Abstract:
A demodulator is provided which includes a filter filtering a signal based on pass frequency bands changed at a time of a channel search and at a time of a normal reception, and a demodulation processing unit performing an orthogonal frequency division multiplex demodulation based on the filtered signal. Also, a demodulator is provided which includes a synchronization processing unit performing a synchronization processing based on an input signal and a demodulation processing unit performing an orthogonal frequency division multiplex demodulation based on the synchronization processed signal, wherein the synchronization processing unit calculates a correlation value of a signal being a delayed input signal and the input signal, and performs the synchronization processing when the correlation value satisfies a condition.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-217216, filed on Jul. 27, 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a demodulator.  
         [0004]     2. Description of the Related Art  
         [0005]      FIG. 10  is a diagram showing a constitutional example of an orthogonal frequency division multiplex (OFDM) demodulator  1001 . In a television receiving apparatus for terrestrial digital broadcast, the OFDM demodulator  1001  is provided. The OFDM demodulator  1001  includes an A/D conversion unit  1002 , a demodulation processing unit  1003 , a synchronization processing unit  1004 , an error correction processing unit  1005 , a frame synchronization processing and TMCC detection unit  1006 , and a control circuit  1007 .  
         [0006]      FIG. 11  is a flowchart showing a processing of a channel search. The channel search is the processing to search a receivable channel, and it is enough that the processing is performed once, for example, at a time of installment of the television receiving apparatus.  
         [0007]     In a step S 1101 , when a start of the channel search is instructed, the following processings are performed. In a step S 1102 , the control circuit  1007  instructs an outside tuner which channel to receive. For example, the control circuit  1007  instructs receiving of Channel One. Then, the tuner outputs a signal of that channel to the A/D conversion unit  1002 . The A/D conversion unit  1002  converts an input signal from an analog format to a digital format, and outputs to the synchronization processing unit  1004  and the demodulation processing unit  1003 .  
         [0008]     Next, in a step S 1103 , the synchronization processing unit  1004  performs a synchronization processing based on an output signal of the A/D conversion unit  1002 . Next, in a step S 1104 , the demodulation processing unit  1003  performs a demodulation processing at a predetermined timing based on output signals of the A/D conversion unit  1002  and the synchronization processing unit  1004 , and outputs to the frame synchronization processing and TMCC detection unit  1006 , and the error correction processing unit  1005 . The demodulation processing includes an OFDM demodulation and a phase demodulation.  
         [0009]     Next, in a step S 1105 , the frame synchronization processing unit  1006  performs a frame synchronization processing based on an output signal of the demodulation processing unit  1003 . Specifically, the frame synchronization processing unit  1006  detects a synchronizing signal (synchronizing byte) in the output signal of the demodulation processing unit  1003 . When the synchronizing signal cannot be detected, that channel is judged to be unreceivable and a process returns to the step S 1002 . In the step S 1002 , a next channel (for example, Channel Two) is set, and the same processings are repeated.  
         [0010]     When the synchronizing signal is detected, the frame synchronization processing is performed and the process proceeds to a step S 1106 . In the step S 1106 , the TMCC detection unit  1006  detects TMCC (Transmission and Multiplexing Configuration Control) information in the output signal of the demodulation processing unit  1003 . The TMCC information includes information of a phase (carrier) modulation scheme.  
         [0011]     Next, in a step S 1107 , it is judged that an ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) signal of that channel is receivable. The ISDB-T signal is a receive signal by a standard of the terrestrial digital broadcast, and adopts an OFDM modulation scheme. Subsequently, the process returns to the step S 1002 . In the step S 1002 , a next channel (for example, Channel Two) is set and the same processings are repeated. By repeating the above processings for all the channels, receivable channels can be searched.  
         [0012]     At a time of a normal reception, when a user designates a channel, the tuner outputs an ISDB-T signal of the designated channel to the OFDM demodulator  1001 . Subsequently, the processings from steps S 1103  to S 1106  in  FIG. 11  are performed. Then, the error correction processing unit  1005  performs an error correction based on the output signals of the demodulation processing unit  1003 , and the frame synchronization processing and TMCC detection unit  1006 . Specifically, the error correction processing unit  1005  error-corrects the output signal of the demodulation processing unit  1003  using the TMCC information detected by the TMCC detection unit  1006 , and outputs an MPEG 2 transport stream. The MPEG 2 transport stream is decoded by an MPEG 2 decoder, and image and audio are reproduced.  
         [0013]     In below-sated Patent Document 1, there is described a broadcast signal identification apparatus which sets a certain band centering on a picture frequency of an NTSC signal, measures a peak value of electric power within this certain band and an electric power value of the entire band, and judges whether a broadcast signal of a received channel is an analog broadcast wave or a digital broadcast wave.  
         [0014]     In below-stated Patent Document 2, there is described a broadcast reception apparatus which tentatively judges presence/absence of digital broadcast in a selected channel by whether or not a TMCC signal is obtained by a demodulation processing, and, in a state that presence of the digital broadcast is tentatively judged, judges presence/absence of the digital broadcast in the selected channel by whether or not section information (NIT) is obtained by a selected channel information extraction processing.  
         [0015]     In below-stated Patent Document 3, there is described a terrestrial digital broadcast reception apparatus which, based on a detection signal detected by an analog broadcast wave detection means, can skip a search in a corresponding channel in a front end unit for digital broadcast, when the apparatus performs a channel search which is to be performed at a time of installation of the terrestrial digital broadcast reception apparatus.  
         [0016]     In below-stated Patent Document 4, there is described a reception apparatus in which an OFDM receiver of ISDB-T standard presets TMCC information to correspond to respective broadcast stations in a memory in advance, and when a user selects a broadcast station, a control circuit reads out the TMCC information corresponding to that broadcast station from the memory, then simultaneously with a start of a reception operation, the above read-out TMCC information is given to the respective circuits, to carry out setting of, for example, a guard interval, a carrier modulation scheme, and so on.  
         [0017]     [Patent Document 1] Japanese Patent Application Laid-open No. 2003-234975  
         [0018]     [Patent Document 2] Japanese Patent Application Laid-open No. 2004-363806  
         [0019]     [Patent Document 3] Japanese Patent Application Laid-open No. 2003-69907  
         [0020]     [Patent Document 4] Japanese Patent Application Laid-open No. 2001-292121  
         [0021]     However, in  FIG. 11 , since it is necessary to perform a synchronization processing of a step S 1103 , a demodulation processing of a step S 1104 , and a frame synchronization processing of a step S 1105  for every channel after the channel is set, search time per one channel becomes long. Additionally, misjudgment may occur in a channel search due to influence of a NTSC (National Television Standard Committee) signal or the like of analog television broadcast.  
       SUMMARY OF THE INVENTION  
       [0022]     The present invention is objected to prevent misjudgment in a channel search and/or to shorten channel search time.  
         [0023]     A demodulator of the present invention includes: a filter filtering a signal based on pass frequency bands changed at a time of a channel search and at a time of normal reception; and a demodulation processing unit performing an orthogonal frequency division multiplex demodulation based on the filtered signal.  
         [0024]     Also, a demodulator of the present invention includes: a synchronization processing unit performing a synchronization processing based on an input signal; and a demodulation processing unit performing an orthogonal frequency division multiplex demodulation based on the synchronization processed signal, wherein the synchronization processing unit calculates a correlation value of a signal being a delayed input signal and the input signal, and performs the synchronization processing when the correlation value satisfies a condition.  
         [0025]     Also, a demodulator of the present invention includes: a synchronization processing unit performing a synchronization processing based on an input signal; and a demodulation processing unit performing an orthogonal frequency division multiplex demodulation based on the synchronization processed signal, wherein the demodulation processing unit performs the orthogonal frequency division multiplex demodulation when frequency allocation of a reference signal in the input signal satisfies a condition.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  is a diagram showing a constitutional example of a television receiving apparatus according to an embodiment of the present invention;  
         [0027]      FIG. 2  is a flowchart showing a processing of a channel search;  
         [0028]      FIG. 3  is a diagram of a frequency spectrum showing pass frequency bands of a FIR type digital filter;  
         [0029]      FIG. 4  is a diagram showing a constitutional example of a part of a synchronization processing unit;  
         [0030]      FIG. 5  is a diagram showing one symbol amounts of an input signal and a delay signal;  
         [0031]      FIG. 6  is a diagram showing a constitutional example of a part of a demodulation processing unit;  
         [0032]      FIG. 7  is a diagram showing a constitutional example of a part of a frame synchronization processing unit;  
         [0033]      FIG. 8  is a diagram showing a constitutional example of a memory recording channel information and TMCC information;  
         [0034]      FIG. 9  is a flowchart showing a processing of a normal reception;  
         [0035]      FIG. 10  is a diagram showing a constitutional example of an OFDM demodulator; and  
         [0036]      FIG. 11  is a flowchart showing a processing of a channel search.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]      FIG. 1  is a diagram showing a constitutional example of a television receiving apparatus  101  according to an embodiment of the present invention. The television receiving apparatus  101  includes an OFDM demodulator  102 , a tuner  103 , an MPEG 2 decoder  104 , and a memory  105 , and can receive an ISDB-T signal of terrestrial digital broadcast. The OFDM demodulator  102  includes an A/D conversion unit  111 , a FIR (Finite Impulse Response) type digital filter  112 , a selector  113 , a demodulation processing unit  114 , a synchronization processing unit  115 , an error correction processing unit  116 , a frame synchronization processing and TMCC detection unit  117 , and a control circuit  118 .  
         [0038]      FIG. 2  is a flowchart showing a processing of a channel search. The channel search is the processing to search a receivable channel, and it is enough that the processing is performed once, for example, at a time of installation of the television receiving apparatus  101 .  
         [0039]     In a step S 201 , when a start of the channel search is instructed, the following processings are performed. In a step S 202 , filter setting is carried out. A control signal  121  designates either a channel search mode or a normal reception mode. The selector  113  sets a filter coefficient Kb to the FIR type digital filter  112  when the control signal  121  designates the channel search mode, and sets a filter coefficient Ka to the FIR type digital filter  112  when the control signal  121  designates the normal reception mode. At a time of the channel search, the filter coefficient Ka is set to the FIR type digital filter  112 , since the channel search mode is designated.  
         [0040]      FIG. 3  is a diagram of a frequency spectrum showing pass frequency bands of the FIR type digital filter  112 . Presently, both terrestrial digital broadcast and terrestrial analog broadcast exist. An ISDB-T signal  303  is a terrestrial digital broadcast signal, while NTSC signals  305  and  306  are terrestrial analog broadcast signals. The NTSC signal  305  is a video carrier, while the NTSC signal  306  is a sound carrier. The ISDB-T signal  303  is a signal to be demodulated and has a plurality of TMCC signals  304  at specified frequency intervals (periodically).  
         [0041]     As for the ISDB-T signal  303 , a frequency bandwidth of one channel CH is 6 MHz. At the time of the normal reception, the selector  113  sets the filter coefficient Ka to the FIR type digital filter  112 . Then, the FIR type digital filter  112 , to which a pass frequency band  301  is set, passes only a signal Ps for one channel CH.  
         [0042]     The frequency bands of the ISDB-T signal  303  and NTSC signals  305 ,  306  are overlapped. At a time of a channel search of the ISDB-T signal  303 , the NTSC signals  305  and  306  become obstacles, and cause misjudgment in the channel search. Therefore, at the time of the channel search, the selector  113  sets the filter coefficient Kb to the FIR type digital filter  112 . Then, the FIR type digital filter  112 , to which a pass frequency band  302  is set, passes only a signal Pb. Since a single or a plurality of TMCC signal(s)  304  is (are) included in the signal Pb, it is possible to perform the channel search based on the signal Pb. By removing (attenuating) the NTSC signals  305  and  306  by the FIR type digital filter  112  and performing the channel search based on the signal Pb, misjudgment in the channel search can be prevented.  
         [0043]     The FIR type digital filter  112  filters a signal based on the pass frequency bands changed at the time of the channel search and at the time of the normal reception. Additionally, the FIR type digital filter  112  can reduce circuit scale since the FIR type digital filter  112  serves as the filter for both the normal reception and the channel search.  
         [0044]     Next, in a step S 203  of  FIG. 2 , the control circuit  118  instructs the tuner  103  which channel to receive. For example, the control circuit  118  instructs receiving of Channel One. Then, the tuner  103  outputs an ISDB-T signal of that channel to the A/D conversion unit  111 . The A/D conversion unit  111  converts an input signal from an analog format to a digital format, and outputs to the FIR type digital filter  112 . The FIR type digital filter  112 , to which the pass frequency band  302  is set, passes only the signal Pb by filtering the input signals, and then outputs to the synchronization processing unit  115  and the demodulation processing unit  114 .  
         [0045]     Next, in a step S 204 , the synchronization processing unit  115  judges whether or not a correlation of guard intervals exists. If the correlation exists, a process proceeds to a step S 205 , and if the correlation does not exist, that channel is judged to be unreceivable and the process returns to the step S 203 . In the step S 203 , a next channel (for example, Channel Two) is set, and the same processings are repeated. As stated above, if the correlation does not exist, the search processing of the next channel can be performed without the synchronization processing of the step S 205 , demodulation processing of a step S 207  and a frame synchronization processing of a step S 208  being performed, so that channel search time can be shortened. Hereinafter, a detailed processing of the synchronization processing unit  115  will be described.  
         [0046]      FIG. 4  is a diagram showing a constitutional example of a part of the synchronization processing unit  115 . An input signal A 411  is an output signal of the FIR type digital filter  112 . A delay processing unit  401  outputs a signal A 412  made by delaying the input signal A 411  by one symbol.  
         [0047]      FIG. 5  is a diagram showing one symbol amounts of the input signal A 411  and the delay signal A 412 . Receive signals include a delay signal (signal arriving after reflected on a building or the like) in addition to a direct signal. In order to separate the direct signal and the delay signal, in the ISDB-T signal A 411 , a rear end portion  502  of the symbol is copied and made to be a front end portion  501 . Making a signal into cyclic construction as described above is the guard interval. Therefore, the rear end portion  502  of the input signal A 411  and the front end portion  501  of the delay signal A 412  are the same, and a correlation value of the both in this guard interval period is high. If the correlation value is high, it is highly possible that the ISDB-T signal of that channel is receivable, and if the correlation value is low, the ISDB-T signal of that channel can be judged to be unreceivable.  
         [0048]     In  FIG. 4 , a correlation processing unit  402  calculates a correlation value of the input signal A 411  and the delay signal A 412  in the guard interval period. A peak detection unit  403  takes a moving average (integral) of the above correlation value and detects its peak value. A comparison unit  405  compares the peak value and a threshold value  404 , and outputs a comparison result. When the peak value exceeds the threshold value  404 , a correlation is judged to exist and the process proceeds to the step S 205  of  FIG. 2 . When the peak value is less than or equal to the threshold value  404 , the correlation is judged not to exist and the process returns to the step S 203  of  FIG. 2 .  
         [0049]     In the step S 205 , the synchronization processing unit  115  performs the synchronization processing based on the output signal of the FIR type digital filter  112 .  
         [0050]     Next, in a step S 206 , the demodulation processing unit  114  judges the number of reference signals in the input signals and whether or not frequency allocation has a correlation with a predetermined one. The reference signal is a TMCC signal and/or an AC (Auxiliary Channel) signal. The A/C signal is a preliminary signal. If the correlation exists, the process proceeds to the step S 207 , and if the correlation does not exist, that channel is judged to be unreceivable and the process returns to the step S 203 . In the step S 203 , the next channel (for example, Channel Two) is set and the same processings are repeated. As stated above, if the correlation does not exist, a search processing of the next channel can be performed without the demodulation processing of the step S 207  and the frame synchronization processing in the step S 208  being performed, so that channel search time can be shortened. Hereinafter, a detailed processing of the demodulation processing unit  114  will be described.  
         [0051]      FIG. 6  is a diagram showing a constitutional example of a part of the demodulation processing unit  114 . A fast Fourier transform (FFT) processing unit  601  transforms an input signal into a frequency component signal by FFT, and outputs to an AC and TMCC processing unit  602 . For example, the signal Pb on a frequency axis of  FIG. 3  is obtained. The AC and TMCC processing unit  602  detects the AC signal and TMCC signal A 611  in the input signal. A correlation processing unit  604  calculates a correlation value of the number and frequency allocation of the AC signal and TMCC signal A 611 , and the number and frequency allocation information  603  of predetermined AC signal and TMCC signal. A peak detection unit  605  takes a moving average (integral) of the above correlation value and detects its peak value. A comparison unit  607  compares the peak value and a threshold value  606 , and outputs a comparison result. When the peak value exceeds the threshold value  606 , a correlation is judged to exist and the process proceeds to the step S 207  of  FIG. 2 . When the peak value is less than or equal to the threshold value  606 , the correlation is judged not to exist and the process returns to the step S 203  of  FIG. 2 .  
         [0052]     In the step S 207 , the demodulation processing unit  114  performs the demodulation processing based on output signals of the FIR type digital filter  112  and the synchronization processing unit  115  at a predetermined timing, and outputs to the frame synchronization processing and TMCC detection unit  117  and the error correction processing unit  116 . The demodulation processing includes an OFDM demodulation and a phase demodulation.  
         [0053]     Next, in the step S 208 , the frame synchronization processing unit  117  performs a frame synchronization processing based on an output signal of the demodulation processing unit  114 . One frame consists of  204  symbols. Specifically, the frame synchronization processing unit  117  detects a synchronizing signal (synchronizing byte) in the output signals of the demodulation processing unit  114 . When the synchronizing signal cannot be detected, that channel is judged to be unreceivable and the process returns to the step S 203 . In the step S 203 , the next channel (for example, Channel Two) is set, and the same processings are repeated. When the synchronizing signal is detected, the frame synchronization processing is performed and the process proceeds to a step S 209 . Hereinafter, a detailed processing of the frame synchronization processing unit  117  will be described.  
         [0054]      FIG. 7  is a diagram showing a constitutional example of a part of the frame synchronization processing unit  117 . Input signals A 611  are the AC signal and TMCC signal which the AC and TMCC processing unit  602  of  FIG. 6  outputs. A comparison unit  702  compares whether or not the synchronizing signal (synchronizing byte) in the AC signal and TMCC signal A 611 , and a predetermined synchronizing signal (synchronizing byte)  701  have the same values, and outputs a comparison result thereof. If the values are the same, it is judged that the synchronizing signal is detected and the process proceeds to the step S 209  of  FIG. 2 . When the values are not the same, it is judged that the synchronizing signal cannot be detected and the process returns to the step S 203  of  FIG. 2 .  
         [0055]     In the step S 209 , the TMCC detection unit  117  detects TMCC information in a TMCC signal of the output signal of the demodulation processing unit  114 . The TMCC information includes information of a phase (carrier) modulation scheme.  
         [0056]     Next, in a step S 210 , an ISDB-T signal of that channel is judged to be receivable. The ISDB-T signal is a receive signal by a standard of terrestrial digital broadcast and adopts an OFDM modulation scheme. The control circuit  118 , when a signal  122  to indicate receivablenss is inputted, performs a recording control to the memory  105  that the channel is receivable. The control circuit  118  also carries out a control such that channel information (number)  801  and TMCC information  802  are recorded correspondingly in the memory  105 , as shown in  FIG. 8 . The TMCC information  802  is the TMCC information detected in the step S 209 .  
         [0057]     Subsequently, the profess returns to the step S 203 . In the step S 203 , the next channel (for example, Channel Two) is set and the same processings are repeated. By repeating the above-described proceesings for all channels, receivable channels can be searched.  
         [0058]     In the steps S 204 , S 206  and S 208 , when the channel is judged to be unreceivable, the control circuit  118  inputs a control signal  122  indicating to that effect and can perform a recording control to the memory  105  that the channel is unreceivable. Judgment result signals  122  of the steps S 204 , S 206  and S 208  are respectively used for channel control of the tuner  103  by the control circuit  118 .  
         [0059]      FIG. 9  is a flowchart showing a processing of the normal reception. After the above-described channel search is finished, the ISDB-T signal is received by the normal reception, and image and audio can be reproduced.  
         [0060]     In the step S 901 , when a start of the normal reception is instructed, the following processings are performed. In a step S 902 , filter setting is carried out. The selector  113  sets the filter coefficient Ka to the FIR type digital filter  112 . Then, the FIR type digital filter  112 , to which the pass frequency band  301  is set, passes only the signal Pa.  
         [0061]     Next, in a step S 903 , when the user designates a channel, the tuner  103  outputs an ISDB-T signal of the designated channel to the A/D conversion unit  111  of the OFDM demodulator  102 . The A/D conversion unit  111  converts an input signal from an analog format to a digital format, and outputs to the FIR type digital filter  112 . The FIR type digital filter  112 , to which the pass frequency band  301  is set, passes only the signal Pa by filtering input signals, and outputs to the synchronization processing unit  115  and the demodulation processing unit  114 .  
         [0062]     Next, TMCC setting is carried out. The control circuit  118  reads out TMCC information  802  ( FIG. 8 ) corresponding to the channel from the memory  105  and set the information. Specifically, the TMCC detection unit  117  can set the TMCC information  802  which has been read out to the error correction processing unit  116 .  
         [0063]     Next, in a step S 904 , the same synchronization processing as that of the step S 205  of  FIG. 2  is performed. Next, in a step S 905 , the same demodulation processing as that of the step S 207  is performed. Next, in a step S 906 , the same frame synchronization processing as that of the step S 208  is performed.  
         [0064]     Next, in a step S 907 , the error correction processing unit  116  performs error correction based on output signals of the demodulation processing unit  114  and the frame synchronization processing and TMCC detection unit  117 . Specifically, the error correction processing unit  116  error-corrects the output signal of the demodulation processing unit  114  using the TMCC information  802  set in the step S 903 , and outputs an MPEG 2 transport stream to the MPEG 2 decoder  104 . The MPEG 2 decoder  104  decodes the MPEG 2 transport stream, and image and audio is reproduced.  
         [0065]     Incidentally, normally, between the steps S 906  and S 907 , the TMCC detection processing of the step S 209  of  FIG. 2  is performed. Subsequently, in the step S 907 , the TMCC information detected there is used to perform the error correction. However, in the present embodiment, since the TMCC setting is carried out in the step S 903 , the TMCC detection processing between the steps S 906  and S 907  becomes unnecessary. Therefore, the normal reception processing can be made simple and fast. The TMCC detection unit  117  detects TMCC information in the input signal at the time of the channel search of  FIG. 2 , and does not detect TMCC information at the time of the normal reception. At the time of the normal reception, the TMCC information in the memory  105  is used.  
         [0066]     As stated above, according to the present embodiment, since the unnecessary NTSC signals  305 ,  306  can be removed by the filter  112 , misjudgment in the channel search can be prevented. The synchronization processing of the step S 205  and/or the demodulation of the step S 207  is (are) carried out only when judgments in the steps S 204  and S 206  of  FIG. 2  satisfy predetermined conditions. When the predetermined condition is not satisfied, the above-described synchronization processing and/or demodulation being (an) additional processing(s) can be omitted, and channel search time can be drastically shortened.  
         [0067]     As for the judgments in the steps S 204  and S 206  of  FIG. 2 , only either one of them can be carried out. Also in that case, an effect of shortened channel search time can be obtained.  
         [0068]     Since the unnecessary signals such as the NTSC signal can be removed by the filter, misjudgment in the channel search can be prevented. Additionally, channel search time can be shortened by performing the synchronization processing or demodulation when the predetermined condition is satisfied.  
         [0069]     The above embodiments are to be considered in all respects illustrative and no restrictive, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof.