback does exist for a test frequency f n , f n+1 , f n+2 , f n+3 , f n+4 , f n+5 , f n+6 , f n+7 , f n+8 , then an interrogation  42  follows interrogation  41 , e.g., whether signal S generated by microphone  30  has already been reduced, with the aid of the bandpass filter, in the environs of this test frequency. 
     If signal S generated by microphone  30  has not already been reduced by the bandpass filter, by signal components around the test frequency, then query  42  is followed by an interrogation  43 , e.g., whether a bandpass filter is available. If a bandpass filter is available, interrogation  43  is followed by a step  47 , in which a bandpass filter is selected and the filter parameters, i.e., the mid-frequency f c  and the quality Q of the bandpass filter, are generated. The mid-frequency f c  is an example of the notch frequency. The notch frequency may be particularly the frequency range about the mid-frequency f c , which the bandpass filter actually filters out of signal S generated by microphone  30 . 
     Mid-frequency f c  may, for example, be equated to the test frequency, for which feedback has been established. In an example embodiment of the present invention, mid-frequency f c  may also be a test frequency having a correction frequency added to it. This correction frequency is formed, for example, as a function of the power of the signal generated by the microphone at the test frequency at which the power generated by the microphone is a maximum, as well as of the power of the signal generated by the microphone at least one test frequency next to this test frequency. Thus, the correction frequency may be generated in accordance with:
 
 fkorr= sign* f dist* P maxneigh/( P max+ P maxneigh);
 
in which:
         fkorr represents the correction frequency;   fdist represents the distance between the test frequency at which the power of the signal generated by the microphone is a maximum, and a test frequency having the greatest power which is directly next to the test frequency at which the power of the signal generated by the microphone is a maximum;   Pmax represents the power of the signal generated by the microphone at the test frequency at which the power of the signal generated by the microphone is a maximum;   Pmaxneigh represents the power of the signal generated by the microphone at the test frequency having the greatest power directly next to the test frequency at which the power of the signal generated by the microphone is a maximum; and   sign represents a sign;   the sign being positive when the test frequency, having the greatest power directly next to the test frequency at which the power of the signal generated by the microphone is a maximum, is greater than the test frequency at which the power of the signal generated by the microphone is a maximum, and the sign otherwise being negative.       

     This is explained in greater detail in the light of the following example:
         192 test frequencies f 1 , f 2 , . . . f 192  are assumed. f 1  is equal to 40 Hz. fdist is 40 Hz for all test frequencies. In addition, for the powers of the signals generated by the microphone at test frequencies f 1   1 , f 2 , . . . f 192 , it is true that:
 
 P ( f   1   , f   2   , . . . f   94 )=1
 
 P ( f   95 )=4
 
 P ( f   96 )=16
 
 P ( f   97 )=2
 
 P ( f   94   , f   99   , . . . f   192 )=1
       

     Then it is true that
 
 fkorr =(−)*40Hz*4/(16+2)=−8Hz
 
     The test frequency at which the power of the signal generated by the microphone is a maximum, is consequently 3840 Hz, and the notch frequency is 3832 Hz. 
     The correction frequency may also be formed according to:
 
 fkorr=Δf* ( P neighright− P neighleft)/( P max+| P neighright− P neighleft|),
 
in which:
         fkorr represents the correction frequency;   Δf represents the difference between two test frequencies;   Pmax represents the power of the signal generated by the microphone at the test frequency at which the power of the signal generated by the microphone is a maximum;   Pneighright represents the power of the signal generated by the microphone at the test frequency directly above the test frequency at which the power of the signal generated by the microphone is a maximum; and       

     Pneighleft represents the power of the signal generated by the microphone at the test frequency directly below the test frequency at which the power of the signal generated by the microphone is a maximum. 
     Based on the above numerical example, it is true in this case that:
 
 fkorr= 40Hz*(2−4)/(16+|4−2|)=−4.44Hz
 
     The test frequency, at which the power of the signal generated by the microphone is a maximum, is consequently 3840 Hz and the notch frequency is 3835.56 Hz. 
     Quality Q is adjusted to a predefined value of, for example, 1/40 Hz. 
     If query  43  results in the statement that no bandpass filter is available, query  43  is followed by a step  48 , in which the power of signal S is reduced by a reduction factor which may be between 2 dB and 5 dB, e.g., at essentially 3 dB. 
     If the result of query  42  is that signal S generated by microphone  30  is already being reduced with the aid of the bandpass filter by signal portions around the test frequency, a query  44  follows query  42 . Using query  44 , the question is whether by a further widening of the frequency range in which the bandpass filter blocks, that is, by a further reduction of its quality Q, a predetermined minimum quality may be undershot. 
     If by a further widening of the frequency range a predetermined minimum quality may be undershot, query  44  is followed by a step  45 , and otherwise by a step  46 . In step  45 , which corresponds to step  48 , the power of signal S is reduced by a reduction factor, which may be between 2 dB and 5 dB, e.g., at essentially 3 dB. In step  46  quality Q is reduced, i.e., the bandpass filter is widened. 
     After steps  45 ,  46 ,  47  and  48  there is a step  49  in which a time between 0.1 s and 3 s is expected. 
       FIG. 7  illustrates an exemplary embodiment for query  41 . In this context, first a query  61  is provided as to whether the power of output signal S′ of bandpass filter  32  exceeds an output threshold value. If the power of output signal S′ of bandpass filter  32  exceeds the output threshold, query  61  is followed by a query  62 , as to whether, for example, the ratio PowerRatio3:
         of the power MaxBinPwrPlusNeighbor of signal S generated by microphone  30  is a maximum at the frequency at which the power of the signal generated by the microphone is a maximum, plus the power of signal S generated by microphone  30  at the test frequency of signal S generated by microphone  30 :
           which is directly adjacent to the test frequency at which the power of signal S generated by microphone  30  is a maximum; and   at which the power is greater than at a test frequency which is also directly adjacent to the test frequency at which the power of signal S generated by microphone  30  is a maximum   
               

     to
         the average value MeanBinPwrRemainder of the power of signal S generated by microphone  30  of all additional test frequencies of signal S generated by microphone  30 
 
is greater than a feedback-power threshold OutGrdRatioThreshold.
       

     Using query  62 , e.g., as provided by this exemplary embodiment, the question is put whether the ratio PowerRatio3:
         of the power MaxBinPwrPlusNeighbor of signal S generated by microphone  30  at the frequency at which the power of signal S generated by microphone  30  is a maximum, plus the power of signal S generated by microphone  30  at the test frequency of signal S generated by microphone  30 :
           which is directly adjacent to the test frequency at which the power of signal S generated by microphone  30  is a maximum; and   at which the power is greater than at a test frequency which is also directly adjacent to the test frequency at which the power of signal S generated by microphone  30  is a maximum   
               

     to
         the average value MeanBinPwrRemainder of the power of signal S generated by microphone  30  of all additional test frequencies of signal S generated by microphone  30 
 
is greater than a feedback-power threshold OutGrdRatioThreshold for longer than a time-ratio-threshold OutBinRatioTimeThreshold. The feedback-power threshold OutGrdRatioThreshold may be between 30 and 40.
       

     It may be provided that query  62  is only answered affirmatively if the global maximum is at a test frequency for longer than a time threshold OutGrdMaxBinTimeThreshold. 
     To carry out query  62 , first of all the local maxima are determined. For this purpose, first of all (for the test frequencies) the first derivative of the power of Signal S with respect to frequency f is determined. From the first derivative of the power of signal S with respect to frequency f a slope signal is subsequently formed, which assumes a first binary value when the first derivative of the power of signal S with respect to the frequency f is greater than or equal to zero, and which assumes a second binary value when the first derivative of the power of signal S with respect to frequency f is less than zero. Subsequently, the first derivative of the slope signal is ascertained. In this context, in an example embodiment of the present invention, the presence of a local maximum of the power of signal S as a function of frequency f is only assumed if the first derivative of the slope signal is less than zero. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 function idx_vec = FinfInfletions(x, flec_thresh) 
               
               
                   
                 dtdx = diff(x); 
               
               
                   
                 dtdx = dtdx &gt; 0; 
               
               
                   
                 dt2dx = diff(dtdx); 
               
               
                   
                 idx_vec = find(dt2dx &lt; flec_thresh); 
               
               
                   
                 idx_vec = idx_vec + 1; 
               
               
                   
                   
               
            
           
         
       
     
     In this context, Table 1 shows an exemplary embodiment of a program written in the language Matlab™, which ascertains the indices idx_vec of the test frequencies at which there are local maxima according to criteria mentioned above. In this context, x denotes a vector having the powers at the individual test frequencies, and flec_thresh denotes a value between 0 and −1. 
     The local maximum having the greatest power is regarded as the global maximum. 
     If query  62  is answered in the affirmative, then query  62  is followed by a query  63 , and otherwise by a step  64 . 
     By query  63 , the question is put as to whether signal S has a strong harmonic component. For this purpose, in an exemplary embodiment, the question is put whether the ratio:
         of the power of signal S generated by microphone  30  at the test frequency, at which the power of signal S generated by microphone  30  is a maximum       

     to
         the average value of the power of signal S generated by microphone  30  at all further test frequencies at which the power of signal S generated by microphone  30  has a local maximum
 
is less than or equal to an additional power threshold RichContentThreshold.
       

     If query  63  reveals that the ratio:
         of the power of signal S generated by microphone  30  at the test frequency, at which the power of signal S generated by microphone  30  is a maximum       

     to
         the average value of the power of signal S generated by microphone  30  at all further test frequencies at which the power of signal S generated by microphone  30  has a local maximum
 
is less than or equal to an additional power threshold RichContentThreshold, then query  63  is followed by step  64 . Otherwise, feedback is assumed.
       

     In step  64 , the sequence is stopped for a predetermined retention time, such as 3 s. After the expiration of the retention time, feedback is negated. 
     If query  61  yields that the power of output signal S′ of bandpass filter  32  does not exceed the output threshold, then query  61  is followed by query  65  which essentially corresponds to query  62 . In this context, however, a different feedback power threshold RatioThresholdis used, and not OutGrdRatioThreshold. However, the feedback-power threshold RatioThreshold may also be between 30 and 40. 
     If query  65  is answered affirmatively, then query  65  is followed by query  66  corresponding to query  63 . Otherwise the presence of feedback is negated. 
     If query  66  reveals that the ratio:
         of the power of signal S generated by microphone  30  at the test frequency, at which the power of signal S generated by microphone  30  is a maximum       

     to
         the average value of the power of signal S generated by microphone  30  at all further test frequencies at which the power of signal S generated by microphone  30  has a local maximum
 
is less than or equal to an additional power threshold RichContentThreshold, then the presence of feedback is negated. Otherwise, feedback is assumed.
       

     The feedback detection is not limited to the example embodiment described above. The feedback detection may, for example, be constituted so that only query  65  is provided. The detection of feedback may also be provided so as to replace the example embodiments in accordance with  FIG. 7  and its binary decision logic by a fuzzy decision logic, e.g., fuzzy logic, or neural networks. 
     Query  63  as in  FIG. 7  will be explained below in the light of two signals  80  and  90  illustrated in  FIGS. 8 and 9  in a power-frequency diagram. Power P of signals  80  and  90  is plotted in dB against the index idx_vec of the test frequencies. It is assumed that query  61  yields for both signals  80  and  90  that the power of output signal S′ of bandpass filter  32  exceeds the output threshold, and that therefore query  62  follows query  61 . It is assumed further that query  62  receives an affirmative response. The + signs in  FIG. 8  and  FIG. 9  denote all test frequencies which have been recognized by the program according to Table 1 as local/global maxima. 
     In  FIG. 8 , reference numeral  81  indicates the global maximum of signal  80 . In  FIG. 9 , reference numeral  91  indicates the global maximum of signal  90 . The test frequencies have a separation distance of 40 Hz. The additional power threshold RichContentThreshold amounts to 37. 
     The ratio:
         of the power of signal  80  at the test frequency at which the power of signal  80  is a maximum       

     to
         the average value of the power of signal  80  at all further test frequencies at which the power of signal  80  has a local maximum
 
amounts to approximately 16, and is consequently clearly less than 37. Thus, query  63  would be answered affirmatively, and so the presence of feedback would be negated.
       

     The ratio:
         of the power of signal  90  at the test frequency at which the power of signal  90  is a maximum       

     to
         the average value of the power of signal  90  at all further test frequencies at which the power of signal  90  has a local maximum
 
amounts to approximately 73, and is consequently clearly greater than 37. Thus, query  63  would be negated and so the presence of feedback would be assumed.
       

     
       
         
           
               
             
               
                   
               
               
                 REFERENCE NUMERAL LIST 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 1 
                 motor vehicle 
               
               
                 2, 3 
                 front seats 
               
               
                 4, 5, 6 
                 rear seats 
               
               
                 7, 8, 9, 10, 11, 12, 
                 loudspeakers 
               
               
                 13, 14, 15, 16, 17, 
                   
               
               
                 18, 19, 20, 31 
                   
               
               
                 21, 22, 23, 24, 30 
                 microphones 
               
               
                 32, 
                 bandpass filter 
               
               
                 33 
                 decision logic 
               
               
                 40, 45, 46, 47, 48, 
                 steps 
               
               
                 49, 64 
                   
               
               
                 41, 42, 43, 44, 61, 
                 queries 
               
               
                 62, 63, 65, 66 
                   
               
               
                 80, 90 
                 signal 
               
               
                 81, 91 
                 global maximum 
               
               
                 BinRatioTimeThreshold 
                 time ratio threshold 
               
               
                 f 
                 frequency 
               
               
                 f n , f n+1 , f n+2 , f n+3 , f n+4 , 
                 frequency points 
               
               
                 f n+5 , f n+6 , f n+7 , f n+8 , f 1 , 
                   
               
               
                 f 2 , f 44 , f 88 , f 94 , f 95 , 
                   
               
               
                 f 97 , f 98 , f 122 , f 192 , 
                   
               
               
                 f c   
                 mid-frequency 
               
               
                 fdist 
                 distance between the test 
               
               
                   
                 frequency at which the power of 
               
               
                   
                 the signal generated by the 
               
               
                   
                 microphone is a maximum, and a 
               
               
                   
                 test frequency having the 
               
               
                   
                 greatest power directly next to 
               
               
                   
                 the test frequency at which the 
               
               
                   
                 power of the signal generated by 
               
               
                   
                 the microphone is a maximum 
               
               
                 fkorr 
                 correction frequency 
               
               
                 MaxBinPwrPlusNeighbor 
                 the power of the signal 
               
               
                   
                 generated by the microphone at 
               
               
                   
                 the frequency at which the power 
               
               
                   
                 of the signal generated by the 
               
               
                   
                 microphone is a maximum, plus 
               
               
                   
                 the power of the signal 
               
               
                   
                 generated by the microphone at 
               
               
                   
                 the frequency of the signal 
               
               
                   
                 generated by the microphone 
               
               
                   
                 which is directly adjacent to 
               
               
                   
                 the frequency at which the power 
               
               
                   
                 of the signal generated by the 
               
               
                   
                 microphone is a maximum, and at 
               
               
                   
                 which the power of the signal 
               
               
                   
                 generated by the microphone is 
               
               
                   
                 greater than at a frequency 
               
               
                   
                 which is also directly adjacent 
               
               
                   
                 to a frequency at which the 
               
               
                   
                 power of the signal generated by 
               
               
                   
                 the microphone is a maximum 
               
               
                 MeanBinPwrRemainder 
                 average value of the power of 
               
               
                   
                 the signal generated by the 
               
               
                   
                 microphone of all further 
               
               
                   
                 (tested) frequencies 
               
               
                 Q 
                 quality 
               
               
                 OutGrdRatioThreshold, 
                 feedback-power threshold 
               
               
                 RatioThreshold 
                   
               
               
                 P 
                 power 
               
               
                 PMax 
                 the power of the signal 
               
               
                 generated by the microphone at 
                   
               
               
                 the test frequency at which the 
                   
               
               
                 power of the signal generated by 
                   
               
               
                 the microphone is a maximum 
                   
               
               
                 Pmaxneigh 
                 the power of the signal 
               
               
                   
                 generated by the microphone at 
               
               
                   
                 which the test frequency having 
               
               
                   
                 the greatest power directly 
               
               
                   
                 adjacent to the test frequency 
               
               
                   
                 at which the power of the signal 
               
               
                   
                 generated by the microphone is a 
               
               
                   
                 maximum 
               
               
                 Pneighleft 
                 the power of the signal 
               
               
                   
                 generated by the microphone at 
               
               
                   
                 the test frequency directly 
               
               
                   
                 below the test frequency at 
               
               
                   
                 which the power of the signal 
               
               
                   
                 generated by the microphone is a 
               
               
                   
                 maximum 
               
               
                 Pneighright 
                 the power of the signal 
               
               
                   
                 generated by the microphone at 
               
               
                   
                 the test frequency directly 
               
               
                   
                 above the test frequency at 
               
               
                   
                 which the power of the signal 
               
               
                   
                 generated by the microphone is a 
               
               
                   
                 maximum 
               
               
                 PowerRatio3 
                 power ratio 
               
               
                 RichContentThreshold 
                 additional power threshold 
               
               
                 S 
                 signal 
               
               
                 S′ 
                 filtered signal 
               
               
                 sign 
                 sign 
               
               
                 V 
                 amplification 
               
               
                 Δf 
                 interval between two test 
               
               
                   
                 frequencies