Patent Publication Number: US-2005143109-A1

Title: Noise eliminating apparatus and receiver

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a noise eliminating apparatus and a receiver having the same. The use of the invention is not limited to the receiver and a diversity receiving method.  
      2. Description of the Related Art  
      Hitherto, a sound processing apparatus having the function of eliminating noise in a sound signal is known (refer to, for example, Japanese Patent Application Laid-open No. 7-177597). The disclosure of the corresponding U.S. Pat. No. 5,568,559A is incorporated by reference in its entirety. In the apparatus disclosed in this patent document, low frequency components in a digital sound signal are extracted by a low pass filter (hereinbelow, abbreviated as LPF). A necessary frequency area of a sound signal is extracted by a band pass filter (hereinbelow, abbreviated as BPF). An output of the LPF and an output of the BPF are compared with each other to thereby detect an amount of wind noise. On the basis of the result of detection, the low frequency component extracted by the LPF is multiplied with a coefficient. By subtracting the result of multiplication from the original sound signal, the wind noise is properly eliminated from the sound signal in accordance with the level.  
     SUMMARY OF THE INVENTION  
      In the case of the conventional technique, the noise of the low frequency component included in the sound signal is extracted and subtracted from the original sound signal. Consequently, there is a problem such that the conventional technique is not adapted to eliminate pulse noise. One object of the invention is to solve the problem.  
      The invention according to claim  1  relates to a noise eliminating apparatus comprising: 
          a noise detector which detects pulse noise included in a signal to be processed;     a threshold generator which generates a threshold on the basis of the pulse noise detected by said noise detector;     a gate pulse generator which compares the pulse noise detected by said noise detector with the threshold generated by said threshold generator and, when the level of said pulse noise exceeds said threshold, generates a gate pulse; and     a gate which inhibits outputting of said signal to be processed during the period in which said gate pulse is output from said gate pulse generator, and interpolates a signal in the period of inhibition to output the resultant signal.        

      The invention according to claim  5  relates to a receiver comprising: 
          a noise detector which detects pulse noise included in a signal to be processed;     a threshold generator which generates a threshold on the basis of the pulse noise detected by said noise detector;     a gate pulse generator which compares the pulse noise detected by said noise detector with the threshold generated by said threshold generator and, when the level of said pulse noise exceeds said threshold, generates a gate pulse;     a gate which inhibits outputting of said signal to be processed during the period in which said gate pulse is output from said gate pulse generator and interpolates a signal in the period of inhibition to output the resultant signal;     a frequency converter which converts a modulated reception signal into an intermediate frequency signal;     an intermediate frequency filter which passes a desired frequency area of an intermediate frequency signal output from said frequency converter; and     a detector which demodulates a signal that has passed through said intermediate frequency filter,     wherein said noise detector detects pulse noise included in an intermediate frequency signal output from said frequency converter as pulse noise included in a signal to be processed, and     said gate inhibits outputting of the signal demodulated by said detector during a period in which a gate pulse is output from said gate pulse generator and interpolates the signal in the inhibition period to output the resultant signal.       

    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram showing the configuration of a noise eliminating apparatus and a receiver in a first embodiment;  
       FIG. 2  is a block diagram showing the configuration of a main portion of the noise eliminating apparatus in the first embodiment;  
       FIG. 3  is a waveform chart illustrating operation of the first embodiment;  
       FIG. 4  is a waveform chart illustrating operation of the first embodiment;  
       FIG. 5  is a block diagram showing the configuration of a main portion of a noise eliminating apparatus in a second embodiment;  
       FIG. 6  is a circuit diagram conceptually showing the configuration of the main portion of the noise eliminating apparatus in the second embodiment;  
       FIG. 7  is a waveform chart illustrating operation of the second embodiment;  
       FIG. 8  is a waveform chart illustrating operation of the second embodiment; and  
       FIG. 9  is a waveform chart illustrating operation of the second embodiment.  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Preferred embodiments of a noise eliminating apparatus according to the invention and a receiver having the noise eliminating apparatus will be described in detail hereinbelow with reference to the appended drawings.  
     First Embodiment  
       FIG. 1  is a block diagram showing the configuration of a noise eliminating apparatus and a receiver of a first embodiment. As shown in  FIG. 1 , a receiver  1  comprises: a receiving device  2  including an antenna  21 , a high frequency amplifier (RF amplifier)  22 , a frequency mixer (MIX)  23 , a local oscillator (OSC)  24 , an intermediate frequency filter (IF filter)  25 , and a detector  26 ; and a noise eliminating apparatus  3  including a noise detector  31 , a threshold generator  32 , a gate pulse generator  33 , and a gate  34 .  
      As the receiving device  2 , a general super heterodyne receiver is used. An electromagnetic wave is received from the space by the receiver  1  via the antenna  21 . The RF amplifier  22  amplifies a high-frequency reception signal S 1  received by the receiver  1  via the antenna  21 . The frequency mixer  23  mixes a high-frequency signal S 2  amplified by the RF amplifier  22  with an oscillation signal S 3  of the local oscillator  24 , and converts the frequency difference between the signals S 2  and S 3  into an intermediate frequency.  
      The local oscillator  24  outputs the oscillation signal S 3  used for frequency conversion. The frequency mixer  23  and local oscillator  24  have the function of frequency converting device. The intermediate frequency filter  25  is a narrow-band filter and eliminates adjacent signals from an intermediate frequency signal S 4  obtained by the frequency converting device. The detector  26  demodulates a signal S 5  passed through the intermediate frequency filter  25 .  
       FIG. 2  is a block diagram showing the configuration of the noise detector  31 , threshold generator  32 , and gate pulse generator  33 . As shown in  FIG. 2 , the noise detector  31  comprises a detector  311  and a high pass filter (hereinbelow, abbreviated as HPF)  312 . The detector  311  demodulates the intermediate frequency signal S 4  obtained by the frequency converting device. The HPF  312  filters an output signal S 6  of the detector  311  so as to pass high frequency components, thereby extracting pulse noise.  
      The threshold generator  32  comprises, as shown in  FIG. 2 , an LPF  321  and a multiplier  322 . A signal (pulse noise) S 7  passed through the HPF  312  is supplied to the multiplier  322  via the LPF  321 . The multiplier  322  multiplies an output signal S 8  of the LPF  321  by “n” and outputs the resultant as a threshold. As the magnification “n” of the multiplier  322 , a value adapted to perform the noise process is selected. When pulse noise continuously enters the threshold generator  32 , the LPF  321  operates as an integrator, so that an output of the LPF  321  increases. With this increase, the threshold increases. That is, the LPF  321  has the function of threshold adjusting device for increasing the threshold in accordance with input of continuous pulse noise.  
      The gate pulse generator  33  comprises, as shown in  FIG. 2 , a comparator  331 . The comparator  331  generates a gate pulse S 10  on the basis of the output signal (pulse noise) S 7  of the noise detector  31  and an output signal (threshold) S 9  of the threshold generator  32 . When the gate pulse S 10  is not generated, the gate  34  outputs a demodulated signal S 11  obtained by the detector  26  as an output signal S 12 . On the other hand, while the gate pulse S 10  is generated, the gate  34  inhibits outputting of the demodulated signal S 11  obtained by the detector  26  and, instead, outputs the output signal S 12  interpolated by any of various interpolating methods such as linear interpolation.  
      The operation of the first embodiment will now be described. Electromagnetic waves such as AM broadcast wave are induced by the antenna  21  and received by the receiver  1 . The high-frequency reception signal S 1  received by the receiver  1  is amplified by the RF amplifier  22  and becomes the intermediate frequency signal S 4  via the frequency mixer  23 . The intermediate frequency signal S 4  is detected by the detector  26  via the narrow-band intermediate frequency filter  25  in the receiving unit  2  and becomes the demodulated signal S 11 . The demodulated signal S 11  is supplied to the gate  34 .  
      The intermediate frequency signal S 4  output from the frequency mixer  23  is, on the other hand, directly detected by the detector  311  without passing through a narrow-band intermediate frequency filter in the noise eliminating apparatus  3 . Therefore, the demodulated signal S 6  obtained by detection in the detector  311  includes pulse noise of a large amplitude. By passing the demodulated signal S 6  via the HPF  312 , the pulse noise S 7  is extracted. The extracted pulse noise S 7  passes through the LPF  321  and is multiplied by “n” by the multiplier  322 .  
      An output of the multiplier  322  is supplied as the threshold S 9  to the comparator  331 . The pulse noise S 7  output from the HPF  312  is also supplied to the comparator  331 . The comparator  331  compares the pulse noise S 7  supplied from the noise detector  31  with the threshold S 9  supplied from the threshold generator  32 . As a result of comparison, when the level of the pulse noise S 7  exceeds the threshold S 9 , the gate pulse S 10  is generated. The generated gate pulse S 10  is supplied to the gate  34 . When the peak value of the pulse noise S 7  decreases and becomes equal to or lower than the threshold S 9 , the gate pulse S 10  is not output.  
       FIG. 3  is a waveform chart showing waveforms of the demodulated signal S 11  of the detector  26 , the output signal (gate pulse) S 10  of the gate pulse generator  33 , and the output signal S 12  of the gate  34 . As shown in  FIG. 3 , the gate  34  inhibits outputting the demodulated signal S 11  of the detector  26  during the period in which the gate pulse S 10  is supplied. During the period of inhibition, the gate  34  interpolates the output signal  12  by any of the various interpolating methods to output the interpolated signal. By the operation, as shown in  FIG. 3 , although the pulse noise exceeding the threshold S 9  is included in the demodulated signal S 11  of the detector  26 , the output signal S 12  having the waveform from which the pulse noise is eliminated is obtained.  
       FIG. 4  is a waveform chart showing waveforms of the output signal (pulse noise) S 7  of the noise detector  31 , the output signal (threshold) S 9  of the threshold generator  32 , and the output signal (gate pulse) S 10  of the gate pulse generator  33  in the case where pulse noise continues. As shown in  FIG. 4 , when the pulse noise continues, the threshold S 9  gradually increases. This is because the LPF  321  operates as an integrator as described above. When the threshold S 9  is lower than the level of the pulse noise S 7 , each time the pulse noise S 7  enters, the gate pulse S 10  is generated.  
      When the threshold S 9  increases and becomes equal to or higher than the level of the pulse noise S 7 , even if the pulse noise S 7  enters, the gate pulse S 10  is not generated. Therefore, when the pulse noise enters continuously, although the output signal S 12  from which noise is eliminated by interpolation is output from the gate  34  initially, and after lapse of some time, the demodulated signal S 11  of the detector  26  is output as the output signal S 12  from the gate  34 . By such operation, at the time of continuous noise, the gate  34  continuously operates to thereby prevent the output signal such as the sound signal from being interrupted.  
      As described above, in the first embodiment, the threshold generator  32  generates the threshold S 9  on the basis of the pulse noise S 7  detected by the noise detector  31 . When the pulse noise S 7  exceeds the threshold S 9 , the gate  34  inhibits outputting of the demodulated signal S 11  of the detector  26  by the gate pulse S 10  generated by the gate pulse generator  33  and interpolates signals during the period. Thus, pulse noise can be eliminated.  
     Second Embodiment  
       FIG. 5  is a block diagram showing the configuration of a main portion of a noise eliminating apparatus of a second embodiment. As shown in  FIG. 5 , the second embodiment is similar to the first embodiment except that a holding circuit  35  and a switch  36  are provided between the threshold generator  32  and the gate pulse generator  33 . The holding circuit  35  retains the threshold S 9  output from the threshold generator  32 . The switch  36  switches between the threshold S 9  output from the threshold generator  32  and a threshold S 13  held in the holding circuit  35  and supplies the selected threshold to the gate pulse generator  33 . The other configuration is the same as that of the first embodiment. The same reference numerals are designated to the same components as those of the first embodiment and their description will not be repeated.  
       FIG. 6  is a circuit diagram conceptually showing the configuration of the holding circuit  35  and the switch  36 . As shown in  FIG. 6 , the holding circuit  35  comprises a D flip flop  351 . To a data terminal D of the D flip flop  351 , the threshold S 9  output from the threshold generator  32  is supplied. By using the rising edge of the gate pulse S 10  as a trigger, the D flip flop  351  latches the threshold supplied to the data terminal D at the time. The D flip flop  351  holds the latched threshold and continuously outputs it from an output terminal Q until the next rising edge of the gate pulse S 10  is input.  
      As shown in  FIG. 6 , the switch  36  comprises two AND gates  361  and  362  and an OR gate  363 . The first AND gate  361  outputs the threshold S 9  output from the threshold generator  32  to the OR gate  363  in the period after the gate pulse S 10  becomes the high level until the gate pulse S 10  becomes the high level again. The second AND gate  362  outputs the threshold S 13  output from the holding circuit  35  to the OR gate  363  in the period after the gate pulse S 10  becomes the high level until the gate pulse S 10  becomes the low level.  
      The OR gate  363  supplies, as a new threshold S 14 , either the threshold S 9  supplied from the first AND gate  361  or the threshold S 13  supplied from the second AND gate  362  to the comparator  331  of the gate pulse generator  33 . The comparator  331  outputs the gate pulse S 10  on the basis of the threshold S 14  supplied from the OR gate  363 .  
      The operation of the second embodiment will now be described. Since the operations of the receiving device  2 , noise detector  31 , threshold generator  32 , and gate  34  are the same as those of the first embodiment, their description will not be repeated.  FIG. 7  is a waveform chart showing waveforms of the output signal (pulse noise) S 7  of the noise detector  31 , output signal (threshold) S 9  of the threshold generator  32 , output signal (new threshold) S 14  of the switch  36 , output signal (gate pulse) S 10  of the gate pulse generator  33 , demodulated signal S 11  of the detector  26 , and output signal S 12  of the gate  34 .  
      In a normal state where there is no pulse noise, the gate pulse S 10  is not output from the gate pulse generator  33 . Therefore, the threshold S 9  output from the multiplier  322  of the threshold generator  32  is supplied as it is from the switch  36  to the comparator  331  via the first AND gate  361  and OR gate  363 . At this time, there is no output from the second AND gate  362  to the OR gate  363 .  
      When the pulse noise S 7  exceeding the threshold S 9  enters the comparator  331 , the gate pulse S 10  is output from the comparator  331 . Synchronously with the rising edge of the gate pulse S 10 , the D flip flop  351  latches the threshold S 9  output from the multiplier  322 . The D flip flop  351  then continuously outputs the latched threshold S 13  to the second AND gate  362 .  
      The second AND gate  362  outputs the threshold S 13  supplied from the D flip flop  351  to the OR gate  363  until the gate pulse S 10  becomes the low level again. At this time, there is no output from the first AND gate  361  to the OR gate  363 . Therefore, in the period after the gate pulse S 10  becomes the high level until the gate pulse S 10  becomes the low level, the OR gate  363  supplies the threshold S 9  at the moment when the gate pulse S 10  becomes the high level, the threshold S 9  being the new threshold S 14  to the comparator  331 .  
      Specifically, in the period after the gate pulse S 10  becomes the high level and until the gate pulse S 10  becomes the low level, as described in the first embodiment, the threshold S 9  output from the threshold generator  32  increases with lapse of time. To the comparator  331 , however, the new threshold S 14  as a constant value is supplied. When the peak value of the pulse noise S 7  decreases and becomes equal to or lower than the new threshold S 14 , the gate pulse S 10  is not output. By such operation, the gate pulse S 10  is output from the comparator  331  until the peak value of the pulse noise S 7  sufficiently decreases.  
      Therefore, a situation can be prevented such that time to generate the gate pulse S 10  is shortened due to increase in the threshold S 9  and noise remains in the output signal S 12  of the gate  34 . In the case where the holding circuit  35  and switch  36  are not provided, as shown in  FIG. 8 , the threshold S 9  becomes high level during the time when the pulse noise S 7  enters. Consequently, time to generate the gate pulse S 10  becomes shorter than time in which the pulse noise S 7  occurs, so that part of the noise may remain in the output signal S 12  of the gate  34  as shown by reference numeral  4  in  FIG. 8 .  
       FIG. 9  is a waveform chart showing waveforms of the output signal (pulse noise) S 7  of the noise detector  31 , output signal (threshold) S 9  of the threshold generator  32 , output signal (new threshold) S 14  of the switch  36 , output signal (gate pulse) S 10  of the gate pulse generator  33 , demodulated signal S 11  of the detector  26 , and output signal S 12  of the gate  34  in the case where pulse noise continues. As shown in  FIG. 9 , when the threshold S 9  is lower than the level of the pulse noise S 7 , the gate pulse S 10  is generated each time the pulse noise S 7  enters.  
      When pulse noise continues, in a manner similar to the first embodiment, the threshold S 9  gradually increases. With this increase, the output signal (new threshold) S 14  of the switch  36  also gradually increases. However, while the gate pulse S 10  is generated, the new threshold S 14  is constant. When the threshold S 9  increases and becomes equal to or higher than the level of the pulse noise S 7 , even if the pulse noise S 7  enters, the gate pulse S 10  is not generated. By such operation, the gate  34  continuously operates at the time of continuous noise, and a situation can be prevented such that the output signal such as sound signal is interrupted.  
      As described above, in the noise eliminating apparatus of the second embodiment, when the gate pulse S 10  is generated due to entry of the pulse noise S 7 , the threshold S 9  at the moment when the gate pulse S 10  is generated is held by the holding circuit  35 . While the gate pulse S 10  is generated by the switching of the switch  36 , the threshold S 13  held by the holding circuit  35  is used as the new threshold S 14 , so that precision of time to generate the gate pulse S 10  improves largely. Therefore, part of the noise can be prevented from remaining.  
      In the foregoing embodiments, the receiver  1  is described as a receiver used for AM broadcast. However, the receiver  1  of the embodiment is not limited to AM broadcast but can be used as a receiver for FM broadcast, television broadcast or other radio receivers. The receiving method of the receiver  1  of the embodiment is not limited to the super heterodyne method. The noise eliminating apparatus  3  of the embodiment is not limited to a radio receiver but can be used to eliminate pulse noise in an apparatus that performs various signal processes.  
      The invention may be embodied in other specific forms without departing from the spirit thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.  
      The entire disclosure of Japanese Patent Application No. 2003-435491 filed on Dec. 26, 2003 including the specification, claims, drawings and abstract is incorporated herein by reference in its entirety.