The present invention relates to a reducing apparatus and method for reducing multipath noise which is suffered in radio wave propagation in receiving radio waves which are transmitted after being modulated by a constant amplitude modulation method.
Conventionally, it is known that a multipath reducing apparatus 1 is used in reception of a radio wave being transmitting after being modulated by a constant amplitude modulation method in order to reduce multipath noise which the radio wave receives during its propagation. The multipath noise reducing apparatus 1 includes an adaptive digital filter (hereinafter, abbreviated as an “ADF”) 2. A receiving apparatus 3 comprising the ADF 2 as the multipath noise reducing apparatus provides a frequency modulation (FM) method or a phase modulation (PM) method, and is originally used for receiving a radio wave having a constant amplitude, or the like. A frequency-modulated radio wave or a phase-modulated radio wave is normally received with constant amplitude, but a change in amplitude is produced due to an effect of the multipath when the radio wave is received in a state where its path is separated into a plurality of paths by reflection or the like. In the receiving apparatus 3, a received signal is converted from an analog signal to a digital signal by an analog/digital converter (hereinafter, abbreviated as an “AD converter”) 4, and the multipath noise on the received signal is reduced by a digital signal processing in the ADF 2. The received signal having the multipath noise reduced in the ADF 2 is detected by a wave detector 5, and the modulated signal is demodulated.
A frequency-modulated radio wave, which is an FM broadcast wave transmitted using a frequency range of, for example, a very high frequency (VHF), is received by an antenna 6, and a radio frequency (RF) signal corresponding to a field strength is outputted. The radio frequency signal is processed in an analog state in an analog receiving processing portion 7 including an RF portion for a radio frequency amplification and a MIX portion for a frequency conversion, and is outputted as, for example, an intermediate frequency (IF) signal having about 10.7 MHz. The IF signal is converted into a digital signal by the AD converter 4 and is supplied to the ADF 2.
The ADF 2 includes a variable filter portion 8, a coefficient memory 9, a filter coefficient calculating unit 10, and an envelope calculating unit 11. In the ADF 2, an amplitude changed portion of the IF signal converted into a digital signal is detected by the envelope calculating unit 11, and a filter coefficient is calculated by the filter coefficient calculating unit 10 so as to correct the amplitude changed portion, and is stored in the coefficient memory 9. The variable filter portion 8 uses the filter coefficient stored in the coefficient memory 9 to remove an effect of the multipath to the IF signal.
The ADF 2 as shown in
A filter property capable of removing the multipath distortion can be obtained by as appropriate updating the filter coefficients a(n) and b(n). The filter coefficient updating portion updates the filter coefficients a(n) and b(n) according to the following expressions (3) and (4):
ak(n+1)=ak(n)−μe(n){xi(n−k−N)yi(n)+xq(n−k−N)yq(n)} (3)
bk(n+1)=bk(n)−μe(n){xi(n−k−N)yq(n)−xq(n−k−N)yi(n)} (4)
In the expressions (3) and (4), a constant μ is called a step size, and is for determining a degree of convergence of an adapting processing. An error e(n) is an error calculated in the envelope error calculating portion, and corresponds to an amplitude changed portion. The ADF 2 operates so as to minimize the envelope curve error, thus making the envelope curve of the IF signal constant. In the envelope curve error calculating portion, the envelope curve error e(n) is calculated using the following expression (5). The above-described operation of an adaptive digital filter is called an LMN algorithm.
e(n)=env−A
env=√{yi(n)2+yq(n)2} (5)
or
env=yi(n)2+yq(n)2
In a multipath removing filter using the above-described ADF, there is disclosed a technique in which a property of a reflected wave causing the multipath is detected and an initial setting is carried out depending on the detected property (refer to Japanese Examined Patent Publication JP-B2 3354286, for example).
In the multipath noise reducing apparatus 1 as shown in
(a) At a time of an overmodulation
At a time of the overmodulation, an amplitude change due to a modulating component is produced in a received signal. The ADF 2 detects the amplitude change and carries out a correcting operation so as to eliminate the amplitude change. Accordingly, a distortion is produced in a sound obtained by a wave detection, or the like, as described later. Further, it is assumed that a state of the overmodulation is caused by the following two factors.
(a1) At a time of inputting an overmodulated received wave, in which a frequency shift amount is increased so as to exceed a maximum bandwidth of the receiving apparatus itself.
(a2) At a time of taking an interference measure in which a bandwidth of the receiving apparatus is controlled so as to remove an interfering wave by an interference measure function of the receiving apparatus under a radio wave condition in which an interference wave is occurring other than a desired receiving wave, and thereby even the frequency shift amount which is not normally regarded as the overmodulation is increased more than the bandwidth.
(b) At a time of a weak electrical field
At a time of the weak electrical field in which a field strength of a received signal is weak, an amplitude change is produced in the received wave by an effect of white noise. The ADF 2 detects the amplitude change and operates, thereby producing a distortion in a sound.
(c) At a time of an antenna switching diversity reception
In order to improve a receiving property such as the multipath, a diversity receiving method is normally advantageous. However, when an antenna switching method is carried out when a diversity reception is carried out, an amplitude change of the received signal is produced in association with the antenna switching, thereby producing a side effect that a sound distortion is produced. The following two cases are assumed as a case in which the antenna switching causes an amplitude change.
(c1) Case in which a difference in a field strength is produced between the antennas to be switched.
(c2) Case in which at least one antenna to be switched has the multipath.
An object of the invention is to provide a reducing apparatus and method and a receiving apparatus, capable of receiving a radio wave signal in a comfortable manner by overcoming defects existing in technique and performance according to the related art related to reducing a multipath noise by use of an adaptive digital filter.
The invention provides a reducing apparatus comprising:
an adaptive digital filter for reducing an effect of a multipath on a received signal; and
a determining unit for determining whether a condition that the received signal is deteriorated by a change in coefficient is fulfilled or not,
the adaptive digital filter including:
In the invention, it is preferable that the determining unit determines whether an overmodulation is produced in the received signal or not.
In the invention, it is preferable that the determining unit includes a modulation degree detecting unit for detecting a modulation degree from a frequency shift amount of the received signal, and a comparing unit for comparing the modulation degree with a predetermined frequency band.
In the invention, it is preferable that the determining unit determines whether the overmodulation is produced or not, based on an interference measure against an interference wave.
In the invention, it is preferable that the reducing apparatus further comprises an interference wave detecting unit for detecting presence or absence of an interference wave, and a bandpass filter for regulating a passband of the received signal based on the interference wave detected by the interference wave detecting unit, and the determining unit includes a modulation degree detecting unit for detecting a modulation degree from a frequency shift amount of the received signal, and a comparing unit for comparing the modulation degree with the passband regulated by the bandpass filter.
In the invention, it is preferable that the determining unit determines whether the overmodulation is produced or not in the received signal, based on a signal changed depending on a time constant.
In the invention, it is preferable that the determining unit determines whether or not a field strength of the received signal is a weak electrical field or less.
In the invention, it is preferable that the determining unit includes an electrical field information obtaining unit for obtaining field strength information from the received signal, and a comparing unit for comparing whether the field strength information represents a weak electrical field or not.
In the invention, it is preferable that the determining unit determines presence or absence of an antenna switching in association with a diversity receiving operation.
In the invention, it is preferable that the determining unit determines whether exogenous noise is produced in the received signal or not.
The invention provides a reducing apparatus comprising:
an adaptive digital filter for reducing an effect of a multipath on a received signal;
an overmodulation determining unit for determining whether overmodulation is produced in the received signal or not; and
a weak electrical field determining unit for determining whether or not a field strength of the received signal is a weak electrical field or less,
the adaptive digital filter including:
In the invention, it is preferable that the overmodulation determining unit includes a modulation degree detecting unit for detecting a modulation degree from a frequency shift amount of the received signal, and a first comparing unit for comparing the modulation degree with a passband regulated by a bandpass filter, and
the weak electrical determining unit includes an electrical field information obtaining unit for obtaining field strength information from the received signal, and a second comparing unit for comparing whether the field strength information represents a weak electrical field or not.
The invention provides a receiving apparatus comprising:
an antenna for receiving a signal;
a converting unit for converting a received signal into a digital signal;
an adaptive digital filter for reducing an effect of a multipath on the received signal; and
a determining unit for determining whether a condition that the received signal is deteriorated by a change of a coefficient is fulfilled or not,
the adaptive digital filter including:
The invention provides a receiving apparatus comprising:
an antenna for receiving a signal;
a converting unit for converting a received signal into a digital signal;
an adaptive digital filter for reducing an effect of a multipath of the received signal; and
an overmodulation determining unit for determining whether an overmodulation is produced in the received signal or not; and
a weak electrical field determining unit for determining whether or not a field strength of the received signal is a weak electrical field or less,
the adaptive digital filter including:
In the invention, it is preferable that the overmodulation determining unit includes a modulation degree detecting unit for detecting a modulation degree from a frequency shift amount of the received signal, and a first comparing unit for comparing the modulation degree with a passband regulated by a bandpass filter, and the weak electrical determining unit includes an electrical field information obtaining unit for obtaining field strength information from the received signal, and a second comparing unit for comparing whether the field strength information represents a weak electrical field or not.
The invention provides a receiving apparatus for a mobile object comprising the reducing apparatus mentioned above.
In the invention, it is preferable that the receiving apparatus for a mobile object is used for an automotive receiving apparatus.
In the invention, it is preferable that the receiving apparatus for a mobile object is used for a portable receiving apparatus.
The invention provides a reducing method comprising:
a coefficient calculating step for calculating a coefficient for reducing an effect of a multipath on a signal;
a determining step for determining whether a condition that the signal is deteriorated by updating the coefficient is fulfilled or not;
an update allowing step for controlling an updating operation of the coefficient based on the determined result; and
an updating step for updating the coefficient based on the calculated coefficient.
The invention provides a reducing method comprising:
a coefficient calculating step for calculating a coefficient for reducing an effect of a multipath on a signal;
an overmodulation determining step for determining whether an overmodulation is produced in the signal or not;
a weak electrical field determining step for determining whether or not a field strength of the signal is a weak electrical field or less;
an update allowing step for controlling an updating operation of the coefficient based on a determined result by either the overmodulation determining step or the weak electrical field determining step; and
an updating step for updating the coefficient based on the calculated coefficient.
In the invention, it is preferable that the overmodulation determining step includes a modulation degree detecting step for detecting a modulation degree from a frequency shift amount of the signal, and a first comparing step for comparing the modulation degree with a passband regulated by a bandpass filter, and
the weak electrical field determining step includes an electrical filed information obtaining step for obtaining a field strength information from the signal, and a second comparing step for comparing whether the field strength information represents a weak electrical field or not.
Other and further objects, features, and advantages of the invention will be more explicit from the following detailed description taken with reference to the drawings wherein:
Hereinafter, referring to the drawings, preferred embodiments of the invention will be described in detail.
In a plurality of embodiments, portions corresponding to the embodiment previously described will be denoted by the same reference numerals and the common description will be omitted in order to prevent overlapped descriptions. Furthermore, embodiments each having no overlapped function may be implemented in combination.
A radio wave received by the receiving apparatus 23, which is, for example, an FM broadcast wave transmitted in a frequency band of a very high frequency (VHF), is received by an antenna (hereinafter, abbreviated as an “ANT”) 26, and a radio frequency (RF) signal is outputted depending on a field strength. The radio frequency signal is processed in an analog signal state in an analog receiving processing portion 27 having an RF portion for a high-frequency amplification and the like and a MIX portion for a frequency conversion, and is then outputted as an intermediate frequency (IF) signal of about 10.7 MHz. The IF signal is converted into a digital signal in the AD converter 24.
The ADF 21 includes a variable filter portion 28 and a coefficient memory 29 as an updating unit; a filter coefficient calculating unit 30 and an envelope calculating unit 31 as a calculating unit; and a coefficient update allowing unit 32 as an update allowing unit. In the ADF 21, an amplitude changed portion of the IF signal converted to a digital signal is detected by the envelope calculating unit 31, and a filter coefficient is calculated so that the amplitude change is corrected by the filter coefficient calculating unit 30, and is then stored in the coefficient memory 29. The variable filter portion 28 uses the filter coefficient stored in the coefficient memory 29 to remove an effect of a multipath to the IF signal. The ADF 21 is provided with the coefficient update allowing unit 32 so as to prohibit a coefficient update operation under such a condition that a side effect is produced by an operation of the ADF 21.
A determined result of whether a coefficient update is allowed by the overmodulation determining unit 22 or not is inputted to the coefficient update allowing unit 32 of the ADF 21. In order to adjust a time required for the determination in the overmodulation determining unit 22, in a delay unit 33, a delay (DELAY) is given to the IF signal inputted from the AD converter 24 to the ADF 21. The overmodulation determining unit 22 includes a wave detector 34, a modulation degree detector 35 as a modulation degree detecting unit, and a comparator 36 as a comparing unit. The wave detector 34 detects the IF signal of a digital signal converted by the AD converter 24, with respect to a frequency, and the modulation degree detector 35 detects a modulation degree from a frequency shift amount. The comparator 36 compares the detected modulation degree with a predetermined reference, determines that an overmodulation is produced when the modulation degree exceeds the reference, and prohibits a coefficient update. The coefficient update can be prohibited by, for example, setting a flag. When the coefficient update is prohibited, for example, the filter coefficient calculating unit 30 recognizes the set flag, and does not calculate a new coefficient. Note that when a coefficient calculation has been already started, the calculated result is stored in a coefficient memory 9, and is used for a filter calculation in the variable filter portion 28 after the coefficient update has been prohibited. As a result, the variable filter 28 is fixed to a coefficient calculated just after the coefficient update has been prohibited. Note that an output of the delay unit 33 may be directly inputted to the wave detector 25 by bypassing the ADF 21 instead of by prohibiting the coefficient update. That is, an effect of an operation of the adaptive digital filter may be suppressed at least more than usual.
A modulation degree of a frequency-modulated radio signal can be represented by comparing a frequency shift amount of the received signal with a standard frequency shift amount. In an FM broadcast or the like, for example, a frequency shift amount of 75 kHz corresponds to a modulation degree of 100%. However, in the receiving apparatus 23, a receiving frequency bandwidth is set to, for example, 150 kHz in order to prevent an effect on the received signal. Therefore, even though an overmodulation exceeding 100% is produced, the immediate effect is not always produced. However, when the overmodulation approaches 200%, a filter property which determines the receiving frequency bandwidth becomes uneven, thereby increasing an effect of the overmodulation.
The overmodulation determining unit 42 includes the wave detector 34, the modulation degree detector 35, and the comparator 46 as the comparing unit. The comparator 46 determines whether a modulation degree detected in the modulation degree detector 35 with respect to the IF signal which passes through the variable BPF 44 and is detected by the wave detector 34 is overmodulated or not, based on cutoff frequency information Fc of the variable BPF 44 for regulating a band based on the adjacent interference wave detected by the interference detecting unit 41. When the comparator 46 determines that the overmodulation is detected, the operation of the ADF 21 is prohibited. That is, since the cutoff frequency information Fc is changed depending on a state of the adjacent interference, the Fc is adapted to be detected.
That is, the overmodulation determining unit 22 recognizes a state of an overmodulation by comparing a modulation degree of a desired signal with a previously known bandwidth of the receiving apparatus 23, and carries out coefficient control for prohibiting the operation of the ADF 21, or the like. When an interfering measure is taken as in a case of the receiving apparatus 43, a bandwidth of the receiving apparatus 43 is changed. For example, assuming that a maximum band width of the receiving apparatus 43 is 150 MHz, and a modulation degree is 75 kHz/deviation when receiving an FM broadcast of 98.1 MHz. In this case, the interference detecting unit 41 detects the interference, and the variable BPF 44 narrows the bandwidth, for example, from 150 kHz to 50 kHz, when an interference station is present at a frequency of 98.3 MHz. In this case, even though the overmodulation is not originally produced, an effect of the variable BPF 44 results in a distortion of the desired signal, and the distortion is further increased when the ADF 21 is operated. At this time, when the ADF is controlled by comparing the Fc information of the variable BPF 44 with the modulation degree, deterioration in a signal distortion can be prevented, allowing an interference measure function and a multipath reducing function at the same time. When presence of the adjacent interference wave having a wide band is detected by the interference detecting unit 41, BW control for narrowing a frequency band BW of the variable BPF 44 is carried out.
In a diversity receiving operation, the IF signal to be inputted to the ADF 21 produces a sudden change at a time of switching the antenna. When the ADF 21 carries out a coefficient update in response to the change, a signal distortion and abnormal noise are produced. Therefore, the operation of the ADF 21 is prohibited at a time of switching operation in the diversity reception such as a switching of the antenna. Accordingly, reduction of the multipath noise and prevention of deterioration in a distortion property in the weak electrical field can be achieved at the same time.
The modulation degree detector 35 carries out detection of the overmodulation using a maximum value (MAXHOLD) of an FM detecting signal level. As shown in
As further still another embodiment of the invention, the operation of the ADF 21 may be prohibited at a time of a sudden change in electrical field by determining a time of the sudden change in electrical field and a time of the occurrence of the multipath noise. As a method for identifying a time of the sudden change in electrical field and a time of the occurrence of the multipath noise, the identification is achieved by determining presence or absence of a relationship between amplitude and a modulation. As a method for recognizing presence or absence of a relationship with the modulation, presence or absence of a singular point of the amplitude of the IF signal based on a modulation degree (a frequency shift) can be used.
The invention may be embodied in other specific forms without departing from the spirit or essential characteristics 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 the range of equivalency of the claims are therefore intended to be embraced therein.
According to the invention, a reducing apparatus comprises an adaptive digital filter, and includes a determining unit. The adaptive digital filter updates a filter coefficient of a variable filter so as to suppress an amplitude change of a received signal while detecting an amplitude changed portion of the received signal, in order to reduce an effect of a multipath produced in a process of propagation of a radio wave when receiving the radio wave modulated by a modulation method in which amplitude is not changed. The determining unit determines whether a condition that a received signal is deteriorated by a change of a coefficient of the adaptive digital filter is fulfilled or not. An update allowing unit suppresses an updating operation of the coefficient based on a determined result by the determining unit. Therefore, defects in technique and performance according to the related art in reducing multipath noise by use of the adaptive digital filter can be overcome so as to allow a radio wave signal to be received in a comfortable manner.
According to the invention, the determining unit determines whether an overmodulation is produced in the received signal or not. Therefore, based on the determined result, the update allowing unit suppresses an updating operation of the coefficient. Accordingly, the defects in technique and performance according to the related art in reducing multipath noise by use of the adaptive digital filter can be overcome so as to allow a radio wave signal to be received in a comfortable manner.
According to the invention, the determining unit determines whether the overmodulation is produced or not, based on comparison between a modulation degree and a predetermined frequency band. Therefore, it is possible to determine whether the overmodulation which produces an actual effect is produced or not.
According to the invention, it is determined whether the overmodulation is produced or not, based on an interference measure against an interference wave. Therefore, an effect of the overmodulation produced by the interference wave can be certainly removed.
According to the invention, the determining unit determines whether the overmodulation is produced or not, based on comparison between the modulation degree and a passband regulated by a bandpass filter. Therefore, it is possible to determine whether the overmodulation which produces an actual effect is produced or not.
According to the invention, the determining unit determines whether the overmodulation is produced or not, based on a signal changed depending on a time constant. Therefore, it is possible to prevent a false detection while the detection has no oversensitivity. When the overmodulation is actually produced, a modulation detecting signal continues for a long time compared with the multipath noise so as to allow the overmodulation to be detected to a satisfactory extent even though the modulation detecting signal is changed depending on the time constant, thereby sufficiently providing an effect to suppress an operation of the adaptive digital filter.
According to the invention, the operation of the adaptive digital filter is suppressed at a time of occurrence of a weak electrical field, as well as at a time of the overmodulation. Therefore, it is possible to avoid a situation in which the received signal is deteriorated all the more by the operation of the adaptive digital filter.
According to the invention, a comparing unit determines whether field strength information from the received signal obtained by an electrical filed obtaining unit represents the weak electrical field or not. When it is determined that the information represents the weak electrical field, the operation of the adaptive digital filter is suppressed. Therefore, the defects in technique and performance according to the related art in reducing multipath noise by use of the adaptive digital filter can be overcome so as to allow a radio wave signal to be received in a comfortable manner.
According to the invention, the determining unit determines whether a condition that the received signal is deteriorated by a change of the coefficient of the adaptive digital filter is fulfilled or not, based on presence or absence of a switching in association with a diversity receiving operation. When it is determined that the switching is present, the operation of the adaptive digital filter is suppressed. Therefore, the defects in technique and performance according to the related art in reducing multipath noise by use of the adaptive digital filter can be overcome so as to allow a radio wave signal to be received in a comfortable manner.
According to the invention, the determining unit determines whether a predetermined condition that the change of the coefficient of the adaptive digital filter deteriorates the received signal is fulfilled or not, based on presence or absence of occurrence of exogenous noise. When it is determined that the exogenous noise is produced, the operation of the adaptive digital filter is suppressed. Therefore, the defects in technique and performance according to the related art in reducing multipath noise by use of the adaptive digital filter can be overcome so as to allow a radio wave signal to be received in a comfortable manner.
According to the invention, the reducing apparatus comprises the adaptive digital filter, and includes an overmodulation determining unit, and a weak electrical field determining unit. The adaptive digital filter updates the filter coefficient of the variable filter so as to suppress an amplitude change of the received signal while detecting an amplitude changed portion of the received signal, in order to reduce an effect of the multipath produced in a process of propagation of the radio wave when receiving the radio wave modulated by a modulation method in which amplitude is not changed. The overmodulation determining unit determines whether the overmodulation is produced in the received signal or not, and the weak electrical field determining unit determines whether or not a field strength of the received signal is the weak electrical filed or less. In the overmodulation determining unit, a first comparing unit compares the modulation degree detected by a modulation degree detecting unit with the passband regulated by the bandpass filter. In the weak electrical determining unit, a second comparing unit determines whether the field strength information from the received signal obtained by an electrical field information obtaining unit represents the weak electrical field or not. The update allowing unit suppresses an updating operation of the coefficient based on a determined result of either of the overmodulation determining unit or the weak electrical field determining unit, thereby allowing control having higher accuracy. Therefore, the defects in technique and performance according to the related art in reducing multipath noise by use of the adaptive digital filter can be overcome so as to allow a radio wave signal to be received in a comfortable manner.
According to the invention, a receiving apparatus uses a converting unit to convert a signal received by an antenna into a digital signal, and carries out digital signal processing using the adaptive digital filter. The determining unit determines whether a condition that the change of the coefficient of the adaptive digital filter deteriorates the received signal is fulfilled or not. The update allowing unit suppresses an updating operation of the coefficient based on a determined result by the determining unit. Therefore, the defects in technique and performance according to the related art in reducing multipath noise by use of the adaptive digital filter can be overcome so as to allow a radio wave signal to be received in a comfortable manner.
According to the invention, the receiving apparatus uses the converting unit to convert a signal received by an antenna into a digital signal, and carries out the digital signal processing using the adaptive digital filter. The overmodulation determining unit determines whether the overmodulation is produced in the received signal or not, and the weak electrical field determining unit determines whether or not a field strength of the received signal is the weak electrical filed or less. In the overmodulation determining unit, the first comparing unit compares the modulation degree detected by the modulation degree detecting unit with the passband regulated by the bandpass filter. In the weak electrical determining unit, the second comparing unit determines whether the field strength information from the received signal obtained by an electrical field information obtaining unit represents the weak electrical field or not. The update allowing unit suppresses an updating operation of the coefficient based on a determined result of either of the overmodulation determining unit or the weak electrical field determining unit, thereby allowing control having higher accuracy. Therefore, the defects in technique and performance according to the related art in reducing multipath noise by use of the adaptive digital filter can be overcome so as to allow a radio wave signal to be received in a comfortable manner.
According to the invention, a receiving apparatus for a mobile object comprises the above-described reducing apparatus, and thereby can be preferably used for an automobile receiving apparatus or a portable receiving apparatus which has a tendency to have an effect of the multipath noise among other things, and there can be realized the receiving apparatus for a mobile object in which the defects in technique and performance according to the related art in reducing multipath noise by use of the adaptive digital filter can be overcome so as to allow a radio wave signal to be received in a comfortable manner.
According the invention, at a determining step, it is determined whether a condition that a signal is deteriorated by the change of the coefficient is fulfilled or not. When it is determined that the condition is fulfilled, an updating operation is suppressed at an update allowing step. Therefore, the defects in technique and performance according to the related art in reducing multipath noise by use of the adaptive digital filter can be overcome so as to allow a radio wave signal to be received in a comfortable manner.
According to the invention, at an overmodulation determining step, it is determined whether the overmodulation is produced in a signal or not, and at a weak electrical determining step, it is determined whether or not a field strength of the signal is the weak electrical field or less. At the overmodulation determining step, a first comparing step compares the modulation degree detected at a modulation degree detecting step with the passband regulated by the bandpass filter. At the weak electrical determining step, a second comparing step determines whether the field strength information from the received signal obtained by an electrical field information obtaining step represents the weak electrical field or not. At an update allowing step, an updating operation of the coefficient is suppressed based on a determined result of either of the overmodulation determining step or the weak electrical field determining step, thereby allowing control having higher accuracy. Therefore, the defects in technique and performance according to the related art in reducing multipath noise by use of the adaptive digital filter can be overcome so as to allow a radio wave signal to be received in a comfortable manner.
Number | Date | Country | Kind |
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2005-105365 | Mar 2005 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2006/306586 | 3/29/2006 | WO | 00 | 11/14/2008 |