Abstract:
A computer-implemented method for reducing a noise signal added to an amplitude modulated (AM) broadcast signal while travelling from a broadcasting antenna to a receiving antenna is provided. The method includes capturing a signal representative of the AM broadcast signal corrupted by the noise signal via the receiving antenna, inverting the captured signal, and determining a carrying frequency of the AM broadcast signal and delaying the inverted waveform by a fraction of a cycle of the carrying frequency. The method further includes generating a difference signal by subtractively combining the captured signal and the delayed inverted signal, generating an estimate noise signal by reducing an amplitude of the generated difference signal using a noise-reduction control multiplier, and minimizing the corrupting noise signal component of the captured signal by subtractively combining the captured signal and the generated estimate noise signal.

Description:
BACKGROUND 
     Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. 
     Amplitude modulation (AM) broadcasting is a process of radio broadcasting that was the first method of impressing sound on a radio signal and is still widely used today. As known to one ordinary skill in the art, AM broadcasting signal has low immunity from interfering signals. As shown in  FIG. 1 , during an AM signal travel from a broadcasting antenna tower  102  to an AM receiver antenna  104  coupled to an AM broadcast receiving device or apparatus  106 , many possible noise signals may become add-on or interference signals to the original AM signal. These interference noise signals can be generated by a number of sources, such as power-line noise, lightning, other wireless communications, etc. . . . . These interference noise signals are captured together with the AM broadcast signal by the receiver circuit to become an in-band noise. 
     In the case, for example, when AM broadcast receiving apparatus  106  is installed in a car, electrical motor noise and electromagnetic interferences generated by the car&#39;s electrical circuits/devices may increase the noise interference to the original AM broadcast signal. 
     Therefore, there is a need for a system and method that can help minimize AM broadcast interferences caused by noise signals. 
     SUMMARY 
     Disclosed herein are improved a method and system for reducing AM noise in AM broadcast signals. 
     In one aspect, a computer-implemented method for reducing a noise signal added to an amplitude modulated (AM) broadcast signal while travelling from a broadcasting antenna to a receiving antenna is provided. The method includes capturing a signal representative of the AM broadcast signal corrupted by the noise signal via the receiving antenna, inverting the captured signal, and determining a carrying frequency of the AM broadcast signal and delaying the inverted waveform by a fraction of a cycle of the carrying frequency. The method further includes generating a difference signal by subtractively combining the captured signal and the delayed inverted signal, generating an estimate noise signal by reducing an amplitude of the generated difference signal using a noise-reduction control multiplier, and minimizing the corrupting noise signal component of the captured signal by subtractively combining the captured signal and the generated estimate noise signal. 
     In another aspect, the computer-implemented method further includes filtering captured signal prior to the signal inversion. 
     In another aspect, the computer-implemented method further includes processing the captured signal through a low noise amplifying unit. 
     In another aspect, the computer-implemented method further includes processing the captured signal through an analog to digital converting unit to generate a digital version of the captured signal prior to the signal inversion. 
     In another aspect, the noise-reduction control multiplier is equal to a rational number 1/n with n being a number that is greater than a first value equal to about one (1) and is less than a second value equal to about two (2). 
     In another aspect, a computer readable storage medium having stored therein instructions executable by a computing element to cause the computing element to perform the above-introduced method. 
     These as well as other aspects, advantages, and alternatives will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings. Further, it should be understood that the disclosure provided in this summary section and elsewhere in this document is intended to discuss the embodiments by way of example only and not by way of limitation. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       In the figures: 
         FIG. 1  is a schematic diagram illustrating an embodiment of an AM broadcast signal corrupted by a number of interfering signals and captured by a receiver antenna; 
         FIGS. 2A-B  are two graphs illustrating an uncorrupted AM broadcast signal and one of its period that has been inverted and delayed by a half-cycle; 
         FIG. 3  is a graph illustrating an AM broadcast signal with a predetermined amplitude modulation on a signal carrier; 
         FIG. 4  is a graph illustrating a zoomed section of the AM broadcast signal of  FIG. 3 ; 
         FIG. 5  is a graph illustrating a near-symmetrical characteristics of an upper half-cycle and of an inverted lower half-cycle of a waveform cycle of the zoomed signal section of  FIG. 4 ; 
         FIG. 6  is a block diagram illustrating an exemplary embodiment of a system, that includes an analog signal processing unit, for reducing AM noise captured by an AM receiver; 
         FIG. 7  is a flow chart illustrating an example embodiment of a method for reducing AM noise using the analog signal processing unit of  FIG. 6 ; 
         FIG. 8  is a block diagram illustrating an exemplary embodiment of a system, that includes a digital signal processing unit, for reducing an in-band AM noise signal captured by an AM receiver; 
         FIG. 9  is a flow chart illustrating an example embodiment of a method for reducing AM noise using the digital signal processing unit of  FIG. 8 ; 
         FIG. 10  is a block diagram illustrating another exemplary embodiment of a system, that includes another digital signal processing unit, for reducing an in-band AM noise signal captured by an AM receiver; 
         FIG. 11A-C  are three graphs that illustrate a corrupted AM broadcast signal, and a demodulated noise signal that corrupted the AM broadcast signal; 
         FIGS. 12A-C  are three graphs that illustrate the corrupted AM broadcast signal of  FIG. 4A  after a reduction of the demodulated noise signal of  FIG. 4C , which has been achieved with a value of an adaptive control factor selected by one of the corresponding systems shown in  FIGS. 2 and 3 ; 
         FIGS. 13A-C  are three graphs that illustrate the corrupted AM broadcast signal of  FIG. 4A  after another reduction of the demodulated noise signal of  FIG. 4C , which has been achieved with another value of the adaptive control factor selected by one of the corresponding systems shown in  FIGS. 2 and 3 ; 
         FIG. 14  is a graph illustrating an embodiment of another uncorrupted AM broadcast signal; 
         FIG. 15  is a graph illustrating the AM broadcast signal of  FIG. 14  as corrupted by a couple of interfering signals; 
         FIG. 16  is a graph illustrating a composite of the signals interfering the AM broadcast signal of  FIG. 15 ; 
         FIG. 17  is a graph illustrating an embodiment of an AM broadcast signal output by one of systems of  FIGS. 6 ,  8 , and  10  after reduction of the interfering signals of  FIG. 16 ; 
         FIG. 18  is a graph illustrating an embodiment of an AM broadcast signal output by one of systems of  FIGS. 6 ,  8 , and  10  after reduction of the interfering signals of  FIG. 16 ; and 
         FIG. 19  is a schematic drawing illustrating a computing network system according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying figures, which form a part hereof. In the figures, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, figures, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein. 
     Overview 
     Some conventional noise suppression systems are known to use a noise generator coupled to a noise canceller. One such noise suppression system may include a tuner configured to selectively receive a radio wave signal and to transform it into an electric signal, a field information detector to detect electric field information of the radio wave signal received by the tuner, a noise data generator that generate a noise pattern on the basis of the detected electric field information, a noise canceler configured to remove a noise component from the signal outputted from the tuner on the basis of the noise pattern generated by the noise data generator. However, these noise data generators are known to lack the accuracy to generate a noise signal that can be considered a substantial reproduction of the captured noise signal. 
     Accordingly, an embodiment of the proposed noise reducing method is configured to process and analyze “near-symmetric” characteristics of a received AM broadcast signal. As such, the proposed method is configured to produce noise signals that are substantially similar to the original add-on noise signals. The reproduced noise signals are then used to cancel substantially all or at least the majority of the add-on noise signals before the AM de-modulation process of the received AM broadcast signal. 
     As known to one of ordinary skill in the art, in telecommunications, a carrier wave or carrier is a waveform (usually sinusoidal) that is modulated (modified) with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal. The purpose of the carrier is usually either to transmit the information through space as an electromagnetic wave (as in radio communication), or to allow several carriers at different frequencies to share a common physical transmission medium by frequency division multiplexing (as, for example, a cable television system). 
     Now referring to  FIG. 2A , an exemplary embodiment  200  of a perfect sinusoidal waveform  202  is illustrated. As an example, waveform  202  represents un-modulated AM carrier waveform at 300 KHz without interference. As shown, waveform  202  is a smooth repetitive oscillating waveform with a periodically constant amplitude, i.e., peak deviation from zero. As shown in  FIG. 2A , waveform  202  includes a positive peak A  204  and a negative peak B  206 . Because waveform  202  is a perfect sine wave, if a half cycle delay is applied to the waveform  202  then, as shown in  FIG. 2B , peak A becomes peak B and peak B becomes peak A, i.e., A=−B. That is, waveform  202  at peak A is the same as at inverted peak B with a half carrier cycle delay. Accordingly, peak A and peak B are considered to be symmetrical with respect to waveform  202 . 
     Now referring to  FIG. 3 , an exemplary embodiment  300  of an AM broadcast signal waveform  302  with a predetermined amplitude modulation on a signal carrier waveform (not shown) is illustrated. As an example, AM broadcast waveform  302  has a frequency of 1.5 KHz and a 95% amplitude-modulation on the waveform carrier with a 300 KHz frequency. 
     Now referring to  FIG. 4 , a waveform  402  representing a zoomed-in section  304  of the waveform carrier of  FIG. 3  is shown. Zoomed-in section  304  corresponds to a waveform section associated with time points T1 and T2, which are close to about 3×10 −4  seconds and about 4×10 −4  seconds, respectively. 
     Now referring to  FIG. 5 , a waveform  502  representing a zoomed-in section  404  of waveform  402  of  FIG. 4  is shown. The zoomed section corresponds to a waveform section associated with time points T3 and T4, which are equal to about 374×10 −6  seconds and about 390×10 −4  seconds, respectively. As shown in  FIG. 5 , waveform  502  includes an upper cycle peak “C” that has a magnitude equal to +4.578062, and an adjacent lower cycle peak “D” that has a magnitude equal to −4.81467. As such, upper cycle peak “C” is close to but not exactly the same as “inverted lower cycle peak “D.” Thus, waveform  502  is a “Near Symmetrical” waveform. As known to one of ordinary skill in the art, a lower modulation index (%) leads to a more symmetrical waveform. Further, a higher audio and carrier frequency ratio leads to a more symmetrical waveform. Also, a lower modulation frequency leads to a more symmetrical waveform. 
     Now referring to  FIG. 6 , a schematic diagram  600  illustrates an exemplary embodiment of an analog system  602  for reducing noise signals added to an AM broadcast signal. As shown, system  602  includes an antenna  604  for capturing an AM broadcast signal  606  augmented with add-on noise signals  608  and  610 . Captured AM broadcast signal  606  is a signal based on airwaves transmitted from a broadcasting station (not shown). System  602  further includes a cable unit  612  for communicating AM broadcast signal  606  to a filter and low-noise amplifier combination unit  614 , hereafter referred to as F&amp;LNA unit  614 , and an analog signal processing unit  616  for AM noise reduction. In one embodiment, the filter of F&amp;LNA unit  614  can be a two pole bandpass filter. As shown in  FIG. 6 , analog signal processing unit  616 , hereafter referred to as analog AM noise reducing unit, includes a signal inverting unit  618 , a signal delaying unit  620 , a signal subtracting and reducing unit  622 , and a signal subtracting unit  624 . 
     Now referring to  FIG. 7 , a flow chart  700  illustrates an example embodiment of a method for reducing/minimizing add-on noises using analog AM noise reducing unit  616 . During operation, upon initiation of the method at step  701 , F&amp;LNA unit  614  processes AM broadcast signal  606  to output AM signal  607 . At step  702 , AM noise reducing unit  616  is configured to provide AM signal  607  to signal inverting unit  618 . Upon receipt of AM signal  607 , signal inverting unit  618  processes it to output inverse AM signal  609 , at step  704 . Then at step  706 , AM noise reducing unit  616  provides AM signal  609  to signal delaying unit  620  that is configured to delay AM signal  609  by about a half carrier cycle and to output resulting AM signal  611 . Subsequently, AM noise reducing unit  616  provides both AM signal  607  and AM signal  611  to signal subtracting and reducing unit  622 , which proceeds to subtractively combine them, at step  708 , and to change an amplitude of the resulting difference signal by multiplying it with a rational number that is less than or equal to one (1), at step  710 . This rational number can be selected to be equal to about 1/n where n satisfies the following inequality: 1≦n≦2. In accordance with one embodiment, the reduced difference signal  613  represents a generated or re-produced noise signal that is substantially similar to combined add-on noise signals  608  and  610 . Then, at step  712 , AM noise reducing unit  616  provides both AM signal  607  and reduced difference signal  613  to signal subtracting unit  624 , which is configured to subtractively combine them and output an AM noise-reduced signal  615 , which is desirably substantially similar to AM broadcast signal  606 . 
     Based on experimental results, AM noise reducing unit  616  substantially reduces add-on noise signals  608  and  610  when n is close to 2. Moreover, an optimal control value of n can be determined adaptively by this noise reduction approach during an on-going processing of AM broadcast signal  606 . This optimal control value of n represents a value that best minimizes add-on noise signals  608  and  610 . 
     Now referring to  FIG. 8 , a schematic diagram  800  illustrates an exemplary embodiment of a digital system  802  for reducing noise signals added to an AM broadcast signal. As shown, system  802  includes an antenna  804  for capturing an AM broadcast signal  806  augmented with add-on noise signals  808  and  810 . System  802  further includes a cable unit  812  for communicating captured AM broadcast signal  806  to a filter and low-noise amplifier combination unit  814 , hereafter referred to as F&amp;LNA unit  814 , an analog to digital (A/D) signal converting unit  819 , and a digital signal processing unit  816  for AM noise reduction. As discussed above, the filter of F&amp;LNA unit  814  can be a two pole bandpass filter. As shown in  FIG. 8 , analog signal processing unit  816 , hereafter referred to as digital AM noise reducing unit, includes a signal inverting unit  818 , a signal delaying unit  820 , a signal subtracting and reducing unit  822 , a delay compensation unit  823 , a signal subtracting unit  824 , an AM demodulating unit  826 , an error control calibration unit  828 , and a digital to analog (D/A) converting unit  830 . 
     Now referring to  FIG. 9 , a flow chart  900  illustrates an example embodiment of a method for reducing/minimizing add-on noises using digital AM noise reducing unit  816 . During operation, upon initiation of the method at step  901 , F&amp;LNA unit  814  processes AM broadcast signal  806  to output AM signal  807 . At step  902 , A/D signal converting unit  819  is configured to convert AM signal  807  to a digital signal  809 . AM noise reducing unit  816  is configured to provide AM digital signal  809  to signal inverting unit  818 , at step  904 . Upon receipt of AM digital signal  809 , signal inverting unit  818  processes it to output inverse AM digital signal  811 , at step  906 . Then, AM noise reducing unit  816  provides AM digital signal  811  to signal delaying unit  820  that is configured to delay AM digital signal  811  by about a half carrier cycle and to output resulting AM signal  813 , at step  908 . Subsequently, AM noise reducing unit  816  provides both AM signal  807  and AM signal  813  to signal subtracting and reducing unit  822 , which proceeds to subtractively combine them, at step  910 , and to change an amplitude of the resulting difference signal by multiplying it with a rational number that is less than or equal to one (1), at step  912 . As discussed above, alternatively, the rational number can be selected to be equal to 1/n where n satisfies the following inequality: 1≦n≦2. In accordance with one embodiment, the reduced difference signal  815  represents a re-produced noise signal that is desirably substantially similar to combined add-on noise signals  808  and  810 . Then, at step  914 , AM noise reducing unit  816  provides AM signal  809  to delay compensation unit  823 , which is configured to apply a compensating time delay to AM signal  809 , and output AM delay-compensated signal  817 . Subsequently, at step  916 , AM noise reducing unit  816  is configured to provide both AM delay-compensated signal  817  and reduced difference signal  815  to signal subtracting unit  824 , which is configured to subtractively combine them and output an AM noise-reduced signal  819 , which is substantially similar to AM broadcast signal  806 . Further, at step  918 , AM noise-reduced signal  819  is demodulated by AM demodulating unit  826 , and the resulting demodulated signal  821  is provided to D/A converting unit  830  that converts it into an analog waveform prior to being outputted as an audio signal by a receiving speaker (not shown). 
     During this noise-reducing process, error control and calibration unit  828  is recruited to analyze demodulated signal  819  and use results of the analysis to adjust as needed the rational number 1/n that is used by signal subtracting and reducing unit  822  in order to improve on the minimization of add-on noise signals  808  and  810 . 
     Now referring to  FIG. 10 , a schematic diagram  800  illustrates another exemplary embodiment of a digital system  1002  for reducing noise signals added to an AM broadcast signal. Digital system  1002  has substantially similar components as those of digital system  802 , except that F&amp;LNA unit  1014  further includes a radio processing unit and error control and calibration unit  1028  is further coupled to signal delaying unit  1020 . In this configuration of Digital system  1002 , F&amp;LNA unit  1014  is configured to identity an intermediate frequency (IF) of AM broadcast signal  1006 , to extract from it a signal, denoted IF signal  1007  having the identified intermediate frequency as its main frequency. In one embodiment, the coupling of error control and calibration unit  1028  to signal delaying unit  1020  serves to control the signal delaying process to further improve on the noise reduction process. That is, based on input received from error control and calibration unit  1028 , signal delaying unit  1020  adaptively adjusts an amount of signal delay that can be different from a half carrier cycle delay and still leads to a better minimization of add-on noise signals  808  and  810 . 
     Now referring to  FIGS. 11A-C , three graphs are shown that illustrate a corrupted AM broadcast signal  1102 , a zoomed section  1104  of AM broadcast signal  1102 , and an add-on noise signal  1106  that corrupted AM broadcast signal  1102 .  FIG. 11A  illustrates AM broadcast signal  1102  that was selected to represent AM broadcast signal waveform  302  of  FIG. 3  corrupted with add-on noise signals. A zoomed section of AM broadcast signal  1102  is illustrated in  FIG. 11B . Subsequent to processing AM broadcast signal  1102  using any one of noise reducing systems  602 ,  802 , and  1002 , the add-noise signal  1106  corresponding to the zoomed  1104  section is substantially determined. 
     During a noise reduction process using any one of noise reducing systems  602 ,  802 , and  1002 , and selecting adaptive control factor “n” to be equal to 2.0,  FIG. 12A  illustrates a resulting AM broadcast signal  1202  that represents AM broadcast signal  1102  with the reduced add-on noise signal  1106 .  FIG. 12B  illustrates the zoomed section of AM broadcast signal  1102  shown in  FIG. 11B  after the noise reduction, and  FIG. 12C  illustrates the reduced version of add-on noise signal  1106 . 
     To further reduce add-on noise signal  1106 , noise reducing systems  602 ,  802 , and  1002  are configured to adaptively vary the value of adjusting control factor n. As such, based on a continuous analysis of outputted noise-reduced AM signals, adjusting control factor n was selected to be equal to 1.5, which lead to a further reduction of add-on noise signal  1106  as illustrated in a further smoother waveform of AM broadcast signal  1102 , and a further reduced amplitude-wise of add-on noise signal  1106 , shown in  FIGS. 13A and 13C . 
     Now referring to  FIG. 14 , a graph  1400  illustrates an embodiment of an uncorrupted AM broadcast signal  1402  provided with a substantially perfect signal modulation. As an example, AM broadcast signal  1402  has a frequency of 1.7 KHz and is amplitude-modulated by a 300 KHz waveform carrier (not shown). During its broadcast travel, AM broadcast signal  1402  is corrupted by a couple of add-on noise signals. These interfering noise signals are both frequency modulated (FM) signals having frequencies equal to 3.33 KHz and 2.0 KHz, respectively, whose composite signal is illustrated by waveform  1602  of  FIG. 16 . The corrupted version of AM broadcast signal  1402  is illustrated by waveform  1502  of  FIG. 15 . By processing the corrupted version of AM broadcast signal  1402  using any one of noise reducing systems  602 ,  802 , and  1002 , a noise-reduced signal version of AM broadcast signal  1402  is generated as illustrated by waveform  1702 , shown in  FIG. 17 . The removed distorting component of waveform  1502  is illustrated by waveform  1802  of  FIG. 18 . 
     In one embodiment, each of noise reducing systems  602 ,  802 , and  1002  include a processing unit and a memory unit. Each of the processing units can be implemented on a single-chip. For example, various architectures can be used including dedicated or embedded microprocessor (μP), a microcontroller (μC), or any combination thereof. Each of the memory units may be of any type of memory now known or later developed including but not limited to volatile memory (such as RAM), non-volatile memory (such as ROM, flash memory, etc.) or any combination thereof, which may store software that can be accessed and executed by the processing units, for example. Each of the memory units are configured to store instructions that correspond to the processing functions of the above discussed noise reducing systems. 
     In some embodiments, the disclosed method may be implemented as computer program instructions encoded on a non-transitory computer-readable storage media in a machine-readable format.  FIG. 19  is a schematic illustrating a conceptual partial view of an example computer program product  1900  that includes a computer program for executing a computer process on a computing device, arranged according to at least some embodiments presented herein. In one embodiment, the example computer program product  1900  is provided using a signal bearing medium  1901 . The signal bearing medium  1301  may include one or more programming instructions  1902  that, when executed by one or more processors may provide functionality or portions of the functionality described above with respect to  FIGS. 7 and 9 . Thus, for example, referring the embodiments shown in  FIGS. 7 and 9 , one or more features of blocks  702 ,  704 ,  706 ,  708  and/or  710  and  902 ,  904 ,  906 ,  908 ,  910  and/or  912 , respectively, may be undertaken by one or more instructions associated with the signal bearing medium  1901 . 
     In some examples, the signal bearing medium  1901  may encompass a non-transitory computer-readable medium  1903 , such as, but not limited to, a hard disk drive, a Compact Disc (CD), a Digital Video Disk (DVD), a digital tape, memory, etc. In some implementations, the signal bearing medium  1901  may encompass a computer recordable medium  1904 , such as, but not limited to, memory, read/write (R/W) CDs, R/W DVDs, etc. In some implementations, the signal bearing medium  1901  may encompass a communications medium  1905 , such as, but not limited to, a digital and/or an analog communication medium (e.g., a fiber optic cable, a waveguide, a wired communications link, a wireless communication link, etc.). 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims, along with the full scope of equivalents to which such claims are entitled. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.