Abstract:
An audio signal processing circuit reduces noise in an incoming audio signal and is particularly useful in telephone communication systems utilizing one or more hands-free microphones. A preferred embodiment of the audio signal processing circuit includes a pre-emphasis circuit receiving the audio signal from a microphone or other transducer, an amplifier circuit receiving the pre-emphasized audio signal from the pre-emphasis circuit and a de-emphasis circuit receiving the amplified signal from the amplifier circuit. An output of the de-emphasis circuit provides the processed audio signal having an improved signal to noise ratio with minimum audible distortion. A preferred embodiment of the amplification circuit includes an amplifier defining a non-linear transfer function therethrough which provides low gain to the lower amplitude noise signals and higher gain to the higher amplitude audio signals.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to audio signal processing circuits, and more specifically to audio signal processing circuits for reducing noise in audio signals.  
       BACKGROUND OF THE INVENTION  
       [0002]     Telephone communications are made easier and sometimes safer through the use of so-called “hands-free” microphones. Such microphones are typically located remote from the talker or talkers and thus pick up many times more noise and echo than with typical a hand-held unit. This problem is typically compounded in cellular telephone communications systems located in a moving vehicle due to added sources of noise such as road noise, for example.  
         [0003]     Noise reduction units are known and are operable to enhance the sound quality and intelligibility of voice recognition systems by providing up to 15-20 dB of effective noise reduction. However, known noise reduction systems have drawbacks associated therewith. For example, some such noise reduction systems using digital signal processing technology create an artificial quality to the reproduced voice and are costly to implement. Known dynamic noise reduction (DNR) circuits, on the other hand, do not appear to work as well in very noisy environments, as “fuzzballs” of modulated noise tend to appear in the received signal.  
         [0004]     What is therefore needed is an improved noise reduction system operable to enhance sound quality and intelligibility in telephone communication systems employing hands-free microphones, wherein such a noise reduction system is inexpensive and easy to implement, and provides a significant improvement in the signal to noise ratio while minimizing audible distortion.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention addresses the foregoing drawbacks of prior art distortion limiting amplifiers. In accordance with one aspect of the present invention, a signal processing circuit for reducing noise in an audio signal comprises a pre-emphasis circuit having an input receiving an audio signal and an output providing a pre-emphasized representation of the audio signal, the audio signal including a higher amplitude audio component and a lower amplitude noise component, an amplifier including a first input coupled to the pre-emphasis circuit output and an output, the amplifier defining a smooth transfer function providing higher gain to the higher amplitude audio component and lower gain to the noise component of the audio signal, and a de-emphasis circuit having an input connected to the amplifier output and an output providing a de-emphasized representation of the audio signal processed by the amplifier in accordance with the transfer function.  
         [0006]     In accordance with another aspect of the present invention, an amplifier circuit for reducing noise in an audio signal comprises a non-inverting input receiving an audio signal, the audio signal including a higher amplitude audio component and a lower amplitude noise component, an inverting input, an output, and feedback circuitry connected between the output and the inverting input. The feedback circuit defines an amplifier transfer function having a smooth non-linear characteristic providing higher gain to the higher amplitude audio component and lower gain to the lower amplitude noise component.  
         [0007]     In accordance with a further aspect of the present invention, an amplifier circuit for reducing noise in an audio signal comprises a non-inverting input connected to ground potential, an inverting input, an output, and feedback circuitry connected between the output and the inverting input, wherein the feedback circuitry receives an audio signal including a higher amplitude audio component and a lower amplitude noise component. The feedback circuitry defines an amplifier transfer function having a smooth non-linear characteristic providing higher gain to the higher amplitude audio component and lower gain to the lower amplitude noise component.  
         [0008]     One object of the present invention is to provide an audio signal processing circuit providing a non-linear gain proportional to signal amplitude.  
         [0009]     Another object of the present invention is to provide such a circuit wherein the non-linear gain is smooth and bi-directional to thereby minimize audible distortion of the processed audio signal.  
         [0010]     Yet another object of the present invention is to provide such a circuit further having the audio signal pre-processed by a pre-emphasis circuit and post-processed by a de-emphasis circuit to thereby further minimize audible distortion of the processed audio signal.  
         [0011]     These and other objects of the present invention will become more apparent from the following description of the preferred embodiment.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a block diagram illustrating one embodiment of the audio signal processing circuit of the present invention.  
         [0013]      FIG. 2  is a plot illustrating a typical audio signal including noise components prior to processing by the circuit of  FIG. 1 .  
         [0014]      FIG. 3  is a schematic diagram illustrating one embodiment of the non-linear gain circuit shown in  FIG. 1 , in accordance with the present invention.  
         [0015]      FIG. 4  is a schematic diagram illustrating another embodiment of the non-linear gain circuit shown in  FIG. 1 , in accordance with the present invention.  
         [0016]      FIG. 5  is a plot illustrating the general shape of the transfer function of either of the gain circuits of  FIG. 3  or  4 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated devices, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.  
         [0018]     Referring now to  FIG. 1 , a block diagram illustrating one embodiment of an audio signal processing circuit  10 , in accordance with the present invention, is shown. Circuit  10  includes a microphone  12  or other known transducer operable to convert a voice or other audible sound to an audio signal  14 . A known pre-emphasis circuit  16  has an input connected to microphone  12  via signal path  18  and an output connected to an input of a non-linear gain circuit  20  via signal path  22 . Non-linear gain circuit  20  has an output connected to a known de-emphasis circuit  24  via signal path  26  and de-emphasis circuit  24  has an output providing the processed audio signal on signal path  28 , which is labeled SIGNAL OUT in  FIG. 1 .  
         [0019]     Pre-emphasis circuit  16  and de-emphasis circuit  24  may be formed in accordance with known techniques and with known circuit components. It is known in the art that desired audio signals in the higher audio-frequency range typically have relatively low amplitude because they are harmonics of the fundamental tones. Therefore, it is desirable to pre-emphasize the amplitude of higher audio frequencies by increasing their relative values prior to amplification. Circuitry for providing such a pre-emphasis function is known and may be manifested as a high pass filter. The result of pre-emphasis, followed by subsequent de-emphasis, is to provide a processed signal having a higher signal to noise ratio. In order to restore the original relative amplitudes, the audio signal is de-emphasized after amplification by de-emphasis circuit  24 . Circuitry for providing such a de-emphasis function is known and may be manifested as a low pass filter.  
         [0020]     The present invention takes advantage of the fact that noise accompanying voice or other audible sound sensed by a suitable transducer is typically lower in amplitude than the desired audible components of the resultant signal. Referring to  FIG. 2 , this is illustrated by a representation of the audio signal  14  of  FIG. 1 , wherein signal  14  includes higher amplitude audio components  30  and lower amplitude noise components  32  during periods when the higher amplitude audio components  30  are not present. In accordance with the present invention, the amplification or gain circuit  20  is provided with a non-linear transfer function that offers low gain to the lower amplitude noise components  32  and higher gain to the higher amplitude audio components  30 . By suppressing noise during periods when the higher audio components  30  are not present, the signals processing circuit  10  of the present invention provides a subjective perceptual effect of continuous noise reduction.  
         [0021]     Referring now to  FIG. 3 , one preferred embodiment of a non-linear gain or amplification circuit  20 ′, which corresponds to the circuit block  20  of  FIG. 1 , is illustrated. Circuit  20 ′ includes an amplifier  40  of known construction, wherein amplifier  40  includes a non-inverting input  42  connected to signal path  22  ( FIG. 1 ) and receiving a signal V IN . When amplifier circuit  20 ′ is inserted into the signal processing circuit  10  of  FIG. 1 , the signal V IN  corresponds to the pre-emphasized audio signal provided by pre-emphasis circuit  16 . However, those skilled in the art will recognize that V IN  may be any communication signal wherein it is desirable to provide lower gain to low amplitude components and higher gain to higher amplitude components of the communication signal. In any case, amplifier  20 ′ also includes an inverting input  44  connected to a first end of a resistor R 1  and a first end of a feedback resistor R 2 . The opposite end of feedback resistor R 2  is connected to an output  46  of amplifier  40  and provides a signal V out . When amplifier circuit  20 ′ is inserted into the signal processing circuit  10  of  FIG. 1 , the signal V OUT  corresponds to the processed audio signal provided to de-emphasis circuit  24 . However, those skilled in the art will recognize that V OUT  may be alternatively provided to other audio signal processing circuits as part of a system wherein it is desirable to provide lower gain to low amplitude components and higher gain to higher amplitude components of a communication signal. The opposite end of resistor R 1  is connected to an anode of a first diode D 1  and to a cathode of a second diode D 2 . Diodes D 1  and D 2  may be of known construction, and the cathode of D 1  is connected to the anode of D 2  and also to ground potential.  
         [0022]     The transfer function of the amplifier circuit  20 ′ illustrated in  FIG. 3  is defined as the instantaneous gain of the circuit which is given by the equation:
 
instantaneous gain=1 +[R   2 /(diode resistance+ R   1 )]  (1),
 
 wherein the diode resistance is given by the equation:
 
diode resistance=[ ln (diode current*2.5×10 14 )/(40×diode current)]  (2).
 
         [0024]     Referring now to  FIG. 4 , an alternate embodiment of a non-linear gain or amplification circuit  20 ″, which corresponds to the circuit block  20  of  FIG. 1 , is illustrated. Circuit  20 ″ includes an amplifier  50  of known construction, wherein amplifier  50  includes a non-inverting input  52  connected to ground potential. An inverting input  54  is connected to one end of a first resistor R 1 , one end of a second feedback resistor R 2 , an anode of a first diode D 1  and a cathode of a second diode D 2 . The opposite end of feedback resistor R 2  is connected to an output  56  of amplifier  50  and provides a signal V out . When amplifier circuit  20 ″ is inserted into the signal processing circuit  10  of  FIG. 1 , the signal V OUT  preferably corresponds to the processed audio signal provided to de-emphasis circuit  24 . Alternatively, a capacitor C may be provided which has a first end connected to inverting input  54  and a second end connected to output  56 . The combination of the capacitor C and the feedback resistor R 2  forms a known de-emphasis circuit so that, if capacitor C is included as shown in  FIG. 4 , de-emphasis circuit  24  of  FIG. 1  is not needed. It will be further recognized by those skilled in the art that V OUT  may be alternatively provided to other audio signal processing circuits as part of a system wherein it is desirable to provide lower gain to low amplitude components and higher gain to higher amplitude components of a communication signal.  
         [0025]     The cathode of D 1  is connected to the anode of D 1  and to a first end of a third resistor R 3 . The opposite end of resistor R 3  is connected to signal path  22  ( FIG. 1 ) and receives a signal V IN . When amplifier circuit  20 ″ is inserted into the signal processing circuit  10  of  FIG. 1 , the signal V IN  corresponds to the pre-emphasized audio signal provided by pre-emphasis circuit  16 . However, those skilled in the art will recognize that V IN  may be any communication signal wherein it is desirable to provide lower gain to low amplitude components and higher gain to higher amplitude components of the communication signal.  
         [0026]     The transfer function of the amplifier circuit  20 ″ illustrated in  FIG. 4  is defined as the instantaneous gain of the circuit which is given by the equation:
 
instantaneous gain= R   2 /{1/[1/( R   3 +diode resistance)+1/ R   1 }  (3),
 
 wherein the diode resistance is given by equation (2). 
 
         [0028]     Referring now to  FIG. 5 , a plot of V OUT  vs V IN    60  is shown which represents the transfer function of either of the amplifier circuit embodiments shown in  FIG. 3  or  4 , wherein the transfer function for each is defined in accordance with equations (1), (2) and (3) as described above. From  FIG. 5 , it can be seen that the transfer function for either amplifier circuit  20 ′ or  20 ″ is a non-linear and smooth transfer function providing lower gain for lower amplitude signals and higher gain to higher amplitude signals. In one preferred embodiment, the transfer function illustrated by plot  60  provides a slope in the diode conduction regions of between approximately one and three, and hence provides a gain in these regions of between approximately one and three. Those skilled in the art will, however, recognize that plot  60  may be configured to provide for other desired gain values.  
         [0029]     Audible distortion of the audio signal  14  processed by the signal processing circuit  10  is minimized by the smooth low to high gain transition of the amplifier circuit transfer function  60  ( FIG. 5 ), the bi-directional nature of the transfer function  60  which cancels even-order harmonics and by the use of the pre-emphasis and de-emphasis circuits  16  and  24  respectively. Pre-emphasis also tends to flatten vehicle noise spectra and thus improve low-frequency noise rejection of signal processing circuit  10 .  
         [0030]     The following table contains typical values for some of the electrical components illustrated in  FIGS. 3 and 4 , although it should be understood that the present invention contemplates configuring amplifier circuits  20 ′ and  20 ″ with alternate component values.  
                                                     FIG.   COMPONENT   VALUE                                3   R1   100 kohms       3   R2   220 kohms       4   R1   220 kohms       4   R2   220 kohms       4   R3   100 kohms                  
 
         [0031]     The present invention is illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.