Patent Publication Number: US-2011063519-A1

Title: Carrier recovery device and method, and demodulator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of PCT International Application PCT/JP2009/002275 filed on May 22, 2009, which claims priority to Japanese Patent Application No. 2008-134760 filed on May 22, 2008. The disclosures of these applications including the specifications, the drawings, and the claims are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     The technology disclosed herein relates to carrier recovery devices used in demodulation of a modulated signal containing a pilot signal. 
     In recent years, digital video has become widespread, and digital broadcasting services have been commenced in many countries in the fields of satellite broadcasting, CATV, and terrestrial broadcasting. The transmission technique has been selected which is suitable for characteristics of each transmission channel. For example, vestigial-sideband (VSB) modulation is used in digital terrestrial broadcasting in the U.S. Systems for demodulating digital modulated signals which are used in such broadcasting are described in a number of documents (see, for example, Taga, Ishikawa, and Komatsu, “A Study On QPSK Demodulation System,” ITEJ Technical Report, August 1991, Vol. 15, No. 46, CE&#39;91-42 (FIG. 3)). 
     For example, when carrier recovery is performed from a VSB modulated signal containing a pilot signal, the pilot signal is extracted, and a frequency error and a phase error are obtained from a difference between the pilot signal and a reference signal. 
     SUMMARY 
     In order to reduce the time required for carrier recovery operation of a carrier recovery device to reach a steady state, it is necessary to optimize demodulation parameters relating to carrier recovery, such as the bandwidth of a pilot extraction filter, the gain of a loop filter, and the like. However, it is difficult to obtain the optimum values under various conditions. Moreover, the demodulation parameters need to be changed, depending on phase noise of a pilot signal, in order to maintain the carrier recovery operation. However, the change of the demodulation parameters affects detection of the phase noise, and therefore, it is difficult to continue correct carrier recovery operation. 
     In some states of the transmission channel, for example, when there is a reflected wave, the pilot signal may be damaged or eliminated. Therefore, it may take a long time for carrier recovery operation to reach a steady state, or demodulation performance may be decreased. 
     The detailed description describes implementations of a technique of reducing the time required for carrier recovery operation of a carrier recovery device to reach a steady state and a technique of continuing correct carrier recovery operation. 
     The detailed description also describes implementations of a technique of reducing or preventing a decrease in demodulation performance when pilot signals cannot be properly received while maintaining good response to phase noise when pilot signals can be properly received. 
     An example carrier recovery device of the present disclosure includes a first carrier recovery unit configured to multiply a baseband signal by a first carrier to obtain a first demodulated signal, extract a pilot signal from the first demodulated signal, and generate the first carrier based on a first phase error in the pilot signal extracted from the first demodulated signal, a second carrier recovery unit configured to multiply the baseband signal by a second carrier to obtain a second demodulated signal, extract a pilot signal from the second demodulated signal, and generate the second carrier based on a second phase error in the pilot signal extracted from the second demodulated signal, and a selector configured to select one of the first and second demodulated signals which has been obtained by one of the first and second carrier recovery units whose carrier recovery operation has reached a predetermined steady state earlier than that of the other, based on the first phase error and the second phase error, and output the selected demodulated signal. 
     With this carrier recovery device, one of the first and second demodulated signals which has been obtained by the carrier recovery unit whose carrier recovery operation has reached a predetermined steady state earlier is selected, whereby the time required for carrier recovery operation of the carrier recovery device to reach a steady state can be reduced. 
     Another example carrier recovery device of the present disclosure includes a multiplier configured to multiply a baseband signal by a carrier, and output the result as a demodulated signal, a pilot signal extractor configured to extract a pilot signal from the demodulated signal, an error detector configured to detect a phase error in the pilot signal extracted from the demodulated signal, a limiter configured to cause the phase error to decrease or remain the same based on the pilot signal extracted from the demodulated signal, and output the resultant phase error, a loop filter configured to smooth the output of the limiter, and output the smoothed output, and a variable frequency oscillator configured to generate a signal corresponding to the output of the loop filter, and output the signal as the carrier. 
     An example demodulator of the present disclosure includes a first carrier recovery unit configured to multiply a baseband signal by a first carrier to obtain a first demodulated signal, extract a pilot signal from the first demodulated signal, and generate the first carrier based on a first phase error in the pilot signal extracted from the first demodulated signal, a second carrier recovery unit configured to multiply the baseband signal by a second carrier to obtain a second demodulated signal, extract a pilot signal from the second demodulated signal, and generate the second carrier based on a second phase error in the pilot signal extracted from the second demodulated signal, a selector configured to select one of the first and second demodulated signals which has been obtained by one of the first and second carrier recovery units whose carrier recovery operation has reached a predetermined steady state earlier than that of the other, based on the first phase error and the second phase error, and output the selected demodulated signal, and an equalizer configured to equalize the demodulated signal selected by the selector. 
     Another example demodulator of the present disclosure includes a multiplier configured to multiply a baseband signal by a carrier, and output the result as a demodulated signal, a pilot signal extractor configured to extract a pilot signal from the demodulated signal, an error detector configured to detect a phase error in the pilot signal extracted from the demodulated signal, a limiter configured to cause the phase error to decrease or remain the same based on the pilot signal extracted from the demodulated signal, and output the resultant phase error, a loop filter configured to smooth the output of the limiter, and output the smoothed output, a variable frequency oscillator configured to generate a signal corresponding to the output of the loop filter, and output the signal as the carrier, and an equalizer configured to equalize the demodulated signal. 
     An example carrier recovery method of the present disclosure includes a first carrier recovery step of multiplying a baseband signal by a first carrier to obtain a first demodulated signal, extracting a pilot signal from the first demodulated signal, and generating the first carrier based on a first phase error in the pilot signal extracted from the first demodulated signal, a second carrier recovery step of multiplying the baseband signal by a second carrier to obtain a second demodulated signal, extracting a pilot signal from the second demodulated signal, and generating the second carrier based on a second phase error in the pilot signal extracted from the second demodulated signal, and a selection step of selecting one of the first and second demodulated signals which has been obtained by one of the first and second carrier recovery steps whose carrier recovery operation has reached a predetermined steady state earlier than that of the other, based on the first phase error and the second phase error. 
     Another example carrier recovery method of the present disclosure includes a multiplication step of multiplying a baseband signal by a carrier, and outputting the result as a demodulated signal, a pilot signal extraction step of extracting a pilot signal from the demodulated signal, an error detection step of detecting a phase error in the pilot signal extracted from the demodulated signal, a limitation step of causing the phase error to decrease or remain the same based on the pilot signal extracted from the demodulated signal, and outputting the resultant phase error, a loop filter step of smoothing the phase error after processing by the limitation step, and a variable frequency oscillation step of generating as the carrier a signal corresponding to the phase error smoothed by the loop filter step. 
     According to the examples of the present disclosure, a plurality of carrier recovery units are provided, whereby the time required for carrier recovery operation of a carrier recovery device to reach a steady state can be reduced, and the carrier recovery operation can be accurately continued. Moreover, when a pilot signal cannot be properly received, a phase error in the pilot signal is utilized with suitable modification, whereby the reduction in demodulation performance can be reduced or prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a demodulator including a carrier recovery device according to a first embodiment of the present disclosure. 
         FIG. 2  is a block diagram showing an example configuration of a loop filter of FIG.  1 . 
         FIG. 3  is a graph showing an example pilot signal amplitude PIA input to a limiter of  FIG. 1 , and an example input phase error EN and an example output phase error EL when the pilot signal amplitude PIA is input. 
         FIG. 4  is a block diagram showing an example configuration of a selector of  FIG. 1 . 
         FIG. 5  is a block diagram showing a variation of the carrier recovery device of  FIG. 1 . 
         FIG. 6  is a block diagram showing a configuration of a demodulator including a carrier recovery device according to a second embodiment of the present disclosure. 
         FIG. 7  is a block diagram showing a configuration of a demodulator including a carrier recovery device according to a third embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will be described hereinafter with reference to the accompanying drawings. Components indicated by reference characters whose last two digits are the same correspond to each other, i.e., are the same or similar components. 
     Functional blocks described herein may each be typically implemented by hardware. For example, each functional block is formed as a part of an integrated circuit (IC) on a semiconductor substrate. As used herein, ICs include large-scale integrated circuits (LSIs), application-specific integrated circuits (ASICs), gate arrays, field programmable gate arrays (FPGAs), and the like. Alternatively, a portion of or all functional blocks may be implemented by software. For example, such functional blocks may each be implemented by a program executable by a processor. In other words, each functional block described herein may be implemented by hardware or software or in any combination thereof. 
     First Embodiment 
       FIG. 1  is a block diagram showing a configuration of a demodulator including a carrier recovery device according to a first embodiment of the present disclosure. The demodulator of  FIG. 1  includes carrier recovery units  10  and  20 , a selector  40 , a clock recovery unit  62 , a roll-off filter  64 , an equalizer  66 , and an error correction unit  68 . The carrier recovery units  10  and  20  and the selector  40  are included in the carrier recovery device. 
     The carrier recovery unit  10  includes a multiplier  11 , a pilot signal extractor  12 , an error detector  14 , a limiter  15 , a loop filter  16 , and a variable frequency oscillator  18 . The carrier recovery unit  20  includes a multiplier  21 , a pilot signal extractor  22 , an error detector  24 , a limiter  25 , a loop filter  26 , and a variable frequency oscillator  28 . 
     It is assumed that a signal compliant with the advanced television systems committee (ATSC) standards is received and subjected to quadrature detection to obtain a baseband signal BI/BQ, and the baseband signal BI/BQ is input to the carrier recovery units  10  and  20  of  FIG. 1 . The received signal, which has been modulated by VSB modulation, contains a pilot signal. The baseband signal BI/BQ, which is a complex signal, contains an inphase signal BI and a quadrature signal BQ. 
     The carrier recovery unit  10  will be described. When quadrature detection is performed upstream from the carrier recovery unit  10 , a carrier used for the quadrature detection does not necessarily always have a correct frequency and a correct phase. Therefore, there remain frequency and phase offsets in the inphase signal BI and the quadrature signal BQ. 
     The baseband signal BI/BQ input to the carrier recovery units  10  and  20  of  FIG. 1  is represented by: 
       (Si+jSq)×exp(j(ΔWt+Δθ))  (1)
 
     ΔW: frequency offset 
     Δθ: phase offset 
     where Si is the inphase signal (I-signal) and Sq is the quadrature signal (Q-signal). 
     The variable frequency oscillator  18  outputs, as a recovered carrier, a signal which is the conjugate of the carrier component exp(j(ΔWt+Δθ)) of the signal represented by expression (1). The conjugate signal is represented by: 
       exp(−j(ΔWt+Δθ))  (2)
 
     The multiplier  11  performs complex multiplication with respect to the output of the variable frequency oscillator  18  and the input baseband signal BI/BQ as represented by: 
       ( Si+jSq )×exp( j (Δ Wt +Δθ))×exp(− j (Δ Wt +Δθ))=( Si+jSq )  (3)
 
     Thus, the multiplier  11  removes the frequency and phase offsets of the input baseband signal BI/BQ and outputs a demodulated signal IA/QA represented by expression (3). 
     The pilot signal extractor  12  extracts a pilot signal from the demodulated signal IA/QA, and outputs the pilot signal to the error detector  14 . The error detector  14  detects and outputs a difference between the phase of the extracted pilot signal and a reference phase as a phase error EN of the pilot signal. When the variable frequency oscillator  18  is outputting the signal of expression (2), the error detector  14  detects zero as the phase error EN. When the variable frequency oscillator  18  is outputting a signal which has a phase error with respect to the signal of expression (2), the error detector  14  detects the phase error. 
     The limiter  15  modifies the phase error EN to have a value which corresponds to the phase error EN and is less than or equal to the phase error EN, based on the pilot signal extracted by the pilot signal extractor  12 , and outputs the modified phase error EL. The loop filter  16  smoothes the phase error EL output from the limiter  15 , i.e., removes high-frequency components from the phase error EL, and outputs the resultant phase error EL as an output signal LA to the variable frequency oscillator  18  and the selector  40 . The variable frequency oscillator  18  generates an oscillating signal having a frequency corresponding to the output signal LA of the loop filter  16 , and outputs the oscillating signal as a recovered carrier to the multiplier  11 . 
     A characteristic of each of the pilot signal extractor  12 , the error detector  14 , and the loop filter  16  is set based on a demodulation parameter PMA which is output from the selector  40 . 
     The phase control loop thus configured is a negative feedback loop. Therefore, by the negative feedback loop, a carrier whose phase is synchronous with that of the received digital modulated signal is recovered by the variable frequency oscillator  18 . The recovered carrier is the conjugate of the carrier component of the baseband signal input to the multiplier  11 , and therefore, there is substantially no frequency and phase errors therebetween, whereby a correct demodulated signal can be obtained. 
     The carrier recovery unit  20  has the same configuration as that of the carrier recovery unit  10 , except that a characteristic of each of the pilot signal extractor  22 , the error detector  24 , and the loop filter  26  is set based on a demodulation parameter PMB which is output from the selector  40 . The carrier recovery units  10  and  20  are assumed to have different characteristics. 
     The selector  40  selects one of the demodulated signal IA/QA output from the carrier recovery unit  10  and a demodulated signal IB/QB output from the carrier recovery unit  20 , and outputs the selected demodulated signal to the clock recovery unit  62 . Here, the selector  40  selects the demodulated signal which is obtained by one of the carrier recovery units  10  and  20  whose carrier recovery operation has reached a predetermined steady state earlier than that of the other. The selector  40  also generates the demodulation parameters PMA and PMB and another demodulation parameter PM based on phase noise of the loop filter output of the carrier recovery unit  10  or  20 . 
     The selected demodulated signal is subjected to timing synchronization by the clock recovery unit  62 , waveform shaping by the roll-off filter  64 , waveform equalization by the equalizer  66 , and demapping and error correction by the error correction unit  68  successively in this stated order. The error correction unit  68  outputs error-corrected data. The equalizer  66  includes, for example, a finite impulse response (FIR) filter and an infinite impulse response (IIR) filter. A loop filter gain of the clock recovery unit  62  and a filter coefficient updating step size of the equalizer  66  are controlled based on the demodulation parameter PM output from the selector  40 . The processes of the clock recovery unit  62 , the roll-off filter  64 , and the equalizer  66  may be performed in an order other than that described above. 
     The demodulator of  FIG. 1  further includes a field synchronizer (not shown). The field synchronizer detects field synchronization from the demodulated signal selected by the selector  40 , and outputs the result of the detection to the selector  40 . 
       FIG. 2  is a block diagram showing an example configuration of the loop filter  16  of  FIG. 1 . The loop filter  16  includes a direct circuit  31 , an integration circuit  32 , and an adder  33 . The direct circuit  31  includes an amplifier  34 . The integration circuit  32  includes an amplifier  36 , an adder  37 , and a delay unit  38 . The adder  33  adds the output of the direct circuit  31  and the output of the integration circuit  32 , and outputs the result of the addition as a control signal LA. 
     The amplifier  34  of the direct circuit  31  amplifies the phase error EL output from the limiter  15  by a gain α. The variable frequency oscillator  18  advances (or delays) the phase of its output signal in proportion to the input control signal LA. Therefore, the direct circuit  31  advances (or delays) the phase of the output signal of the variable frequency oscillator  18  linearly with respect to the phase error EL. In other words, the direct circuit  31  corrects a phase error in the carrier recovery process. 
     On the other hand, in the integration circuit  32 , the amplifier  36  amplifies the input phase error EL by a gain β. The adder  37  adds the output of the amplifier  36  and the output of the delay unit  38 , and outputs the result of the addition. The delay unit  38  delays the output of the adder  37 , and outputs the delayed output to the adders  33  and  37 . A loop which is formed by the adder  37  and the delay unit  38  has an integration function. Therefore, the integration circuit  32  controls a frequency of the output signal of the variable frequency oscillator  18  based on the phase error signal. In other words, the integration circuit  32  corrects a frequency error in the carrier recovery process. 
     The gain α of the amplifier  34  and the gain β of the amplifier  36  are set based on the demodulation parameter PMA. The loop filter  26  has the same configuration as that of the loop filter  16 , except that the amplifier gains α and β are set based on the demodulation parameter PMB. Note that only the gain α or β may be set based on the demodulation parameter PMA or PMB. 
       FIG. 3  is a graph showing an example pilot signal amplitude PIA input to the limiter  15  of  FIG. 1 , and an example input phase error EN and an example output phase error EL when the pilot signal amplitude PIA is input. 
     The limiter  15  compares the pilot signal amplitude PIA (a component (I-axis signal) having the same phase as the reference phase, of the pilot signal extracted by the pilot signal extractor  12 ) with a set threshold (here, the threshold is assumed to be 100). When the pilot signal amplitude PIA is less than the threshold, the limiter  15  determines that the reliability of the phase error EN output from the error detector  14  is low, modifies and reduces the value of the phase error EN by a half, and outputs the modified phase error EN as the phase error EL. When the pilot signal amplitude PIA is greater than or equal to the threshold, the limiter  15  determines that the reliability of the phase error EN output from the error detector  14  is high, and outputs the phase error EN directly as the phase error EL. 
     Thus, when the pilot signal amplitude PIA is less than the threshold, the limiter  15  reduces the value of the phase error EN to a value corresponding to that value. Therefore, even when the pilot signal is damaged or eliminated and therefore cannot be properly received, it is possible to reduce or prevent the reduction in demodulation performance which is caused by a residual phase error remaining in the negative feedback loop of the carrier recovery unit. Moreover, it is possible to prevent reduced response to phase noise when the pilot signal can be properly received. 
     The limiter  15  may compare the pilot signal amplitude PIA with a plurality of thresholds. For example, the limiter  15  may modify and reduce the value of the phase error EN by a half when the pilot signal amplitude PIA is less than a threshold TAA, and may modify and reduce the value of the phase error EN by a factor of four when the pilot signal amplitude PIA is less than a threshold TAB (TAB&lt;TAA). 
     The threshold may have a value other than that described above. When the pilot signal amplitude PIA is less than the threshold, the limiter  15  may modify the value of the phase error EN to have a value other than ½ of that value. Specifically, when the pilot signal amplitude PIA is less than the threshold, the limiter  15  may modify the value of the phase error EN to have a reduced absolute value. 
     The limiter  15  can be easily constructed by combining an amplifier and a selector, and therefore, the specific configuration of the limiter  15  will not be described. Note that the limiters  15  and  25  of  FIG. 1  may be removed. 
       FIG. 4  is a block diagram showing an example configuration of the selector  40  of  FIG. 1 . The selector  40  includes synchronization determiners  41  and  42 , a determiner  44 , selectors  46 ,  48 , and  56 , a phase noise detector  52 , a parameter setter  54 , and an averager  58 . 
     The synchronization determiner  41 , when the range of fluctuation of the control signal LA output from the carrier recovery unit  10  is less than or equal to a set threshold THA, determines that the operation of the carrier recovery unit  10  has reached the steady state, and outputs the result of the determination. The synchronization determiner  42 , when the range of fluctuation of the control signal LB output from the carrier recovery unit  20  is less than or equal to a set threshold THB, determines that the operation of the carrier recovery unit  20  has reached the steady state, and outputs the result of the determination. 
     In its initial state, the determiner  44  outputs the determination result so that the selector  46  selects the output signal of the carrier recovery unit  10 , for example. The determiner  44  determines in which of the carrier recovery units  10  and  20  the carrier recovery operation has reached the steady state earlier than that of the other, based on the determination results of the synchronization determiners  41  and  42 , and outputs the result of the determination. Based on the determination result of the determiner  44 , the selector  46  selects the output signal (the demodulated signal IA/QA or IB/QB) of one of the carrier recovery units  10  and  20  whose carrier recovery operation has reached the steady state earlier than that of the other, and outputs the selected output signal to the clock recovery unit  62 . After field synchronization is detected by the field synchronizer, the determiner  44  fixes its output. 
     Note that the determiner  44  selects the carrier recovery unit  10 , which has been selected since the initial state, with higher priority. Specifically, when the carrier recovery operation has reached the steady state at the same time in the carrier recovery unit  10  and  20 , the determiner  44  outputs the result of the determination so that the selector  46  selects the demodulated signal IA/QA of the carrier recovery unit  10 . Alternatively, even when it is determined that the operation of the carrier recovery unit  20  has reached the steady state earlier, the determiner  44  may output the result of the determination so that the selector  46  selects the demodulated signal IA/QA of the carrier recovery unit  10  until a predetermined time has passed. 
     As described above, the carrier recovery device of  FIG. 1  includes the carrier recovery units  10  and  20  which have different characteristics, and selects a demodulated signal output from one of them whose carrier recovery operation has reached the steady state earlier, whereby the time required for carrier recovery operation of the carrier recovery device to reach the steady state can be reduced, and the use of a stable demodulated signal can be more quickly started. Although the case where the carrier recovery device includes two carrier recovery units has been described, three or more carrier recovery units may be provided, and a demodulated signal of one of the carrier recovery units whose carrier recovery operation has reached the steady state earliest may be selected. 
     The selector  48  selects the loop filter output LA of the carrier recovery unit  10  or the loop filter output LB of the carrier recovery unit  20  based on the output of the determiner  44 , and outputs the selected loop filter output LA or LB to the phase noise detector  52 . Here, the selector  48  selects the loop filter output LA or LB of the carrier recovery unit  10  or  20  which has not been selected by the selector  46 . For example, when the selector  46  has selected the output of the carrier recovery unit  10 , the selector  48  selects the loop filter output LB of the carrier recovery unit  20 . 
     The phase noise detector  52  calculates the amount of phase noise from the loop filter output selected by the selector  48 , and outputs the phase noise amount to the parameter setter  54 . In its initial state, the parameter setter  54  outputs predetermined parameters as the demodulation parameters PMA, PMB, and PM. After field synchronization is detected, the parameter setter  54  obtains and outputs the demodulation parameters PMA, PMB, and PM based on the phase noise amount calculated by the phase noise detector  52 . 
     The demodulation parameter PMA is used to set the bandwidth of the pilot extraction filter of the pilot signal extractor  12  and the gains α and β of the loop filter  16  in the carrier recovery unit  10 . The demodulation parameter PMB is used to set the bandwidth of the pilot extraction filter of the pilot signal extractor  22  and the gains of the loop filter  26  in the carrier recovery unit  20 . 
     The parameter setter  54  generates the demodulation parameter PMA or PMB so that the bandwidth of the pilot extraction filter of the pilot signal extractor  12  or  22  increases, or the gains of the loop filter  16  or  26  increase, with an increase in phase noise. The parameter setter  54  also generates the demodulation parameter PM so that the loop filter gain of the clock recovery unit  62  increases, and the filter coefficient updating step size of the equalizer  66  increases, with an increase in phase noise. 
     As a result, the response of the carrier recovery device to phase noise when phase noise is large can be improved. When phase noise is small, the bandwidth of the pilot extraction filter of the pilot signal extractor  12  or  22  is narrow, the loop filter gain of the clock recovery unit  62  and the gains of the loop filter  16  or  26  are small, and the filter coefficient updating step size of the equalizer  66  is small. Therefore, it is possible to reduce or prevent the reduction in demodulation performance which is caused by a residual phase error remaining in the negative feedback loop of the carrier recovery device. Note that the characteristic of only one of the clock recovery unit  62  and the equalizer  66  may be controlled based on the demodulation parameter PM. 
     The demodulation parameters PMA and PMB are input to the carrier recovery units  10  and  20 , respectively. The parameter setter  54  continues to update, based on the determination result of the determiner  44 , (i) one of the demodulation parameter PMA or PMB corresponding to the carrier recovery unit  10  or  20 , which has been selected by the selector  46 , and (ii) the demodulation parameter PM. 
     Thus, the carrier recovery device of  FIG. 1  uses the output of the carrier recovery unit selected by the selector  46  as a demodulation output to the clock recovery unit  62 , and also uses the loop filter output of the carrier recovery unit not selected by the selector  46  for detection of phase noise. In other words, the demodulation parameter of the selected carrier recovery unit which generates the demodulation output is changed based on the detected phase noise, but the change does not affect the result of detection of phase noise by the other carrier recovery unit. Therefore, the demodulation parameter can be maintained at an appropriate value corresponding to the state of the transmission channel while phase noise detection is correctly performed, whereby carrier recovery operation can be correctly continued. 
     Note that instead of changing the gains of the loop filters of the carrier recovery units  10  and  20 , the same effect may be achieved in another manner. For example, the amplitude of the baseband signal BI/BQ may be changed based on the demodulation parameter PMA (or PMB) before the baseband signal BI/BQ is input to the carrier recovery unit  10  (or  20 ). 
     The parameter setter  54  may update the demodulation parameter PMA or PMB which will be input to one of the carrier recovery unit  10  and  20  which has not been selected by the selector  46  so that phase noise can be more easily detected. 
     The selector  56  selects the pilot signal amplitude (I-axis signal) PIA or PIB of one of the carrier recovery units  10  and  20  which has not been selected by the selector  46 . For example, when the selector  46  selects the output of the carrier recovery unit  10 , the selector  56  selects the pilot signal amplitude PIB of the carrier recovery unit  20 . The averager  58  performs an averaging process with respect to the pilot signal amplitude selected by the selector  56 , and outputs the resultant average value to the parameter setter  54 . 
     The parameter setter  54  may obtain the demodulation parameters PMA, PMB, and PM based on the average value obtained by the averager  58  instead of the phase noise amount obtained by the phase noise detector  52 . In this case, the parameter setter  54  generates the demodulation parameter PMA or PMB so that the bandwidth of the pilot extraction filter of the pilot signal extractor  12  or  22  increases, and the gains of the loop filter  16  or  26  increase, with an increase in the obtained average value. The parameter setter  54  also generates the demodulation parameter PM so that the loop filter gain of the clock recovery unit  62  increases, and the filter coefficient updating step size of the equalizer  66  decreases, with an increase in the calculated average value. 
     As a result, the response of the carrier recovery device to phase noise when the pilot signal amplitude is large can be improved. When the pilot signal amplitude is small (i.e., the pilot signal is damaged or eliminated, and therefore, the pilot signal cannot be properly received), the bandwidth of the pilot extraction filter of the pilot signal extractor  12  or  22  is narrow, the loop filter gain of the clock recovery unit  62  and the gains of the loop filter  16  or  26  are small, and the filter coefficient updating step size of the equalizer  66  is large. Therefore, it is possible to reduce or prevent the reduction in demodulation performance which is caused by a residual phase error remaining in the negative feedback loop of the carrier recovery unit. 
     Note that the parameter setter  54  may obtain the demodulation parameter PMA, PMB and PM based on both the phase noise amount obtained by the phase noise detector  52  and the average value obtained by the averager  58 . 
     Alternatively, the parameter setter  54  may generate the demodulation parameter PMA, PMB, or PM so that at least one (but not all) of the bandwidths of the pilot extraction filters of the pilot signal extractors  12  and  22 , the gains of the loop filters  16  and  26 , the loop filter gain of the clock recovery unit  62 , and the filter coefficient updating step size of the equalizer  66  has a value corresponding to the phase noise amount obtained by the phase noise detector  52  or the average value obtained by the averager  58 . 
       FIG. 5  is a block diagram showing a variation of the carrier recovery unit  10  of  FIG. 1 . The carrier recovery unit of  FIG. 5  is different from that of  FIG. 1  in that a limiter  115  is provided instead of the limiter  15 . 
     The limiter  115  is different from the limiter  15  in that the limiter  115  compares, with a set threshold, the power of the pilot signal instead of the pilot signal amplitude PIA. The limiter  115  obtains, as the pilot signal power, the sum of the square of the pilot signal amplitude PIA and the square of a pilot signal amplitude PQA (a component (Q-axis signal) in quadrature with the reference phase, of the pilot signal extracted by the pilot signal extractor  12 ). The limiter  115  also sets the threshold, and a factor by which the phase error EN is modified, to appropriate values. The limiter  115  has the same configuration as that of the limiter  15 , except for the foregoing. Also in the carrier recovery unit  20  of  FIG. 1 , a limiter similar to the limiter  115  is used instead of the limiter  25 . 
     When the limiter  115  is used, then if the pilot signal is unstable, the phase error can be detected with higher accuracy than when the pilot signal amplitude (I-axis signal) is used. Therefore, when the response of the carrier recovery device to phase noise when the phase noise is large can be improved. Moreover, even when the pilot signal is damaged or eliminated and therefore cannot be properly received, it is possible to reduce or prevent the reduction in demodulation performance which is caused by a residual phase error remaining in the negative feedback loop of the carrier recovery device. 
     Second Embodiment 
       FIG. 6  is a block diagram showing a configuration of a demodulator including a carrier recovery device according to a second embodiment of the present disclosure. The demodulator of  FIG. 6  includes a carrier recovery unit  10 , a phase noise detector  52 , a parameter setter  254 , an averager  58 , a clock recovery unit  62 , a roll-off filter  64 , an equalizer  66 , and an error correction unit  68 . The carrier recovery unit  10 , the phase noise detector  52 , the parameter setter  254 , and the averager  58  are included in the carrier recovery device. The same components as those described in the first embodiment are indicated by the same reference characters. 
     The phase noise detector  52  calculates the amount of phase noise from the loop filter output LA, and outputs the phase noise amount to the parameter setter  254 . The averager  58  performs an averaging process with respect to the pilot signal amplitude PIA, and outputs the resultant average value to the parameter setter  254 . The parameter setter  254  obtains the demodulation parameters PMA and PM based on at least one of the phase noise amount obtained by the phase noise detector  52  or the average value obtained by the averager  58 , as does the parameter setter  54  of  FIG. 4 . 
     Although the demodulator of  FIG. 6  includes a single carrier recovery unit, the carrier recovery unit includes the limiter  15 . Therefore, even when the pilot signal is damaged or eliminated, it is possible to reduce or prevent the reduction in demodulation performance which is caused by a residual phase error remaining in the negative feedback loop of the carrier recovery unit. Moreover, it is possible to prevent reduced response to phase noise when the pilot signal can be properly received. 
     Third Embodiment 
       FIG. 7  is a block diagram showing a configuration of a demodulator including a carrier recovery device according to a third embodiment of the present disclosure. The demodulator of  FIG. 7  is configured to be able to receive not only VSB modulated signals, but also quadrature amplitude modulation (QAM) modulated signals. 
     The demodulator of  FIG. 7  includes carrier recovery units  310  and  320 , a selector  340 , a clock recovery unit  362 , a roll-off filter  364 , an equalizer  366 , and an error correction unit  368 . The carrier recovery units  310  and  320  and the selector  340  are included in the carrier recovery device. The same components as those described in the first embodiment are indicated by the same reference characters. 
     The carrier recovery unit  310  of  FIG. 7  is different from the carrier recovery unit  10  of  FIG. 1  in that a QAM error detector  13  and a selector  17  are provided instead of the limiter  15 . The carrier recovery unit  320  is different from the carrier recovery unit  20  of  FIG. 1  in that a QAM error detector  23  and a selector  27  are provided instead of the limiter  25 . 
     The QAM error detector  13  detects a phase error in a received QAM modulated signal using the demodulated signal IA/QA output from the multiplier  11 , and outputs the detected phase error. The selector  17  selects the phase error obtained by the QAM error detector  13  or the phase error obtained by the error detector  14  based on a VSB/QAM switch signal VQS, and outputs the selected phase error to the loop filter  16 . 
     The QAM error detector  23  detects a phase error in a received QAM modulated signal using the demodulated signal IB/QB output from the multiplier  21 , and outputs the detected phase error. The selector  27  selects the phase error obtained by the QAM error detector  23  or the phase error obtained by the error detector  24  based on the VSB/QAM switch signal VQS, and outputs the selected phase error to the loop filter  26 . 
     The clock recovery unit  362 , the roll-off filter  364 , the equalizer  366 , and the error correction unit  368  are the same as the clock recovery unit  62 , the roll-off filter  64 , the equalizer  66 , and the error correction unit  68  of  FIG. 1 , except that the demodulated signal obtained from the QAM modulated signal can also be processed. 
     In the carrier recovery device of  FIG. 7 , most of the components which are used when the VSB modulated signal is received can also be used when the QAM modulated signal is received. Therefore, the scale of the device which is capable of receiving both the VSB modulated signal and the QAM modulated signal can be reduced. 
     Note that the QAM error detectors  13  and  23  may detect a phase error using the output of the equalizer  366  instead of the demodulated signals IA/QA and IB/QB. 
     Moreover, characteristics of the loop filters  16  and  26  may be switched based on the VSB/QAM switch signal VQS. 
     Moreover, each of the QAM error detectors  13  and  23  of the carrier recovery unit  310  and  320  of  FIG. 7  may be replaced with a national television system committee (NTSC) error detector which detects an error in an NTSC signal. 
     Thus, the carrier recovery device of  FIG. 7  includes two carrier recovery units in order to improve reception performance. This configuration is preferable when VSB modulated signals used in terrestrial broadcasting are received. However, when QAM modulated signals used in cable broadcasting are received, the transmission channel is in good conditions, and therefore, only a single carrier recovery unit may be used. Therefore, when QAM modulated signals are received, the carrier recovery units may receive signals having different frequencies. 
     In this case, the delay time of delayed waves is not as long as that in terrestrial broadcasting, and the number of taps in the filter included in the equalizer may be less than when VSB modulated signals are received. Therefore, each filter included in the equalizer is divided into two parts, which are in turn used by the two respective carrier recovery units. As a result, a circuit having substantially the same scale as that of the demodulator of  FIG. 7  can receive signals on a single channel using two carrier recovery units when receiving VSB modulated signals, and can simultaneously receive signals on two channels when receiving QAM modulated signals. 
     The many features and advantages of the present disclosure are apparent from the written description, and thus, it is intended by the appended claims to cover all such features and advantages of the present disclosure. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the present disclosure to the exact construction and operation as illustrated and described. Hence, all suitable modifications and equivalents may be resorted to as falling within the scope of the present disclosure. 
     As described above, according to the embodiments of the present disclosure, the time required for carrier recovery operation of a carrier recovery device to reach a steady state can be reduced. Therefore, the present disclosure is useful for carrier recovery devices, demodulators, and the like.