Abstract:
An adaptive equalizer includes: an adaptive filter; and a control unit. The adaptive filter performs an adaptive equalization processing for an input signal modulated by a modulation method that produces a modulation signal with constant amplitude characteristics so as to make an amplitude of an equalized output signal constant. The control unit controls stop and execution of the adaptive equalization processing of the adaptive filter in accordance with characteristics of at least one of the input signal and the output signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a technique of adaptive equalizing radio signals such as a frequency modulated (FM) signal, and more specifically to an adaptive equalizer with a function of stopping an adaptive equalization processing. 
     2. Description of Related Art 
     In a radio broadcasting system such as an FM radio or a wireless communication system, a transmission signal is deteriorated due to multipath distortion or noises in a wireless transmission path. To that end, an equalizer for decoding the received transmission signal that is deteriorated after propagating through the wireless transmission path is used on the wireless receiver side. In general, a state of the wireless transmission path is uncertain on the receiver side. Further, a wireless receiver of high mobility should follow change in characteristics of the wireless transmission path. Thus, an adaptive equalizer capable of following a change in characteristics of the wireless transmission path has been used. 
     In general, the adaptive equalizer is configured by a digital filter such as an IIR (Infinite Impulse Response) filter or an FIR (Finite Impulse Response) filter, and an adaptation algorithm such as an LMS (Least Mean Square) algorithm or an NLMS (Normalized LMS) algorithm has been employed to optimize filter coefficients. 
     To optimize the filter coefficients based on the adaptation algorithm, a reference signal for estimating characteristics of the wireless transmission path is necessary. As one conceivable method, a training signal with a known signal pattern is sent earlier than an information signal and used as a reference signal to determine filter characteristics. As another method, a blind equalization method has been well known. The blind equalization method generates a reference signal from a received signal and thus does not need to previously send a training signal. Examples of the blind equalization method include a CMA (Constant Modulus Algorithm). 
     The CMA is an algorithm in general, which sets a statistical quantity regarding the filter output signal such as an envelope of a filter output signal and higher-order statistical quantity thereof as an index, and updates the filter coefficients to approximate the index to a target value. The CMA is effective for a modulation system where an amplitude of a modulation signal such as an FM (Frequency modulation) signal or PM (Phase modulation) signal is constant, in other words, a modulation signal has constant amplitude characteristics. 
     Japanese Unexamined Patent Application Publication No. 2005-167717 discloses an improved one of the CMA-based blind adaptive equalizer. To be specific, a multipath distortion eliminating filter is disclosed, which aims at eliminating multipath distortion of an FM signal or PM signal, and controls filter coefficients of a digital filter such that an error between an envelope of an input signal and an output of a digital filter (hereinafter referred to as “correction error”) approximates to 0. 
     Further, Japanese Unexamined Patent Application Publication No. 2005-167717 describes that if it is difficult to converge the correction error to 0 in the case where correction error exceeds a predetermined threshold value, for example, in such a case that a delay of a multipath signal increases beyond the total delay time of delay elements in the multipath distortion eliminating filter, fluctuations of the filter coefficients are constrained by restricting the maximum value of the correction error. The filter operation can be stabilized based on the LMS algorithm if it is difficult to make a correction error converge to 0. 
     Further, as another configuration example of the multipath distortion eliminating filter, Japanese Unexamined Patent Application Publication No. 2005-167717 describes the configuration for stabilizing filter operations by approximating all tap factors of a digital filter to 0 if a correction error exceeds a predetermined threshold value (paragraphs [0065] to [0067]). 
     As described above, the adaptive equalizer (multipath distortion eliminating filter) as disclosed in Japanese Unexamined Patent Application Publication No. 2005-167717 determines a difficult situation in which the correction error between an envelope of an input signal and a digital filter output converges to 0 based on whether or not the correction error exceeds a predetermined threshold value. Further, the adaptive equalizer has a feature that, if the correction error exceeds a threshold value, an operational mode of the digital filter is changed to facilitate convergence of the correction error to 0. That is, additional processing is performed to facilitate convergence of the correction error to 0. 
     However, the adaptive equalizer as disclosed in Japanese Unexamined Patent Application Publication No. 2005-167717 continuously performs adaptive control of filter coefficients in response to an input signal before and after an operational mode of a digital filter is changed. This causes a problem that operational stability of the digital filter is not secured. 
     For example, input signal intensity is changed at a small interval, it is impossible to make the correction error to converge even through the CMA to approximate the correction error to 0, leading to unstable operations such as oscillations in filter coefficients. 
     In short, the adaptive equalization based on the CMA can adaptively configure a filter having characteristics opposite to characteristics of a wireless transmission path unless a factor of signal deterioration such as multipath distortion in a wireless transmission path, an interfering wave, and noise hinders estimation of a modulation signal of a constant amplitude from a received signal. However, if there is a factor of signal deterioration that hinders estimation of a modulation signal of a constant amplitude from a received signal, adaptive equalization processing is no longer performed stably through the CMA. 
     Japanese Unexamined Patent Application Publication No. 2005-167717 includes no description about configuration and operation to deal with an unstable behavior such as non-convergence and oscillations of filter coefficients of a digital filter if a factor of signal deterioration that hinders estimation of a modulation signal of a constant amplitude from a received signal. 
     SUMMARY 
     In one embodiment, there is provided an adaptive equalizer that includes: an adaptive filter; and a control unit. The adaptive filter performs an adaptive equalization processing for an input signal modulated by a modulation method that produces a modulation signal with constant amplitude characteristics so as to make an amplitude of an equalized output signal constant. The control unit controls stop and execution of the adaptive equalization processing of the adaptive filter in accordance with characteristics of at least one of the input signal and the output signal. 
     According to this configuration, for example, it is possible to determine an environment involving non-convergence, oscillation, or divergence of filter coefficients of the adaptive filter in accordance with characteristics of at least one of the input signal and the output signal, and stop the adaptive equalization processing of the adaptive filter under the determined environment. As a result, it is possible to prevent an output signal of the adaptive filter from deteriorating due to an unstable behavior such as oscillations of filter coefficients, and to improve an operational stability of the adaptive filter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, advantages and features of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram of an FM receiver according to a first embodiment of the present invention; 
         FIG. 2  is a block diagram of a channel equalizer of the first embodiment; 
         FIG. 3  shows a configuration example of an individual coefficient calculating unit provided in a channel equalizer according to a first embodiment of the present invention; 
         FIG. 4  shows a configuration example of the individual coefficient calculating unit provided in the channel equalizer of the first embodiment; 
         FIG. 5  shows a configuration example of the individual coefficient calculating unit provided in the channel equalizer of the first embodiment; 
         FIG. 6  shows a configuration example of the individual coefficient calculating unit provided in the channel equalizer of the first embodiment; 
         FIG. 7  is a flowchart of operations of an equalizer control unit provided in an FM receiver of the first embodiment; and 
         FIG. 8  illustrates how to control adaptive equalization processing executed by the FM receiver of the first embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposes. 
     Detailed description is given of embodiments of the present invention below with reference to the accompanying drawings. The same components are denoted by identical reference numerals throughout the drawings, and repetitive description is omitted if not necessary for clear description. Incidentally, the following embodiments of the present invention relate to an FM receiver for receiving an FM signal that is frequency-modulated by a sound signal with an encoded carrier. 
     First Embodiment 
       FIG. 1  shows the configuration of an FM receiver  1  according to a first embodiment of the present invention. Referring first to  FIG. 1 , components of the FM receiver  1  are described. In  FIG. 1 , an RF-IF converting unit  101  receives an RF signal through an antenna  100 , and combines the input RF signal and a signal generated with a local oscillator (not shown) to generate an IF signal. 
     The IF signal generated with the RF-IF converting unit  101  is converted into a digital signal by an A/D converter  102  and input to a channel selection filter  103 . The channel selection filter  103  is a band pass filter to extract a desired channel from the input signal. 
     The IF signal subjected to bandwidth selection with the channel selection filter  103  (hereinafter referred to as “FM signal”) is input to the channel equalizer  107 . The channel equalizer  107  is an adaptive equalizer to compensate for signal distortion of an FM signal due to multipath delay wave, interfering wave, and noise. The equalizer performs adaptive equalization processing for the purpose of stabilizing an amplitude of an output signal. The channel equalizer  107  is configured by, for example, a FIR filter to optimize filter coefficients based on an LMS algorithm. Incidentally, a specific configuration example of the channel equalizer  107  is described later in detail. The FM signal equalized by the channel equalizer  107  (hereinafter referred to as “equalized FM signal”) is input to an FM detection unit  109 . The FM detection unit  109  performs FM detection, that is, demodulates a sound signal encoded through frequency-voltage conversion. A stereo demodulation unit  112  demodulates the encoded sound signal to stereo sound signals (L signal and R signal) and outputs the stereo sound signals. 
     An adjacent-channel determination unit  104  determines whether an adjacent-channel signal mixes into the FM signal, and selects a transmission bandwidth of the channel selection filter  103  in accordance with a determination result. For example, whether or not the adjacent-channel signal mixes into the FM signal can be determined by extracting an adjacent-channel band through a band pass filter, full-wave rectifying the extracted signal, calculating time mean intensity of the extracted signal, and then determining whether or not the thus-calculated signal intensity exceeds a threshold value. If signal intensity of an adjacent channel exceeds a predetermined threshold value, it is likely that an adjacent-channel signal is included in an FM signal subjected to bandwidth selection with the channel selection filter  103 . Thus, a transmission bandwidth of the channel selection filter  103  is decreased. On the other hand, if the signal intensity of the adjacent channel is below a predetermined threshold value, a transmission bandwidth of the channel selection filter  103  is increased. Incidentally, the intensity may be compared with plural threshold values to select a desired transmission bandwidth of the channel selection filter  103  from three or more levels. Further, whether or not the adjacent-channel signal is included can be determined by another method. 
     The signal intensity detecting unit  105  detects signal intensity of the FM signal. To be specific, the FM signal may be full-wave rectified to calculate time mean intensity. 
     The multipath determination unit  106  detects signal intensity of a multipath signal. In addition, the multipath determination unit  106  may detects a delay spread of the multipath signal. To be specific, the multipath determination unit  106  extracts a pilot signal from the received signal through a band pass filter, and detects signal intensity and delay spread of the pilot signal. 
     A signal intensity variation detecting unit  110  detects variations in signal intensity of the equalized FM signal output from the channel equalizer  107 . The FM signal has constant amplitude characteristics, so it is possible to determine a level of convergence through adaptive processing of the channel equalizer  107  in accordance with variations in signal intensity of the equalized FM signal. That is, if variations in signal intensity of the equalized FM signal are too large, it is supposed that the FM signal is not equalized as expected, and operations of the channel equalizer  107  are unstable. 
     A DC offset detecting unit  111  detects DC offset components of an output signal from the FM detection unit  109 . An amplitude of DC offset of the signal subjected to the FM detection means an amplitude of offset from a channel center frequency of an equalized FM signal frequency. That is, a large absolute value of the DC offset means that a signal having a frequency that largely deviates from a channel center frequency such as an adjacent-channel signal is demodulated with the FM detection unit  108 . 
     The equalizer control unit  108  determines whether to execute or stop adaptive equalization processing of the channel equalizer  107  based on measurements or determination results from the above adjacent-channel determination unit  104 , the signal intensity detecting unit  105 , the multipath determination unit  106 , signal intensity variation detecting unit  110 , and the DC offset detecting unit  111  to output a control signal (hereinafter referred to as “equalizer control signal”) to the channel equalizer  107 . To be specific, it is determined whether to execute or stop adaptive equalization processing of the channel equalizer  107  in accordance with measurements of an adjacent-channel signal or a threshold value determination result based on the measurements, signal intensity of the FM signal or a threshold value determination result based on the signal intensity, signal intensity of a multipath signal or a threshold value determination result based on the signal intensity, variations in signal intensity of the equalized FM signal or a threshold value determination result based on the variations, and an amount of DC offset components in an output signal subjected to FM detection or a threshold value determination result based on the amount. A processing of determining whether to execute or stop adaptive equalization processing of the equalizer control unit  108  is described in detail below. 
     The above channel equalizer  107  converges filter coefficients in the channel equalizer  107  to predetermined values if the equalizer control signal instructs to stop the adaptive equalization processing. To be specific, one coefficient among the filter coefficients is made to converge to “1”, and the other coefficients are made to converge to “0”. As a result of converging the filter coefficients this way, the channel equalizer  107  gives a delay to the input FM signal and outputs the input FM signal without modification, and thus does not function as an adaptive equalizer. On the other hand, if the equalizer control signal instructs to execute adaptive equalization processing, the channel equalizer  107  cancels convergence the filter coefficients to “1” or “0” and adaptively update the filter coefficients. 
     Subsequently, a specific configuration example of the channel equalizer  107  is described with reference to  FIGS. 2 to 6 .  FIG. 2  is a block diagram of a configuration example of the channel equalizer  107 . The channel equalizer  107  of  FIG. 2  employs a general transversal type FIR filter as an adaptive digital filter, and updates filter coefficients of the FIR filter based on the LMS algorithm every sampling period. In  FIG. 2 , an input terminal  201  is an input terminal for the FM signal, and receives the FM signal from the channel selection filter  103 . An input terminal  203  is an input terminal for the equalizer control signal, and receives the equalizer control signal from the equalizer control unit  108 . An output terminal  202  is an output terminal for the equalized FM signal. 
     N−1 delay devices  204   — 1 to  204 _N−1 each give a delay of predetermined sampling periods to the input FM signal and output the delay signals. The cascaded connected delay devices  204   — 1 to  204 _N−1 constitute a shift-memory, and values stored in the delay devices  204   — 1 to  204 _N−1 are shifted per sampling period. 
     N multipliers  205   — 0 to  205 _N−1 multiply signals x(m) to x(m−N+1) at N tap points between the input terminal  201  and the delay devices  204   — 1 to  204 _N−1 by filter coefficients C (m, 0) to C (m, N−1). The N values obtained through the multiplication by the filter coefficients are added by N−1 adders  206   — 1 to  206 _N−1 and output to the output terminal  202 . That is, the N multipliers  205   — 0 to  205 _N−1, and the N−1 adders  206   — 1 to  206 _N−1 perform convolutional encoding of the input FM signals x(m) to x(m−N+1) and the filter coefficients C (m, 0) to C (m, N−1). 
     N individual coefficient calculating units  207   — 0 to  207 _N−1 calculate filter coefficients C (m, 0) to C (m, N−1). To be specific, N individual coefficient calculating units  207   — 0 to  207 _N−1 each calculates a new filter coefficient based on an update value input from a common coefficient calculating unit  208 , a filter coefficient before one sampling period and a sampling value of the input FM signal before one sampling period. 
     A common coefficient calculating unit  208  calculates the update value of the filter coefficient based on the LMS algorithm and outputs the value to the individual coefficient calculating units  207   — 0 to  207 _N−1. The LMS algorithm executed by the individual coefficient calculating units  207   — 0 to  207 _N−1 and the common coefficient calculating unit  208  is expressed by Expression (1).
 
 {right arrow over (h)} ( m+ 1)= {right arrow over (h)} ( m )+μ  e ( m ) {right arrow over (u)} ( m )  (1)
 
     In Expression (1), a vector h(m) is a vector including N filter coefficients C (m, 0) to C (m, N−1) at an m-th sample and is described from Expression (2). Further, the vector u(m) is an input signal vector representing tapped FM signals x(m) to x(m−N+1) and is derived from Expression (3). Further, p represents a scalar value called a “step size”. Further, e(m) represents an error amount of a filter coefficient expressed by Expression (4). 
     
       
         
           
             
               
                 
                   
                     
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     In Expression (4), d(m) represents a reference signal. In this embodiment, a target value of an envelope amplitude calculated based on an input FM signal is a reference signal d(m) by the utilization of the fact that the FM signal has a constant amplitude. The common coefficient calculating unit  208  calculates the reference signal d(m) based on the input FM signal, and calculates an error amount e(m) based on a difference between the reference signal d(m) and an envelope amplitude V(m) derived from the filter output value. Moreover, the common coefficient calculating unit  208  outputs a value calculated by multiplying the error amount e(m) by a predetermined step size μ to the individual coefficient calculating units  207   — 0 to  207 _N−1. 
     As described above, adaptive equalization processing of the channel equalizer  107  is stopped in accordance with an equalizer control signal output from the equalizer control unit  108 . The configuration of the individual coefficient calculating units  207   — 0 to  207 _N−1 to stop the adaptive equalization processing of the channel equalizer  107  in accordance with an equalizer control signal is described next. In the case of stopping the adaptive equalization processing, a filter coefficient output from one of the N individual coefficient calculating units  207   — 0 to  207 _N−1 is made to converge to “1”, and filter coefficients output from the remaining units, that is, the N−1 individual coefficient calculating units are made to converge to “0”. The individual coefficient calculating unit conveging the factor to “1” may be any one of the N individual coefficient calculating units. The following description is directed to the case where a filter coefficient calculated by the individual coefficient calculating unit  207   — 0 is made to converge to “1” and filter coefficients calculated by the other individual coefficient calculating units are made to converge to “0” by way of example. 
       FIG. 3  shows a configuration example of the individual coefficient calculating unit  207   — 1 that approximates a filter coefficient to “0” at the time of stopping the adaptive equalization processing. The multiplier  301  multiplies a scalar value μe(m) input from the common coefficient calculating unit  108  by a sampling value x(m−1) of an input signal. The multiplier  304  multiplies a filter coefficient given a delay of one sampling period with the delay device  303  by the equalizer control signal K. The adder  302  adds an output value from the multiplier  302  and an output value from the multiplier  304 , and outputs an update value C (m+1, 1) of the filter coefficient. That is, the update value C (m+1, 1) of the filter coefficient output from the multiplier  302  is derived from Expression (5).
 
 C ( m+ 1,1)= K C ( m, 1)+μ  e ( m ) x ( m− 1)  (5)
 
     If the adaptive equalization processing is performed, the equalizer control signal K output from the equalizer control unit  108  is set to 1. In this case, Expression (5) shows an updated algorithm similar to Expression (1) above. On the other hand, if the adaptive processing is stopped, the equalizer control signal K output from the equalizer control unit  108  is set larger than 0 and smaller than 1 (0&lt;K&lt;1), and an output signal value of the common control unit  208  is set to 0. The output signal value of the common control unit  208  can be set to 0 by setting, for example, step size μ to 0. Thus, a filter coefficient during a period where the adaptive equalization processing is stopped gradually decreases to converge to 0 upon each update. The individual coefficient calculating units  207   — 2 to  207 _N−1 that should approximate the filter coefficient to “0” at the time of stopping the adaptive equalization processing may be configured as illustrated in  FIG. 3 . 
       FIG. 4  shows a configuration example of the individual coefficient calculating unit  207   — 0 that makes a filter coefficient converge to “1” at the time of stopping the adaptive equalization processing. The individual coefficient calculating unit  207   — 0 of  FIG. 4  differs from the individual coefficient calculating unit  207   — 1 of  FIG. 3  in that a −1 adder  305  and a +1 adder  306  are provided upstream of and downstream of the multiplier  304 . In this example, an update value C(m+1, 0) of the filter coefficient output from the multiplier  302  is expressed by Expression (6). That is, at the time of stopping the adaptive equalization processing, if the equalizer control signal K is larger than 0 and smaller than 1 (0&lt;K&lt;1), and the step size μ is 0, the filter coefficient gradually approximate to 1 upon each update.
 
 C ( m+ 1,0)= K{C ( m, 0)−1 }+μ e ( m ) x ( m )+1  (6)
 
     According to the configuration of  FIGS. 3 and 4 , the filter coefficients can gradually converge to a target value at the time of stopping the adaptive equalization processing of the channel equalizer  107 , instead of instantly changing the filter coefficients to 1 or 0. Thus, large discontinuous variations of an output signal from the channel equalizer  107  can be suppressed, making it possible to prevent noise generated in a stereo sound signal due to such variations. 
     Incidentally, in the above configuration of  FIGS. 3 and 4 , the common filter control signal K is input to the individual coefficient calculating unit  207   — 0 and the individual coefficient calculating unit  207   — 1, but a different signal value may be input. Further, even if an adaptive filter that performs complex number operation is used as the channel equalizer  107 , a complex arithmetic unit is used as each operation unit of  FIGS. 3 and 4  to thereby apply these configurations. That is, the configuration of  FIG. 3 , which makes the filter coefficient converge to 0 at the time of stopping the adaptive equalization processing can approximate a real part and an imaginary part of the filter coefficient to 0. Further, the configuration of  FIG. 4 , which makes the filter coefficient converge to 1 at the time of stopping the adaptive equalization processing can converge a real part of the filter coefficient to 1 and an imaginary part to 0. 
     Next, another configuration example of the individual coefficient calculating unit is described.  FIG. 5  shows another configuration example of the individual coefficient calculating unit  207   — 0 that makes the filter coefficient converge to “1” at the time of stopping the adaptive equalization processing. The configuration of  FIG. 5  adds a fixed value of 1 to the filter coefficient by the adder  307 , and the other configuration is the same as that of  FIG. 3 . With such configuration, the adaptive equalization processing can be also performed, and the filter coefficient converges to 1 at the time of stopping the adaptive equalization processing (0&lt;K&lt;1) depending on a value added by the adder  307 . 
       FIG. 6  shows another configuration example of the individual coefficient calculating unit  207   — 1 that makes the filter coefficient converge to “0” at the time of stopping the adaptive equalization processing. In the configuration of  FIG. 6 , the equalizer control signal L is a binary logic signal of 0 or 1. Further, the multiplier  308  multiplies the filter coefficient by 2 −m  before one sampling period. Here, m represents a positive integer. Such multiplication can be performed through signed m-bit shift operation. 
     The multiplier  309  multiplies the equalizer control signal L by an output from the multiplier  308 . Here, L is 0 or 1, so such multiplication can be replaced by selective operation or AND operation. The adder  310  calculates a difference between an output value of the multiplier  309  and a filter coefficient before one sampling period. An update value C(m+1, 1) of the filter coefficient in the above configuration is expressed by Expression (7). That is, the configuration of  FIG. 6  differs from the configuration of  FIG. 3  in that a multiplier factor K is limited to 1-2 −m . L=0 corresponds to K=1, and L=1 corresponds to 0&lt;K&lt;1. In the configuration of  FIG. 6 , the number of multiplying processings is not larger than that of  FIG. 3 . The configuration of  FIG. 6  is especially effective for realizing an adaptive filter through fixed-point calculation.
 
 C ( m+ 1,1)=(1 −L× 2 −m ) C ( m, 1)+μ  e ( m ) x ( m− 1)  (7)
 
     Subsequently, how to control the adaptive equalization processing of the channel equalizer  107  by the equalizer control unit  108  is described with reference to a flowchart of  FIG. 7 .  FIG. 7  shows an example of a control sequence periodically executed in the equalizer control unit  108 . In step S 11 , if variations in equalized signal intensity of the FM signal detected by the signal intensity variation detecting unit  110  exceed a predetermined threshold value, that is, an equalized FM signal that needs to have constant amplitude is unstable, the adaptive equalization processing of the channel equalizer is preferentially stopped irrespective of the other conditions (step S 14 ). Further, if a DC offset detected with the DC offset detecting unit  111  is large, that is, if the FM detection unit  109  demodulates a signal having a frequency that largely deviates from a channel center frequency, the adaptive equalization processing of the channel equalizer is preferentially stopped (steps S 11  and S 14 ). 
     Next, in step S 12 , if conditions of stopping the adaptive equalization processing of step S 11  are not met and signal intensity of an adjacent-channel signal detected with the adjacent-channel determination unit  104  exceeds a predetermined threshold value, the adaptive equalization processing is preferentially carried out regardless of the signal intensity of a multipath signal detected with the multipath determination unit  106  (step S 15 ). Further, if signal intensity of the FM signal detected with the signal intensity detecting unit  106  is in a predetermined range, the adaptive equalization processing is preferentially carried out regardless of the signal intensity of a multipath signal detected with the multipath determination unit  106  (steps S 12  and S 15 ). 
     If both of the conditions of stopping the adaptive equalization processing of step S 11  and the conditions of executing the adaptive equalization processing of step S 12  are not met, it is determined whether to stop or perform the adaptive equalization processing based on determination in step S 13 . To be specific, if the signal intensity of a multipath signal detected with the multipath determination unit  106  is below a predetermined threshold value, the channel equalizer  107  carries out the adaptive equalization processing. If the intensity is above a predetermined threshold value, the adaptive equalization processing is stopped. 
       FIG. 8  is a time chart of an temporary transit example in the case of repeatedly executing and stopping the adaptive equalization processing of the channel equalizer  107  under the control of the equalizer control unit  108 . In  FIG. 8 , the adaptive equalization processing is stopped as a result of determination of the equalizer control unit  108  at time T 1 . Hence, the filter coefficients converge to a predetermined values (0 or 1) from time T 1  onward, and after the convergence of the filter coefficients, the delay input FM signal is directly output to the channel equalizer  107  as it is. Subsequently, at time T 2 , the adaptive equalization processing is performed as a result of determination with the equalizer control unit  108 . Hence, from time T 2  onward, the filter coefficients converge to the optimized values with an aim to set an amplitude of the equalized FM signal constant. 
     An interval at which the equalizer control unit  108  performs determination, that is, at which the control sequence of  FIG. 7  is executed should be determined based on a period necessary for the filter coefficients of the channel equalizer  107  converge to a target value and an interval at which a phenomenon that requires control of the equalizer occurs. The period necessary for the filter coefficient to converge to a target value and the interval at which the control sequence is executed are considered below. 
     The period necessary for the filter coefficient to converge to a target value at the time of stopping the adaptive equalization processing is determined by the prefix K n  of the multiplier factor K as represented by Expression (5). The larger the value K, the longer the period necessary for convergence. Here, n represents a sampling number. Here, K=1−ε (0&lt;C&lt;1), and the sampling number n is set such that K n  is smaller than ε, that is, set to satisfy Expression (8) below.
 
(1−ε) n &lt;ε  (8)
 
     If ε is much smaller than 1, the left side of Expression (8) is binomial expanded; if the term of second or more orders of ε is ignored, n that satisfies Expression (8) is substantially the inverse of ε as apparent from the expression. That is, if n exceeds 1/ε, the filter coefficient is smaller than n×ε. Thus, if ε is too small, it is expected to approximate the filter coefficient to substantially 0. For example, if ε=2 −m , and a value of the multiplier factor K is 1-2 −m , it is expected that the filter coefficient converges to substantially 0 after sampling number n exceeding 1/ε=2 m . 
     To elaborate, the FM signal is processed while an intermediate frequency of 10.7 MHz is converted into a lower intermediate frequency. Thus, assuming that an intermediate frequency of 1 MHz is used, a clock having a frequency 4 times higher than the intermediate frequency, that is, 4 MHz is used for sampling of the IF signal, and m=12, the filter coefficient supposedly converges to substantially 0 in a period corresponding to the number of sampling processings 2 12 , that is, after the elapse of about 1 millisecond from when the adaptive equalization processing is stopped. 
     Incidentally, a period necessary for convergence of the filter coefficient needs to be short enough to prevent form outputting noise to a demodulated sound signal. Considering a convergence period of the adaptive filter, the period is desirably approximately 10 microseconds. On the other hand, considering how to follow multipath null due to Doppler shift of an FM signal received by a mobile device, a convergence period of more than 10 milliseconds is not desirable. As described above, a value of the multiplier factor K or power m is determined in consideration of the convergence period of the filter coefficient; for example, a system may be designed such that a filter coefficient converges within a period at which a control sequence for controlling the equalizer is executed. Incidentally, the above consideration is given by way of example, and the system may be designed such that a convergence period of the filter coefficient exceeds a period at which the control sequence is executed. 
     Subsequently, detailed description is given of beneficial effects attained by performing and stopping the adaptive equalization processing under the control of the channel equalizer  107  and the equalizer control unit  108 . The adaptive equalizer as disclosed in Japanese Unexamined Patent Application Publication No. 2005-167717 continuously executes adaptive control of filter coefficients in accordance with an input signal before and after an operational mode of a digital filter is changed. This results in a problem that operational stability of the digital filter is not secured. Further, the adaptive equalizer as disclosed in Japanese Unexamined Patent Application Publication No. 2005-167717 aims at eliminating multipath distortion, and neither considers deterioration of an input signal due to the other factors such as interference of the adjacent channel nor determines an influence of a multipath and an influence of adjacent channel interference in order of priority. 
     In contrast, according to the FM receiver  1  of this embodiment, the equalizer control unit  108  evaluates reception environments of the FM receiver  1  based on variations in signal intensity of the FM signal, multipath signal intensity, adjacent channel signal intensity, and equalized signal intensity of the FM signal, and a DC offset of the FM detection signal to determine whether to stop or execute the adaptive equalization processing of the channel equalizer  107  based on the evaluation result. With the above configuration, even in such environments that non-convergence, oscillations, or divergence of filter coefficients of the channel equalizer  107  occurs due to disturbance elements other than the multipath, the adaptive equalization processing can be stopped. Hence, it is possible to prevent an unstable equalized FM signal from being output from the channel equalizer  107  due to an unstable behavior of the channel equalizer  107  resulting from the disturbance elements other than the multipath. Further, it is possible to prevent noise that sounds unusual on human ears being output from the stereo demodulation unit  112  due to unstable equalized FM signals. 
     Further, as shown in the flowchart of  FIG. 7 , the equalizer control unit  108  prioritizes and determines plural measurements for evaluating reception environments of the FM receiver  1 . That is, the equalizer control unit  108  executes a determination based on the equalized signal intensity of the FM signal variation and a determination based on DC offset after the FM detection in priority to a determination based on a determination of the signal intensity of a multipath signal. As a result, it is possible to detect an unstable adaptive processing of the channel equalizer  107  regardless of the multipath signal, so the channel equalizer  107  can be promptly shifted to a stable state. 
     Further, the equalizer control unit  108  executes based on the signal intensity of the adjacent-channel signal and signal intensity of the FM signal in priority to a determination based on the the signal intensity of a multipath signal. That is, if the adjacent-channel signal is included in the FM signal to be modulated or if the signal intensity of the FM signal exceeds a predetermined threshold value, the channel equalizer  107  performs adaptive equalization processing irrespective of existence of the multipath signal. The adaptive equalization processing of the FM signal is effective not only for compensating for multipath distortion but also for restoring the FM signal deteriorated by a general factor of signal deterioration to the original FM signal, so even if the multipath signal is included, the adaptive equalization processing can be continued through the operation of the equalizer control unit  108 , and quality of the demodulated signal can be improved. 
     Further, the configuration illustrated in  FIGS. 2 to 6  is employed to prevent increase in operational amounts of the channel equalizer  107  necessary for switchably execute and stop the adaptive equalization processing of the channel equalizer  107 , so an increase in integration scale and processing load of the channel equalizer  107  can be suppressed. 
     Other Embodiments 
     The determination flow of the equalizer control unit  108  of the first embodiment as shown in  FIG. 7  is described for illustrative purposes. That is, a combination and priority of determination conditions in the control sequence with the equalizer control unit  108  are not uniquely determined, and various modifications can be employed to stabilize operations of the channel equalizer  107 . 
     For example, in the flowchart of  FIG. 7 , if a DC offset of the output signal of the FM detection unit  109  detected with the DC offset detecting unit  111  is large, the adaptive equalization processing of the channel equalizer  107  is stopped. The determination conditions are changed, and determination based on the signal intensity of the FM signal detected with the signal intensity detecting unit  105  is performed in combination, with the result that if the intensity of the FM signal is small and a DC offset after FM detection is large, the adaptive equalization processing of the channel equalizer  107  can be stopped. 
     Further, as another example, the determination of the signal intensity of a multipath signal in the flowchart of  FIG. 7  and the determination of the signal intensity variation of the signal output from the channel equalizer  107  may be performed in combination. These processings are combined to stop and perform the adaptive equalization processing under control to thereby more precisely control stability if the multipath signal is included in the FM signal. 
     The optimum combination and priority of the determination condition of the equalizer control unit  108  vary depending on the environments of the FM receiver  1 . Therefore, the combination and priority of the determination conditions of the equalizer control unit  108  may be determined based on measurements under actual use environments and after it is confirmed whether or not an erroneous operation occurs. 
     Further, the first embodiment describes the example where the present invention is applied to the FM receiver. However, the present invention is effective for a device that receives a signal modulated by a modulation system where a modulation signal has a constant amplitude, to be specific, a phase modulation (PM), and FSK (Frequency Shift Keying) and PSK (Phase Shift Keying) as a digital modulation method. 
     Further, additional determination conditions such as signal intensity of an adjacent-channel signal or a DC offset after the FM detection are used in combination with determination conditions such as an influence of the multipath signal as in the control sequence of  FIG. 7 , and these determination conditions are prioritized for determination. This is effective for configurations other than the configuration to stop the adaptive equalization processing of the channel equalizer  107  in accordance with the determination result. For example, the above is effective for a configuration to switch operational modes of the channel equalizer  107  in accordance with the determination result. To be specific, it is possible to switch an operational mode that allows large variations of the filter coefficients each time the filter coefficients are updated in accordance with the determination result of the equalizer control unit  108  and an operational mode that suppresses variations in filter coefficients each time the filter coefficients are updated. 
     It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention.