CA2265259C - Digital broadcast receiver - Google Patents

Abstract

A digital broadcast receiver is provided which can reliably perform a seek operation for an ensemble. When a NULL symbol is detected by a NULL detector 20 while a front end 2 sequentially tunes in frequencies of a plurality of ensembles, a system controller 37A checks whether the NULL detector 20 detects a transmission mode permitted for the band of the ensemble to be presently sought. If detected, the seek operation is terminated, and the automatic frequency adjusting system including a frequency error detector 33A, integrator 34, D/A converter 35, reference oscillator 13, and PLL circuits 7, 12, 17 adjusts a frequency. If the NULL detector 20 does not detect a transmission mode permitted for the band of the ensemble to be presently sought, the seek operation continues.

Description

10 15 20 25 CA 02265259 1999-03-l2 DIGITAL BROADCAST RECEIVER BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital broadcast receiver, and more particularly to 21 digital broadcast receiver with a seek function of the type that when a seek is instructed, a plurality of digital broadcast frequencies are sequentially tuned in, and when a receivable digital broadcast station is found, _the seek operation is terminated. 2. Description of the Related Art In Europe, so—called digital audio broadcasting (DAB) is prevailing in practice. DAB uses orthogonal frequency division. multiplex (OFDM) which is one Ikind of multi- carrier modulation methods. Each transmission symbol is constituted of a guard interval and an effective symbol to thereby allow reception highly resistant to ghosts. Each carrier 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 1.492 GHz band). The band II and III utilize a transmission mode 1 having a transmission frame period of 96 ms and a carrier interval of 1 kHz. The transmission mode 1 is highly 10 15 20 25 CA 02265259 l999-03- l2 resistant to multi—path and suitable for a single frequency network (SFN), and is limited to only for use with the bands II and III. The L band utilizes transmission modes 2, 3, and 4. The transmission mode 2 has a frame period of 24 ms and a carrier interval of 4 kHz and is suitable for mobil reception. The transmission mode 3 has a frame period of 24 ms and a carrier interval of 8 kHz and is suitable for satellite broadcast or the like. The transmission mode 4 has a frame period of 48 ms and a carrier interval of 2 kHz. The format of a transmission frame signal in the transmission mode 1 of DAB is shown in the upper portion of Fig. 3. There are a sync signal constituted of a NULL symbol of 1.297 ms and a phase reference symbol (PRS: Phase Reference Symbol) in the initial field, and seventy five OFDM symbols each of 1.246 ms in the following fields." Symbols other than the NULL symbol are transmission symbols. A start period of 0.246 ms of each transmission symbol constitutes 21 guard interval, and the remaining period 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 being obtained inter—carrier through adjacent differential modulation of a predetermined and specific code (called a CAZAC (Constant Amplitude Zero Auto Correlation) code). 10 15 20 25 ' CA 02265259 1999-03-12 The transmission symbols of S = 2 to 4 are FIC's (Fast Information Channels) for transmission of information necessary for a receiver to tune in to a desired program, auxiliary information for a program, and the like. The transmission symbols of S = 7 to 76 are MSC's (Main Service Channels) for transmission of multiplexed sub-channels of voices and data. Generally, one sub—channel corresponds to one program. Information on how sub-channels are multiplexed in MSC is contained in FIG. Therefore, by referring to FIC, a sub-channel of a program desired by a user can be located. In the transmission mode 2, each symbol period shown in Fig. 3 is shortened by 1/4. In the transmission mode 3, each symbol period shown in Fig. 3 is shortened by 1/8 and the number of OFDM symbols is increased. In the transmission mode 4, each symbol period shown in Fig. 3 is shortened by 1/2. Fig. 4 is a block diagram of a DAB receiver with a seek function. A DAB broadcast signal‘ (called ensemble) of, for example, in the band II, band III, or L band caught with an antenna 1 is sent to a front end 2, and the reception signal of the band II or III is input to an a terminal of an RF switch 3. The reception signal of the L band is subject to a band limitation by a BPF 4, and is passed 10 15 20 25 CA 02265259 l999-03- 12 through an AGC amplifier 5 to be input to a mixer 6 whereat it is mixed with a local oscillation signal Lo input from a PLL circuit 7 to be frequency—converted into a band of the band III. The signal L2 output from the PLL circuit 7 has a frequency of f1° (no/mo), where fl is a frequency of a reference oscillation signal input from a reference oscillator 13 to be described later, and mo and no take fixed values. An output of the mixer 6 is applied to a Q terminal of the switch 3. An envelope of an output of the mixer 6 is detected by an envelope detector 9 and output as an AGC voltage to the AGC amplifier 5. The AGC amplifier 5 lowers or increases its gain in accordance with the AGC voltage so that an output of the mixer 6 has generally a constant level irrespective of the antenna input level. An output of the RF switch 3 is RF—amp1ifier by an RF amplifier 10 capable of changing its gain with the AGC voltage and mixed at a mixer with a first local oscillation frequency L1 supplied from a PLL circuit 12 to be converted into a first intermediate frequency signal having a center frequency of f ml. The output L1 of the PLL circuit has a frequency of f1- (nl/ml), where fl is the frequency of a reference oscillation signal supplied from a reference oscillator 13, ng takes a fixed value, and n1 takes a value which is changed by a system controller made of a _ 4 _ 10 15 20 25 CA 02265259 l999-03- l2 microcomputer to be described later, n1 being used for changing the tuned at a step of, for example, 16 kHz. The reference oscillator 13 is a VCXO which changes its oscillation frequency in accordance with an automatic frequency adjusting control voltage. The first intermediate frequency signal is supplied to a SAW filter (elastic surface wave filter) 14 to limit a pass-band to 1.536 MHz. An output of the SAW filter 14 is supplied via an AGC amplifier 15 to a nfixer 16 whereat it is mixed with a second local oscillation signal L2 input from a PLL circuit 17 to be converted into a second intermediate frequency signal having a center frequency of f”? (< fnl). The signal Iv output from the PLL circuit 17 has a frequency of f1’ (n-2/m2) . where fl is a reference frequency of a oscillation signal input from the reference oscillator 13, take and both m2 fixed values. and n2 The second intermediate frequency signal is supplied to an anti- aliasing filter 18 to limit a pass-band to 1.536 MHz. An envelope of the second intermediate frequency signal output from the anti—aliasing filter 18 is detected by an envelope detector 19 and output as the AGC voltage to the RF amplifier 10 and AGC circuit 15 (refer to Q in Fig. 3). The RF amplifier 10 and AGC circuit 15 lower or increase their gains in accordance with the AGC voltage so 10 15 20 25 CA 02265259 l999-03- 12 that the second intermediate frequency signal having generally a constant level independent from the antenna input level can be obtained. An output of the envelope detector 19 is input to a NULL detector 20 to detect a NULL symbol. NULL symbol (refer to Q in Fig. 3), and measures a low The NULL detector 20 shapes the waveform of the level time Td which corresponds to the NULL symbol period. If this low level time is coincident with a NULL symbol length of any transmission mode defined by DAB, the NULL detector 20 outputs a. NULL symbol detection signal ND (refer to g in Fig. 3) to a timing sync circuit 21, system controller, and the like, synchronously with the rise timing of the envelope signal. According to the measured time period Td, the NULL detector 20 also outputs a transmission mode detection signal TM which represents the transmission mode (refer to Q in Fig. 3. It is assumed that Td = 1.297 ms so that the transmission mode detection signal TM indicates the transmission mode 1). The timing sync circuit 21 generates various timing signals during an ordinary operation, by receiving carrier- components in the phase reference symbol PRS (effective symbol period) input from an FFT circuit to be described later, calculating a carrier—power, detecting a frame sync from a cepstrum obtained through IFFT of the carrier—power, and outputting this sync detection signal to an 10 15 20 25 CA 02265259 l999-03- l2 unrepresented timing signal generator. However, immediately after the start of ensemble reception, the timing sync circuit 21 detects the frame sync by using the NULL symbol detection signal ND input from the NULL detector 20, and outputs a sync detection signal. An output of the anti—aliasing filter 18 is A/D converted by an A/D converter 30. An I/Q demodulator 31 demodulates I/Q components to recover the transmission frame signal shown in Fig. 3. The demodulated I/Q components are subject to a FFT (Fast Fourier Transform) process by an FFT circuit 32 constituted of a dedicated processor to thereby derive carrier—dependent components (complex number data representative of an amplitude and phase of each carrier) of each of g carriers constituting an OFDM modulated wave, in the unit of symbol, where n = 1536 for the transmission mode 1, n = 384 for the transmission mode 2, n = 192 for the transmission mode 3, and n = 768 for the transmission mode 4. The FFT circuit 32 outputs the carrier—dependent components during the effective symbol period of PRS to a frequency error detector 33 in response to predetermined timing signals. The frequency error detector 33 comprises a digital signal processor having a decoding software and decodes the carrier—dependent components of PRS through inter-carrier differential demodulation (for PRS, a predetermined fixed 10 15 20 25 CA 02265259 l999-03- 12 code was subject to the inter-carrier differential modulation on the transmission side), and thereafter calculates a correlation function between the decoded carrier—components and a predetermined reference code (e.g., conjugate of CAZAC code). The correlation function is shown in the graph of Fig. 7). A frequency error of the tuned frequency from the DAB broadcast signal is calculated from this correlation function. While AFC is enabled by the system controller, the frequency error detector 33 outputs frequency error data to an integrator 34 (while AFC is disabled, data indicating that the frequency error is zero is output). Data integrated by the integrator 34 is D/A converted by a D/A converter and output to the reference oscillator 13 as the automatic frequency adjusting control voltage. In accordance with this control voltage, the reference oscillator 13 changes its oscillation frequency to thereby change the reference oscillation signal frequency fl and cancel the frequency error. The FFT circuit 32 outputs FFT carrier—components (complex nmnber data representative of an amplitude and phase of each carrier) of each symbol (effective symbol period) of S = 2 to 76 shown in Fig. 3 to a channel decoder 36. The channel decoder 36 performs frequency deinterleaving, DQPSK symbol demapping, and FIC/MSC 10 15 20 25 CA 02265259 l999-03- l2 separation, and outputs packet data called an FIG (Fast Information Group) to the system controller, the FIG including twelve FIB‘s (Fast Information Blocks) obtained through error detection/correction (Viterbi decoding) and descrambling of three effective FIC symbols each divided into four. MSC effective symbols are classified into eighteen symbols to reconfigure four CIF's (Common Interleaved Frames). Each CIF contains a plurality of sub—channels each corresponding to one program. when a user selects a desired program by using a program select key of an operation panel 40, the system controller 37 performs a predetermined program selection control, and outputs information of designating a sub- channel corresponding to the desired program, by referring to FIC information. The channel decoder 36 derives the sub-channel designated by the system controller 37 from four CIF's, and thereafter performs time deinterleaving, error detection/correction (Viterbi decoding), error count, and descrambling to output the demodulated DAB audio frame data to a MPEG decoder 38. The MPEG decoder 38 decodes the DAB audio frame data and outputs audio data of two channels. This audio data is D/A converted by a D/A converter 39 and output as an analog audio signal. 10 15 20 25 CA 02265259 l999-03- 12 The operation panel 40 is also provided with a seek key. A memory 41 stores therein broadcast frequency data of a plurality of ensembles. when a seek command is given upon depression of the seek key of the operation panel 40, the system controller 39 performs a seek control. This seek control process will be described with reference to the flow chart of Fig. 5. Upon reception of a seek command, the system controller 37 supplies an AFC disable command to the frequency error detector 33 to make the latter output data indicating that the frequency error is zero and to fix the oscillation frequency of the reference oscillator 13 (Step S1 in Fig. 5). Broadcast frequency data of the first ensemble is read from the memory 41, and if the reception signal is the band II 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 to the Q terminal. Their corresponding to the first ensemble frequency is set to the PLL circuit 12 to tune in to the first ensemble (Step S2). Next, it is checked whether the NULL symbol detection signal is supplied from the NULL detector 20 (Step S3). If the ensemble is captured at the present reception frequency, an output of the envelope detector 19 lowers at the NULL symbol. The NULL detector 20 shapes the waveform of the output of the envelope _ 10 _ 10 15 20 25 CA 02265259 l999-03- l2 detector 19, and outputs the NULL symbol detection signal at the rise timing of the envelope signal. Upon reception of the NULL symbol detection signal, the system controller 32 judges as YES at Step S3. Since there is a DAB broadcast signal at the present reception frequency, the system controller 37 supplies an AFC enable signal to the frequency error detector 33 to thereafter terminate the seek control process (Step S4). An output of the front end 2 is I/Q demodulated by the I/Q demodulator 31, and is subject to FFT at the FFT circuit 32. The carrier—components of PRS are decoded through inter-carrier differential demodulation by the frequency error detector 33, and thereafter a correlation function between the decoded carrier—components and a predetermined reference code is calculated. An example of this correlation function is shown in the graph of Fig. 7 whose abscissa represents a frequency and ordinate represents a correlation value. In accordance with this correlation function, a frequency error of the tuned frequency from the DAB broadcast signal frequency can be calculated. If the center of spectrum distribution of a received ensemble relative to the first intermediate frequency is shifted toward a frequency higher than the normal center frequency fml, as shown by a solid line A in Fig. 6 (one-dot _ 11 _ 10 15 20 25 CA 02265259 l999-03- 12 indicates the attenuation chain line in I Fig. 6 characteristics of the SAW filter 7), the corresponding correlation function becomes as shown in the graph of Fig. 7. while the AFC enable command is supplied to the frequency error detector 33, it outputs frequency error data representative of the frequency error calculated from the correlation function. This frequency error data is integrated by the integrator 34, D/A converted by the D/A converter 35, and supplied to the reference oscillator 13. The reference oscillator 13 changes its oscillation frequency in accordance with the supplied control voltage, and changes the first and second local oscillation signals L1 and L2 so as to cancel the frequency error. Therefore, the spectrum distribution of the received ensemble relative to the first intermediate frequency signal shifts to the lower frequency (refer to an arrow C in Fig. 6), and ultimately enters the pass—band of the SAW filter 7 as indicated at A‘ in Fig. 8. It is therefore possible for the channel decoder 36 to correctly recover the information of FIC and MSC. As a user selects a desired program by using the operation panel 40, the system controller 37 instructs the channel decoder 36 to supply the DAB audio frame 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 _ 10 15 20 25 CA 02265259 l999-03- 12 being received at the presently tuned frequency, and the system controller 37 checks by referring to the memory 41 whether there is broadcast frequency data of the next ensemble (Step S5). If not, the seek control process is terminated, whereas if present, a corresponding nlis set to the 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 seek function described above, when the band II or III is to be received, 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 because high AGC is incorporated in order to receive an antenna input of a minimum of — 90 dBm according to the DAB specification. In the case of an ensemble A shown in Fig. 9A, although the reception signal frequency—converted by the mixer 6 is attenuated by 50 dB by the RF switch 3 (refer to B in Fig. 9B), it is amplified by the RF amplifier 10 and AGC amplifier 15 (refer to C in Fig. 9C). While an ensemble of the band II or III is sought at some tuning frequency, an ensemble in the L band cannot be tuned with this frequency. However, if a reception signal of an ensemble of the L band frequency—converted by the mixer 6 enters a pass band of the SAW filter 14, the receiver operates to erroneously pull in this ensemble of -13.. 10 15 20 25 CA 02265259 l999-03- 12 the L band and the ensemble of the band II or III cannot be correctly sought. SUMMARY OF THE INVENTION A digital broadcast receiver according to the present invention comprises reception means for tuning a selected broadcast frequency to receive a digital broadcast signal of an OFDM modulated wave in the tuned broadcast frequency; deriving’ means for deriving carrier-components from an output of the reception means; program information demodulating means for demodulating information part (FIC, MSC) of the derived carrier-components to recover a program desired by ea user; frequency error detecting means for detecting a tuning frequency error by referring to a correlation function calculated from control part (PRS) of the derived carrier—component and a reference code; frequency adjusting means for adjusting the tuning frequency in the reception means to eliminate the detected tuning frequency error; NULL detecting means for detecting a NULL symbol in the output of the reception means; and control means for in response to a seek instruction controlling the reception means to sequentially tune each of broadcast frequencies of the digital broadcast signal and stop the seek operation when the NULL detecting means detects the NULL symbol at one of the sequentially tuned -14.. 10 15 20 25 CA 02265259 l999-03- l2 broadcast frequencies and then controlling the frequency adjusting means to conduct the tuning frequency adjustment at said one of broadcast frequency, wherein said control means is response to the NULL symbol detection further examines the transmission mode and controls the reception means to stop the seek operation when the examined transmission mode is a transmission mode aimed by the seeking. In the above a digital broadcast receiver according, said NULL symbol detecting means generates a transmission mode signal which represents the NULL symbol period and send the transmission mode signal to said control means. In the above digital broadcast receiver, said control means further judges whether or not the tuning frequency error adjusted by the frequency adjusting means when the seek operation is stopped is less than a pmedetermined value after a preselected time period has elapsed, and resumes the seek operation if the tuning frequency error is not less so that the reception means tunes the next broadcast frequency. In the above digital broadcast receiver, said control means turns off the tuning frequency adjustment operation by the frequency adjusting means during the seek operation. 10 15 20 25 CA 02265259 1999-03-l2 BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a DAB receiver with a seek function according to an embodiment of the invention. Fig. 2 is a flow chart illustrating a seek control process to be executed by a system controller shown in Fig. 1. Fig. 3 is a diagram illustrating the format of a DAB transmission frame signal and an operation of detecting a NULL symbol. Fig. 4 is a block diagram of a conventional DAB receiver with a seek function. Fig. 5 is a flow chart illustrating a seek control process to be executed by a system controller shown in Fig. 4. Fig. 6 is a graph showing a frequency spectrum of an ensemble relative to a first intermediate frequency signal. Fig. 7 is a graph illustrating an operation of an frequency error detector. Fig. 8 is a graph showing a frequency spectrum of an ensemble relative to the first intermediate frequency signal. Figs. 9A to 9C are graphs showing a frequency spectrum of an ensemble of the L band. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS -16- 10 15 20 25 CA 02265259 l999-03- 12 An embodiment of the invention will be described with reference to Fig. 1. Fig. 1 is a block diagram of a DAB receiver with a seek function according to the embodiment of the invention. In Fig. 1, like elements to those shown in Fig. 4 are represented by using identical reference numerals. A system controller 37A constituted of a microcomputer performs a predetermined seek control process upon reception of a seek instruction entered by depressing the seek key of an operation panel 40, and performs a predetermined program selection control upon reception of a program selection instruction entered by the program select key. The conditions of terminating the seek control process are that a NULL symbol is detected, and that the transmission mode of an ensemble to be sought is coincident with the transmission mode detected by a NULL detector 20. The other structures are quite the same as those shown in Fig. 4. The seek operation of the above embodiment will be described with reference to Fig. 2 which is a flow chart illustrating the seek control process to be executed by the system controller 37A. It is assumed that a memory 41 stores in advance broadcast frequency data of ten ensembles of the bands II and III and the L band, in memory channels CH1 to CH10. -17.. 10 15 20 25 CA 02265259 l999-03- 12 Upon reception of a seek command entered from a user by depressing the seek key of the operation panel 40, the system controller 37A supplies an AFC disable command to a frequency error detector 33A to make the latter output data indicating that the frequency error is zero and to fix the oscillation frequency of a reference oscillator 13 (Step S11 in Fig. 2). Broadcast frequency data of the first ensemble is read from the memory 41 in the memory channel CH1, and if the reception signal corresponds to the band II or III, an RF switch 3 is turned to Q terminal, whereas if it corresponds to the L band, the switch 3 is turned to a Q terminal. A value nl corresponding to the broadcast frequency data of the first ensemble is set to a PLL circuit 12 to tune in to the first ensemble (Step S12). Next, it is checked whether the NULL symbol detection signal ND is supplied from a NULL detector 20 (Step S13). If No, there is no possibility that the ensemble is received at the present reception frequency. Therefore, broadcast frequency data of the next ensemble stored in the memory 41 in the memory channel CH2 is read, and if the reception signal corresponds to the band II or III, the RF switch 3 is turned to a terminal, whereas if it corresponds to the L band, the switch 3 is turned to a Q terminal. The value n1 corresponding to the broadcast frequency data of the second ensemble is set to -18- 10 15 20 25 CA 02265259 l999-03- 12 the PLL circuit 12 to tune in to the second ensemble (Steps S14 and S15). when the ensemble or DAB broadcast signal is captured at the present reception frequency, the front end 2 outputs the second intermediate frequency signal, and an output of the NULL symbol from an envelope detector 19 lowers. The NULL detector 20 shapes the waveform of the output of the envelope detector 19, and measures a low level time Td. If this low level time is coincident with a NULL symbol length of any transmission mode defined by DAB, the NULL detector 13 outputs a NULL symbol detection signal ND synchronously with the rise timing of the envelope signal (refer to Fig. 3). By using the NULL symbol detection signal ND, a timing sync circuit 21 detects a frame sync, and outputs a sync detection signal to an unrepresented timing signal generator 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 the band II or III, or the L band as viewed from the output of the front end 2. After Step S13, the system controller 37A fetches the transmission mode detection signal TM from the NULL detector 20. If the ensemble to be sought is the band II or III, the transmission mode 1 is used (if a transmission -19- 10 15 20 25 CA 02265259 l999-03- l2 distance of a radio wave is long and SFN is used, the transmission mode 1 is used in order to have a sufficient length of the guard interval). It is therefore checked whether the transmission mode designated by the transmission detection signal TM is the transmission mode 1 (Step S16). If not, it is judged that the NULL symbol was accidentally detected because an ensemble of the L band was frequency—converted to the band III, and the flow advances to Step S14. If the ensemble to be sought is the band II or III and the transmission mode designated by the transmission detection signal TM is the transmission mode 1, there is a high possibility that the presently received ensemble is an ensemble to be sought. The AFC enable command is therefore supplied to the frequency error detector 33A, and a timer for counting up a predetermined time is made to start (Steps S17 and S18). If the ensemble to be sought is the L .band, the permitted transmission modes are modes 2, 3, and 4. It is therefore judged at Step S16 whether the transmission mode designated by the transmission detection signal TM is coincident with one of the transmission modes 2, 3, and 4. If not coincident, it is judged that the NULL symbol was accidently detected because some ensemble of the band II or III leaked to the output side of the RF switch 3, and the -20.. 10 15 20 25 CA 02265259 l999-03- l2 flow advances to Step S14. If the ensemble to be sought is the L band and the transmission mode designated by the transmission detection signal TM is one of the transmission modes 2, 3, and 4, there is a high possibility that the presently received ensemble is an ensemble to be sought. The AFC enable command is therefore supplied to the frequency error detector 33A, and the timer for counting up a predetermined time is made to start (Steps S17 and S18). An output of the front end 2 is I/Q demodulated by an I/Q demodulator 31, and is subject to a FFT process by a FFT circuit 32. Each time the carrier—dependent components of PRS are received from the FFT circuit, the frequency error detector 33A received the AFC enable command decodes the carrier—dependent components through inter—carrier differential demodulation and calculates a correlation function between the carrier—dependent components and a predetermined. reference code. In accordance with the calculated correlation function, a frequency error is calculated, and the calculated frequency error data is supplied to an integrator 34. The frequency error data is integrated by the integrator 34, D/A converted by a D/A converter 35, and output as an automatic frequency adjusting control voltage to the reference oscillator 13. The reference oscillator 13 changes its oscillation 10 '15 20 25 CA 02265259 l999-03- l2 frequency fl with this control voltage to change the frequencies of the first and second local oscillation signals L1 and L2 to cancel the frequency error. If the center frequency of the received ensemble is originally away from the center frequency fml of the SAW filter 7 relative to the first intermediate frequency and the frequency pull—in by AFC is impossible, then the frequency error does not become small even if a time lapses after the AFC enable command and the ensemble cannot be received correctly. If the detection of the NULL symbol is originated‘ not from an ensemble but from a dip formed during a mobile reception on the time axis of a TV broadcast signal or the like other than DAB broadcast signals, because of fading phenomenon or the like and if the maximum correlation value accidentally becomes equal to or higher than the reference value Sc, the frequency error does not become small even if a time lapses after the AFC enable command. When the timer counts up the predetermined time, the system controller 37A checks whether the current frequency error data fetched from the frequency error detector 33A has converged into a predetermined value or lower (Steps S19 and S20). If NO, it is judged that the NULL symbol of the transmission mode 1 was detected because, for example, a dip formed during a mobile reception on the time axis of -22.. 10 15 20 25 CA 02265259 l999-03- 12 a TV broadcast signal because of fading phenomenon or the like was erroneously detected as the NULL symbol. Then, the system controller 37A supplies the AFC disable command to the frequency error detector 33A (step S21), and the flow advances to Step S14 whereat the next ensemble corresponding to the memory channel CH2 is tuned in to repeat the above processed. In this manner, a seek can be speeded up and performed correctly, without a wasteful frequency pull—in operation. On the contrary to the above, if YES at Step S20, the seek operation is terminated. because a program of the sought ensemble can be listen. A channel decoder 36 recovers information of FIC and MSC from the carrier-independent components of each symbol input from the FFT circuit 32. when a user selects a desired program by using the operation panel 40, the system controller 37A instructs the channel decoder 36 to output the DAB audio frame data of the desired program to a MPEG decoder 38. In this manner, the desired program can be listened. In this embodiment, when the NULL symbol is detected at some reception frequency of an ensemble during the seek operation and if the transmission mode detected by the NULL detectar 20 is coincident with the transmission mode 1 because if the ensemble to be sought is the band II or III, _ 23 _ 10 15 20 25 CA 02265259 l999-03- 12 the transmission mode is the mode 1, then the AFC is enabled and the seek operation is terminated if the frequency error converges to the predetermined value or lower in the predetermined time. In this manner, the ensemble to be sought can be correctly received. If the ensemble to be sought is the L band, the mode is only the transmission modes 2, 3, and 4. Therefore, only if the transmission. mode detected. by the NULL detector 20 is coincident with the transmission mode permitted for the L band, the AFC is enabled and the seek operation is terminated if the frequency error converges to the predetermined value or lower in the predetermined time. In this manner, the ensemble to be sought can be correctly received. In the above-described embodiment and modifications, DAB broadcasting in Europe is used. The invention is not limited only to the DAB broadcasting, but is also applicable to other broadcasting and communications such as digital ground wave TV broadcasting and digital satellite broadcasting. According to the invention, when the NULL symbol is detected at some reception frequency during the seek operation and if the transmission mode detected by the transmission mode detecting means is coincide with the transmission mode permitted for the digital broadcast __24 _ £ CA 02265259 1999-03-l2 signal to be sought, the seek operation is terminated to correctly receive the ensemble to be sought. 10 15 20 25

Claims (3)

CLAIMS:

1. A digital broadcast receiver comprising:
reception means (3-11) for receiving a digital broadcast signal of an OFDM modulated wave in the tuned broadcast frequency;
deriving means (30-32) for deriving carrier-components from an output of the reception means;
program information demodulating means (36-40) for demodulating information part (FIC,MSC) of the derived carrier-components to recover a program desired by an user;
frequency error by referring to a correlation function calculated from control part (PRS) of the derived carrier-component and a reference code;
frequency adjusting means (35) for adjusting the tuning frequency in the reception means to eliminate the detected tuning frequency error;
NULL detecting means or detecting a NULL symbol in the received signal; and control means (37A) for in response to a seek instruction controlling the reception means to sequentially tune each of broadcast frequencies of the digital broadcast signal and stop the seek operation when the NULL detecting means detects the NULL
symbol at one of the sequentially tuned broadcast frequencies and then controlling the frequency adjusting means to conduct the tuning frequency adjustment at said one of broadcast frequency, CHARACTERIZED IN THAT
said NULL detecting means measures the whole time period (Td) of NULL symbol, determines a transmission mode according to the measured time period of NULL symbol, and outputs a transmission mode signal indicative of the determined transmission mode, said control means in response to the transmission mode signal controls the reception means to stop the seek operation when the determined transmission mode coincides with a transmission mode of the digital broadcast signal aimed in the seeking.

2. A digital broadcast receiver according to claim 1, wherein said control means further judges whether or not the tuning frequency adjustment by the frequency adjusting means while the seek operation is being stopped is within a predetermined tuning frequency error (.DELTA.f) after a preselected time period has elapsed, and controls the seek operation according to the further judgment.

3. A digital broadcast receiver according to claim 1 or claim 2, wherein said control means controls the frequency adjusting means to turn off the tuning frequency adjustment operation by the frequency adjusting means during the seek operation.