Abstract:
Device for detecting noise in a signal (AI) with a bandwidth-limited useful-signal component, including a first filter for separating a spectral component of the signal (AI) lying outside the bandwidth of the useful-signal component, and a comparator for comparing the power of the separated spectral component with a limit (E1) and for detecting a noise when the power exceeds the limit (E1).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to a device and method for detecting noise in an electronic device, especially noise in impulse form, in a signal with a bandwidth-limited useful component, and a device and method for suppressing such noise.  
           [0002]    Probably the most important example of this type of bandwidth-limited signal is an audio signal. The useful component of such an audio signal lies within the audible frequency range of human hearing, or approximately 40 Hz to 16,000 Hz. However, noise impulses that propagate through a line along with the useful component may have frequency components that extend beyond the audible frequency range.  
           [0003]    Noise impulses in such an audio signal may occur for a number of reasons. For example, electrical equipment in the vicinity of a radio receiver may become noticeable as noise, such as ignition pulses from a vehicle engine in a vehicle. In addition, processing of the audio signal itself (e.g., from clipping of a digital signal or overflow in a digital demodulator) may result in impulse noise.  
           [0004]    To efficiently implement noise suppression, it is first necessary to reliably detect the noise. A variety of techniques have been proposed for detecting and eliminating noise components in a signal.  
           [0005]    U.S. Pat. No. 5,479,440 discloses a technique that is based on conversion of time data from the signal to be cleared of noise to the frequency domain, the determination of a threshold below which all frequency components are canceled, and the back-conversion of the spectrum obtained to the time domain. This technique requires a double fast Fourier transform (FFT) and is thus very complex and expensive, and accordingly poorly suited for use in devices intended for a broad range of consumers.  
           [0006]    U.S. Pat. No. 4,622,520 discloses a method for eliminating noise from an audio signal modulated from a high-frequency carrier. In this method, the strength of the signal component is compared with that of the carrier, and signal components that are stronger than the carrier are detected as noise and suppressed. However, this technique cannot be applied to signals in the baseband since no carrier is available there as a reference for comparison. In addition, the application of this technique to digital signals is problematic due to the required high sampling rates.  
           [0007]    U.S. Pat. No. 4,143,333 proposes detecting noise impulses in an audio signal using a differentiating filter. For the duration of the noise the signal is blanked out. This type of noise detection directly from the audio signal is error-prone because it must first be ensured that a loud, high tone in the audio signal is not erroneously detected as impulse noise. In addition, there is the danger that noise will go undetected and thus unattenuated if the noise detection threshold is too high.  
           [0008]    Therefore, there is a need to for a technique of detecting and suppressing noise in a bandwidth-limited signal that are relatively simple and inexpensive to implement, yet provide reliable detection and suppression.  
         SUMMARY OF THE INVENTION  
         [0009]    A device for noise detection includes a first filter for separating a spectral component lying outside the bandwidth of the useful-signal component, and a comparator for comparing the power of this separated spectral component with a limit and for detecting a noise when the power exceeds the limiting value.  
           [0010]    The limit is suitably derived form a power mean value of the signal. This allows the sensitivity of the noise detection to be automatically adjusted to the level of the signal. The power mean value may be determined for the power of the entire frequency spectrum of the signal or also only for the power of the spectral component separated from it.  
           [0011]    The lower cut-off frequency of the first filter lies above the upper cut-off frequency of the bandwidth-limited signal. When the bandwidth-limited signal is an audio signal the first filter appropriately has a lower cut-off frequency that depending on the base audio standard, may lie in the range between 4 kHz and 24 kHz. The upper cut-off frequencies may be for example for signals of telephone quality in the 4 kHz range, around 15 kHz for TV audio signals, around 22.05 kHz for audio CD(s), and around 24 kHz for digital audio. Depending on the type of audio signal, a lower cut-off frequency for the first filter is selected just above the cut-off frequency of the signal. The first filter may be designed as a bandpass filter with an upper cut-off frequency in the range 1.5 times to 2 times the lower cut-off frequency.  
           [0012]    A device for noise suppression in a bandwidth-limited signal may include a device for noise detection of the above-defined type as well as a noise elimination filter with a transmission characteristic controllable by the detection result provided by the detection device.  
           [0013]    In the event no noise is detected, the noise elimination filter here is transmissive. When noise is detected, the noise elimination filter appropriately assumes a low-pass filter characteristic. In the following discussion, this is also called the first transmission characteristic, as opposed to the second transmission characteristic applied in the event of no detection of noise. The corner frequency of the first transmission characteristic or the low-pass characteristic is readjusted by a control circuit for the upper cut-off frequency of the signal. Preferably, this adjustment occurs by having the power of the low-pass-filtered signal converge on a predetermined percentage of the power of the unfiltered signal, appropriately between 50% and 95%, preferably around 85%.  
           [0014]    The noise elimination filter with a controllable transmission characteristic may be constructed out of a first controllable weighting element for weighting the signal with the controllable factor and, in parallel to this element, a series circuit with a second controllable weighting element with a second filter.  
           [0015]    Preferably, a ramp generator is provided to control the two weighting elements with two weighting factors such that the sum of the two weighting factors is constant. When a noise is detected, a ramp generator of this type enables progressive, gradual, and thus listener-audible switching between two transmission characteristics of the noise elimination filter.  
           [0016]    In one embodiment, between the noise detection device and the noise elimination filter, a delay element is provided.  
           [0017]    These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0018]    [0018]FIG. 1 is a block diagram illustration of a noise suppression device;  
         [0019]    [0019]FIG. 2 is a block diagram illustration of a first embodiment of a noise detection device;  
         [0020]    [0020]FIG. 3 is a block diagram illustration of a second embodiment of a noise detection device;  
         [0021]    [0021]FIG. 4 is a block diagram illustration of a first embodiment of a noise elimination filter;  
         [0022]    [0022]FIG. 5 is a block diagram illustration of a second embodiment of a noise elimination filter; and FIG. 6 is a plot of an output signal from a ramp generator associated with noise elimination filter illustrated in FIG. 5. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    [0023]FIG. 1 is a block diagram of a device  100  for noise suppression in an audio signal on a line  102 . The device  100  includes a noise detection device  104  that receives the audio signal to be cleared of noise, and delivers an output signal on a line  106  that indicates the presence or absence of a noise. A delay element  108  receives the audio signal on the line  102 , and provides a delayed audio signal on a line  109  to a noise elimination filter  110 . The noise elimination filter  110  is switchable between two transmission characteristics as controlled by output signal on the line  106 . In a first state, the noise elimination filter  110  operates as a low-pass characteristic for filtering out the impulse noises. A second state of the noise elimination filter  110  corresponds to an essentially unchanged transmission of the delayed audio signal on the line  109  through the noise elimination filter.  
         [0024]    [0024]FIG. 2 is a block diagram of a first embodiment of the noise detection device  104 . The noise detection device includes a bandpass filter  202 , which receives the audio signal on the line  102 . The lower corner frequency of the bandpass filter  202  lies above an upper cut-off frequency of the audio signal on the line  102 . Therefore, when the bandwidth of the audio signal is limited to a maximum of 15 kHz, the lower cut-off frequency of the bandpass filter  202  may be about 17 kHz. A suitable value for the upper cut-off frequency is about 31 kHz. When the audio signal on the line  102  has impulse noise, the result is that its spectrum temporarily has a significant power component outside the intended useful-signal bandwidth, here a maximum of 15 kHz. The noise component traverses the bandpass filter  202  and reaches an absolute-value circuit  204 , whose output signal on a line  206  is proportional to the power of the signal received at its input. The output of the absolute-value circuit  204  is input to a comparator  208 , which also receives a fixed limit signal value E1 on a line  210 . The comparator  208  provides the noise detection signal on the line  106 , which assumes one of two different values depending on whether the power measured by the absolute-value circuit  204  of the noise signal component is greater or less than the threshold value E1.  
         [0025]    For the sake of simplicity, it may be assumed in the following discussion that the output signal of the comparator  208  has a value of one in the event of a noise, and otherwise has the value zero—with the understanding that this assignment of numerical values is purely arbitrary.  
         [0026]    [0026]FIG. 3 shows a second embodiment  300  of a noise detection device. Similar to the embodiment of FIG. 2, the embodiment in FIG. 3 also includes a bandpass filter  302 , an absolute-value circuit  304 , and a comparator  308 . In addition, an averaging circuit  310  is provided whose input is connected to the output of absolute-value circuit  304 , and whose output on a line  312  is connected to the second input of comparator  308 . The averaging circuit  310  generates a variable mean of the signal from absolute-value circuit  304 , and sends this mean multiplied by a weighting factor value N 1  (N 1 &lt;1) as the limit signal E1 on the line  312  to the comparator  308 .  
         [0027]    The averaging circuit  310  adjusts the threshold value E1 on the line  312  over time with the mean amplitude of the noise signal component transmitted by the bandpass filter  302 . When the audio signal contains only a small power component outside its useful bandwidth, the threshold value E1 on the line  312  decreases with time, while the sensitivity of the noise detection device increases. However, when the audio signal continues to be rich in high-frequency spectral components which, if they continue to be present, by definition cannot represent impulse noises, the sensitivity of the noise detection device is automatically reduced. This reduces the likelihood of erroneously detecting an intensive, continuous high-frequency signal as noise that might be contained in the audio signal AI on the line  102  due to prior digital processing.  
         [0028]    Instead of connection to the output of absolute-value circuit  304 , the input of the averaging circuit  310  may also be connected to the input of the bandpass filter  302  to generate a mean of the power of audio signal AI on the line  102 . In this type of embodiment, the threshold value E1 on the line  312  follows the mean power of audio signal AI on the line  102 . That is, the noise detection becomes more sensitive as the amplitude of the audio signal AI on the line  102  decreases. Given a quiet audio signal in which even noises of lesser amplitude become audible, a more sensitive detection and thus a more sensitive suppression becomes possible than is the case given a relatively loud signal. On the other hand, given a loud signal only those noises are detected that are intensive enough to become noticeably objectionable to a listener.  
         [0029]    [0029]FIG. 4 shows a first embodiment of the noise elimination filter  110 . The noise elimination filter  110  includes two switches  402 .  404 . Switch  402  receives the delayed audio signal on the line  109 . A low pass filter  406  also receives the delayed audio on the line  102  and provides a low pass filtered signal on a line  408  to the second switch  404 . Both switches  402 ,  404  are controlled by the noise detection signal C on the line  106 . The second switch  404  is responsive to the noise detection signal on the line  106 , while the first switch  402  is connected to the noise detection signal on the line  106  through an inverter  410 . As a result, when one of the two switches  402 ,  404  is open the other is closed, and visa versa. In the event no noise is detected the switch  402  is closed, and the audio signal passes through the switch  402  and is output on a line  412 . When a noise is detected the switch  404  is closed, and the low pass filtered signal on the line  408  is output on the line  412 .  
         [0030]    Referring again to FIG. 1, the delay time for the audio signal in the delay element  108  is chosen as a function of the reaction time of noise detection device  104  such that, upon detection of noise the noise detection signal C on the line  106  changes state before the disturbed audio signal AI arrives at the noise elimination filter  110 . This ensures that even the beginning of a noise is filtered.  
         [0031]    Referring again to FIG. 4, the noise eliminator filter also includes a first absolute value detector  414  that receives the filtered signal on the line  408  and provides a first absolute value signal indicative of the absolute value on a line  416 . A multiplier  417  receives the delayed audio signal on the line  109  and a constant value X1 on a line  418 , and provides the resultant product signal on a line  420 . The product signal on the line  420  is input to a second absolute value detector  422  that provides a second absolute value signal indicative thereof on a line  424 . The constant value X1 on the line  418  may be in the range of about 0.5 to 0.9, and preferably about 0.85. The absolute value signals are indicative of power.  
         [0032]    The noise elimination filter  110  also includes a control circuit  426  that provides a cut-off frequency control signal f c  on a line  428  to the low pass filter  406 . The control signal f c  sets the lower frequency for the low pass filter  406 .  
         [0033]    The control circuit  426  includes a comparator (not shown) that cyclically performs a comparison of the signals from the absolute-value circuits  416 ,  422 . When the output level of the absolute-value circuit  422  is larger, this indicates that the output power of the low-pass filter  406  is less than X1 times the input power. In this case, the control circuit  426  increases the value of the corner frequency f c  by a predetermined increment d 1  in order to increase the transmission bandwidth of the low-pass filter  406  and thus its output power. When the level of absolute-value circuit  414  larger, the value of the corner frequency f c  is reduced by the same increment d 1 .  
         [0034]    When the audio signal has only few high-frequency components and the frequency components of an impulse noise lying in the upper audible range of the frequency spectrum are especially noticeable to the listener, the dynamic adjustability feature of the corner frequency f c  allowing the frequency to be reduced enabled impulse noises to be effectively suppressed and frequency components of the noise in the audible frequency range to be effectively suppressed as well. However, when the audio signal has a large number of high-frequency components the value of the corner frequency f c  on the line  428  is shifted upward so that the high-frequency components of the audio signal AI essentially pass through the low-pass filter  406  and are preserved. In this case, while the spectral components of the noise extending into the same frequency range are not suppressed to the same degree as they would be if corner frequency f c  were lower, nevertheless this factor generally does not become noticeable as noise since these spectral components are masked by the audio signal.  
         [0035]    [0035]FIG. 5 illustrates an alternative embodiment noise elimination filter  500 . The elements already described in FIG. 4 and designated by the same reference numbers in FIG. 5 shall not be described again in the interest of brevity. In the noise elimination filter  500  of FIG. 5, the switches are replaced by weighting/multiplier elements  502 ,  504  that are connected though an  502 ,  504  that are connected through an adding element  506 , which provides an output signal on a line  508 . The inverter  410  is an analog inverter, and a ramp generator  510  is connected between the signal on the line  106  and the weighting elements  502 ,  504 . The ramp generator  510  provides an output signal K on a line  512 .  
         [0036]    [0036]FIG. 6 shows the characteristic of the output signal K on the line  512  as a function of time when noise is detected. At time t 1  the onset of a noise is detected and the noise detection signal C on the line  106  changes its level to indicate the presence of noise. In response, the output signal K on the line  512  begins to rise linearly from 0 to 1. Therefore, the weighting factor by which weighting element  504  multiplies the low pass filtered signal on the line  408  changes linearly over time from 0 to 1. The weighting factor (1−K) on line  514  applied by weighting element  502  to the unfiltered audio signal on the line  109  moves from 1 toward 0. Shortly before the disturbed audio signal AI on the line  109  is input to the filter  500  at time t 2 , the switch-over is completed so that the disturbed audio signal passes completely through the low pass filter  406 . No noise is any longer detected at time t 3 , and the level of noise detection signal C on the line  106  changes again. After a time interval that takes into account the delay of the signal in the delay element  108  (FIG. 1), the ramp generator  510  begins to generate a ramp declining from 1 to 0, in the course of which the transmission characteristic of noise elimination filter  500  gradually returns from that of low-pass filter to complete transmission.  
         [0037]    Of course the noise detection device described with reference to FIGS. 2 and 3 may be employed in connection with other types of noise elimination filters. For example, in one embodiment of the noise elimination filter the low-pass filter  406  and its control circuit  426  may be eliminated, and instead, the input of the switch  404  or of the multiplier  504  may be connected to ground to respond to noise by simply temporarily blanking out the disturbed audio signal in the case of noise. Another approach would be to connect that input of the switch  404  or the multiplier  504  to the output of a sample-and-hold circuit, which for the duration of the noise constantly delivers the instantaneous value of the audio signal as the output level that this signal had before the noise began. These two variants enable suppression of single, spot noises. However, when noises last several milliseconds or occur in rapid succession, these embodiments may result in dropouts in the audio signal that are objectionable to the listener. For this reason, the noise elimination filters described in FIGS. 4 and 5 are preferred.  
         [0038]    Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.