Note: Descriptions are shown in the official language in which they were submitted.
10152025CA 02265259 1999-03-l2DIGITAL BROADCAST RECEIVERBACKGROUND OF THE INVENTION1. Field of the InventionThe present invention relates to a digital broadcastreceiver, and more particularly to 21 digital broadcastreceiver with a seek function of the type that when a seekis instructed, a plurality of digital broadcast frequenciesare sequentially tuned in, and when a receivable digitalbroadcast station is found, _the seek operation isterminated.2. Description of the Related ArtIn Europe, soâcalled digital audio broadcasting (DAB)is prevailing in practice. DAB uses orthogonal frequencydivision. multiplex (OFDM) which is one Ikind of multi-carrier modulation methods. Each transmission symbol isconstituted of a guard interval and an effective symbol tothereby allow reception highly resistant to ghosts. Eachcarrier of DAB is DQPSK modulated.DAB uses three bands: band II (87 to 108 MHz band),band III (175 to 250 MHz band), and L band (1.452 to 1.492GHz band). The band II and III utilize a transmission mode1 having a transmission frame period of 96 ms and a carrierinterval of 1 kHz. The transmission mode 1 is highly10152025CA 02265259 l999-03- l2resistant to multiâpath and suitable for a single frequencynetwork (SFN), and is limited to only for use with thebands II and III. The L band utilizes transmission modes2, 3, and 4. The transmission mode 2 has a frame period of24 ms and a carrier interval of 4 kHz and is suitable formobil reception. The transmission mode 3 has a frameperiod of 24 ms and a carrier interval of 8 kHz and issuitable for satellite broadcast or the like. Thetransmission mode 4 has a frame period of 48 ms and acarrier interval of 2 kHz.The format of a transmission frame signal in thetransmission mode 1 of DAB is shown in the upper portion ofFig. 3. There are a sync signal constituted of a NULLsymbol of 1.297 ms and a phase reference symbol (PRS: PhaseReference Symbol) in the initial field, and seventy fiveOFDM symbols each of 1.246 ms in the following fields."Symbols other than the NULL symbol are transmissionsymbols. A start period of 0.246 ms of each transmissionsymbol constitutes 21 guard interval, and the remainingperiod of 1 ms constitutes an effective symbol.The transmission symbol of S = 1 is PRS used for AFC(Automatic Frequency Control) or the like, PRS beingobtained interâcarrierthrough adjacent differentialmodulation of a predetermined and specific code (called aCAZAC (Constant Amplitude Zero Auto Correlation) code).10152025' CA 02265259 1999-03-12The transmission symbols of S = 2 to 4 are FIC's (FastInformation Channels) for transmission of informationnecessary for a receiver to tune in to a desired program,auxiliary information for a program, and the like. Thetransmission symbols of S = 7 to 76 are MSC's (Main ServiceChannels) for transmission of multiplexed sub-channels ofvoices and data. Generally, one subâchannel corresponds toone program. Information on how sub-channels aremultiplexed in MSC is contained in FIG. Therefore, byreferring to FIC, a sub-channel of a program desired by auser can be located.In the transmission mode 2, each symbol period shownin Fig. 3 is shortened by 1/4. In the transmission mode 3,each symbol period shown in Fig. 3 is shortened by 1/8 andthe number of OFDM symbols is increased. In thetransmission mode 4, each symbol period shown in Fig. 3 isshortened by 1/2.Fig. 4 is a block diagram of a DAB receiver with aseek function.A DAB broadcast signalâ (called ensemble) of, forexample, in the band II, band III, or L band caught with anantenna 1 is sent to a front end 2, and the receptionsignal of the band II or III is input to an a terminal ofan RF switch 3.The reception signal of the L band issubject to a band limitation by a BPF 4, and is passed10152025CA 02265259 l999-03- 12through an AGC amplifier 5 to be input to a mixer 6 whereatit is mixed with a local oscillation signal Lo input from aPLL circuit 7 to be frequencyâconverted into a band of theband III. The signal L2 output from the PLL circuit 7 hasa frequency of f1° (no/mo), where fl is a frequency of areferenceoscillation signal input from a referenceoscillator 13 to be described later,and mo and no takefixed values. An output of the mixer 6 is applied to a Qterminal of the switch 3.An envelope of an output of the mixer 6 is detected byan envelope detector 9 and output as an AGC voltage to theAGC amplifier 5. The AGC amplifier 5 lowers or increasesits gain in accordance with the AGC voltage so that anoutput of the mixer 6 has generally a constant levelirrespective of the antenna input level.An output of the RF switch 3 is RFâamp1ifier by an RFamplifier 10 capable of changing its gain with the AGCvoltage and mixed at a mixer with a first local oscillationfrequency L1 supplied from a PLL circuit 12 to be convertedinto a first intermediate frequency signal having a centerfrequency of fml. The output L1 of the PLL circuit has afrequency of f1- (nl/ml), where fl is the frequency of areference oscillation signal supplied from a referenceoscillator 13, ng takes a fixed value, and n1 takes a valuewhich ischanged by a system controller made of a_ 4 _10152025CA 02265259 l999-03- l2microcomputer to be described later, n1 being used forchanging the tuned at a step of, for example, 16 kHz. Thereference oscillator 13 is a VCXO which changes itsoscillation frequency in accordance with an automaticfrequency adjusting control voltage. The firstintermediate frequency signal is supplied to a SAW filter(elastic surface wave filter) 14 to limit a pass-band to1.536 MHz.An output of the SAW filter 14 is supplied via an AGCamplifier 15 to a nï¬xer 16 whereat it is mixed with asecond local oscillation signal L2 input from a PLL circuit17 to be converted into a second intermediate frequencysignal having a center frequency of fâ? (< fnl). The signalIv output from the PLL circuit 17 has a frequency off1â (n-2/m2) .where fl is a referencefrequency of aoscillation signal input from the reference oscillator 13,takeand both m2 fixed values.and n2 The secondintermediate frequency signal is supplied to an anti-aliasing filter 18 to limit a pass-band to 1.536 MHz.An envelope of thesecond intermediate frequencysignal output from the antiâaliasing filter 18 is detectedby an envelope detector 19 and output as the AGC voltage tothe RF amplifier 10 and AGC circuit 15 (refer to Q in Fig.3). The RF amplifier 10 and AGC circuit 15 lower orincrease their gains in accordance with the AGC voltage so10152025CA 02265259 l999-03- 12that the second intermediate frequency signal havinggenerally a constant level independent from the antennainput level can be obtained. An output of the envelopedetector 19 is input to a NULL detector 20 to detect a NULLsymbol.NULL symbol (refer to Q in Fig. 3), and measures a lowThe NULL detector 20 shapes the waveform of thelevel time Td which corresponds to the NULL symbol period.If this low level time is coincident with a NULL symbollength of any transmission mode defined by DAB, the NULLdetector 20 outputs a. NULL symbol detection signal ND(refer to g in Fig. 3) to a timing sync circuit 21, systemcontroller, and the like, synchronously with the risetiming of the envelope signal. According to the measuredtime period Td, the NULL detector 20 also outputs atransmission mode detection signal TM which represents thetransmission mode (refer to Q in Fig. 3. It is assumedthat Td = 1.297 ms so that the transmission mode detectionsignal TM indicates the transmission mode 1).The timing sync circuit 21 generates various timingsignals during an ordinary operation, by receiving carrier-components in the phase reference symbol PRS (effectivesymbol period) input from an FFT circuit to be describedlater, calculating a carrierâpower, detecting a frame syncfrom a cepstrum obtained through IFFT of the carrierâpower,and outputting this syncdetection signal to an10152025CA 02265259 l999-03- l2unrepresented timing signal generator. However,immediately after the start of ensemble reception, thetiming sync circuit 21 detects the frame sync by using theNULL symbol detection signal ND input from the NULLdetector 20, and outputs a sync detection signal.An output of the antiâaliasing filter 18 is A/Dconverted by an A/D converter 30. An I/Q demodulator 31demodulates I/Q components to recover the transmissionframe signal shown in Fig. 3. The demodulated I/Qcomponents are subject to a FFT (Fast Fourier Transform)process by an FFT circuit 32 constituted of a dedicatedprocessor to thereby derive carrierâdependent components(complex number data representative of an amplitude andphase of each carrier) of each of g carriers constitutingan OFDM modulated wave, in the unit of symbol, where n =1536 for the transmission mode 1, n = 384 for thetransmission mode 2, n = 192 for the transmission mode 3,and n = 768 for the transmission mode 4. The FFT circuit32 outputs the carrierâdependent components during theeffective symbol period of PRS to a frequency errordetector 33 in response to predetermined timing signals.The frequency error detector 33 comprises a digital signalprocessor having a decoding software and decodes thecarrierâdependent components of PRS through inter-carrierdifferential demodulation (for PRS, a predetermined fixed10152025CA 02265259 l999-03- 12code was subject to the inter-carrier differentialmodulation on the transmission side), and thereaftercalculates a correlation function between the decodedcarrierâcomponents and a predetermined reference code(e.g., conjugate of CAZAC code). The correlation functionis shown in the graph of Fig. 7). A frequency error of thetuned frequency from the DAB broadcast signal is calculatedfrom this correlation function. While AFC is enabled bythe system controller, the frequency error detector 33outputs frequency error data to an integrator 34 (while AFCis disabled, data indicating that the frequency error iszero is output). Data integrated by the integrator 34 isD/A converted by a D/A converter and output to thereference oscillator 13 as the automatic frequencyadjusting control voltage. In accordance with this controlvoltage, thereference oscillator 13changes itsoscillation frequency to thereby change the referenceoscillation signal frequency fl and cancel the frequencyerror.The FFT circuit 32 outputs FFT carrierâcomponents(complex nmnber data representative of an amplitude andphase of each carrier) of each symbol (effective symbolperiod) of S = 2 to 76 shown in Fig. 3 to a channel decoder36. The channel decoder 36 performs frequencydeinterleaving, DQPSK symbol demapping, and FIC/MSC10152025CA 02265259 l999-03- l2separation, and outputs packet data called an FIG (FastInformation Group) to the system controller, the FIGincluding twelve FIBâs (Fast Information Blocks) obtainedthrough error detection/correction (Viterbi decoding) anddescrambling of three effective FIC symbols each dividedinto four.MSC effective symbols are classified into eighteensymbols to reconfigure four CIF's (Common InterleavedFrames). Each CIF contains a plurality of subâchannelseach corresponding to one program.when a user selects a desired program by using aprogram select key of an operation panel 40, the systemcontroller 37 performs a predetermined program selectioncontrol, and outputs information of designating a sub-channel corresponding to the desired program, by referringto FIC information. The channel decoder 36 derives thesub-channel designated by the system controller 37 fromfour CIF's, and thereafter performs time deinterleaving,error detection/correction (Viterbi decoding), error count,and descrambling to output the demodulated DAB audio framedata to a MPEG decoder 38.The MPEG decoder 38 decodes the DAB audio frame dataand outputs audio data of two channels. This audio data isD/A converted by a D/A converter 39 and output as an analogaudio signal.10152025CA 02265259 l999-03- 12The operation panel 40 is also provided with a seekkey. A memory 41 stores therein broadcast frequency dataof a plurality of ensembles. when a seek command is givenupon depression of the seek key of the operation panel 40,the system controller 39 performs a seek control. Thisseek control process will be described with reference tothe flow chart of Fig. 5.Upon reception of a seek command, the systemcontroller 37 supplies an AFC disable command to thefrequency error detector 33 to make the latter output dataindicating that the frequency error is zero and to fix theoscillation frequency of the reference oscillator 13 (StepS1 in Fig. 5).Broadcast frequency data of the first ensemble is readfrom the memory 41, and if the reception signal is the bandII or III, the RF switch 3 is turned to the terminal Q,whereas if it is the L band, the RF switch 3 is turned tothe Q terminal. Their corresponding to the first ensemblefrequency is set to the PLL circuit 12 to tune in to thefirst ensemble (Step S2). Next, it is checked whether theNULL symbol detection signal is supplied from the NULLdetector 20 (Step S3). If the ensemble is captured at thepresent reception frequency, an output of the envelopedetector 19 lowers at the NULL symbol. The NULL detector20 shapes the waveform of the output of the envelope_ 10 _10152025CA 02265259 l999-03- l2detector 19, and outputs the NULL symbol detection signalat the rise timing of the envelope signal. Upon receptionof the NULL symbol detection signal, the system controller32 judges as YES at Step S3. Since there is a DABbroadcast signal at the present reception frequency, thesystem controller 37 supplies an AFC enable signal to thefrequency error detector 33 to thereafter terminate theseek control process (Step S4).An output of the front end 2 is I/Q demodulated by theI/Q demodulator 31, and is subject to FFT at the FFTcircuit 32. The carrierâcomponents of PRS are decodedthrough inter-carrier differential demodulation by thefrequency error detector 33, and thereafter a correlationfunction between the decoded carrierâcomponents and apredetermined reference code is calculated. An example ofthis correlation function is shown in the graph of Fig. 7whose abscissa represents a frequency and ordinaterepresents a correlation value. In accordance with thiscorrelation function, a frequency error of the tunedfrequency from the DAB broadcast signal frequency can becalculated.If the center of spectrum distribution of a receivedensemble relative to the first intermediate frequency isshifted toward a frequency higher than the normal centerfrequency fml, as shown by a solid line A in Fig. 6 (one-dot_ 11 _10152025CA 02265259 l999-03- 12indicates the attenuationchain line in I Fig. 6characteristics of the SAW filter 7), the correspondingcorrelation function becomes as shown in the graph of Fig.7. while the AFC enable command is supplied to thefrequency error detector 33, it outputs frequency errordata representative of the frequency error calculated fromthe correlation function. This frequency error data isintegrated by the integrator 34, D/A converted by the D/Aconverter 35, and supplied to the reference oscillator 13.The reference oscillator 13 changes its oscillationfrequency in accordance with the supplied control voltage,and changes the first and second local oscillation signalsL1 and L2 so as to cancel the frequency error. Therefore,the spectrum distribution of the received ensemble relativeto the first intermediate frequency signal shifts to thelower frequency (refer to an arrow C in Fig. 6), andultimately enters the passâband of the SAW filter 7 asindicated at Aâ in Fig. 8. It is therefore possible forthe channel decoder 36 to correctly recover the informationof FIC and MSC. As a user selects a desired program byusing the operation panel 40, the system controller 37instructs the channel decoder 36 to supply the DAB audioframe data of the desired program to the MPEG decoder 38.In this manner, the desired program can be listened.If NO at Step S3, there is no ensemble capable of_ 12 _10152025CA 02265259 l999-03- 12being received at the presently tuned frequency, and thesystem controller 37 checks by referring to the memory 41whether there is broadcast frequency data of the nextensemble (Step S5). If not, the seek control process isterminated, whereas if present, a corresponding nlis set tothe PLL circuit 12, and after the new ensemble is tuned in,the above processes are repeated (Step S6).with the DAB receiver with a conventional seekfunction described above, when the band II or III is to bereceived, the RF switch 3 is turned to the contact a.Isolation between the terminals Q and g is about 50 dB.This isolation of the RF switch 3 is not sufficient becausehigh AGC is incorporated in order to receive an antennainput of a minimum of â 90 dBm according to the DABspecification. In the case of an ensemble A shown in Fig.9A, although the reception signal frequencyâconverted bythe mixer 6 is attenuated by 50 dB by the RF switch 3(refer to B in Fig. 9B), it is amplified by the RFamplifier 10 and AGC amplifier 15 (refer to C in Fig. 9C).While an ensemble of the band II or III is sought atsome tuning frequency, an ensemble in the L band cannot betuned with this frequency. However, if a reception signalof an ensemble of the L band frequencyâconverted by themixer 6 enters a pass band of the SAW filter 14, thereceiver operates to erroneously pull in this ensemble of-13..10152025CA 02265259 l999-03- 12the L band and the ensemble of the band II or III cannot becorrectly sought.SUMMARY OF THE INVENTIONA digital broadcast receiver according to the presentinvention comprises reception means for tuning a selectedbroadcast frequency to receive a digital broadcast signalof an OFDM modulated wave in the tuned broadcast frequency;derivingâ means for deriving carrier-components from anoutput of the reception means; program informationdemodulating means for demodulating information part (FIC,MSC) of the derived carrier-components to recover a programdesired by ea user; frequency error detecting means fordetecting a tuning frequency error by referring to acorrelation function calculated from control part (PRS) ofthe derived carrierâcomponent and a reference code;frequency adjusting means for adjusting the tuningfrequency in the reception means to eliminate the detectedtuning frequency error; NULL detecting means for detectinga NULL symbol in the output of the reception means; andcontrol means for in response to a seek instructioncontrolling the reception means to sequentially tune eachof broadcast frequencies of the digital broadcast signaland stop the seek operation when the NULL detecting meansdetects the NULL symbol at one of the sequentially tuned-14..10152025CA 02265259 l999-03- l2broadcast frequencies and then controlling the frequencyadjusting means to conduct the tuning frequency adjustmentat said one of broadcast frequency, whereinsaid control means is response to the NULL symboldetection further examines the transmission mode andcontrols the reception means to stop the seek operationwhen the examined transmission mode is a transmission modeaimed by the seeking.In the above a digital broadcast receiver according,said NULL symbol detecting means generates a transmissionmode signal which represents the NULL symbol period andsend the transmission mode signal to said control means.In the above digital broadcast receiver, said controlmeans further judges whether or not the tuning frequencyerror adjusted by the frequency adjusting means when theseek operation is stopped is less than a pmedeterminedvalue after a preselected time period has elapsed, andresumes the seek operation if the tuning frequency error isnot less so that the reception means tunes the nextbroadcast frequency.In the above digital broadcast receiver, said controlmeans turns off the tuning frequency adjustment operationby the frequency adjusting means during the seek operation.10152025CA 02265259 1999-03-l2BRIEF DESCRIPTION OF THE DRAWINGSFig. 1 is a block diagram of a DAB receiver with aseek function according to an embodiment of the invention.Fig. 2 is a flow chart illustrating a seek controlprocess to be executed by a system controller shown in Fig.1.Fig. 3 is a diagram illustrating the format of a DABtransmission frame signal and an operation of detecting aNULL symbol.Fig. 4 is a block diagram of a conventional DABreceiver with a seek function.Fig. 5 is a flow chart illustrating a seek controlprocess to be executed by a system controller shown in Fig.4.Fig. 6 is a graph showing a frequency spectrum of anensemble relative to a first intermediate frequency signal.Fig. 7 is a graph illustrating an operation of anfrequency error detector.Fig. 8 is a graph showing a frequency spectrum of anensemble relative to the first intermediate frequencysignal.Figs. 9A to 9C are graphs showing a frequency spectrumof an ensemble of the L band.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS-16-10152025CA 02265259 l999-03- 12An embodiment of the invention will be described withreference to Fig. 1.Fig. 1 is a block diagram of a DAB receiver with aseek function according to the embodiment of the invention.In Fig. 1, like elements to those shown in Fig. 4 arerepresented by using identical reference numerals.A system controller 37A constituted of a microcomputerperforms a predetermined seek control process uponreception of a seek instruction entered by depressing theseek key of an operation panel 40, and performs apredetermined program selection control upon reception ofa program selection instruction entered by the programselect key. The conditions of terminating the seek controlprocess are that a NULL symbol is detected, and that thetransmission mode of an ensemble to be sought is coincidentwith the transmission mode detected by a NULL detector 20.The other structures are quite the same as those shownin Fig. 4.The seek operation of the above embodiment will bedescribed with reference to Fig. 2 which is a flow chartillustrating the seek control process to be executed by thesystem controller 37A.It is assumed that a memory 41 stores in advancebroadcast frequency data of ten ensembles of the bands IIand III and the L band, in memory channels CH1 to CH10.-17..10152025CA 02265259 l999-03- 12Upon reception of a seek command entered from a userby depressing the seek key of the operation panel 40, thesystem controller 37A supplies an AFC disable command to afrequency error detector 33A to make the latter output dataindicating that the frequency error is zero and to fix theoscillation frequency of a reference oscillator 13 (StepS11 in Fig. 2).Broadcast frequency data of the first ensemble is readfrom the memory 41 in the memory channel CH1, and if thereception signal corresponds to the band II or III, an RFswitch 3 is turned to Q terminal, whereas if it correspondsto the L band, the switch 3 is turned to a Q terminal. Avalue nl corresponding to the broadcast frequency data ofthe first ensemble is set to a PLL circuit 12 to tune in tothe first ensemble (Step S12). Next, it is checked whetherthe NULL symbol detection signal ND is supplied from a NULLdetector 20 (Step S13). If No, there is no possibilitythat the ensemble is received at the present receptionfrequency. Therefore, broadcast frequency data of the nextensemble stored in the memory 41 in the memory channel CH2is read, and if the reception signal corresponds to theband II or III, the RF switch 3 is turned to a terminal,whereas if it corresponds to the L band, the switch 3 isturned to a Q terminal. The value n1 corresponding to thebroadcast frequency data of the second ensemble is set to-18-10152025CA 02265259 l999-03- 12the PLL circuit 12 to tune in to the second ensemble (StepsS14 and S15).when the ensemble or DAB broadcast signal is capturedat the present reception frequency, the front end 2 outputsthe second intermediate frequency signal, and an output ofthe NULL symbol from an envelope detector 19 lowers. TheNULL detector 20 shapes the waveform of the output of theenvelope detector 19, and measures a low level time Td. Ifthis low level time is coincident with a NULL symbol lengthof any transmission mode defined by DAB, the NULL detector13 outputs a NULL symbol detection signal ND synchronouslywith the rise timing of the envelope signal (refer to Fig.3). By using the NULL symbol detection signal ND, a timingsync circuit 21 detects a frame sync, and outputs a syncdetection signal to an unrepresented timing signalgenerator which generates various timing signals.Upon reception of the NULL symbol detection signal ND,the system controller 37A judges as YES at Step S13.However, it is uncertain that the received ensemble is theband II or III, or the L band as viewed from the output ofthe front end 2.After Step S13, the system controller 37A fetches thetransmission mode detection signal TM from the NULLdetector 20. If the ensemble to be sought is the band IIor III, the transmission mode 1 is used (if a transmission-19-10152025CA 02265259 l999-03- l2distance of a radio wave is long and SFN is used, thetransmission mode 1 is used in order to have a sufficientlength of the guard interval). It is therefore checkedwhether the transmission mode designated by thetransmission detection signal TM is the transmission mode1 (Step S16). If not, it is judged that the NULL symbolwas accidentally detected because an ensemble of the L bandwas frequencyâconverted to the band III, and the flowadvances to Step S14.If the ensemble to be sought is the band II or III andthe transmission mode designated by the transmissiondetection signal TM is the transmission mode 1, there is ahigh possibility that the presently received ensemble is anensemble to be sought. The AFC enable command is thereforesupplied to the frequency error detector 33A, and a timerfor counting up a predetermined time is made to start(Steps S17 and S18).If the ensemble to be sought is the L .band, thepermitted transmission modes are modes 2, 3, and 4. It istherefore judged at Step S16 whether the transmission modedesignated by the transmission detection signal TM iscoincident with one of the transmission modes 2, 3, and 4.If not coincident, it is judged that the NULL symbol wasaccidently detected because some ensemble of the band II orIII leaked to the output side of the RF switch 3, and the-20..10152025CA 02265259 l999-03- l2flow advances to Step S14.If the ensemble to be sought is the L band and thetransmission mode designated by the transmission detectionsignal TM is one of the transmission modes 2, 3, and 4,there is a high possibility that the presently receivedensemble is an ensemble to be sought. The AFC enablecommand is therefore supplied to the frequency errordetector 33A, and the timer for counting up a predeterminedtime is made to start (Steps S17 and S18).An output of the front end 2 is I/Q demodulated by anI/Q demodulator 31, and is subject to a FFT process by aFFT circuit 32. Each time the carrierâdependent componentsof PRS are received from the FFT circuit, the frequencyerror detector 33A received the AFC enable command decodesthe carrierâdependent components through interâcarrierdifferential demodulation and calculates a correlationfunction between the carrierâdependent components and apredetermined. reference code. In accordance with thecalculated correlation function, a frequency error iscalculated, and the calculated frequency error data issupplied to an integrator 34. The frequency error data isintegrated by the integrator 34, D/A converted by a D/Aconverter 35, and output as an automatic frequencyadjusting control voltage to the reference oscillator 13.The reference oscillator 13 changes its oscillation10'152025CA 02265259 l999-03- l2frequency fl with this control voltage to change thefrequencies of the first and second local oscillationsignals L1 and L2 to cancel the frequency error.If the center frequency of the received ensemble isoriginally away from the center frequency fml of the SAWfilter 7 relative to the first intermediate frequency andthe frequency pullâin by AFC is impossible, then thefrequency error does not become small even if a time lapsesafter the AFC enable command and the ensemble cannot bereceived correctly. If the detection of the NULL symbol isoriginatedâ not from an ensemble but from a dip formedduring a mobile reception on the time axis of a TVbroadcast signal or the like other than DAB broadcastsignals, because of fading phenomenon or the like and ifthe maximum correlation value accidentally becomes equal toor higher than the reference value Sc, the frequency errordoes not become small even if a time lapses after the AFCenable command.When the timer counts up the predetermined time, thesystem controller 37A checks whether the current frequencyerror data fetched from the frequency error detector 33Ahas converged into a predetermined value or lower (StepsS19 and S20). If NO, it is judged that the NULL symbol ofthe transmission mode 1 was detected because, for example,a dip formed during a mobile reception on the time axis of-22..10152025CA 02265259 l999-03- 12a TV broadcast signal because of fading phenomenon or thelike was erroneously detected as the NULL symbol. Then,the system controller 37A supplies the AFC disable commandto the frequency error detector 33A (step S21), and theflow advances to Step S14 whereat the next ensemblecorresponding to the memory channel CH2 is tuned in torepeat the above processed.In this manner, a seek can be speeded up and performedcorrectly, without a wasteful frequency pullâin operation.On the contrary to the above, if YES at Step S20, theseek operation is terminated. because a program of thesought ensemble can be listen.A channel decoder 36 recovers information of FIC andMSC from the carrier-independent components of each symbolinput from the FFT circuit 32. when a user selects adesired program by using the operation panel 40, the systemcontroller 37A instructs the channel decoder 36 to outputthe DAB audio frame data of the desired program to a MPEGdecoder 38. In this manner, the desired program can belistened.In this embodiment, when the NULL symbol is detectedat some reception frequency of an ensemble during the seekoperation and if the transmission mode detected by the NULLdetectar 20 is coincident with the transmission mode 1because if the ensemble to be sought is the band II or III,_ 23 _10152025CA 02265259 l999-03- 12the transmission mode is the mode 1, then the AFC isenabled and the seek operation is terminated if thefrequency error converges to the predetermined value orlower in the predetermined time. In this manner, theensemble to be sought can be correctly received. If theensemble to be sought is the L band, the mode is only thetransmission modes 2, 3, and 4. Therefore, only if thetransmission. mode detected. by the NULL detector 20 iscoincident with the transmission mode permitted for the Lband, the AFC is enabled and the seek operation isterminated if the frequency error converges to thepredetermined value or lower in the predetermined time. Inthis manner, the ensemble to be sought can be correctlyreceived.In the above-described embodiment and modifications,DAB broadcasting in Europe is used. The invention is notlimited only to the DAB broadcasting, but is alsoapplicable to other broadcasting and communications such asdigital ground wave TV broadcasting and digital satellitebroadcasting.According to the invention, when the NULL symbol isdetected at some reception frequency during the seekoperation and if the transmission mode detected by thetransmission mode detecting means is coincide with thetransmission mode permitted for the digital broadcast__24 _£ CA 02265259 1999-03-l2signal to be sought, the seek operation is terminated tocorrectly receive the ensemble to be sought.10152025
